Electronic Properties of the Axial Co−C and Co−S Bonds in B12 Systems − A Density Functional Study

Abstract

The numerous accurate structural data of cobalamins now available allows us to optimize the geometry of these systems, based on a simplified model by using density functional theory (DFT) calculations. This approach, which reproduces the trend of the experimental distances derived from EXAFS and X-ray crystal structures in the corrin macrocycle, permit us to interpret the electronic properties in the NB3−Co−X axial system. In particular, the results are analyzed for cobalamins containing a sulfur ligand which exhibits a ''regular'' trans influence, i.e., when the Co−S bond shortens, the trans Co−NB3 bond lengthens. This feature appears in contrast with an anomalous effect (''inverse'' trans [a]

Combined Spectroscopic/Computational Studies of Vitamin B12 Precursors: Geometric and Electronic Structures of Cobinamides

“…While the equatorial Co–N eq bonds are slightly overestimated computationally, by up to 0.03 Å, these error values are consistent with those reported previously in the literature for pure DFT calculations of truncated corrinoid models. 31, 32, 34–36 The distance between the cobalt ion and the upper axial ligand (R) is accurately reproduced for MeCbl and only slightly underestimated for CNCbl (Δ QM/MM-Xtal = −0.03 Å). In the case of H 2 OCbl + , however, the upper axial Co–O bond is significantly overestimated by our calculations, with Δ QM/MM-Xtal = 0.17 Å.…”

NMR spectroscopy and molecular modelling studies of nitrosylcobalamin: further evidence that the deprotonated, base-off form is important for nitrosylcobalamin in solution

“…Interestingly, the Co–NB3 bond distance for AdoCbl (2.24 Å72) is considerably longer than that for MeCbl (2.16 Å66). Finally, like NOCbl, sulfitocobalamin (SO 3 Cbl) also has a longer Co–NB3 bond distance (2.13–2.15 Å73,74) compared with other Cbls with β-axial inorganic ligands. The longer Co–NB3 for SO 3 Cbl and NOCbl can be attributed to the strong σ donor properties of these ligands, in addition to NO being a moderately good π acceptor ligand 75…”

Probing the nature of the Co(III) ion in cobalamins: The reactions of aquacobalamin (vitamin B12a), aqua-10-chlorocobalamin and aqua-10-bromocobalamin with anionic and neutral ligands

“…We have previously shown that a cobalamin will usually coordinate an ambident nucleophile through the softer of the two potential donor atoms, as in SeCN À , NO 2 À and S 2 O 3 2À [3]. SO 3 2À is S-bound as in sulfito-cobalamin [67,68]. An exception we found was SCN À which is N-bound in the solid state; however, we obtained NMR evidence to show that in solution it existed as a nearly equal mixture of two species which we took to be the N-and S-bound thiocyanato complexes.…”