A major 34 w i and b schwerderski 39 55 of b ioinorm

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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 pp. ganic Chemistlyrlnorgunic Elements in the Chemistry o Life, f Wiley, Chichester, 1994; L. R. MILGROM, hem. in Brit 31, C 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 represented as: 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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This note was uploaded on 10/12/2012 for the course CHEMISTRY CHM1040 taught by Professor Jhon during the Spring '12 term at Florida A&M.

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