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Heterofunctional carboxylic acids.

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Title: Heterofunctional carboxylic acids.


1
  • Lecture ? 9
  • Heterofunctional carboxylic acids.

Prepared by ass. Medvid I.I., ass. Burmas N.I.
2
  • Outline
  • Physical and chemical properties of oxoacids.
    Acetoacetic ester.
  • Physical and chemical properties of halogenacids
  • Physical and chemical properties of hydroxyacids.
  • Physical and chemical properties of phenolacids.
  • Physical and chemical properties of aminoacids.
  • Chloranhydrides of carbonic acid
  • a) Physical and chemical properties of a phosgene
  • 7. Amides of carbonic acid
  • Physical and chemical properties of an urea
  • Physical and chemical properties of a guanidine
  • 8. Sulfoacids
  • aliphatic sulfoacids
  • aromatic sulfoacids

3
  • 9. Aminoacids
  • 1). a-Aminoacids as structure components of
    proteins.
  • 2). Classification and structure of aminoacids.
  • 3). Chirality of aminoacids.
  • 4). Acid - base properties of aminoacids.
  • 5). Chemical properties of a-aminoacids.
  • 6). Indentification of aminoacids.
  • 7). Proteins. Structure of ? protein.
  • 8). Globular and fibrous proteins
  • 9). Simple and conjugated proteins.
  • 10.Peptides
  • a) acid-base properties
  • b) optical properties of peptides
  • c) chemical properties of peptides
  • d) synthesis of peptides.

4
  • The most important heterofunctional carboxylic
    acids are halogenocarboxylic acid (halogenoacid),
    hydroxycarboxylic acid (hydroxyacid)
    oxocarboxylic acid (aldehydo- and ketonoacid) and
    amino- carboxylic acids (aminoacids)

5
  • 1. Oxoacids
  • To oxoacids include aldehydo- and ketonoacids.
    These compounds include in the structure of the
    carboxyl group, aldehyde functional group or
    ketone functional group.

pyroracemic acid, 2-oxopropanoic acid
acetoacetic acid, 3-oxobutanoic
acid, ß-ketobutyric acid
glyoxylic acid, oxoethanoic acid
?-ketovaleric acid, 4-oxopentanoic
acid, levulinic acid
oxalacetic acid, oxobutanedioic
acid, ketosuccinic acid
6
  • Methods of extraction of oxoacids
  • Oxidation of hydroxyacids
  • Hydrolysis dihalogenocarboxylic acids

lactic acid
pyroracemic acid
2,2-dichlorpropanoic acid pyroracemic
acid (pyruvic acid)
7
  • Chemical properties of oxoacids
  • Decarboxylation of a-oxoacids
  • Decarboxylation of ß-oxoacids

8
  • Acetoacetic ester
  • Acetoacetic ester synthesis is a chemical
    reaction where ethyl acetoacetate is alkylated at
    the a-carbon to both carbonyl groups and then
    converted into a ketone, or more specifically an
    a-substituted acetone.
  • Acetoacetic ester is a tautomeric substance. He
    characterized keto-enol tautomery.

9
  • The enol form of "acetoacetic ester" stand by
    formation of hydrogen bond

10
  • Chemical properties of acetoacetic ester
  • Reactions of ketone form

11
  • 2. Reactions to enol form
  • interaction of acetoacetic ester with metallic
    sodium
  • interaction of acetoacetic ester with NaOH
  • c) interaction acetoacetic ester with PCL5

ethyl-3-chlorbutene-2-oate
12
  • d) interaction of acetoacetic ester with
    bromine water.
  • The discolouration of bromine water, that
    explained unsaturated of "acetoacetic ester.
  • e) interaction of acetoacetic ester with FeCL3

13
  • The characteristic feature of acetoacetic
    ester is the ability to ketone decomposition and
    acid decomposition .
  • Ketone decomposition occurs when heated in the
    presence of the dilute solutions of acids or
    alkalis.
  • Acid decomposition of acetoacetic ester

14
  • An acetoacetic ester used in the organic
    synthesis for the extraction of difference
    ketones and carboxylic acids.

15
  • 2. Halogenoacids
  • Halogenoacids are the derivatives of carboxyl
    acids that contain halogen radical (1 or more).
  • a-bromopropanoic acid
  • 2-bromopropanoic acid

2-bromo-3-methylbutanoic acid, a -
bromoisovaleric acid
16
  • Methods of extraction of halogenocarboxylic acid
  • Halogenation of saturated carboxylic acids
  • Hydrohalogenation of unsaturated carboxylic acids
  • Halogenation of aromatic carboxylic acids

acrylic acid
ß-chloropropanoic acid

m-chlorobenzoic acid
17
  • Physical and chemical properties of
    halogenocarboxylic acid
  • For physical properties of halogencarboxylic
    acids are colorless liquids or crystalline
    substance, soluble in water.
  • Chemical properties in the molecule of
    halogenoacids either carboxyl group or halogen
    radical can react.
  • As the halogen atom separation of carboxyl
    group inductive effect decreases, and so the
    acidity decreases. In the transition from mono-
    to di- and polyhalogencarboxylic acids the
    acidity increases. The most powerful of
    carboxylic acid is trifluoroacetic acid
    CF3-COOH (pKa 0,23)

18
  • I. Carboxyl group can react with formation of
  • Salts

chloroacetat sodium
19
  • b) complex ethers
  • c) amides

methyl ether of ß-chloropropanoic acid
amide ß-chloropropanoic acid
20
  • II. Halogen radical can react with
  • ammonium
  • b) NaOH (water solution)
  • 1) for a-halogenoacids

ammonium salt of ß-aminopropanoic acid
lactic acid
21
  • 2) for ß-halogenoacids
  • 3) for ?,s-halogenoacids

ß-chloropropanoic acid ß-hydroxypropanoic
acid acrylic acid
?-butyrolactone
22
  • Representatives of halogenocarboxylic acid

Monochloroacetic acid
Trichloroacetic acid
Dichloroacetic acid
These acids are used in organic synthesis
Ureide of a-bromisovaleric acid (bromisoval) used
in medical practice as a hypnotic.
23
  • 3. Hydroxyacids
  • Hydroxyacids are the derivatives of carboxyl
    acids that contain OH group (1 or more).

ß a
2-hydroxypropanoic acid a-hydroxypropanoic acid
24
glycolic acid, hydroxyacetic acid, hydroxyethanoic
acid
tartaric acid a,a-dihydroxysuccinic
acid, 2,3-dihydroxybutandioic acid,
lactic acid, a- hydroxypropanoic acid, 2-
hydroxypropanoic acid
malic acid, hydroxysuccinic acid hydroxybutanedioi
c acid
citric acid, 2-hydroxy-1,2,3-propantricarboxylic
acid
25
  • In a row of hydroxyacids often found the optical
    isomery.

