Stability cobalamins

As will be noted later, it is commonly thought that homolytic cleavage of the Co—C bond is an important initial stage in the reactions of cobalamins. Accordingly, there has been much interest in the formation of Co—C bonds, the factors that determine their stability, and the cleavage of these Co—C bonds. 

Co3+ complexes of cobalamin have been cryoreduced by y-irradiation of aqueous-organic solutions at 77K, and the resulting Co2+ complexes have been characterized by EPR spectroscopy.73,74 Various nonequilibrium forms kinetically stabilized at low temperature gradually relaxed to the normal reduced complexes after annealing at 15 OK and higher. 

However, some indirect indication of the strength of alkyl-Co bonds in organo-cobalamin, relative to those in other alkyl-cobalt compounds, is provided by observations concerning the stability of ben-zylcobalamin. Attempts to prepare benzylcobalamin by either the Bi2s route (Reaction 31) (32) or the Bi2r route (Reactions 32-34) (15) have yielded spectroscopic evidence for its initial formation in solution. However, benzylcobalamin proved to be too unstable for isolation and... 

Finally, the apparent thermal stabilities of alkyl-cobalamins, as well as of some of the other transition-metal-alkyl compounds that have been examined in the course of these studies, generally are higher than would correspond to their metal-C bond-dissociation energies. The most probable explanation for this is that, in the absence of effective radical scavengers, homolytic dissociation of metal-alkyl bonds occurs reversibly because of selective recombination of the initially produced radicals and metal complexes. 

It is interesting that E. coli contains two genes that code for methionine synthase metH for the cobalamin-dependent enzyme and metE for a cobalamin-independent enzyme that depends on an active site Zn + to stabilize deprotonated homocysteine (24). This thiolate species demethylates A -methyl-tetrahydrofolate, which is activated by proton transfer to N-5. MetE is less active ( 100 x ) than MetH, and so in the absence of Bi2 E. coli it produces much more MetE to compensate for the lack of MetH. 

Strong bases (pKa > 11) also convert alkyl cobaloximes and alkyl cobalamins into -complexes such as 73. This is usually followed by further decomposition to olefins and alkanes. The stability of complexes such as 73 depends very much upon X and the nature of the axial ligand in the cobalt chelate.98-218-227 230 Strong nucleophiles such as RS or CN can cause decomposition of LCo—R as well.98-231 Under the normal conditions of radical polymerization, Markovnikov organocobaloxime should form whenever the hydride, LCoH, appears in the polymerization mixture. If 1,2-vinylidene monomers are being polymerized, then thermally unstable tert-alkyl-cobaloximes are obtained. These species are expected to undergo homolytic Co—C cleavage to yield tertiary radicals. 

NO adds reversibly to reduced cobalamin, Cbl(II).156 It does not react directly with aquacobalamin(III), (0blni(H2O)), but it does add to Cbl,n(N02 ) and Cblm(NO).175 Acid hydrolysis of the dinitroso species releases nitrite, and binding of nitrite to Cblln(H20) generates Cbln,(. 02 ). This sequence thus affords a nitrite-catalyzed mechanism for NO substitution at Cblln(H20). The reaction of NO with Com porphyrins is quite complex.176 In the first step, NO displaces an axial water ligand to form a weakly bound mono NO complex this mono NO complex reacts with a second molecule of NO to form nitrite and a reduced Co-NO complex. This latter process is called reductive nitrosylation. Manganese(II) porphyrins bind NO very rapidly.177 Stability constants have been measured for the formation of mono and bis NO complexes of Cun(dithiocarbamate)2.157... 

It has been proposed for the reactions of MCM and GM that the methylene radicals in the mechanisms are stabilized by magnetic interactions with low-spin Co in cob(II)alamin. " To date, the importance of such stabilization has not been established. The amount of stabilization has not allowed methylene radical intermediates such as the 5 -deoxyadenosyl radical to be observed by EPR spectroscopy. The only observable radicals to date have been the most chemically stable radicals in the mechanisms, so magnetic stabilization could not have led to leveling of the stabilities of the radical intermediates. The rates and activation parameters measured for the model radical isomerizations in Figure 24 were compatible with enzymatic rates. The results showed that participation of the cobalamin portion of the coenzyme would not be required to explain substrate isomerization. The energy of the magnetic interaction between cob(II)alamin and the free radical at the active site of MCM is 4000 G (0.37 cm ) or 1 cal mol (G. H. Reed, personal communication). This amount of energy would not be an important contribution to stabilization in a radical. 

It is likely that specific cobalamin-protein and adenosyl-protein interactions serve to weaken the Co-C bond, through stabilization of the cleaved state, thereby facilitating formation of the 5 -deoxyadenosyl radical. A clear understanding of this process will require much more structural work in the future. 

The cobalt atom in the cobalamin pentadentate ligand system, as well as in all the other simpler model systems capable of stabilizing carbon-cobalt bonds, may exist... 

Cobalamin has an extraordinary affinity for cyanide— greater, perhaps, than any of the aerobic respiratory enzymes—and the complex of cobalamin with cyanide is more stable than with other anions, with the possible exception of — SH. Cobalamin isolated as the cyano complex might, therefore, be partly an artifact, for (1) non-cyano cobalamins probably have full biological activity if allowance is made for their lesser stability, and (2) cyanide may have functions independent of Bu. 

Cyanocobalamin (Formula 6.17) was isolated in 1948 from Lactobacillus lactis. Due to its stability and availability, it is the form in which the vitamin is used most often. In fact, cyanocobalamin is formed as an artifact in the processing of biological materials. Cobalamins occur naturally as adenosylcobalamin and methylcobalamin, which instead of the cyano group contain a 5 -deoxyadenosyl residue and a methyl group respectively. 

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