Models cobalamin chemistry

Model studies in nitrogen fixation and cobalamin chemistry Nitrogen-fixing plants... 

Model Studies in Nitrogen Fixation and Cobalamin Chemistry... 

Cobalamins are essential enzymatic cofactors in human biochemistry. Coba-lamins chemical structure is based on the tetrapyrrole ring, while the chemical properties of the Co bond located in the centre of their moiety have been the focus of extensive research. Cyanocobalamin, the most known form of cobalamins, is rarely found in nature. Methylcobalamin and adenosylcobalamin are the two active forms of cobalamins in vivo in humans. Weakening of the Co C bond and its homolytic or heterolytic cleavage have been uncovered as an essential mechanism in the biochemical role of cobalamins as cofactors in humans. Recent studies using modem computational methods and application of quantum chemistry models have widened our knowledge of cobalamins biochemistry and are expected to contribute to our further understanding of cobalamin-dependent enzymes. 

The mechanistic and structural chemistry of B12 may be separated into (i) investigations of cobalamin cofactors both apart from and in complex with their enzymes, and (ii) biomimetic model complexes, both structural and functional. 

The preceding qualitative picture requires further investigation of both the energetics and structural changes in models and cobalamins. However, if it represents a reasonable approximation of the interrelationships among structure, coordination number, and axial bond strengths, then the intermediacy of 5-coordinate ComR species in the enzymatic process involving Co" and radical chemistry of coenzyme B -dependent enzymes appears unlikely. 

Numerous papers deal with the chemistry of vitamin B]2> cobalamins, cobinamides, and their synthetic models, the cobaloximes [Co(dmgH)2(R)(L)], including the synthesis of new species,225-228 structural studies,229-239 the stereochemistry of formation and isomerisation,240-244 and reactivity.245-248 jhe effects of viscosity,249 pressure, temperature,250,25l and base coordination252... 

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