Heteronuclear NMR studies of cobalt corrinoids—17. characterization of neopentylcobinamide and neopentyl-13-epicobinamide by 1H and 13C NMR spectroscopy: Inferred corrin ring conformations from chemical shift differentials

Abstract

While the 1H and 13C NMR spectra of neopentylcobalamin (NpCbl) and its epimer at corrin ring C(13), neopentyl-13-epiCbl (Np-13-epiCbl), are extremely broad, apparently due to chemical exchange between the base-on and base-off species, the cobinamide derivatives, neopentyl cobinamide (NpCbi +) and neopentyl-13-epicobinamide (Np-13-epiCbi +), have sharp, well-resolved NMR spectra. The 1H and 13C NMR spectra of NpCbi + and Np-13-epiCbi + have now been completely assigned by modern two-dimensional NMR methodologies. Comparison of the 13C spectra of these two complexes shows that significant chemical shift differences occur at a variety of corrin ring and peripheral carbon atoms and are not localized near the site of epimerization. Similarly, comparison of the 13C NMR spectra of NpCbi + and 5′-deoxyadenosylcobinamide (AdoCbi +) shows differences at many corrin ring and peripheral carbons. A first attempt at discerning differences in corrin ring conformation from such differences in 13C chemical shift has been made by comparing the X-ray crystal structures and 13C NMR spectra of 5′-deoxyadenosylcobalamin (coenzyme B 12, AdoCbl) and cyanocobalamin (vitamin B 12, CNCbl). After elimination of carbon atoms whose chemical shifts are likely to be significantly affected by differences in the inductive effect of the Ado and CN ligands, and after consideration of differential anisotropic shielding effects in the two complexes due to the presence or absence of the Ado ligand, the difference in magnetic anisotropy of the central cobalt atom, the change in position of the axial nucleotide and differences in the magnetic anisotropy of the corrin ligand, 15 peripheral carbon atoms [C(2), C(18), C(20), C(25), C(26), C(30), C(36), C(37), C(41), C(46), C(47), C(48), C(54), C(55), C(60)] emerge as candidate “reporter” carbons whose 13C chemical shifts may be useful in deducing conformational differences in cobalt corrinoids. Application of this method to adeninylpropylcobalamin (AdePrCbl), for which the X-ray crystal structure and absolute NMR assignments are known, correctly predicts the gross conformational differences between the corrin ring of AdePrCbl and that of CNCbl. Use of these reporter carbon chemical shifts suggests that in NpCbi +, the fold angle, defined as the angle between the “northern” and “southern” planes of the corrin ring, is reduced relative to AdoCbi +. Comparison of the chemical shifts of the reporter carbon atoms in NpCbi + and Np-13-epiCbi + suggests that the fold angle in the former is larger than that for the latter.