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Carbohydrates Lecture 2

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Define stereochemistry and explain its importance in the structures of carbohydrates ... Axial groups can cause steric hindrance if they are on same side of the ring ... – PowerPoint PPT presentation

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Title: Carbohydrates Lecture 2


1
CarbohydratesLecture 2
2
Objectives
  • Explain why carbohydrates are important
  • Define what a carbohydrate molecule is
  • Recognise and classify carbohydrate molecules
  • Define stereochemistry and explain its importance
    in the structures of carbohydrates

3
Lecture Outline
  • Lecture 1 Monosaccharides
  • Definition
  • Nomenclature
  • Fischer projections
  • Lecture 2 Monosaccharides
  • Haworth projections
  • Furanose Pyranose rings
  • Modified Sugars
  • Sugars in DNA
  • Lecture 3 Dissaccharides Energy storage
  • Glycosidic bond
  • Glycolysis
  • Lecture 4 Complex Carbohydrates Function
  • Starch glycogen
  • Cellulose
  • Blood groups
  • Glycoproteins

4
What is a Carbohydrate?
5
What is a Carbohydrate?
  • Aldehyde or ketone compounds with multiple
    hydroxyl groups (OH)
  • Ketones both groups attached to carbonyl group
    are carbon
  • Aldehydes one carbon and one hydrogen attached
    to carbonyl group

Acetaldehyde
Acetone
6
Monosaccharides
  • Monosaccharides Polyhydroxyaldehydes or
    Polyhydroxyketones
  • Stereoisomers with identical functional groups
    linked in the same order but with different
    spatial orientation
  • Enantiomer Nonsuperimposable mirror images

7
Fischer Projections
  • Acyclic compounds drawn so that vertical bonds
    represent bonds pointing back and horizontal
    bonds are bonds pointing forward

8
Which classification fits this sugar?
9
Which classification fits this sugar?
Galactose
10
Which classification fits this sugar?
11
Which classification fits this sugar?
12
Carbohydrate Ring Structures
  • Many monosaccharides (most commonly pentoses and
    hexoses) have cyclic structures in aqueous
    solution

13
Haworth Projections
  • Cyclic structures depicted by 3D drawing adhering
    to a set of rules
  • Carbon atoms not explicitly drawn
  • Ring perpendicular to plane of paper heavy line
    projects towards reader
  • Haworth projection named after Walter Haworth who
    confirmed the structures of many complex sugars

14
Fischer Haworth Projections
  • Bonds pointing right in Fischer projection
    bonds pointing down in Haworth projection

15
Pyranose Ring Structure
An aldehyde can react with alcohol to form a
hemiacetal
16
Pyranose Ring Structure
C1 aldehyde reacts with C-5 hydroxyl to form
intramolecular hemiacetal
17
Ring Structure Stereoisomers
  • A new chiral carbon formed when monosaccharide
    cyclizes
  • 2 new stereoisomers are possible
  • ? means -OH points down
  • ? means OH points up
  • New chiral carbon anomeric carbon

18
Ring Structure Stereoisomers
  • ? means -OH points down
  • ? means OH points up

19
Furanose Ring Structure
A ketone reacts with an alcohol to form
intramolecular hemiketal
20
Furanose Ring Structure
C2 carbon reacts with C-5 hydroxyl to form
intramolecular hemiketal
21
Furanose Pyranose
C1 aldehyde can react with C-4 or C5 hydroxyl
to form intramolecular hemiketal
22
Carbohydrate Ring Structures
  • Most common 5 carbon ring furanose
  • Furan is derived from the furan ring
  • Furan 5 membered ring with 1 oxygen
  • Ose ending indicates multiple hydroxyls
  • Most common 6 carbon ring pyranose
  • Pyranose derived from pyran ring
  • Pyran ring is 6 membered ring with 1 oxygen

Furan
Pyran
23
Ring Structure Conformation
  • Pyranose ring is not planar due to geometry of
    saturated C atoms
  • Adopt 2 classes of conformation

24
Pyranose Chair Conformation
  • Groups on ring have 2 possible orientations
  • Axial nearly perpendicular to average plane
    of the ring
  • Equatorial nearly parallel to average plane
    of the ring

25
Pyranose Chair Conformation
  • Axial groups can cause steric hindrance if they
    are on same side of the ring
  • Chair form of ?-D-glucopyranose most prevalent as
    all axial positions occupied by hydrogen atoms

26
Furanose Ring Conformation
  • Furanose ring is not planar due to geometry of
    saturated C atoms
  • Ring is puckered with 4 atoms approx coplanar and
    5th 0.5Å away

27
Furanose Ring Conformation
  • Envelope form as structure resembles opened
    envelope with the flap raised
  • Either C2 or C3 out of plane on same side as C5

5
5
3
2
C3-endo
C2-endo
28
Which 3 general terms apply to this sugar?
29
What have you learnt?
Which 3 general terms apply to this sugar?
30
Which 3 general terms apply to this sugar?
31
Which 3 general terms apply to this sugar?
32
Rasmol
33
Modified Monosaccharides
  • Monosaccharides can be modified by natural or
    laboratory processes into compounds that retain
    basic configuration but have different functional
    groups
  • Commonly get reaction of monosaccharides with
    alcohols or amines
  • Modified sugars are called adducts

34
Modified Monosaccharides Sugar Alcohols
  • Sugar alcohols polyols, polyhydric alcohols,
    polyalcohols
  • Hydrated forms of aldoses or ketoses
  • Glucitol (sorbitol) has same linear structure as
    glucose but aldehyde group replaced by CH2OH

sorbitol
glucose
35
Modified Monosaccharides Sugar Alcohols
  • Sugar alcohols have approx half calories of
    sugars
  • Frequently used in low-calorie or sugar-free
    products

36
Sugars in DNA
  • Deoxyribose Nucleic Acid
  • Comprised of nucleotides

37
Sugars in DNA
  • Sugar in DNA is 2-Deoxyribose

38
Sugars in DNA
  • 4 different bases commonly found in DNA
  • Pyrimidines thymine and cytosine (uracil)
  • Purines adenine and guanine
  • Deoxyribose sugar joined to bases

39
Sugars in DNA
  • Sugar in DNA is 2-Deoxyribose 5 carbon sugar
    missing the OH at C2
  • C1 of deoxyribose connected to N1 of pyrimidines
    N9 of purines

40
Sugars in DNA
  • 5 O of one nucleotide linked to the 3 of next
    phosphate
  • Linked through a phosphate group
  • Sugar-phosphate backbone

41
Sugars in DNA
42
Lecture 2 Summary
  • Carbohydrate Ring Structures
  • Pyranose Ring
  • Furanose Ring
  • Drawing sugars
  • Haworth projections
  • Stereo chemistry
  • Modified Sugars
  • Sugar Alcohols
  • Amino Sugars
  • Sugar in DNA
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