D-, or (R,R)-tartaric acid
L-, or (S,S)-tartaric acid
mezo-, or (R,S)-tartaric acid
26
  • Methods of extraction of hydroxyacids
  • Hydrolysis of a-halogenoacids
  • Oxidations of diols and hydroxyaldehydes
  • Hydration of a,ß-unsaturated carboxylic acids

lactic acid
ß-hydroxypropanoic acid
27
  • 4. Hydrolysis of hydroxynitriles (cyanohydrins)

28
  • Physical and chemical properties of
    hydroxycarboxylic acid
  • For physical properties of hydroxycarboxylic
    acids are colorless liquids or crystalline
    substance, soluble in water.
  • Chemical properties in the molecule of
    hydroxyacids ether OH group or carboxyl group
    can react.
  • Carboxyl group can react forming
  • a) salts

sodium ß-hydroxypropanoic acid
29
  • b) complex ethers

methyl ether of ß-hydroxypropanoic acid
30
  • c) amides
  • II. OH group can react with
  • hydrohalogens (HCl, HBr, HI, HF)
  • b) can oxidize

amide of ß-hydroxypropanoic acid
ß-oxopropanoic acid
31
Related to heat of
1. a-hydroxyacids
lactic acid
lactide
2. ß-hydroxyacids
heating
3-hydroxybutanoic acid
butene-2-onic (crotonic) acid
32
  • 3. ?-hydroxyacids

heating
4-hydroxybutanic acid
?-butyrolacton
33
Decomposition of a-hydroxyacids
acetic acid
formic acid
34
  • Representatives of hydroxyacids
  • Milk acid . Milk acid is
    a trivial name because at
    first it was extracted from milk. It is present
    in kefir yogurt, sour milk and
    other milk products. It can form in muscles
    during hard and prolonged work. That is why
    peoples can feel ache in their muscles after
    physical training. Salts of milk acid are used in
    medicine.
  • Apple acid . It is present in green apples
    and some berries. It takes part in biological
    processes in human organisms and organisms of
    other alive creatures. In industry it is used for
    manufacturing of wine, fruit waters and sweets.
    It is used in medicine for synthesis of some
    medical preparations.
  • Tartaric acid . It is present in grape. It is
    used in medicine for synthesis of some
    medical preparations.

35
  • Citric acid . It is present in orange,
    lemon and other citric fruits. It takes part
    in biological processes in human
    organism.

36
  • 4. Phenolacids.

Phenolacids are the derivatives of aromatic
carboxyl acids that contain OH group (1 or more).
salicylic acid, 2-hydroxybenzoic acid
o-hydroxycinnamic acid
4-hydroxybenzoic acid
3,4,5-trihydroxybenzoic acid, gallic acid
37
  • Methods of phenolacids extraction
  • Carboxylation of phenols by carbon oxide (IV)
  • In the Kolbe synthesis, also known as the
    KolbeSchmitt reaction, sodium phenoxide is
    heated with carbon dioxide under pressure, and
    the reaction mixture is subsequently acidified to
    yield salicylic acid
  • 2. Hydroxylation of arencarboxylic acids

38
3. Alloying of sulphobenzoic acid with alkalis
m-sulphobenzoic acid potassuim
salt of
3-hydroxybenzoic acid
39
Chemical properties of phenolacids Chemical
properties of phenolacids due to the presence in
their structure of carboxyl group, phenolic
hydroxyl and the aromatic nucleus.
Decarboxylation
40
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41
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42
  • The best known aryl ester is O-acetylsalicylic
    acid, better known as aspirin. It is prepared by
    acetylation of the phenolic hydroxyl group of
    salicylic acid
  • Aspirin possesses a number of properties that
    make it an often-recommended drug. It is an
    analgesic, effective in relieving headache pain.
    It is also an antiinflammatory agent, providing
    some relief from the swelling associated with
    arthritis and minor injuries. Aspirin is an
    antipyretic compound that is, it reduces fever.
    Each year, more than 40 million lb of aspirin is
    produced in the United States, a rate equal to
    300 tablets per year for every man, woman, and
    child.

43
5. Aminoacids
  • An aminoacid is an organic compound that contains
    both a amino (N?2) group and a carboxyl (-????)
    group. The amino acids found in proteins are
    always a-amino acids.

44
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45
  • Methods of aminoacids extraction
  • Effects of ammonia on halogencarboxylic acids
  • Effects of ammonia and HCN on aldehydes

a-chlorpropanoic acid
a-aminopropanoic acid
a-aminopropanonitrile
acetalaldehyde
aldimine
a-aminopropanoic acid
46
3. Accession of ammonia to the a, ßunsatured
acids
acrylic acid
ß-aminopropanoic acid
4. Reduce of nitrobenzoic acid
n-nitrobenzoic acid n-aminobenzoic acid
47
Optical properties
48
Physical and chemical properties of aminoacids
  • Both an acidic group (-????) and ? basic group
    (-N?2) are present on the same carbon in an
    a-amino acid.
  • The net result is that in neutral solution, amino
    acid molecules have the structure
  • ? zwitter-ion is ? molecule that has ? positive
    charge on one atom and ? negative charge on
    another atom.

49
  • Reactions on a amino-group

50
  • Reactions to a carboxylic group

51
  • Heating of
  • a-aminoacids
  • ß-aminoacids

a-aminopropanoic acid
3,6-dimethyl-2,5-diketopiperazine
ß-aminooil acid
crotonic acid
52
3. ?-aminoacids
?-aminooil acid ?-lactam
53
  • React a-aminoacids with ninhydrin

54
  • Carbonic acid (ancient name acid of air or
    aerial acid) has the formula H2CO3. It is also a
    name sometimes given to solutions of carbon
    dioxide in water, which contain small amounts of
    H2CO3. The salts of carbonic acids are called
    bicarbonates (or hydrogen carbonates) and
    carbonates. It is a weak acid.
  • Carbonic acid is diprotic it has two hydrogen
    atoms which may dissociate from the parent
    molecule. Thus there are two dissociation
    constants, the first one for the dissociation
    into the bicarbonate (also called hydrogen
    carbonate) ion HCO3
  • H2CO3 ? HCO3 H
  • Ka1 2.5104  pKa1 3.45 /- 0,15 at 25
  • The second for the dissociation of the
    bicarbonate ion into the carbonate ion CO3²
  • HCO3 ? CO3 H
  • Ka2 5.6110-11  pKa2 10.25 at 25 C.

