Two-Dimensional ESEEM Study of VO2+ Complexes with Imidazole and Histidine: Histidine Is a Polydentante Ligand

Abstract

The oxovanadium cation, VO{sup 2+}, has been successfully employed as a spin probe in electron-nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) studies of metalloprotein active sites. Here we demonstrate that the complex of VO{sup 2+} with histidine has been misinterpreted and that the dominant ESEEM pattern is produced by the {alpha}-amino group. Nevertheless, we show that histidine side chain ligands can be identified from either nitrogen hyperfine or quadrupole couplings. The application of 2D ESEEM to the [{sup 15}N]imidazole complex shows, in the same spectrum, both the coordinated and the remote ring nitrogens with very different isotropic hyperfine couplings. In the case of histidine ligation, equatorial coordination by both imine and amine nitrogen is demonstrated. The reproducible differences between both the hyperfine and the quadrupole constants of the amine and imine nitrogens are reflected in the shape and position of the lines in the 2D spectra and allow them to be distinguished in proteins. With both imine and amine coordination, the VO{sup 2+}/histidine complex cannot be considered an adequate model for coordination by the histidine side chain in proteins. The imidazole, as a monodentante ligand, is better for this role. 16 refs., 3 figs.