Carbohydrates Lecture 2

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CarbohydratesLecture 2
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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

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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

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What is a Carbohydrate?
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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
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Monosaccharides

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

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Fischer Projections

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

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Which classification fits this sugar?
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Which classification fits this sugar?
Galactose
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Which classification fits this sugar?
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Which classification fits this sugar?
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Carbohydrate Ring Structures

• Many monosaccharides (most commonly pentoses and
  hexoses) have cyclic structures in aqueous
  solution

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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

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Fischer Haworth Projections

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

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Pyranose Ring Structure
An aldehyde can react with alcohol to form a
hemiacetal
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Pyranose Ring Structure
C1 aldehyde reacts with C-5 hydroxyl to form
intramolecular hemiacetal
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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

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Ring Structure Stereoisomers

• ? means -OH points down
• ? means OH points up

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Furanose Ring Structure
A ketone reacts with an alcohol to form
intramolecular hemiketal
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Furanose Ring Structure
C2 carbon reacts with C-5 hydroxyl to form
intramolecular hemiketal
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Furanose Pyranose
C1 aldehyde can react with C-4 or C5 hydroxyl
to form intramolecular hemiketal
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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
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Ring Structure Conformation

• Pyranose ring is not planar due to geometry of
  saturated C atoms
• Adopt 2 classes of conformation

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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

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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

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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

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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

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5
3
2
C3-endo
C2-endo
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Which 3 general terms apply to this sugar?
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What have you learnt?
Which 3 general terms apply to this sugar?
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Which 3 general terms apply to this sugar?
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Which 3 general terms apply to this sugar?
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Rasmol
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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

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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
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Modified Monosaccharides Sugar Alcohols

• Sugar alcohols have approx half calories of
  sugars
• Frequently used in low-calorie or sugar-free
  products

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Sugars in DNA

• Deoxyribose Nucleic Acid
• Comprised of nucleotides

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Sugars in DNA

• Sugar in DNA is 2-Deoxyribose

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Sugars in DNA

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

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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

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Sugars in DNA

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

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Sugars in DNA
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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