55
Functional derivates of carbonic acid.
56
6. Chloranhydrides of carbonic acid
Produces phosgene by interaction of carbon oxide
(II) with chlorine on the light.
57
Physical and chemical properties of a
phosgene Phosgene is the chemical compound with
the formula COCl2. This colorless gas gained
infamy as a chemical weapon during World War I,
and is also a valued industrial reagent and
building block in organic synthesis. In low
concentrations, its odor resembles freshly cut
hay or grass. Some soldiers during the First
World War stated that it smelled faintly of May
Blossom. In addition to its industrial
production, small amounts occur naturally by the
breakdown of chlorinated compounds and the
combustion of chlorine-containing organic
compounds
1. Hydrolysis of phosgene
58
  • 2. Interaction of phosgene with alcohols
  • 3. Interaction of phosgene with ammoium

59
7. Amides of carbonic acid
Esters of carbamic acid are named urethanes
60
Meprothan used in a medicine as a medicament,
which has tranquilization and hypnotic effects.
61
Urea or carbamide is an organic compound with the
chemical formula (NH2)2CO. The molecule has two
amine (-NH2) residues joined by a carbonyl (-CO-)
functional group. Urea was first discovered from
urine in 1773 by the French chemist Hilaire
Rouelle. In 1828, the German chemist Friedrich
Wöhler obtained urea by treating of silver
isocyanate with ammonium chloride in a failed
attempt to prepare ammonium cyanate
AgNCO NH4Cl ? (NH2)2CO AgCl
In the industry urea produces by interaction of
an ammonia with carbon oxide (IV)
62
Physical and chemical properties of an urea The
urea molecule is planar. Each carbonyl oxygen
atom accepts four N-H-O hydrogen bonds. This
dense and energetically favourable hydrogen-bond
network is probably established at the cost of
efficient molecular packing. The structure is
quite open, the ribbons forming tunnels with
square cross-section. The carbon in urea is
described as sp² hybridized, the C-N bonds have
significant double bond character, and the
carbonyl oxygen is basic compared to
formaldehyde. Its high solubility is due to
extensive hydrogen bonding with water up to
eight hydrogen bonds may form - two from the
oxygen atom, one from each hydrogen atom and one
from each nitrogen atom.
1. Interaction of an urea with strong acids
2. Hydrolysis of an urea during to heating
63
3. Interaction of an urea with halohenalkanes
(alkylation)
4. Interaction of an urea with halohenanhydrides
of carboxylic acids (acylation)
64
Dicarboxylic acids can form with an urea cycle
ureides. For example, barbituric acid or
malonylurea or 6-hydroxyuracil is an organic
compound based on a pyrimidine heterocyclic
skeleton. It is an odorless powder soluble in hot
water. Barbituric acid is the parent compound of
barbiturate medicine, although barbituric acid
itself is not pharmacologically active.
65
5. Interaction of an urea with HNO2
6. Interaction of an urea with water solution of
hypobromides. This reaction as the previous can
be used to quantitative determination of an urea.
66
5. Biuret reaction. Used for qualitative
determination of an urea and proteins, as
containing in its structure of a group?O-NH-.
67
By-product of a biuret reaction is the
isocyanuric acid, which forms as a result of
trimerazation of cyanuric acid.
68
Physical and chemical properties of a guanidine
1. Interaction a guanidine with acids
2. Interaction a guanidine with bifunctional
compounds (diesters, diketones)
69
The remain of guanidine is the structural
components of many compounds. For example
  • Arginine plays an important
    role in cell division, the healing of
    wounds, removing
    ammonia from the body,
    immune function, and the release
    of hormones.

Guanine is one of the five main nucleobases
found in the nucleic acids DNA and RNA.
70

  • Streptomycin is an antibiotic
    drug, the
    first of a class of drugs
    called aminoglycosides
    to be
    discovered, and was the first
    antibiotic remedy for
    tuberculosis.

Streptomycin - (IUPAC) name 5-(2,4-diguanidino- 3,
5,6-trihydroxy-cyclohexoxy)- 4-4,5-dihydroxy-6-(h
ydroxymethyl) -3-methylamino-tetrahydropyran-2-yl
oxy-3-hydroxy-2-methyl -tetrahydrofuran-3-carbald
ehyde
71
  • 8. Sulfoacids called the derivatives of
    organic compounds in which an atom of hydrogen
    replaced by the residue of sulfuric acid
    sulfogroup SO3H.
  • Aliphatic sulfoacids
  • ?H3-SO2OH
    ?H3-?H2-SO2OH
  • methanesulfonic acid
    ethanesulfonic acid
  • (methanesulfoacid)
    (ethanesulfoacid)

72
  • Functional derivatives of sulfoacids
  • CH3-SO2Cl - choranhydride of methanesulfoacid
    (methane
    sulfonylchloride)
  • CH3-SO2ONa sodium salt of methanesulfoacid
    (methanesulfate sodium)
  • CH3-SO2NH2 amide of methanesulfoacid
    (methanesulfonamide
    )
  • CH3-SO2-OC2H5 ethyl ester of methanesulfoacid
    (ethylmetanesulfonat
    e)

73
  • Sulfonic acids are typically much stronger acids
    than their carboxylic equivalents, and have the
    unique tendency to bind proteins and
    carbohydrates tightly most "washable" dyes are
    sulfonic acids (or have the functional sulfonyl
    group in them) for this reason. They are also
    used as catalysts and intermediates for a number
    of different products. Sulfonic acids and their
    salts (sulfonates) are used extensively in
    obtaining such diverse products like detergents,
    antibacterial drugs, medicine with derivatives of
    sulfonic acid, anion exchange resins (water
    purification), and dyes. The simplest example is
    methanesulfonic acid, CH3SO2OH, which is a
    reagent regularly used in organic chemistry.
    p-toluenesulfonic acid is also an important
    reagent.

74
  • Extraction of aliphatic sulfoacids
  • Sulfochlorination
  • Sulfooxidation
  • 2R-H 2SO2 O2 2R-SO2OH

  • alkanesulfonic acid
  • 3. Oxidation of thiols

75
  • 4. Sulfonation of alkanes by conc. H2SO4
  • 5. Accession of hydrosulfites to alkenes

76
  • Chemical properties of aliphatic sulfoacids
  • 1. Formation salts of sulfoacids
  • C2H5-SO2-OH NaOH C2H5-SO2-ONa H2O
  • 2 C2H5-SO2-OH 2 Na 2 C2H5-SO2-ONa H2
  • 2. Formation of sulfonylchlorides
  • R-SO2-OH PCl5 R-SO2Cl POCl3 HCl
  • 3. Formation of sulfonamides
  • R-SO2-Cl 2 NH3 R-SO2-NH2 NH4Cl
  • 4. Formation esters of sulfoacids
  • R-SO2-Cl 2 NaO-R' R-SO2-O-R' NaCl

