[4] Polarography of compounds of the vitamin B12 series in dimethylformamide

This chapter discusses the polarography of compounds of the vitamin B12 series in dimethylformamide (DMF). The chapter describes the ac technique, which manifests considerable advantages in the case of electrochemically reversible processes, for example those of oxycobalamin (vitamin B12a). Polarography in an aprotic solvent— namely, DMF, seems to provide better opportunities for assay of vitamin B12 derivatives, particularly, 5′-deoxyadenosylcobalamin. In the assay method, cyanocobalamin, oxycobalamin, organocobalamins, and organocobinamides are reduced at a dropping mercury electrode in DMF solution, diffusion cathodic waves being observed. Reduction of oxycobalamin involves two reversible electron transfers. Each of the other corrinoids gives one irreversible two-electron wave. The cathodic waves of organocobalamins and organocobinamides as well as the well-defined second cathodic wave of oxycobalamin can be used for assay. In particular, if mixtures of oxycobalamin and an organocobalamin must be analyzed, a combination of dc and ac polarography is quite effective. Sensitivity of the analysis can be improved considerably by means of certain advanced polarographic techniques. Thus, gains by two orders of magnitude may well be expected if differential pulse polarography is used.