33654_26b - Previous Page 1138 Cobalt Rhodium and Iridium...

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Ch. 26 1138 Cobalt, Rhodium and Iridium such redox series are known for these and other elements. 26.3.4 The biochemistry of ~obalt(~~) The wasting disease in sheep and cattle known variously as “pine” (Britain), “bush sickness” (New Zealand), “coast disease” (Australia), and “salt sick” (Florida) has been recognized since the late eighteenth century. When it was realized to be an anaemic condition it was thought to be due to iron deficiency and was therefore treated, with mixed success, by administering iron salts. Then, in the 1930s, it was found by workers in Australia and New Zealand that the efficacious principle in the iron treatment was actually an impurity (cobalt) but its role was not understood. This became more evident when vitamin BIZ was extracted from raw liver Vitamin B12: (a) a corrin ring showing a square-planar set of N atoms and and shown to be responsible for the latter’s well-known effectiveness in treating pernicious a replaceable H, and (b) simplified anaemia. It is now known that vitamin B12 structure of BI2. In view of the is a coenzyme? in a number of biochemical H displaced from the comn ring, the Co-C bond, and the charge on the ribose phosphate, the cobalt is processes, the most important of which is the formation of erythrocytes (red blood-cells). It formally in the +3 oxidation state. This obviously functions extremely effectively, the and related molecules are conveniently human body for instance containing a mere represented as: R 2-5 mg, concentrated in the liver. The structure of the diamagnetic, cherry-red vitamin BIZ is shown in Fig. 26.6 and it can be seen that the coordination sphere of the difference is apparent, however, in the sixth cobalt has many similarities with that Of iron cowdination position which, in haemoglobin, is in haem (See Fig. 25.7)- In both cases the either vacant or occupied by 02. Here it is filled metal is coordinated to 4 nitrogen atoms Of an by a a-bonded carbon,(35) making vitamin BI2 unsaturated macrocycle (in this case Part of a the first, and so far the only, fully established ‘‘Corrin” ring which is less Symmetrical and not naturally occurring organometallic compound. SO Unsaturated as the Porphyrin in haem) With an The usual methods of isolation lead to a product imidazole nitrogen in the fifth position. A major known as cyanocobalamin, which is the same as vitamin BIZ itself but with CN- instead 34 W. ~IM and B. SCHWERDERSKI, pp. 39-55 of Bioinor- of deoxyadenosine in the sixth coordination ganic Chemistlyrlnorgunic Elements in the Chemistry of Life, Wiley, Chichester, 1994; L. R. MILGROM, Chem. in Brit 31, 923-7 (1994). Figure 26.6 I 11113 position. This is a labile site, and other derivatives such as aquocobalarnin can be prepared. t Enzymes are proteins which act as very specific catalysts Incorporation of cobalt into the corrin ring in biological systems. Their activity may depend on the system modifies the reduction potentials of presence of substances, often metal complexes, of much lower molecular weight. These activators are known as “coenzymes”.
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33654_26b - Previous Page 1138 Cobalt Rhodium and Iridium...

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