77
Aromatic sulfoacids
78
Extraction of aromaric sulfoacids
1. Sulfonation of aromatic ring
79
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80
  • Chemical properties of aromatic sulfoacids
  • Reactions of the sulfogroup
  • a) formation salts of sulfoacids
  • C6H5SO2OH NaOH C6H5SO2Na H2O
  • b) formation of sulfonylchlorides
  • C6H5SO2OH PCl5 C6H5-SO2-Cl POCl3 HCl
  • C6H5 2 HO-SO2Cl C6H5-SO2Cl H2SO4 HCl
  • c) formation of sulfonamides
  • C6H5SO2Cl 2 NH3 C6H5-SO2-NH2 NH4Cl
  • d) formation esters
  • C6H5SO2Cl HO-C2H5 C6H5-SO2-O-C2H5 HCl
  • e) reduced of the sulfogroup

81
f) synthesis of a saccharin
82
II. Reactions SE, SN of sulfogroup
a) desulfonation
b) a reaction of alkalic floating
83
III. Substitution reaction of atom H in the
benzene nucleus
84
7.Sulphanylamidic preparations
  • Sulfanilamide is a molecule containing the
    sulfonamide functional group attached to an
    aniline. Sulfanilamide is a sulfonamide
    antibiotic. The sulfonamides are synthetic
    bacteriostatic antibiotics with a wide spectrum
    against most gram-positive and many gram-negative
    organisms. However, many strains of an individual
    species may be resistant. Sulfonamides inhibit
    multiplication of bacteria by acting as
    competitive inhibitors of p-aminobenzoic acid in
    the folic acid metabolism cycle. Bacterial
    sensitivity is the same for the various
    sulfonamides, and resistance to one sulfonamide
    indicates resistance to all. Most sulfonamides
    are readily absorbed orally. However, parenteral
    administration is difficult, since the soluble
    sulfonamide salts are highly alkaline and
    irritating to the tissues. The sulfonamides are
    widely distributed throughout all tissues. High
    levels are achieved in pleural, peritoneal,
    synovial, and ocular fluids. Although these drugs
    are no longer used to treat meningitis, CSF
    levels are high in meningeal infections. Their
    antibacterial action is inhibited by pus.
    Mechanism of action Sulfanilamide is a
    competitive inhibitor of bacterial
    para-aminobenzoic acid (PABA), a substrate of the
    enzyme dihydropteroate synthetase. The inhibited
    reaction is necessary in these organisms for the
    synthesis of folic acid. Indication For the
    treatment of vulvovaginitis caused by Candida
    albicans

85
  • Sulphanylamidic preparations. All
    sulphanylamidic medicines contain the next
    fragment
  • Albucyde (sulphacyl) is an antibacterial mean,
    is a part of eye-drops.
  • Urosulphane is an antibacterial mean by
    infection of urinal canals.
  • Norsulphazol is used by pneumonia, meningitis,
    staphylococcal and streptococcal sepsis,
    infectious diseases.
  • Bucarbane is a hypoglycemic mean.

Albucyde Urosulphane Norsulphazol
Bucarbane (sulphacyl)
86
9. Aminoacids. 1). a-Aminoacids as structure
components of proteins.
  • Next to water, proteins are the most abundant
    substances in most cells - from 10 to 20 of the
    cells mass. All proteins contain the elements
    carbon, hydrogen, oxygen, and nitrogen most also
    contain sulfur. The presence of nitrogen in
    proteins sets them apart from carbohydrates and
    lipids, which generally do not contain nitrogen.
    The average nitrogen content of proteins is 15.4
    by mass. Other elements, such as phosphorus and
    iron, are essential constituents of certain
    specialized proteins. Casein, the main protein of
    milk, contains phosphorus, an element very
    important in the diet of infants and children.
    Hemoglobin, the oxygen-transporting protein of
    blood, contains iron.

87
  • The word protein comes from the Greek proteios,
    which means "of first importance." This
    derivation alludes to the key role that proteins
    play in life processes.
  • ? protein is in polymer in which the monomer
    units are aminoacids. Thus the starting point for
    ? discussion of proteins is an understanding of
    the structures and chemical properties of
    aminoacids.
  • An aminoacid is an organic compound that contains
    both an amino (N?3) group and a carboxyl (-????)
    group. The aminoacids found in proteins are
    always a-aminoacids - that is, aminoacids in
    which the aminogroup is attached to the a-carbon
    atom of the carboxylic acid carbon chain.

88
  • The general structural formula for an a-aminoacid
    is
  • The R group present in an a-aminoacid is called
    the aminoacid side chain. The nature of this side
    chain distinguishes ?-aminoacids from each other.
    Side chains vary in size, shape, charge, acidity,
    functional groups present, hydrogen-bonding
    ability, and chemical reactivity.

89
  • 2). Classification and structure of aminoacids.
  • Over 700 different naturally occurring
    aminoacids are known, but only 20 of them, called
    standard aminoacids, are normally present in
    proteins. ? standard aminoacid is one of the 20
    a-aminoacids normally found in proteins.
    Aminoacids are grouped according to side-chain
    polarity. In this system there are four
    categories (1) nonpolar aminoacids, (2) polar
    neutral aminoacids, (3) polar acidicamino acids,
    and (4) polar basic aminoacids. This
    classification system gives insights into how
    various types of aminoacid side chains help
    determine the properties of proteins.

90
  • Nonpolar aminoacids contain one amino group, one
    carboxyl group, and a nonpolar side chain. When
    incorporated into ? protein, such aminoacids are
    hydrophobic (water fearing) that is, they are
    not attracted to water molecules. They are
    generally found in the interior of proteins,
    where there is limited contact with water. There
    are eight nonpolar aminoacids. The three types of
    polar aminoacids have varying degrees of affinity
    for water. Within ? protein, such aminoacids are
    said to be hydrophilic ("water-loving").
    Hydrophilic aminoacids are often found on the
    surfaces of proteins.

91
  • Polar neutral aminoacids contain one amino
    group, one carboxyl group, and ? side chain that
    is polar but neutral. The side chain is neutral
    in that it is neither acidic nor basic in
    solution at physiological pH. There are seven
    polar neutral aminoacids. Polar acidic aminoacids
    contain one amino group and two carboxyl groups,
    the second carboxyl group being part of the side
    chain. In solution at physiological ??, the side
    chain of ? polar acidic aminoacid bears ?
    negative charge the side-chain carboxyl group
    has lost its acidic hydrogen atom. There are two
    polar acidic aminoacids aspartic acid and
    glutamic acid. Polar basic aminoacids contain one
    amino groups and one carboxyl group, the second
    amino group being part of the side chain. In
    solution at physiological ??, the side chain of ?
    polar basic aminoacid bears ? positive charge
    the nitrogen atom of the amino group has accepted
    ? proton. There are three polar basic aminoacids
    lysine, arginine, and histidine.

