CHE 4341 Exam 3

Carbohydrates (Sugars) are

Polyhydroxy Aldehydes and Ketones

Carbohydrates (Carbo-hydrates) b/c they have the

general formula (C●H₂O)_n. (= water + carbon), n ≥ 3

Classes of Carbohydrates

Monosaccharides, Oligosaccharides and Polysaccharides

Monosaccharides

single sugars freely soluble in water. Sweet taste.

Oligosaccharides

short chains of single sugars covalently linked

Polysaccharides

long chains (hundreds or thousands) of single sugars covalently linked.

All monosaccharides (except dihydroxyacetone) are

chiral (i.e. have 1 or more asymmetric carbon atoms )

Fisher notation determined to a standard reference compound = glyceraldehyde; most →Most naturally occurring carbohydrates are

D; although L- occur

Recognizing D vs L → Only mirror image isomer (enantiomer) gives exact opposite Fischer projection!

Look at first chiral carbon; if OH on left → L-glucose (Same applies for Haworth Projections)

Monosaccharides D-aldoses:

NEED TO KNOW BOXED STRUCTURES!

D-Glyceraldehyde

Aldose Structure Mnemonic

D-Glucose

Most important sugar in nature; blood sugar

D-Mannose

D-Galactose

Constituent of lactose

D-Lyxose

All in a row are diastereomers of each other

D-Ribose

D-Xylose

Plant Polysaccharides

D-Threose

All D sugars b/c OH on right for last chiral C

Epimers

A special subset of diastereoisomers when 2 sugars differ only in the configuration around one carbon.

eg of Epimer

D-mannose is a C-2 epimer of D-glucose; D-galactose is a C-4 epimer of D-glucose → D-galactose and D-mannose are NOT epimers

Monosaccharides General Structure:

• Same as for aldoses

• except switch C1 C=O and C2 OH

• (results in one less chiral C)

Cyclization of Glucose and Fructose

Ring Structures- Anomers → isomers forned when

the ring forms: OH up = a & OH down = b; the new chiral hemiacetal/hemiketal C is referred to as the anomeric carbon

Anomers Example

In solution get equilibrium between anomers through

linear form (>C=O)

mutarotation (or anomerization) is the

interconversion in solution between a and b forms via the open-chain structure.

The predominant anomer of glucose in soln. is the

pyranose b form!

To modify sugars,

Just convert OH to H at specified position!

Reactions of Carbohydrates: Acetal (Glycosidic linkage) Formation

Condensation of two monosaccharides gives disaccharide

Glycosidic linkage naming is based on the anomeric carbon in the

1st sugar & C-OH position # of the 2nd sugar

Disaccharides consist of

two monosaccharides joined covalently by an O-glycosidic bond/linkage

Characteristics of Disaccharides

• Hemiacetal OH can be a or b

• Only glycosidic bond is different– but different shaped molecules w/ very different properties!

Maltose is made

breakdown of starch of germinated cereal grains (malt)

Maltose and Cellobiose

Cellobiose

Made from breakdown of cellulose

Lactose

Milk sugar

Sucrose (cane sugar, table sugar)

composed of a-glucose and b-fructose

Maltose Summary

Glca(1→ 4)Glc

reducing

maltase

Plants (starch) Animals (glycogen)

Cellobiose Summary

Glcb(1→ 4)Glc

reducing

cellulase

Plants (cellulose dimer)

Lactose Summary

Gal b(1 → 4)Glc

reducing

Lactase

Milk (major energy source)

Sucrose Summary

Glca(1 → 2) bFru

non-reducing

Sucrase (invertase)

Fruits seeds roots and honey

Anomeric bond, whether a or b, greatly influences the

shape of the molecule, which in turn influences enzyme activity on disaccharide

Mono/Disaccharides: Chemical & Physical Properties

• Almost all monosaccharides and disaccharides taste sweet

• undergo hydrogen bonding with themselves as well as with water

• solids at room temperature and very soluble in water

• Highly concentrated solutions of monosaccharides like honey, maple syrup and molasses are very viscous liquids.