Electron spin echo envelope modulation (ESEEM) studies of copper(II) model complexes and the copper(II) site structure of metalloproteins in frozen solution

Abstract

Electron Spin Echo Envelope Modulation (ESEEM) spectroscopy has been used to study the active site structures of Cu(II) proteins. The {dollar}\sp{lcub}14{rcub}{dollar}N modulations obtained are attributed to the remote nitrogen of coordinated histidine imidazole. ESEEM studies of Cu(II)-imidazole in d{dollar}\sb{lcub}\rm x2-\rm y2{rcub}{dollar} and d{dollar}\sb{lcub}\rm z2{rcub}{dollar} ground state model complexes allowed us to determine the electron-nuclear coupling between the unpaired electron and remote imidazole {dollar}\sp{lcub}14{rcub}{dollar}N, and the {dollar}\sp{lcub}14{rcub}{dollar}N nuclear quadrupole parameters. By applying a modified Townes-Dailey model, we were able to relate the change of the nuclear quadrupole interaction to a change of the electron occupancy in the {dollar}sp\sp2{dollar} orbital of the remote nitrogen along the N-H bond and to a change of the N-H bond polarization. This suggests that the difference in ESEEM spectra observed for some Cu(II) proteins may arise from changes in hydrogen bonding at the remote nitrogen of the coordinated histidine imidazole side chain. This effect was duplicated in Cu(II) model studies by altering the local environment of the N-H bond.;Spectral simulation provides a tool for quantifying the electron-nuclear coupling for multi imidazole systems. In Cu(II)-isopenicillin synthase, we recognize two histidine imidazoles and water molecules as equatorial ligands and demonstrate that substrate binding replaces a water ligand and rearranges the histidine binding geometry.;Cu(II)-nitrite complexes have been implicated in a number of enzymes and in the nitrite product of hemocyanin. We have observed for the first time modulations from {dollar}\sp{lcub}14{rcub}{dollar}NO{dollar}\sb2\sp-.{dollar} Because of the large anisotropy of the electron-nuclear coupling, we were able to determine both the magnitude and the orientation of the electron-nuclear hyperfine and nuclear quadrupole tensors. This allows us to determine the Cu(II)-nitrite structure in solution, which is consistent with its structure in the crystal.