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  • According to the chemical origin of the residue
    connected with a-aminoacid fragment
    CH(NH2)COOH, a-aminoacids divided on aliphatic,
    aromatic and heterocyclic.
  • In heterocyclic a-aminoacids proline and
    oxyproline a-aminoacids fragment presents in
    hetecyclic structure.

According to the quantity of NH2 and COOH
groups in molecule a-aminoacids divided on
monoaminocarbonic, monoaminodicarbonic and
diaminomonocarbonic.
93
  • Classification and structure of amino acids

94
  • The names of the standard aminoacids are often
    abbreviated using three-letter codes. Except in
    four cases, these abbreviations are the first
    three letters of the aminoacids name. In
    addition, ? new one-letter code for aminoacid
    names is currently gaining popularity
    (particularly in computer applications). These
    abbreviations are used extensively when
    describing peptides and proteins, which contain
    tens and hundreds of aminoacid units.

95
  • The essential aminoacids. All of the 20
    aminoacids are necessary constituents of human
    protein. Adequate amounts of 11 of the 20
    aminoacids can be synthesized from carbohydrates
    and lipids in the body if ? source of nitrogen is
    also available. Because the human body is
    incapable of producing 9 of these 20 acids fast
    enough or in sufficient quantities to sustain
    normal growth, these 9 aminoacids, called
    essential aminoacids, must be obtained from food.
    Essential aminoacids are aminoacids that must be
    obtained from food. An adequate human diet must
    include foods that contain these essential
    aminoacids. The human body can synthesize small
    amounts of some of the essential aminoacids, but
    not enough to meet its needs, especially in the
    case of growing children. The 9 essential
    aminoacids for adults are histidine, isoleucine,
    leucine, lysine, methionine, phenylalanine,
    threonine, tryptophan, and valine. (In addition,
    arginine is essential for children).

96
  • ? complete dietary protein contains all the
    essential aminoacids in the same relative amounts
    in which human being require them. ? complete
    dietary protein may or may not contain all the
    nonessential aminoacids. Most animal proteins,
    including casein from milk and proteins found in
    meat, fish, and eggs, are complete proteins,
    although gelatin is an exception (it lacks
    tryptophan). Proteins from plants (vegetables,
    grains, and legumes) have quite diverse aminoacid
    patterns, and some tend to be limiting in one or
    more essential aminoacids. Some plant proteins
    (for example, corn protein) are far from
    complete. Others (for example, soy protein) are
    complete. Thus vegetarians must eat ? variety of
    plant foods to obtain all of the essential
    aminoacids in appropriate quantities.

97
3). Chirality of aminoacids.
  • Four different groups are attached to the
    a-carbon atom in all of the standard aminoacids
    except glycine, where the R group is ? hydrogen
    atom. This means that the structures of 19 of the
    20 standard aminoacids possess ? chiral center at
    this location, so enantiomeric forms (left- and
    right-handed forms) exist for each of these
    aminoacids. With few exceptions (in some
    bacteria), the aminoacids found in nature and in
    proteins are isomers. Thus, as is the case with
    monosaccharides, nature favors one mirror-image
    form over the other. Interestingly, for
    aminoacids the L isomer is the preferred form,
    whereas for monosaccharides the n isomer is
    preferred.

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  • The rules for drawing Fischer projections for
    aminoacid structures follow
  • 1. The - ???? group is put at the top of the
    projection, the R group at the bottom. This
    positions the carbon chain vertically.
  • 2. The N?2 group is in ? horizontal position.
    Positioning it on the left denotes the L isomer,
    and positioning it on the right denotes the D
    isomer.

100
4). Acid - base properties of aminoacids.
  • In pure form, aminoacids are white crystalline
    solids with relatively high decomposition points.
    (Most amino acids decompose before they melt.)
    Also most aminoacids are not very soluble in
    water because of strong intermolecular forces
    within their crystal structures. Such properties
    are those often exhibited by compounds in which
    charged species are present. Studies of
    aminoacids confirm that they are charged species
    both in the solid state and in solution. Both an
    acidic group (-????) and ? basic group (-N?2) are
    present on the same carbon in an a-aminoacid.

101
  • We learned that in neutral solution, carboxyl
    groups have ? tendency to lose protons (?),
    producing ? negatively charged species
  • ???? ???- ?
  • We learned that in neutral solution, amino groups
    have ? tendency to accept protons (?), producing
    ? positively charged species
  • NH2 H NH3
  • As is consistent with the behavior of these
    groups, in neutral solution, the ???? group of
    an aminoacid donates ? proton to the NH2 of the
    same aminoacid. We can characterize this behavior
    as an internal acid base reaction. The net
    result is that in neutral solution, aminoacid
    molecules have the structure.

102
  • Such ? molecule is known as ? zwitterion, from
    the German term meaning double ion. ?
    zwitterion is ? molecule that has ? positive
    charge on one atom and ? negative charge on
    another atom. Note that the net charge on ?
    zwitterion is zero even though parts of the
    molecule carry charges. In solution and also in
    the solid state, a-aminoacids are zwitterions.
    Zwitterion structure changes when the pH of ?
    solution containing an aminoacid is changed from
    neutral either to acidic (low pH) by adding an
    acid such as ??1 or to basic (high pH) by adding
    ? base such as NaOH. In an acidic solution, the
    zwitterion accepts ? proton (?) to form ?
    positively charged ion.

103
  • In basic solution, the NH3 of the zwitterion
    loses ? proton, and ? negatively charged species
    is formed.
  • Thus, in solution, three different aminoacid
    forms can exist (zwitterion, negative ion, and
    positive ion). The three species are actually in
    equilibrium with each other, and the equilibrium
    shifts with pH change. The overall equilibrium
    process can be represented as follows
  • In acidic solution, the positively charged
    species on the left predominates nearly neutral
    solutions have the middle species (the
    zwitterion) as the dominant species in basic
    solution, the negatively charged species on the
    right predominates.

104
  • The previous discussion assumed that the side
    chain (R group) of an aminoacid remains unchanged
    in solution as the pH is varied. This is the case
    for neutral aminoacids but not for acidic or
    basic ones. For these latter compounds, the side
    chain can also acquire ? charge, because it
    contains an amino or ? carboxyl group that can,
    respectively, gain or lose ? proton. Because of
    the extra site that can be protonated or
    deprotonated, acidic and basic aminoacids have
    four charged forms in solution. The existence of
    two low-pH forms for aspartic acid results from
    the two carboxyl groups being deprotonated at
    different pH values. For basic aminoacids, two
    high-pH forms exist because deprotonation of the
    amino groups does not occur simultaneously. The
    side-chain amino group deprotonates before the
    a-amino group.

