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Cyclic Structure of Fructose

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Recall that Benedict's reagent (CuSO4) can oxidize aldehydes with adjacent ... group of an aldose is oxidized under basic conditions to a carboxylate anion ... – PowerPoint PPT presentation

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Title: Cyclic Structure of Fructose


1
Cyclic Structure of Fructose
  • As a ketohexose, fructose forms a 5-membered ring
    when the hydroxyl on C-5 reacts with the carbonly
    on C-2

?-D-Fructose
?-D-Fructose
2
Dehydration of Carbohydrates The Molisch Test
  • Carbohydrates, like most alcohols, undergo
    dehydration reactions in the presence of
    concentrated sulfuric acid. Pentoses (five
    carbon sugars) give furfural, and ketohexoses and
    aldohexoses give substituted furfurals.

3
Oxidation of Monosaccharides
  • Recall that Benedicts reagent (CuSO4) can
    oxidize aldehydes with adjacent hydroxyl groups
  • The blue Cu2 ions in the Benedicts reagent are
    reduced to form a brick-red precipitate, Cu2O
  • Normally, ketones are not oxidized, however
    ketones with an adjacent hydroxyl group can
    rearrange to the aldehyde during reaction with
    Benedicts reagent
  • So, both aldoses and ketoses, in open chain form,
    can be oxidized by Benedicts reagent to form
    carboxylic acids
  • Sugars that can be thus oxidized are called
    reducing sugars

4
Oxidation to Aldonic Acids
  • the aldehyde group of an aldose is oxidized under
    basic conditions to a carboxylate anion
  • the oxidation product is called an aldonic acid
  • any carbohydrate that reacts with an oxidizing
    agent to form an aldonic acid is classified as a
    reducing sugar (it reduces the oxidizing agent)

5
Oxidation to Uronic Acids
  • Enzyme-catalyzed oxidation of the primary alcohol
    at C-6 of a hexose yields a uronic acid
  • enzyme-catalyzed oxidation of D-glucose, for
    example, yields D-glucuronic acid

6
D-Glucuronic Acid
  • D-glucuronic acid is widely distributed in the
    plant and animal world
  • in humans, it is an important component of the
    acidic polysaccharides of connective tissues
  • it is used by the body to detoxify foreign
    phenols and alcohols in the liver, these
    compounds are converted to glycosides of
    glucuronic acid and excreted in the urine

7
Reduction of Monosaccharides
  • Reduction of the carbonyl group of a
    monosaccharide (in open-chain form) produces a
    sugar alcohol, or alditol
  • D-Glucose is reduced to D-glucitol (also called
    D-sorbitol) using hydrogenation (H2 and a metal
    catalyst)

8
Reduction to Alditols
  • sorbitol is found in the plant world in many
    berries and in cherries, plums, pears, apples,
    seaweed, and algae
  • it is about 60 percent as sweet as sucrose (table
    sugar) and is used in the manufacture of candies
    and as a sugar substitute for diabetics
  • these three alditols are also common in the
    biological world

9
Formation of glycosides
  • Recall that an alcohol can react with a
    hemiacetal to form an acetal (a di-ether)
  • When an alcohol reacts with a cyclic hemiacetal
    of a monosaccharide the cyclic acetal product is
    called a glycoside
  • The new ether bond is called a glycosidic bond
  • Monosaccharides are linked together by glycosidic
    bonds to form disaccharides and polysaccharides
  • Alkyl glycosides can not undergo mutarotation,
    and so are not reducing sugars

?-D-Glucose Methanol
Methyl-?-D-glucoside
10
Formation of Glycosides
  • a cyclic acetal derived from a monosaccharide is
    called a glycoside
  • the bond from the anomeric carbon to the -OR
    group is called a glycosidic bond
  • mutarotation is not possible in a glycoside
    because an acetal, unlike a hemiacetal, is not in
    equilibrium with the open-chain
    carbonyl-containing compound
  • glycosides are stable in water and aqueous base,
    but like other acetals, are hydrolyzed in aqueous
    acid to an alcohol and a monosaccharide
  • glycosides are named by listing the alkyl or aryl
    group bonded to oxygen followed by the name of
    the carbohydrate in which the ending -e is
    replaced by -ide

11
Disaccharides
  • A disaccharide is formed when a hydroxyl group on
    one monosaccharide reacts with the anomeric
    carbon of another monosaccharide to form a
    glycosidic bond
  • Each disaccharide has a specific glycosidic
    linkage (depending on which hydroxyl reacts with
    which anomer)
  • The three most common disaccharides are maltose,
    lactose and sucrose
  • When hydrolyzed using acid or an enzyme, the
    following monosaccharides are produced

