Kinetic Redox Properties of Cobamides

With a one-electron transfer reaction of a cobalt-corrinoid complex cleavage or formation of a bond to an axial ligand occurs. A reduction is accompanied by an expulsion, and an oxidation by the coordination, of the ligand [90]. The electron transfer step accordingly takes place either in a concerted fashion or in a rapid associated step with coordination or dissociation of the axial Hgand. 

Electron transfer in the protonated Co(ll)-/Co(I)-couple Bi2r-H+ (23-H+)/ Bi2s-H (40-H) is fast in aqueous solution 0.1 cms ) as the presmned axial water hgand is only kinetically weakly bound in the base-off Co(ll)-corrin 23-H+ [90,95[. However, when the aquo hgand in 23-H+ is substituted by a stronger axial hgand, e.g., by the nucleotide base as in base-on B12, the electron transfer is slowed down sufficiently so that its kinetics can be conveniently measured by cychc voltammetry [90,97,98]. For example in the Co(ll)-/Co(l)-redox couple 23/40 ks = 0.0002 cm s [90], the electron transfer is at least a thousand times slower than in the base-off forms 23-H+/40-H. 

The trend in kinetics for Co(III)-/Co(II)-couples follows the same trend as those for the corresponding Co(II)-/Co(I)-couples, albeit much slower. The Co(lll)-Co(ll)-couple aquocob(lll)alamin (4+)/Bi2r (23) has a rate constant for heterogeneous electron transfer of about 10 cms [90]. The electron transfer steps for the cyano-cob(III)- and cyano-cob(II)alamins 1-CN and 23-CN are slower still [90,104]. 

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