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  • The isoelectric point for an aminoacid is the
    pH at which the total charge on the aminoacid is
    zero. Every aminoacid has ? different isoelectric
    point. Fifteen of the 20 amino acids, those with
    nonpolar or polar neutral side chains, have
    isoelectric points in the range of 4.8 - 6.3. The
    three basic aminoacids have higher isoelectric
    points (His 7.59, Lys 9.74, Arg 10.76), and
    the two acidic aminoacids have lower ones (Asp
    2.77, Glu 3.22).
  • ? ?? below the isoelectric point favors the
    positively charged form of the aminoacid.
    Conversely, ? ?? above the isoelectric point
    favors the negatively charged form of the
    aminoacid.
  • When two electrodes (one positively charged and
    one negatively charged) are immersed in ?
    solution containing an aminoacid, molecules with
    ? net positive charge are attracted to the
    negatively charged electrode, and negatively
    charged aminoacid molecules migrate toward the
    positively charged electrode. The zwitterion form
    exhibits no net migration toward either
    electrode. This behavior is the basis for the
    measurement of isoelectric points. The pH of the
    solution is adjusted until no net migration
    occurs.
  • Mixtures of aminoacids in solution can be
    separated by using their different migration
    patterns at various pH values. This type of
    analytical separation is called electrophoresis.
    Electrophoresis is the process of separating
    charged molecules on the basis of their migration
    toward charged electrodes.

106
Methods of aminoscids obtaining
  • Proteins hydrolysis. It can be alkaline, acidic
    or fermentative hydrolysis. Widely use
    fermentative hydrolysis, for separation of
    a-aminoacids ionchange chromatography is used.
  • Microbiological synthesis. In these method use
    special microorganisms that produce a-aminoacids.

107
5). Chemical properties of a-aminoacids
  • Reaction on amino-group
  • 1) Formation of N-acylderivatives. This reaction
    use for blocking (protection) of aminogroup at
    the synthesis of peptides. As acylation agents
    use benzoxycarbonylchloride (a) or
    tret-butoxycarboxazide (b)

Blocked carbobenzoxygroup removed by catalytic
hydrogenolysis or by action of HBr in acetic acid
in cold.
108
  • Tret-butoxycarbonyl group destroyed by action of
    triftoracetic acid

109
  • 2) Deamination
  • oxidation deamination important pathway for the
    biodegradation of a-aminoacids
  • hydrolytic deamination reaction with nitrous
    acid. Aminoacids react with nitrous acid to give
    hydroxyacid along with the evolution of nitrogen.
  • The nitrogen can be collected and measured.
    Thus this reaction constitutes one of the methods
    for the estimation of amino acids.

110
  • c) intramolecular deamination - unsaturated acids
    are formed
  • d) redaction deamination saturated carboxylic
    acid formation

111
  • 3) Tranceamination. Reaction goes under the
    present of enzymes tranceaminases and coenzyme
    pyridoxalphosphate

4) Interaction with carbonyl compounds
112
  • 5) Reaction with phenylisothiocyanate (Edmane
    reaction). Form derivatives of 3-phenyl-2-thiohyda
    ntoine (derivatives of phenylthiohydantoine)

6) Interaction with 2,4-dinitroftorbenzol
(Senhers reagent)
113
B. Reaction on carboxyl group
  • 1) Formation of helate compounds ( complex salts
    with ions of heard metals)
  • 2) Reaction with alcohols difficult esters
    formation

114
  • 3) Reaction with ammonia amides formation. The
    amides of aspartic and glutamic acid acids,
    asparagine and glutamine, play important role in
    the transport of ammonia in the body.

4) Formation of halogenanhydrides and anhydrides
( like carbonyl acids). Before these reaction
blocked aminogroup by formation of
N-acylderivatives.
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  • 5) Decarboxylation. Aminoacids may be
    decarboxylated by heat, acids, bases or specific
    enzymes to the primary amines
  • Some of the decarboxylation reaction are of great
    importance in the body, decarboxylation of
    histidine to histamine
  • In the presence of foreign protein introduced
    into the body, very large quantities of histamine
    are produced in the body and allergic reactions
    become evident. In extreme cases shock may
    result. The physiological effects of histamine
    may be neutralized or minimized by the use of
    chemical compounds known as antihistamines.

116
  • C. Formation of salts. All aminoacids can react
    with some inorganic acids and bases and form two
    kind of sold

117
  • D. Peptide formation. Two aminoacids can react in
    ? similar way - the carboxyl group of one
    aminoacid reacts with the amino group of the
    other aminoacid. The products are ? molecule of
    water and ? molecule containing the two
    aminoacids linked by an amide bond. Removal of
    the elements of water from the reacting carboxyl
    and amino groups and the ensuing formation of the
    amide bond are better visualized when expanded
    structural formulas for the reacting groups are
    used.

118
In aminoacid chemistry, amide bonds that link
aminoacids together are given the specific name
of peptide bond. ? peptide bond is ? bond between
the carboxyl group of one aminoacid and the amino
group of another aminoacid. Under proper
conditions, many aminoacids can bond together to
give chains of aminoacids containing numerous
peptide bonds. For example, four peptide bonds
are present in ? chain of five aminoacids.
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  • Short to medium-sized chains of aminoacids are
    known as peptides. ? peptide is ? sequence of
    aminoacids, of up to 50 units, in which the
    aminoacids are joined together through amide
    (peptide) bonds. ? compound containing two amino
    acids joined by ? peptide bond is specifically
    called ? dipeptide three aminoacids in ? chain
    constitute ? tripeptide and so on. The name
    oligopeptide is loosely used to refer to peptides
    with 10 to 20 aminoacid residues and polypeptide
    to larger peptides. In all peptides, the
    aminoacid at one end of the aminoacid sequence
    has ? free H3N group, and the aminoacid at the
    other end of the sequence has ? free ???- group.
    The end with the free H3N group is called the
    N-terminal end, and the end with the free ???-
    group is called the ?-terminal end. By
    convention, the sequence of aminoacids in ?
    peptide is written with the N-terminal end
    aminoacid at the left. The individual aminoacids
    within ? peptide chain are called aminoacid
    residues.

120
  • The structural formula for ? polypeptide may be
    written out in full, or the sequence of
    aminoacids present may be indicated by using the
    standard three-letter aminoacid abbreviations.
    The abbreviated formula for the tripeptide
  • which contains the aminoacids glycine, alanine,
    and serine, is Gly Ala Ser. When we use
    this abbreviated notation, by convention, the
    aminoacid at the N-terminal end of the peptide is
    always written on the left.

