Equilibrium constants are reported for several ligand replacement reactions of cobalamins and cobinamides (see Figure ) and are interpreted in terms of the thermodynamic trans-effect of the ligand which is not replaced. It is shown t h a t as this ligand is changed in the order H,O, NC-, HCEC-, H,C=CH-, CH,-, so the trans cobalt-cyanide bond becomes less stable relative t o the cobalt-benziminazole bond which in turn becomes less stable relative to the cobalt-water bond. The work involved the identification of certain cobalamins and cobinamides.* DBC is cr-(5,6-diniethylbenziminazolyl)cobinamide coenzyme.
Formation constants have been determined for the substitution of co-ordinated water in diaquocobinamide by ammonia and imidazole, in aquocobalamin by ammonia, and in cyanoaquocobinamide by ammonia, pyridine, imidazole, benzimidazole, and adenine; the last three bases can co-ordinate in the conjugate anion form. It is shown that the formation constants for the binding of ammonia are more sensitive to the nature of the trans-ligand than those for the binding of imidazole, and that the relative order of their formation constants can change as the frans-ligand is varied. Pseudo-first-order rate constants for the substitution of co-ordinated ammonia by water a t 30" have been obtained for corrinoids with the following trans-ligands: H,O, 1.4 x s-l; 5.6-dimethylbenzimidazole, 8.6 x 10J s-l; CN-, SO,*-, and CH,-, 2 3 x 10-1 s-l. The order of ligands observed in the kinetic trans-effect parallels that for the thermodynamic trans-effect.
Die Bildungskonstanten für die Substitution des koordinierten Wassers in dem Cobinamid (Ia) durch Ammoniak und Imidazol bzw. im Cobinamid (Ib) durch Ammoniak und Pyridin oder Imidazol, Benzimidazol und Adenin (neutral oder anionisch) wurden bestimmt.
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