Magnetic Circular Dichroism Spectroscopy as a Probe of Geometric and Electronic Structure of Cobalt(II)-Substituted Proteins:  Ground-State Zero-Field Splitting as a Coordination Number Indicator

Variable-temperature magnetic circular dichroism (VT MCD) is used as a probe of the ground-state electronic structure in Co(II)-substituted liver alcohol dehyrogenase, carbonic anhydrase, carboxypeptidase, substrate and inhibitor complexes of these enzymes, and four- and five-coordinate Co(II) model complexes. VT MCD was used to determine the magnitude of the ground-state zero-field splitting (ZFS) in these samples. The four-coordinate Co(II) species had ZFS's that ranged from 2.3 to 30 cm-1 and the five-coordinate species had ZFS's that ranged from 2.7 to 98 cm-1, values which fall outside of the ranges previously suggested for distinguishing 4-coordinate and 5-coordinate Co(II) (Makinen, M. W.; Kuo, L. C.; Yim, M. B.; Wells, G. B.; Fukuyama, J. M.; Kim, J. E. J. Am. Chem. Soc. 1985, 107, 5245−5255). The magnitude of the ZFS is not very sensitive to ligand heterogeneity but can be very sensitive to bond angles. Since protein active sites are in general highly angularly distorted from ideal four- or five-coordinate geometries, ZFS is rendered useless as a sole indicator of coordination number in Co(II)-substituted proteins. The MCD and absorption (or diffuse reflectance) spectra and the values of the ZFS for the proteins and model compounds are used in angle overlap method (AOM) calculations. These calculations support the conclusion that there is significant overlap in the ranges of ZFS for four- and five-coordinate Co(II) compounds.