A SINGLE-CRYSTAL CO-59 NMR-STUDY OF VITAMIN-B-12

A Co-59 solid-state NMR study of a single crystal of vitamin B-12 was performed at 11.75 T. The results showed that the cobalt nucleus in th is biologically relevant molecule experiences a highly anisotropic che mical shift spanning 1200 ppm about the isotropic chemical shift of 45 49 ppm. The large electric field gradient of the cobalt site results i n a quadrupolar coupling constant of 27.31 MHz with asymmetry of 0.243 . The two tensors describing the orientation dependence of these NMR i nteractions exhibit distinct orientations with respect to the molecule , the quadrupolar coupling lying close to the axial direction, and the chemical shift tensor tilted 41 degrees away from this direction. Thi s study demonstrates the importance and feasibility of single-crystal NMR to provide this orientation information in metal-centered biologic al assemblies, and supports the prospects for solid-state NMR of quadr upolar metal nuclei to contribute further to the elucidation of metal participation in biological systems.