Describe the isomerism shown by carbohydrates. Explain epimers, anomers and mutarotation with their significance.

Introduction

• A chiral carbon carries four different groups
• Number of possible isomers = 2n, where n is the number of chiral carbons
• Glucose has 4 chiral carbons → 24 = 16 isomers; fructose has 3 → 8

Types of Isomerism

D and L Isomerism

• Determined by the –OH on the penultimate (highest-numbered chiral) carbon — C-5 in a hexose
• –OH on the right = D form; on the left = L form, by reference to D- and L-glyceraldehyde
• Almost all naturally occurring sugars are of the D series
• D and L refer to configuration, not to the direction of optical rotation — D-fructose is in fact laevorotatory, which is why it was once called laevulose

Epimers

• The clinical importance is that galactose must be epimerised to glucose before it can be used — failure at any step of this pathway causes galactosaemia

Ring Structure, Anomers and Mutarotation

Open-chain glucose → The C-5 hydroxyl attacks the C-1 aldehyde → Forms an intramolecular hemiacetal → Six-membered pyranose ring (five-membered = furanose) → C-1 becomes a new chiral centre — the anomeric carbon → Two forms: α (–OH below the plane) and β (above)

• The anomeric carbon is C-1 in an aldose and C-2 in a ketose
• β-D-glucose is the more stable and predominates (about 64%) at equilibrium, since all its bulky groups are equatorial
• Haworth projections represent the ring; the chair conformation is the true shape

Mutarotation

• α-D-glucose has a specific rotation of +112°, β of +19°; the equilibrium mixture settles at +52.7°
• It proves that the ring opens and closes freely — which is exactly why the anomeric carbon can act as a reducing group
• Accelerated by acid, alkali and the enzyme mutarotase

Biological Significance of Configuration

• Enzymes are absolutely stereospecific — hexokinase phosphorylates D-glucose but not L-glucose
• α versus β determines digestibility: man can hydrolyse the α-1,4 bonds of starch but not the β-1,4 bonds of cellulose — the same monosaccharide, entirely different nutritional fate
• Only β-D-glucose is a substrate for glucose oxidase, which is why strips contain mutarotase
• Blood group antigens differ only in their terminal sugars and linkages

Optical Activity and its Measurement

• A solution of a chiral compound rotates the plane of polarised light
• Dextrorotatory (+ or d) rotates it clockwise; laevorotatory (− or l) anticlockwise
• Measured with a polarimeter; expressed as specific rotation
• Racemic mixture — equal amounts of both, giving no net rotation
• Examples — D-glucose is dextrorotatory (+52.7°), D-fructose is laevorotatory (−92°)
• Invert sugar — hydrolysis of sucrose (+66°) gives an equimolar mixture of glucose and fructose whose net rotation is negative; the sign inverts, hence the name. Honey is largely invert sugar

Applied Aspects

• Galactosaemia arises from failure to convert galactose to glucose — the clinical consequence of an epimerisation pathway defect
• L-sugars are not metabolised and have been explored as non-caloric sweeteners
• Polarimetry was historically used to measure urine glucose, exploiting optical rotation
• Enzymes distinguish stereoisomers absolutely, which is why L-glucose passes through the body unmetabolised while D-glucose is the central fuel of life
• Thalidomide is the classical warning from stereochemistry outside carbohydrate chemistry — one enantiomer sedative, the other teratogenic
• Glucose oxidase strips contain mutarotase, because the enzyme acts only on the β anomer and the sample contains both
• Honey is sweeter than sucrose because it is largely invert sugar, and fructose is the sweetest natural sugar
• Osazone crystals distinguish sugars in the laboratory, but glucose, fructose and mannose give identical crystals since they differ only at carbons involved in the reaction