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BIOC 2580 (94)

Carbohydrate Chemistry: simple sugars, rings and glycosides

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Department
Biochemistry
Course
BIOC 2580
Professor
E P Wijekoon
Semester
Fall

Description
BIOC 2580 Final Exam Notes 2. Carbohydrates Functions • Energy metabolism • Essential components of nucleic acids (DNA and RNA) Structure • Glycoproteins: sugars attached to proteins Monosaccharides Structure • Combine 2 organic chemical functional groups:  Carbonyl (C=O):  Ketone: Bonded to 2 other carbons  Aldehyde: bonded to 1 other Carbon and 1 Hydrogen  Hydroxyl groups (OH): alcohol groups  Have at least 2 C-OH • Therefore sugars are polyhydroxy aldehydes or polyhydroxy ketones • Ketoses: ketone sugars • Aldoses: aldose sugars aldehyde ketone Properties • Hydrophilic, polar • Poorly soluble in organic solvents (ether, hexane) • Colourless • Sweet to taste • Standard formula: (CH2O)n Trioses • Simplest monosaccharides • 3 C atoms: 1 carbonyl group, 2 Cs bearing hydroxyl groups • Hexoses are the most common monosaccharides in nature Chirality • Asymmetry or handedness • Needs to have 4 different constituents (cannot superimpose) *dihydroxyacetone in not chiral Enantiomers • Mirror images (left handed or right handed) • Have identical chemical properties • Have different optical activity  Plane of polarization when polarized light is passed through each is bent in different directions Right handed Left handed Diastereomers • Stereoisomers that differ in handedness at some C atoms but not others • Do not have identical chemical properties  Spatial arrangements among atoms are different *red C = chiral Number of Possible Stereoisomers • 2 – n = number of chiral centres 3 • E.g. 5 carbons, 3 Chiral centres = 2 =8 • Half will be D sugars, half will be L • Most sugars are D • Everytime C atom is added, # of isomers doubles D and L Sugars • D: chiral atom furthest away from carbonyl group has same configuration as D-glyceraldehyde  OH to the right • L: chiral atom furthest from carbonyl group has OH to the left Epimers • Pair of sugars identical except at 1 C atom • Just special case of diastereomers Fischer Projection Formula • Emil Fischer (1852-1911)  Studied analysis, synthesis, and stereochemistry of simple sugars  Introduced much of the terminology/notation we use • Use projection formula to represent 3D sugar structures on paper  C atoms in vertical row numbered from top to bottom  Vertical bonds project behind the plane, horizontal bonds project out Perspective Formula • Another way of representing sugar structures • Solid wedge shapes project in front • Dashed lines project away Reducing Sugars 2+ • Cup2+c ion (Cu ) can o+idize carbonyl groups of sugars to corresponding acids • Cu is reduced to Cu which forms red precipitate • Provides basis for detecting presence and concentration of certain sugars Reducing Sugars • Reacts with oxidants (to form red precipitate Non-reducing Sugars • Do not react with oxidants • Only open forms of sugars can undergo oxidation by cupric ions • If anomeric C is associated with in glycosidic bond, ring sugar cannot assume linear form Aldehydes and Ketones React with Alcohols • Hemiacetal: product of an aldehyde reacting with an alcohol • Hemiketal: product of a ketone reacting with an alcohol • Creates new chiral C atom (*) • Reactions are reversible Cyclization of Sugars • Su
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