Electron spin resonance studies of biologically significant cobalt complexes, bio-organic radicals and radical-cations.
thesisposted on 19.11.2015, 08:47 by Ramakrishna D. N. Rao
This work is described in three major sections. (1) A Range of cobalamins and cobaloximes have been studied by electron-addition and photolysis methods with special reference to alkyl derivatives because of its role in several coenzymeB 12-dependent enzyme reactions. Photolysis of a range of alkylcobalamins and alkylcobaloximes at 77K resulted in the homolysis of Co-C bond to give Co(II) derivatives and the respective alkyl radicals. Methylcobalamin gave a radical-pair where the alkyl(solvent) radical was trapped at distance of 8.3A from Co(II), however radicals were not so precisely trapped in the case of methylcobaloxime. Electron-addition at 77K gave different results for cobalamins and cobaloximes. All alkylcobaloximes gave Co(I) derivatives upon annealing above ca.150K while cobalamins gave Co (II) derivatives. Several derivatives of cobalamins and cobaloximes have been studied by electron addition and photolysis and the nature of intermediates and products have been identified by e.s.r. The relevence of our work with respect to the role of cobalamins in enzymatic reactions and cobalt chemistry is discussed. Some studies on cobalt complexes of porphyrins, peptides and phthalocyanins have been done to compare with cobalamins and cobaloximes. (2) Studies on electron-addition to a range of disulphide containing proteins at 77K showed that electrons are efficiently trapped by disulphides to form RS-SR- type of radicals. (3) Radical-cations of some aromatic, heterocyclic, aliphatic and inorganic derivatives have been prepared using a recently developed technique of radiolysis of frozen freon solutions, and, they were studied by e.s.r spectroscopy. This method offers an interesting comparison to PES and theoretical results and thus it has helped to understand their electronic structure better.