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Unformatted text preview: nitrogen in the fifth position. A major
34 W. ~
I and B . SCHWERDERSKI, 39-55 of B ioinorM
ganic Chemistlyrlnorgunic Elements in the Chemistry o Life,
Wiley, Chichester, 1994; L. R. MILGROM, hem. in Brit 31,
9 23-7 (1994).
t Enzymes are proteins which act as very specific catalysts
in biological systems. Their activity may depend on the
presence of substances, often metal complexes, of much
lower molecular weight. These activators are known as
“coenzymes”. Figure 26.6 Vitamin B12: (a) a corrin ring showing
a square-planar set of N atoms and
a replaceable H, and (b) simplified
structure of B I2. In view of the
H displaced from the comn ring,
the Co-C bond, and the charge on the ribose phosphate, the cobalt is
formally in the +3 oxidation state. This
and related molecules are conveniently
R I 11113 difference is apparent, however, in the sixth
cowdination position which, in haemoglobin, is
either vacant or occupied by 0 2 . Here it is filled
by a a-bonded carbon,(35) making vitamin BI2
the first, and so f ar the only, fully established
naturally occurring organometallic compound.
The usual methods of isolation lead to a product
known as cyanocobalamin, which is the same
as vitamin BIZ itself but with CN- instead
of deoxyadenosine in the sixth coordination
position. This is a labile site, and other derivatives
such as aquocobalarnin can be prepared.
Incorporation of cobalt into the corrin ring
system modifies the reduction potentials of
35 D. C. HODGKIN,roc. Roy. SOC.A 288, 2 94-305 (1965).
P Organometalliccompounds S635
2.. 1139 Bizs, are amongst the most powerful nucleophiles
known (hence, “supernucleophi...
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