121
  • The repeating chain of peptide bonds and
    a-carbon atoms in ? peptide is referred to as the
    backbone of the peptide. The R group side chains
    are substituents on the backbone. Peptides that
    contain the same aminoacids but in different
    order are different molecules (structural
    isomers) with different properties. For example,
    two different dipeptides can be formed from one
    molecule of alanine and one molecule of glycine.
  • In the first dipeptide, the alanine is the
    N-terminal residue, and in the second molecule,
    it is the ?-terminal residue. These two compounds
    are isomers with different chemical and physical
    properties.

122
  • The number of isomeric peptides possible
    increases rapidly as the length of the peptide
    chain increases. Let us consider the tripeptide
    Ala Ser Cys as another example. In addition
    to this sequence, five other arrangements of
    these three components are possible, each
    representing another isomeric tripeptide Ala
    Cys Ser, Ser Ala Cys, Ser Cys Ala, Cys
    Ala Ser, and Cys Ser Ala. For ?
    pentapeptide containing 5 different aminoacids,
    120 isomers are possible.
  • More than two hundred peptides have been
    isolated and identified as essential to the
    proper functioning of the human body. In general,
    these substances serve as hormones or
    neurotransmitters. Their functions range from
    controlling pain to controlling muscle
    contraction or kidney fluid excretion.

123
  • Two important hormones produced by the
    pituitary gland are oxytocin and vasopressin,
    Each hormone is ? nonapeptide (nine amino acid
    residues) with six of the residues hells in the
    form of ? loop by ? disulfide bond formed from
    the interaction of two cysteine residues.
  • Oxytocin regulates uterine contractions and
    lactation. Vasopressin regulates the excretion of
    water by the kidneys it also affects blood
    pressure. The structure of vasopressin differs
    from that of oxytocin at only two aminoacid
    positions the third and eighth aminoacid
    residues. The result of these variations is ?
    significant difference in physiological action.

124
  • 6). Indentification of aminoacids.
  • Biuret test. The protein is warmed gently with
    10 solution of sodium hydroxide and then ? drop
    of very dilute copper sulphate solution is added,
    the formation of reddish - violet colour
    indicates the presence of peptide link, ?? NH
    . The test is given by all proteins, peptones
    and peptides. Its name is derived from the fact
    that the test is also positive for the compound
    biuret, ?2? CONH CONH2 obtained from urea by
    heating. It should be noted that dipeptides do
    not give the biuret test, while all other
    polypeptides do so. Hence biuret test is
    important to know whether hydrolysis of proteins
    is complete or not. If the biuret test is
    negative, hydrolysis is complete, at least to the
    dipeptide stage.

125
  • Xanthoprotein test. On treatment with
    concentrated nitric acid, certain proteins give
    yellow color. This yellow color is the same that
    is formed on the skin when the latter comes in
    contact with the concentrated nitric acid. The
    test is given only by the proteins having at
    least one mole of aromatic aminoacid, such as
    tryptophan, phenylalanine, and tyrosine which are
    actually nitrated during treatment with
    concentrated nitric acid. When you add after
    conc. HNO3 conc. NaOH forms light orange color
    (hynoid structure).

126
  • Millon's test. Protein on adding Millon's
    reagent (? solution of mercuric and mercurous
    nitrates in nitric acid containing ? little
    nitrous acid) followed by heating the solution
    give ? red precipitate or colour. The test is
    responded by the proteins having tyrosine. The
    hydroxyphenyl group of tyrosine is the structure
    responsible for this test. Moreover, the
    non-proteinous material having phenolic group
    also responds the test.
  • Foll reaction. This reaction reveals the sulfur
    containing aminoacids (cysteine, cystine).
    Treatment of the sulfur containing aminoacids
    with salt of lead and alkali yields a black
    sediment.
  • Adamkevich reaction. This reaction detects the
    amino acid tryptophan containing indol ring. The
    addition of the concentrated acetic and sulfuric
    acids to the solution of tryptophan results in
    the formation of red-violet ring appearing on the
    boundary of different liquids.

127
  • Ninhydrin test. The ninhydrin colour reaction
    is the most commonly test used for the detection
    of aminoacids. This is an extremely delicate
    test, to which proteins, their hydrolytic
    products, and a-aminoacids react. Although the
    test is positive for all free amino groups in
    aminoacids, peptides, or proteins, the test is
    much weaker for peptides or proteins because not
    as many free groups are available as in
    aminoacids. For certain aminoacids the test is
    positive in dilutions as high as 1 part in
    100,000 parts of water. When ninhydrin is added
    to ? protein solution and the mixture is heated
    to boil, blue or violet color appears on cooling.
    The colour is due to the formation of ? complex
    compound.

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129
  • The test is also given by ammonia, ammonium
    salts, and certain amines. Ninhydrin is also used
    as ? reagent for the quantitative determination
    of free carboxyl groups in solutions of
    aminoacids.
  • Nitroprusside test. Proteins containing free -SH
    groups (of cysteine) give ? reddish colour with
    sodium nitroprusside in ammonical solution.

130
7). Proteins. Structure of the proteins.
  • Proteins are polypeptides that contain more
    than 50 aminoacid units. The dividing line
    between ? polypeptide and ? protein is arbitrary.
    The important point is that proteins are polymers
    containing ? large number of aminoacid units
    linked by peptide bonds. Polypeptides are shorter
    chains of aminoacids. Some proteins have
    molecular masses in the millions. Some proteins
    also contain more than one polypeptide chain. To
    aid us in describing protein structure, we will
    consider four levels of substructure primary,
    secondary, tertiary, and quaternary. Even though
    we consider these structure levels one by one,
    remember that it is the combination of all four
    levels of structure that controls protein
    function.

131
  • The primary structure of ? protein is the
    sequence of aminoacids present in its peptide
    chain or chains. Knowledge of primary structure
    tells us which aminoacids are present, the number
    of each, their sequence, and the length and
    number of polypeptide chains.
  • The first protein whose primary structure was
    determined was insulin, the hormone that
    regulates blood-glucose level ? deficiency of
    insulin leads to diabetes. The sequencing of
    insulin, which took over 8 years, was completed
    in 1953. Today, thousands of proteins have been
    sequenced that is, researchers have determined
    the order of amino acids within the polypeptide
    chain or chains.

132
  • The primary structure of ? specific protein is
    always the same, regardless of where the protein
    is found within an organism. The structures of
    certain proteins are even similar among different
    species of animals. For example, the primary
    structures of insulin in cows, pigs, sheep, and
    horses are very similar both to each other and to
    human insulin. Until recently, this similarity
    was particularly important for diabetics who
    required supplemental injections of insulin.
  • An analogy is often drawn between the primary
    structure of proteins and words. Words, which
    convey information, are formed when the 26
    letters of the English alphabet are properly
    sequenced. Proteins, which function biologically,
    are formed from the proper sequence of 20
    aminoacids. The proper sequence of letters in ?
    word is necessary for it to make sense, just as
    the proper sequence of aminoacids is necessary to
    make biologically active protein.

