HETERONUCLEAR NMR-STUDIES OF COBALT CORRINOIDS .18. CORRELATION OF STRUCTURE AND MAGNETIC-RESONANCE PARAMETERS IN BASE-ON COBALAMINS

Recent X-ray crystal structure determinations (including a new X-ray d etermination of the structure of cyano-13-epicobalamin reported herein ) create a series of seven base-on cobalamins structurally characteriz ed by modern crystallographic techniques in which the intramolecular e quilibrium constant for coordination of the axial benzimidazole ligand (Bzm) varies from 76.6 to 4.90 x 10(7). For the five normal, unepimer ized cobalamins, the free energy change for this equilibrium correlate s linearly with the axial Co-N bond length (r(2) = 0.99). Absolute ass ignment of the H-1 and C-13 NMR spectra of two of these structurally c haracterized cobalamins (CH(3)Cbl and CN-13-epiCbl) together with lite rature assignments for the other complexes now provides reliable C-13 NMR assignments and chemical shifts for all seven complexes. The magne tic anisotropies of the central cobalt atom of all seven complexes, es timated by a method described earlier, are well correlated with the ax ial Co-N bond distance (r(2) = 0.97) and the free energy of coordinati on of the Bzm ligand-(r(2) = 0.95). The P-31 NMR chemical shift of the phosphodiester moiety of the nucleotide loop is excellently correlate d to the axial Co-N bond length (r(2) = 0.996) of the unepimerized cob alamins and provides a reliable method of estimating this bond length. The N-15 chemical shifts of the axially coordinated Bun nitrogen vary strongly with the axial Co-N bond distance and correlate linearly wit h this structural parameter (P = 0.991) except for the case of H(2)OCb l(+), which deviates substantially. However, there is a good linear co rrelation (r(2) = 0.98) of this N-15 chemical shift with the free ener gy of Bzm coordination for the five unepimerized cobalamins. Attempts to correlate C-13 NMR chemical shifts with structural, thermodynamic, and corrin ring conformational parameters are discussed.