Perturbed angular correlation studies of Hf binding to cyanocobalamin (vitamin B12)

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Abstract
Gamma ray perturbed angular correlation (PAC) experiments have been carried out with 181Hf labeled cyanocobalamin. Evidence is presented which strongly indicates that Hf binds to vitamin B12 at the phosphate group linking the sugar residue to a side chain of the corrin ring system. Analysis of the time‐differential PAC spectra for the crystalline Hf–B12 complex indicates a static electric quadrupole interaction at the Hf nucleus, corresponding to the electric field gradient generated by the chemical bonding. The magnitudes of the derived interaction parameters are similar to those found in Hf phosphate compounds. In aqueous solution, the Hf–B12 complex exhibits PAC spectra which appear to originate from two sources. Approximately 3/4 of the Hf nuclei experience a static electric quadrupole interaction with the same characteristic interaction frequency as in the solid, but with an increased asymmetry parameter. Approximately 1/4 of the Hf signal strength is attributable to a time‐dependent quadrupole interaction with a relaxation constant indicative of an effective molecular entity comparable in size to the B12 molecule. This effect may be related to molecular motion in the solution. These results demonstrate the utility of the PAC experimental method for the study of macromolecular species in both the solid and solution forms, and opens possibilities for obtaining new information concerning the structure, orientation, and behavior of macromolecules.