133
  • The secondary structure of ? protein is the
    arrangement in space of the atoms in the backbone
    of the protein. Three major types of protein
    secondary structure are known the alpha helix,
    the beta pleated sheet, and the triple helix. The
    major force responsible for all three types of
    secondary structure is hydrogen bonding between ?
    carbonyl oxygen atom of ? peptide linkage and the
    hydrogen atom of an amino group (-NH) of another
    peptide linkage farther along the backbone. This
    hydrogen-bonding interaction may be diagrammed as
    follows

134
  • The Alpha Helix The alpha helix (a-helix)
    structure resembles ? coiled helical spring, with
    the coil configuration maintained by hydrogen
    bonds between N ? and ? ? groups of every
    fourth aminoacid, as is shown diagrammatically in
    Figure.2.

135
  • Figure. Three representations of (?) the ?
    helix protein structure. Hydrogen bonds between
    amide groups (peptide linkages) are shown in (b)
    and (?). (d) The top view of an ? helix shows
    that amino acid side chains (R groups) point away
    from the long axis of the helix.
  • Figure. Two representations of the p pleated
    sheet protein structure. (?) ? representation
    emphasizing the hydrogen bonds between protein
    chains. (b) ? representation emphasizing the
    pleats and the location of the R groups. Proteins
    have varying amounts of a-helical secondary
    structure, ranging from ? few percent to nearly
    100 . In an a-helix, all of the aminoacid side
    chains (R groups) lie outside the helix there is
    not enough room for them in the interior.
    Figure.3d illustrates this situation. This
    structural feature of the a-helix is the basis
    for protein tertiary structure.

136
  • The beta pleated sheet (ß-pleated sheet)
    secondary structure involves aminoacid chains
    that are almost completely extended. Hydrogen
    bonds form between two different side-by-side
    protein chains (interchain bonds) as shown in
    Figure.3, or between different parts of ? single
    chain that folds back on itself (intrachain
    bonds). The term pleated sheet arises from the
    repeated zigzag pattern in the structure
    (Figure.3b). Aminoacid side chains are located
    above and below the plane of the sheet. Very few
    proteins have entirely n helix or p pleated sheet
    structures. Instead, most proteins have only
    certain portions of their molecules in these
    conformations. The rest of the molecule assumes ?
    "random structure." It is possible to have both ?
    helix and p pleated sheet structures within the
    same protein.

137
Secondary ß-structure of proteins contains
parallel (a) and antiparallel (b) fragment
138
  • Collagen, the structural protein of connective
    tissue (cartilage, tendon, and skin), has ?
    triple-helix structure. Collagen molecules are
    very long, thin, and rigid. Many such molecules,
    lined up alongside each other, combine to make
    collagen fibers. Cross-linking gives the fibers
    extra strength.

139
  • The tertiary structure of ? protein is the
    overall three-dimensional shape that results from
    the attractive forces between aminoacid side
    chains (R groups) that are widely separated from
    each other within the chain. ? good analogy for
    the relationships among the primary, secondary,
    and tertiary structures of ? protein is that of ?
    telephone cord. The primary structure is the
    long, straight cord. The coiling of the cord into
    ? helical arrangement gives the secondary
    structure. The supercoiling arrangement the cord
    adopts after you hang up the receiver is the
    tertiary structure.

140
  • Interactions responsible for the tertiary
    structure.
  • Four types of attractive interactions contribute
    to the tertiary structure of ? protein
  • 1) covalent disulfide bonds,
  • 2) electrostatic attractions (salt bridges),

141
  • 3) hydrogen bonds,

4) hydrophobic attractions.
142
  • All four of these interactions are interactions
    between aminoacid R groups. This is ? major
    distinction between tertiary-structure
    interactions and secondary-structure
    interactions. Tertiary-structure interactions
    involve the R groups of aminoacids
    secondary-structure interactions involve the
    peptide linkages between aminoacid units.
    Disulfide bonds, the strongest of the
    tertiary-structure interactions, result from the
    SH groups of two cysteine molecules reacting
    with each other to form ? covalent disulfide.
    This type of interaction is the only one of the
    four tertiary-structure interactions that
    involves ? covalent bond. That SH groups are
    readily oxidized to give ? disulfide bond, S
    S . Disulfide bonds may involve two cysteine
    units in the same chain or in different chains.

143
  • Figure. Four types of interactions between
    aminoacid R groups produce thetertiary structure
    of ? protein. (?) Disulfide bonds. (b)
    Electrostatic interactions (salt bridges). (?)
    Hydrogen bonds. (d) Hydrophobic interactions.
    Electrostatic interactions, also called salt
    bridges, always involve aminoacids with charged
    side chains. These aminoacids are the acidic and
    basic aminoacids. The two R groups, one acidic
    and one basic, interact through ion ion
    attractions. Figure.b shows an electrostatic
    interaction.

144
  • Hydrogen bonds can occur between aminoacids
    with polar R groups. ? variety of polar side
    chains can be involved, especially those that
    possess the following functional groups
  • Hydrogen bonds are relatively weak and are
    easily disrupted by changes in pH and
    temperature. Hydrophobic interactions result when
    two nonpolar side chains are close to each other,
    In aqueous solution, many proteins have their
    polar R groups outward, toward the aqueous
    solvent (which is also polar), and their nonpolar
    R groups inward (away from the polar water
    molecules). The nonpolar R groups then interact
    with each other. Hydrophobic interactions are
    common between phenyl rings and alkyl side
    chains. Although hydrophobic interactions are
    weaker than hydrogen bonds or electrostatic
    interactions, they are a significant force in
    some proteins because there are so many of them
    their cumulative effect can be greater in
    magnitude than the effects of hydrogen bonding.

145
  • An example of ? protein with quaternary
    structure is hemoglobin, the oxygen-carrying
    protein in blood. It is ? tetramer in which there
    are two identical a chains and two identical ß
    chains. Each chain enfolds ? heme group, the site
    where oxygen binds to the protein.
  • Figure. ? schematic diagram showing the
    quaternary structure of the oxygen-carrying
    protein hemoglobin.

146
  • 8). Globular and fibrous proteins.
  • On the basis of structural shape, proteins can
    be classified into two major types fibrous
    proteins and globular proteins. ? fibrous protein
    is ? protein that has ? long, thin, fibrous
    shape. Such proteins are made up of long
    rod-shaped or string-like molecules that can
    intertwine with one another and form strong
    fibers. They are water-insoluble and generally
    have structural functions within the human body.
    ? globular protein is ? protein whose overall
    shape is roughly spherical or globular. Globular
    proteins either dissolve in water or form stable
    suspensions in water, which allows them to travel
    through the blood and other body fluids to
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