STUDY OF CU(II) BINDING TO CHIRAL TRIPODAL LIGANDS BY ELECTRON-SPIN ECHO SPECTROSCOPY

In an attempt to provide models for copper binding in proteins, symmetric and nonsymmetric chiral ligands designed to bind copper in a controlled geometry were synthesized. These ligands are assembled from trifunctional anchors extended by donors containing amino acids such as histidine and methionine. The copper coordination of these complexes was studied by orientation selective electron spin echo envelope modulation (ESEEM) experiments. The three-pulse FT-ESEEM spectra consist of peaks at 0.7, 1.4, 2.1, and 3.0 MHz, which positions are practically independent on the resonant magnetic field, and a peak at about 4 MHz, which shows significant field dependence. The relative intensities of all lines vary with the resonant field. These lines are typical for the remote nitrogen in the imidazole ring. By using computer simulations of the FT-ESEEM spectra recorded at the various resonant magnetic fields along the powder pattern and taking into account the selected excited orientations, the N-14 isotropic and anisotropic hyperfine interactions were determined. The simulations also gave the relative orientation of the N-14 hyperfine and quadrupole tensor principal axes with respect to the g tensor principal axis. From these parameters it was concluded that in the complexes consisting of three histidyl residues all three imidazoles are coordinated to the copper, not in a coplanar structure but in a propeller-like arrangement. No other nitrogens, such as the pivotal nitrogen, were found to be coordinated to the copper.