Tuesday, July 3, 2007

Vitamin C

The structure of vitamin C, designated as a hex uronic
acid, was established in 1933 at the University of
Birmingham in England by Walter Haworth and
his associates, who also accomplished its synthesis.
Szent-Györgyi and Haworth renamed hexuronic acid
‘L-ascorbic acid’ to convey its antiscorbutic properties;
the new name was offi cially accepted in 1965.
Both Szent-Györgyi and Haworth were to be awarded
the Nobel Prize in 1937, the former for Physiology
and Medicine and the latter for Chemistry. Synthetic
ascorbic acid proved to have identical physicochemical
and biological properties to the vitamin C isolated
from plant or animal tissues, and there was no difference
in biological potency between the synthetic and
natural products. In 1934, Reichstein and Grüssner
in Switzerland worked out a chemical route for synthesizing
ascorbic acid commercially, starting from

The term ‘vitamin C’ refers to both ascorbic acid
and dehydroascorbic acid, since the latter oxidation product is reduced back to ascorbic acid in the body.
The principal natural compound with vitamin C activity
is L-ascorbic acid. There are two enantiomeric
pairs (mirror images) of the 2-hexenono-1,4-lactone
structure; namely, L- and D-ascorbic acid and L- and
D-isoascorbic acid (Fig. 19.1). D-Ascorbic acid and
L-isoascorbic acid are devoid of vitamin C activ- ity and do not occur in nature. D-Isoascorbic acid
(commonly known as erythorbic acid) is an epimer
of L-ascorbic acid, the structural difference being the
orientation of the hydrogen and hydroxyl group at
the fi fth carbon atom. D-Isoascorbic acid is also not
found in natural products, apart from its occurrence
in certain microorganisms. It possesses similar reductive
properties to L-ascorbic acid, but exhibits only
5% of the antiscorbutic activity of L-ascorbic acid in
guinea pigs (Pelletier & Godin, 1969).
At around neutral pH, ascorbic acid exists as the
ascorbate anion due to facile ionization of the hydroxyl
group on C-3. Ascorbate is easily and reversibly
oxidized to dehydro-L-ascorbic acid, forming the
ascorbyl radical (also known as semidehydroascorbate)
as an intermediate (Fig. 19.2). The delocalized
nature of the unpaired electron in the ascorbyl radical
makes it a relatively unreactive free radical and two
ascorbyl radicals can react together non-enzymatically
to produce ascorbate and dehydroascorbic acid. In
the body, enzymes are available to reduce the ascorbyl
radical and dehydroascorbic acid back to ascorbate.
Dehydroascorbic acid is not a true organic acid as
it contains no readily ionizable protons. In aqueous
solution, dehydroascorbic acid exists not as the
2,3-diketo compound, but as the bicyclic hemiketal
hydrate. In buffered solution at neutral or alkaline
pH, dehydroascorbic acid undergoes a non-reversible
oxidation in which the two rings open to give 2,3-
diketogulonic acid in a straight-chain structure.

Dietary sources of Vitamin C
Fresh fruits (especially citrus fruits and blackcurrants)
and green vegetables constitute rich sources
of vitamin C. Potatoes contain moderate amounts
but, because of their high consumption, represent the
most important source of the vitamin in the British
diet. Liver (containing 10–40 mg per 100 g), kidney
and heart are good sources, but muscle meats and cereal
grains do not contain the vitamin in measurable
amounts. Human milk provides enough ascorbic acid
to prevent scurvy in breast-fed infants, but preparations
of cow’s milk are a poor source owing to oxidative
losses incurred during processing.

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