Redetermination of the X-ray structure of nitroxylcobalamin: base-on nitroxylcobalamin exhibits a remarkably long Co–N(dimethylbenzimidazole) bond distance

Abstract

The X-ray structures of three new crystals of nitroxylcobalamin (NOCbl) have been determined. Unlike our earlier reported structure in which NOCbl was partially oxidized , the O atom of the nitroxyl ligand is located in a single position with a N=O bond distance of 1.12-1.14 Å, consistent with a double bond. The Co-N-O angle is in the 118.9-120.3 Å range. The α-axial Co-N(dimethylbenzimidazole) (Co-NB3) bond distance is a remarkable 2.32-2.35 Å in length, ~0.1 Å longer than that reported for all other cobalamin structures. The change in the Gibbs free energy for the base-on/base-off equilibrium now correlates extremely well with the Co-NB3 bond distance, as observed for other cobalamins.

Trans and cis influences and effects in cobalamins and in their simple models

“…Very recently, Brasch et al have shown that extension of this correlation to a larger number of cobalamins having distances with e.s.d. 's up to 0.01 Å, gave an R 2 value of 0.84 [39,40,43]. However, when the correlation is limited to Co\NB3 distances with e.s.d.…”

“…2. The distance increases from 1.925(2) Å, when X is the very weak σ-donor H 2 O, to 2.351(2) Å, when X is the strong σ-donor NO: the latter is a significantly stronger trans influencing ligand than even an alkyl group [39,40]. In contrast to NOCbl, nitrosylcobaloxime is pentacoordinate and represents, to our knowledge, the only example so far known of a crystallographically characterized pentacoordinate Co(III) dioximate [41].…”

Probing the nature of the Co(III) ion in cobalamins: The ligand substitution reactions of aquacyanocobester, aquacyano(10-nitro)cobester and aquacyano(10-amino)cobester

“…Hence K cyanide = K obs K azide , where K obs is the observed equilibrium constant for binding of CN À in the presence of excess N 3 À . By analogy with [ACCbs] + and [ACSYCbs] + [1,27], both [AC(10-NO 2 )Cbs] + and [AC(10-NH 2 )Cbs] + exist in solution as an equilibrium of two diastereomers, viz., a-cyano, b-aqua and a-aqua, b-cyano. The two diastereomers should have similar, but not identical, equilibrium constants.…”

“…Di-aqua (or aquahydroxo) species due to CN À ligand exchange reaction may be present since aquacyano cobesters interchange between the a-CN À , b-H 2 O and b-CN À , a-H 2 O isomers; this presumably occurs through intermediate formation of a bimolecular species with CN À acting as a bridging ligand, leading to the dicyano and diaqua (or aquahydroxo) species [1,17,27]. The observations of pK a s at around 8.5 and between 3 and 5.5, and the relatively small absorbance changes associated with these is evidence for the existence of di-aqua and aquahydroxo species in solution (see above, and Fig.…”

“…We prepared the corresponding aquacyano complexes of the cobester and the stable yellow cobester, [ACCbs] + and [ACSYCbs] + , and showed that they exist as a mixture of interconverting diastereomers (i.e., Coa-aqua,Cob-cyano and Coa-cyano,Cob-aqua) through intermediacy of the diaqua and dicyano forms; the diastereomers therefore cannot be isolated [17]. We compared the thermodynamics ( [27].…”