12
Maltose
  • Maltose (malt sugar or corn sugar) consists of
    two glucose molecules linked by an
    ?-1,4-glycosidic bond
  • It comes from partial hydrolysis of starch by the
    enzyme amylase, which is in saliva and also in
    grains (like barley)
  • Maltose can be fermented by yeast to produce
    ethanol
  • Maltose is also used in cereals, candies and
    malted milk
  • Because one of the glucose molecules is a
    hemiacetal, it can undergo mutorotation, and so
    maltose is a reducing sugar

13
Lactose
  • Lactose (milk sugar) consists of one glucose
    molecule and one galactose molecule linked by a
    ?-1,4 glycosidic bond
  • It comes from milk products (about 4-5 of cows
    milk)
  • Because the glucose is a hemiacetal, it can
    undergo mutorotation, and so lactose is a
    reducing sugar

14
Hydrolysis of Lactose
  • Some people dont produce enough lactase, the
    enzyme that hydrolyzes lactose, and so cant
    digest lactose
  • Many adults become lactose intolerant, and
    develop abdominal cramps, nausea and diarrhea
  • Lactase can be added to milk products (or taken
    as a supplement) to combat this problem

15
Sucrose
  • Sucrose (table sugar) consists of one glucose
    molecule and one fructose molecule linked by an
    ?,?-1,2-glycosidic bond
  • Sucrose is the most abundant disaccharide and is
    commercially produced from sugar cane and sugar
    beets
  • Because the glycosidic bond in sucrose involves
    both anomeric carbons, neither monosaccharide can
    undergo mutorotation, and so sucrose is not a
    reducing sugar

16
Hydrolysis of Sucrose
  • Sucrose is hydrolyzed by the enzyme sucrase,
    which is secreted in the small intestine
  • The glucose and fructose can then be absorbed
    into the bloodstream (disaccharides are too large
    to be absorbed)

17
Fermentation
  • A fermentation is defined as an energy-yielding
    metabolic pathway with no net change in the
    oxidation state of products as compared to
    substrates
  • Yeast can ferment glucose, fructose, maltose and
    sucrose
  • Ultimately, glucose is converted to pyruvate
    through glycolosis, and the pyruvate is then
    converted to CO2 and ethanol by a two-step
    enzymatic process
  • The net reaction is
  • C6H12O6 ? 2C2H5OH 2CO2

18
Polysaccharides
  • A polysaccharide is a polymer consisting of
    hundreds to thousands of monosaccharides joined
    together by glycosidic linkages
  • Three biologically important polysaccharides are
    starch, glycogen and cellulose
  • - all three are polymers of D-glucose, but they
    differ in the type of glycosidic bond and/or the
    amount of branching
  • Starch and glycogen are used for storage of
    carbohydrates
  • - starch is found in plants and glycogen in
    animals
  • - the polymers take up less room than would the
    individual glucose molecules, so are more
    efficient for storage
  • Cellulose is a structural material used in
    formation of cell walls in plants

19
Plant Starch (Amylose and Amylopectin)
  • Starch contains a mixture of amylose and
    amylopectin
  • Amylose is an unbranched polymer (forms ?-helix)
    of D-glucose molecules linked by ?-1,4-glycosidic
    bonds
  • Amylopectin is like amylose, but has extensive
    branching, with the branches using
    ?-1,6-glycosidic bonds

20
Glycogen and Cellulose
  • Glycogen (animal starch) is like amylopectin,
    except its even more highly branched
  • - animals store glycogen in the liver (about a
    one-day supply in humans) and use it to maintain
    fairly constant blood sugar levels between meals
  • Cellulose is an unbranched polymer of D-glucose
    molecules linked by ?-1,4-glycosidic bonds
  • - cellulose forms ?-sheets of parallel strands
    held together by hydrogen bonding
  • - we dont have the enzyme to break down
    cellulose
  • - some animals have microorganisms that do have
    the enzyme

21
Iodine Test for Starch
  • The presence of starch can easily be identified
    using iodine (I2)
  • Rows of iodine atoms form in the core of the
    ?-helix of amylose, forming a dark blue complex
  • Because amylopectin, glycogen and cellulose do
    not form ?-helices, they do not complex well with
    iodine, so do not show the blue color (they show
    a purple or brown color)
  • Monosaccharides do not interact with the iodine,
    so no color is produced
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