Organic Chemistry

Section 22.20 Carbohydrates on Cell Surfaces 951

Although L-ascorbic acid does not have a carboxylic acid group, it is an acidic com-
pound because the of the C-3 OH group is 4.17. L-Ascorbic acid is readily oxidized
to L-dehydroascorbic acid, which is also physiologically active. If the lactone ring is
opened by hydrolysis, all vitamin C activity is lost. Therefore, not much intact vitamin
C survives in food that has been thoroughly cooked. Worse, if the food is cooked in
water and then drained, the water-soluble vitamin is thrown out with the water!

pKa

CH 2 OH

HOH

O

H H

CH 2 OH

HOH

O

O O

HO OH

oxidizing

enzyme

reducing

enzyme

rotate 180°

lactonase

oxidation oxidase

the synthesis of L-ascorbic acid

HO H

OH

HOH

HOH

CH 2 OH

H

HC O

HO H

OH

HOH

HOH

COOH

H

HC O

HO H

OH

HOH

HOH

COOH

H

CH 2 OH

HO H

H

HOH

HO H

CH 2 OH

HO

COOH

CH 2 OH

HOH

O

O O

OHHO

O

L-ascorbic acid

pK vitamin C

a^ = 4.17

L-dehydroascorbic acid a -lactone

D-glucose L-gulonic acid

L-configuration

VITAMIN C

Vitamin C traps radicals formed in aqueous envi-

ronments (Section 9.8). It is an antioxidant be-

cause it prevents oxidation reactions by radicals. Not all the

physiological functions of vitamin C are known. What is

known, though, is that vitamin C is required for the synthesis of

collagen, which is the structural protein of skin, tendons, con-

nective tissue, and bone. If vitamin C is not present in the diet

(it is abundant in citrus fruits and tomatoes), lesions appear on

the skin, severe bleeding occurs about the gums, in the joints,

and under the skin, and wounds heal slowly. The disease caused

by a deficiency of vitamin C is known as scurvy. British sailors

who shipped out to sea after the late 1700s were required to eat

limes to prevent scurvy. This is how they came to be called

“limeys.”Scurvy was the first disease to be treated by adjusting

the diet. Scorbutusis Latin for “scurvy”; ascorbic, therefore,

means “no scurvy.”

PROBLEM 28

Explain why the C-3 OH group of vitamin C is more acidic than the C-2 OH group.

22.20 Carbohydrates on Cell Surfaces

The surfaces of many cells contain short polysaccharide chains that allow the cells to
interact with each other, as well as to interact with invading viruses and bacteria. These
polysaccharides are linked to the cell surface by the reaction of an OH or an
group of a protein with the anomeric carbon of a cyclic sugar. Proteins bonded to poly-
saccharides are called glycoproteins. The percentage of carbohydrate in glycoproteins
is variable; some glycoproteins contain as little as 1% carbohydrate by weight, where-
as others contain as much as 80%.

NH 2