Studies on Cobalamins

A number of carbon skeletal rearrangements paralleling (in outline thou not in detail) the isomerizations of the B12 enzyme systems are known with various cobalt models. The methylmalonyl to succinyl type rearrangement, equation (9), 

The transfer of methyl from methylcobalamin to HgClg has been shown to involve Me transfer and initially MeHg+. The activation energy for the reaction was determined as 52.4 kJ mol, with 49.9, A5a —58.7 kJ mol . Alkylation of tin by alkylcobaloximes has also been studied in aqueous HCl-NaCl, when one equivalent of an oxidizing agent [Fe i or aquocobalaminfm) suffice] is required. The stoicheiometry with aquocobalamin is shown in equation (10). 

The mechanism proposed involves the generation by the oxidizing agent of a transient Sn species which cleaves the Co—C bond.  

A study of the electrochemistry of methylcobalamin reveals the existence of two one-electron reduction products both with the Co—C bond intact, which are presumably the base-on and base-off forms. In agreement with this view, methylcobinamide (in which the axial base is not present) shows a single such product. Methylcobalamin reacts with 2-mercaptoethanesulphonate (coenzyme M) in the pH range 6—14 with predominant iS-methylation, but at lower pH reductive cleavage of the Co—C bond affords methane. In contrast methyl-cobaloximes give methane almost exclusively and insignificent S -methylation.  

Some synthetic routes to methylcobalamin have been compared, and infrared studies on adenosylcobalamin and methylcobalamin contrasted with other derivatives. The coenzyme capabilities of Co-a[-a-(5-hydroxybenzamid-azolyl)]-Co-j3-adenosylcobamide and Co- -[- -(5-trifluoromethylbenzimid-azolyl)]-Co-i -adenosylcobamide with respect to the propane-1,2-diol system were studied and found to resemble the natural coenzyme. The co-ordinating 

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Early paper chromatographic studies on plasma cobalamin extracts used Whatman 2 paper with s c-butanol-acetic acid-water (100 3 50) as the mobile phase (Lindstrand and Stahlberg, 1963). More recent studies on cobalamin separations have used silica and cellulose TLC. For instance, Firth et al. (1968) separated a large number of organocobalamins by cellulose TLC with four different solvent systems. Lindemans and Abels (1985) provided tabular data on TLC of vitamin B derivatives on cellulose. 

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