Chemistry of Vitamin B12
Vitamin B12 is the only naturally occurring organic
compound which contains cobalt (35%). It is water soluble, deep red, tasteless,
crystalline compound with a molecular weight of 1355 and is hence stable at
neutral pH but not at alkaline pH.
The chemical Structure of vitamin B12 is given below
The salient features of the structure of Vitamin B12 are
given below.
There are many closely related compounds having B12
activity. All are cobalamins and contain in their molecules a portion called
corrin ring which to a large extent resembles the tetrapyrrole ring structure
of porphyrins.
A single cobalt atom having one positive charge is present
in the center of this ring. The Co atom is attached to all the four N atoms of
corrin ring in the same way as Fe++ is attached to 5,6,
dimethylbenzimidazole ribose which is attached to the side chain on ring IV
through Phosphate and aminopropanol.
The positive charge on Co atom is balanced by one of the
several groups which give rise to more than one type of vitamin B12. These
different types of vitamin B12 are given below along with their characteristic
groups attached to the Co atom.
B12: Cyanocobalamin:
It has CN. It is the commonly used form of the vitamin B12
B12-a: hydroxocobalamin: It has OH. It is claimed to be retained in the body
B12-b: aquocobalamin: It has H2O
B12-c: nitrocobalamin: It has NO2
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