Chemistry Reference
In-Depth Information
12
Selected Vitamins
Vitamins represent a series of organic compounds, which are very crucial to people's
health. Some of them can be synthesized by the host organism, while many of them
must be obtained from food and are considered essential vitamins. Based on the
differences in biological and chemical activities, vitamins are classified into several
groups. Each group normally contains a set of compounds and possesses a corre-
sponding function in the body.
The name vitamin was coined as a term derived from “vitamine,” a compound
word from “vital” and “amine.” The classification of vitamin is based on biological
and chemical activity but not structure. There are mainly 13 types of vitamins (Vit):
A, B
1
, B
2
, B
3
, B
5
, B
6
, B
7
, B
9
, B
12
, C, D, E, and K. In this chapter, five vitamins are
discussed: Vit C, Vit D, Vit E, Vit B
6
, and Vit B
12
.
12.1 VITAMIN C
Vit C, also named ascorbic acid, is an essential nutrient for human health. Vit C
includes the L-enantiomer of ascorbic acid and its oxidized forms, which have activ-
ity in animals, including ascorbic acid and its salts, and some oxidized forms of
the molecule. For example, dehydroascorbic acid. D-ascorbate has equal antioxidant
power, but is not found in nature, and has no physiological significance, and far less
vitamin activity.
Ascorbic acid is a primary metabolite that is directly involved in normal growth,
development, and reproduction, and also plays a role as a redox cofactor and catalyst
in a broad array of biochemical reactions and processes. Vit C is present in veg-
etables; fruits; and animal organs such as liver, kidney, and brain.
Vit C can be absorbed in the body by active transportation to all types of cells
with the help of several biological transporters as well as simple diffusion. Ascorbate
and ascorbic acid are both natural forms in the body, and both of them can intercon-
vert depending on the pH of the environment. Vit C is a cofactor involved in at least
eight enzymatic reactions. Furthermore, due to its antioxidant properties, Vit C has
been widely used as a food additive, to prevent oxidation, and Vit C can also act as a
reducing agent, donating electrons to various enzymatic and nonenzymatic reactions.
12.2 STRUCTURAL FEATURES OF VITAMIN C
The structure of Vit C, with elaboration of the two stereochemical centers, is shown
in Figure 12.1. In the
1
H spectrum (300 MHz) of Vit C taken in D
2
O, there are
three peaks corresponding to three types of protons attached to their respective
135
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