NO Binding to Cobalamin:  Influence of the Metal Oxidation State

Abstract

Density functional and molecular orbital theory calculations on models for cobalamin suggest that NO binds similarly to the Co(II) and Co(III) oxidation states. However, Co(III) can bind water far more strongly than Co(II) as a sixth ligand, so that the competition between water and NO complexation strongly favors water for Co(III) in the gas phase. Although the Co(II) oxidation state is found to bind water slightly more strongly than NO in the gas phase, the inclusion of solvation effects via the polarizeable continuum model makes NO binding more favorable. Thus, the experimentally observed ability of cob(II)alamin to bind NO in aqueous solution is the result of its weak complexation with water and the relatively poor solvation of NO. Calculated vibrational frequencies support the interpretation of the cob(II)alamin-NO complex as being cob(III)alamin-NO-, although the DFT calculations underestimate the degree of charge transfer in comparison to Hartree-Fock calculations.

Nitroxylcob(III)alamin: Synthesis and X‐ray Structural Characterization

“…[13,14] The oxidation state of the Co center of NOCbl is also of considerable interest. [11,12,[15][16][17] Herein, we report the synthesis and X-ray structural characterization of NOCbl, which was prepared by reacting hydroxycobalamin hydrochloride with the NO donor 2-(N,Ndiethylamino)diazenolate-2-oxide (DEA-NONOate) under strictly anaerobic, alkaline conditions (pH 8.9).…”

Catalysis of the Quadricyclane to Norbornadiene Rearrangement by SnCl2 and CuSO4

“…4 shows the calculated (UB3LYP) spin densities for 5, 6 ‡ , and 7. We use both spin densities and atomic charges, because the latter have been shown [32] not to reveal electron transfer in transition-metal complexes, whereas the calculated spin density gives a detailed and accurate picture of the electronic features of reactants, transition states and products. Table 6 shows the calculated NBO [33,34] charges for the species described above.…”

Mechanistic Studies on the Activation of NO by Iron and Cobalt Complexes

“…[74] Experimental and theoretical work from our laboratory proved that Cbl(H 2 O) does not react with NO at pH 7. [75,76] Nevertheless, it was reported later that Cbl(H 2 O) reacts with NO at low pH, under conditions in which the dimethylbenzimidazole group is protonated and subsequently dechelates. [77] The final product of this reaction was not a Co III -(NO) complex but Co II -(NO), or better described as Co III -(NO -).…”

“…As mentioned in the previous section, aquacobalamin, [Cbl(H 2 O)], does not react with NO at pH 7, [75,76] nevertheless, it was reported that it can undergo reductive nitrosylation at low pH to generate a Co III -NO -complex or Cbl(NO -) [77] in a manner similar to that of Co porphyrins. [79] As a natural extension of the kinetic and mechanistic studies on Co porphyrins, [80] the reductive nitrosylation of Cbl(H 2 O) at low pH was revisited.…”

“…[83] Earlier studies concluded that Cbl(H 2 O) cannot interact with NO in aqueous solution. [76] The more recent work focused on the effect of the coordination of NO, NO 2 -, H 2 O and OH -on the lengthening of the Co-dimethylbenzimidazole bond. [83] On deprotonation of the coordinated water molecule in the Cbl(H 2 O) complex, significant lengthening of the Co-dimethylbenzimidazole bond occurs.…”