The kinetics of benzimidazole dissociation in methylcobalamin

Unusually detailed insight into the dynamic behavior of coordinated ambidentate ligands. Factors influencing the binding mode of 2-aminopyridines in B12 models

“…The shortening of the latter bond as well as the widening of the Co-C-C angle is typical of Co-CH2R moieties in cobaloximes. 28 These values adhere to a trend in which the CH2-CH3 bond length decreases and the Co-C-C bond angle increases as L bulk increases (e.g., 1.519 (8) Á and 117.8 (4)°f or L = 4(MeO)C-(NH)py;14 1.479 (6) A and 117.8 (3)°f or L = PhNH2;23 1.317 (9) A and 126.2 (5)°f or L = Ph3P28). No significant trend is observed for Co-C bond lengths, however.…”

“…In all cases isomerization rates exceeded dissociation rates by at least an order of magnitude (Table I-III). Thus, ambidentate isomerization is mainly an intramolecular process with only a small contribution from ligand exchange processes as given by eq [4][5][6][7][8] and depicted in Figure 9 (with L = L')• For all compounds studied, changes in kobsd as R is varied follow the usual reactivity patterns already observed11•24•26•27 for a variety of other L; e.g., ¿obsd increases by a factor of ~2 X 103 when CH2CF3 is replaced by CH2CH3 (Table I).…”

Structural basis for carbapenamC-alkylation by TokK, a B12-dependent radical SAM enzyme

“…In TsrM, this structural feature is essential for the atypical polar methylation of its substrate, Trp, which requires heterolytic cleavage of the Co(III)-carbon bond of MeCbl. The bottom face of the Co ion in TsrM is adjacent to Arg69 but not directly coordinated, likely promoting nucleophilic attack of MeCbl by Trp by blocking coordination of a sixth ligand and destabilizing the Co(III)-C bond owing to charge-charge repulsion (Figure 3A) (23)(24)(25). In TokK, a different side chain, Trp76, occupies the lower axial face of the Cbl, residing 3.8 Å from the metal ion (Figure 3A).…”

Thermodynamic and Kinetic Data for the Base‐On/Base‐Off Equilibration of Alkylcobalamins

“…At the same time, the DMBz dissociation rate of these compounds can be monitored with stopped‐flow techniques, since, from the electron‐inductive effect of these X, the rate of DMBz dissociation is expected to be much slower for these complexes than for those with strongly donating X. The DMBz dissociation rates of other XCbl’s such as MeCbl and CF 3 CH 2 Cbl are much too fast to be monitored by the stopped‐flow technique 23,24…”