Adocobinamide, the Base-off Analog of Coenzyme B₁₂ (Adocobalamin). 2.¹ Probing the “Base-on” Effect in Coenzyme B₁₂ via Cobalt−Carbon Bond Thermolysis Product and Kinetic Studies as a Function of Exogenous Pyridine Bases

Abstract

The thermolysis of the Co-C bond in adocobinamide (AdoCbi + BF 4 -) in anaerobic ethylene glycol has been studied as a function of a series of para-substituted pyridine axial bases using the TEMPO radical-trapping method. In contrast to the slower rates of Co-C cleavage previously found for benzylcobinamide, neopentylcobinamide, and the (R-phenylethyl)cobaloxime coenzyme B 12 models, for AdoCbi + the rate of total Co-C cleavage becomes faster as the para-substituted pyridines become more electron-donating. Specifically, the 110 °C k obsd for AdoCbi + BF 4 -total Co-C cleavage increased 23-fold on going from 1 M pyridine (py) to 1 M p-(dimethylamino)pyridine (Me 2 N-py). However, HPLC product studies reveal that the percentage of abiological Co-C heterolysis increases (to a limiting value); that is, Co-C heterolysis is a major reason for the observed rate increase seen for Me 2 N-py. Deconvolution of the k obsd rate constant into its heterolysis and homolysis components yields values of the 110 °C k heterolysis and k homolysis for AdoCbi + ‚base for Me 2 N-py and pyridine. These data in turn reveal that the AdoCbi + BF 4 -base-on homolysis rate constant does not increase within experimental error as one goes from py to the more basic Me 2 N-py (k on,h ) 8(3) × 10 -4 and 7(1) × 10 -4 s -1 , respectively), but that the base-on heterolysis rate constant changes by 17-fold (k on,het ) 0.4(0.1) × 10 -4 and 7(1) × 10 -4 s -1 for py and Me 2 N-py, respectively). The plausible biological significance of these results is then discussed, notably the heretofore unsubstantiated idea, first suggested by Mealli, Sabat, and Marzilli, that one major evolutionary pressure for selecting and appending the 5,6-dimethylbenzimidazole axial base in coenzyme B 12 is because this base limits Co-C bond heterolysis, thereby promoting the biologically relevant Co-C cleavage reaction, Co-C homolysis.

Spectroscopic Studies of the Corrinoid/Iron−Sulfur Protein from Moorella thermoacetica

“…24 The appearance of this band at 450 nm (22,200 cm −1 ) in the protein spectrum eliminates the possibility that the methoxybenzimidazolyl (MBI) moiety of Factor III m directly coordinates to the cobalt center, as the α-bands of alkylated corrinoids with N-donor ligands in the lower axial position consistently peak between 535 and 515 nm (18,700-19,500 cm −1 ). While this result suggests that the MBI ligand is displaced by a water molecule in the enzyme-bound cofactor, the α-band in the Me-Co 3+ CFeSP Abs spectrum is blue-shifted by 8 nm (390 cm −1 ) relative to that of free AdoCbi + (Figure 4), signaling, perhaps, a previously unknown mode of cofactor binding.…”

“…16,18 Numerous thermodynamic studies of alkylated corrinoid model complexes have shown that the nature of the lower ligand has a considerable influence on both the mode (i.e., homolytic vs heterolytic) and the rate of Co-C bond cleavage. [19][20][21][22][23] Consequently, the lower axial ligand is expected to play a key role in controlling both the reaction of Co 1+ CFeSP with CH 3 -H 4 folate and the transfer of the methyl cation from Me-Co 3+ CFeSP to the ACS A-cluster, and it is therefore of considerable interest to unambiguously establish the binding scheme of the corrinoid cofactor in CFeSP. To this end, we have employed our recently developed combined spectroscopic and computational methodology [24][25][26][27] to probe the electronic structures, and thus indirectly the axial ligand interactions, of the methylated and Co 2+ -containing forms of Factor III m in the CFeSP active site.…”

Cobalamin‐Catalysed Chemical Reactions: Probing the Role of the Nucleotide Loop

“…The structure-activity relationship studies reveal that in the studied cobalamin-catalysed reaction, the reduction step is not rate-determining. [14][15][16][17][18][19] Finke et al focused on the radical chemistry behind the cleavage of the Co-C bond and found that the DMB base-on form favours Co-C homolysis over heterolysis. Furthermore, it has been reported that with an increasing basicity of an α-axial ligand, the energy required to break the Co-C bond increases.…”

Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. III. Photoracemization of 1-cyanoethyl cobaloxime complexes

“…The B 12 -dependent enzymes are known to catalyze various intramolecular rearrangements of methyl, hydroxyl, amino and carboxyl groups, which are accompanied by the migration of H atoms. The hydrogen migration mechanism has generated interest in chemistry and biochemistry (Dolphin, 1982;Garr et al, 1996;Walker et al, 1998;Dong et al, 1999). Since the photoracemization of a cobaloxime complex with the chiral 1-cyanoethyl group was found to proceed with retention of the single-crystal form (Ohashi & Sasada, 1977), such crystallinestate reactions have been highly regarded because their reaction processes at the initial, intermediate and ®nal stages have been analyzed by single-crystal X-ray diffraction.…”