Vitamin B12 or cyanocobalamin was first isolated as an anti-pernicious anemia factor. It has long fascinated chemists because of its unique complex structure and diverse catalytic activities. Vitamin B12 is biologically inactive and its active forms are known as B12 coenzymes or cofactors which play important roles in the essential enzymatic reactions related to nucleic acid, protein, and lipid syntheses. Vitamin B12 is produced by microorganism (bacteria/fungi) and this is only vitamin which contain metal (cobalt). Plants do not produce or contain vitamin B12 and food sources include egg, meat, fish, milk and other dairy products.
Lenhert and Crowfoot-Hodgkin in 1961 discovered that B12 coenzyme contains an adenosyl group linked to the cobalt center by a direct CCo bond, which indicated, for the first time, presence of a metal-carbon bond in the biological systems (Fig. 1). The CoC bond in B12 coenzyme is considered to be one of the most stable σ-organocobalt bond ever reported. It was seen that, the macrocyclic ligand system in the vitamin B12, actually influences and modifies the properties of cobalt significantly, enabling it to form a highly stable Co–C bond. Crowfoot-Hodgkin was rewarded with the Nobel Prize in chemistry (1964) for determining the complex solid-state structure of the vitamin B12 (along with the structures of other molecules of biological significance, like, penicillin and cholesterol) by using X-ray diffraction spectroscopy.
The known B12 coenzymes/cofactors are alkyl cobalamins (RCbl), consisting of a cobalt complex of tetrapyrrole macrocyclic ligand (corrin ring) with a pendent nucleotide (intra-molecularly bound 5,6-dimethylbenzimidazole) which occupies the five coordination site of an octahedral Co(III), and the sixth position being occupied by different R groups in different cofactors, methylcobalamin (MeCbl, R = CH3) and coenzyme B12 (5’-deoxyadenosylcobalamin, AdoCbl, R = 5′-deoxyadenosyl). In vitamin B12, the sixth position is occupied by a CN ligand (cyanocobalamin, CNCbl, R = CN) and it is a biologically inactive species. The corrin ring system in vitamin B12 coenzymes is roughly planar and the short side chains (acetamide) extend above the corrin ring plane while the long side chains (propionamide) extend below the plane of ring. From the chemical point of view, alkyl cobalamins are stable and acid resistant, but thermo- and photo-labile organocobalt complexes. Vitamin B12 coenzymes show high reactivity only in presence of corresponding apoenzymes and their rate of labilization of Co–C bond could be increased by a factor 1013, this indicates that protein conformational changes play a major role toward Co–C bond stability and reactivity.
All known reactions of B12-dependent enzymes involve the making and breaking of the Co–C bond. The different modes of cleavage of the CoC σ bonds have been postulated for the two vitamin B12 coenzymes, AdoCbl and MeCbl. Enzymes containing adenosylcobalamin (AdoCbl) coenzyme require the homolytic cleavage of Co–C bond resulting in the formation of cob(II)alamin and 5′-deoxyadenosyl radical, e.g., isomerase and mutase enzymes, which catalyze the intramolecular 1,2-shift of a hydrogen atom and an electronegative group.
Methylcobalamin (MeCbl) is a cofactor in methyltransferase enzymes which require heterolytic cleavage of Co–C bond leaving both electrons on cobalt which results the formation of methyl carbocation and Co(I)alamin. These enzymes participate in the methyl transfer reactions, e.g., methionine synthase. (Fig. 3).
Vitamin B12 Coenzyme Models
After the pioneer work of Hodgkin, a large number of organocobalt compounds have been reported and some of these compounds have been proposed as models of vitamin B12. G. N. Schrauzer and Kohnle in 1964 reported that the reaction of B12 coenzyme can be simulated with much simpler Co(III) complexes of the monoanionic dimethylglyoxime (dmgH), R group and B. (Fig. 4).
Here, R is an organic group σ-bonded to cobalt, e.g., alkyls, and B is a neutral axial base ligand trans to Co–C bond, e.g., pyridine, H2O, etc. These compounds have been named as “cobaloximes” to stress their similarity with cobalamins. Numerous cobaloximes with different equatorial dioximes and axial ligands (R and B) have been synthesized to study the effect of steric and electronic nature of ligands on the Co–C bond stability. In addition, numerous other vitamin B12 analogues have been synthesized with variety of Schiff-base type ligands, e.g., BAE and SALEN (Figs. 5a and 5b). Complexes of the type [RCoIII((DO)(DOH)pn)B]+ (Fig. 5c) with a monoanionic tetradentate ligand was reported by Costa, et al. Cobalt complexes with porphyrins and tetraaza macrocyclic ligands (1,4,8,11-tetraazacyclotetradecane) (Fig. 5d) have also been studied as B12 model compounds.
Cobaloxime as Better Model of B12 Coenzyme
Though many model compounds are reported, it has been observed that the simple cobaloximes simulate the reactions of vitamin B12 coenzymes more closely. Equatorial dioximes model the corrin ring of B12 coenzymes and crystallographic data available on cobalamins suggested the structural effects of change in axial R group are similar to those found in cobaloximes. Theoretical calculations have also showed a close similarity between the cobalamin and cobaloximes. The two electron reduction of cobaloxime, [ClCo(dmgH)2py] produces Co(I), as super nucleophile, which upon reaction with CH3I gives [CH3Co(dmgH)2py]. This reaction is very similar to the B12 coenzyme chemistry. Also, the studies of effects imposed by equatorial dioximes and axial base on the properties of Co–C bond give insight to the homolysis and heterolysis cleavage of Co¬¬–C bond in B12 coenzymes. Apart from these, cobaloximes can easily be synthesized by one step alkylation of Co(I), generated in situ from the readily available and inexpensive starting materials (dimethylglyoxime and pyridine), whereas most of the other chelate systems demand the synthesis of ligand followed by metal complexation.
It is easy to incorporate ligands with diverse properties into the alkyl cobaloxime and this cannot be introduced so readily into other model systems. More importantly, the cobaloximes are ideal system for structural determination by NMR spectroscopy. All these advantages have led to extensive study of cobaloximes' properties to mimic the characteristics of B12 coenzymes. However, recent studies on cobaloximes have indicated that these have outgrown their initial relevance as a B12 model. They have acquired an independent research field because of their rich chemistry and versatile applications as precursors in organic synthesis and as catalysts in various organic transformation reactions including polymerization reactions.