Electron spin-echo envelope modulation study of multicrystalline Cu2+-insulin: Effects of Cd2+ on the nuclear quadrupole interaction of the Cu2+-coordinated imidazole remote nitrogen

A comparison of electron spin-echo envelope modulation (ESEEM) spectra from multicrystalline Cu2+-insulin with and without additional Cd2+ show a dram atic change in the quadrupole coupling parameters of the remote nitrogens o f the two histidine imidazoles that ligate to copper. Without Cd2+, the qua drupole parameters are Like those observed in blue copper proteins and in c opper substituted lactoferrin. With Cd2+ soaked into the Cu2+-insulin cryst als, the quadrupole parameters are similar to those found in galactose oxid ase. Theoretical simulations of ESEEM spectra guided by structure modeling suggest that these changes originate from differences in the hydrogen bondi ng environments of the imidazole remote nitrogen. In addition, a compilatio n of results from previous ESEEM studies of copper proteins reveals that th e asymmetry parameter, eta, may be an indicator of type of hydrogen bond th e imidazole remote nitrogen makes. When eta greater than or equal to 0.9, t he nitrogen hydrogen bonds to water, whereas when eta < 0.9, the nitrogen h ydrogen bonds to the protein.