Structure of Copper(II)-histidine based complexes in frozen aqueous solutions as determine dfrom high-field pulsed electron nuclear double resonance

摘要

W-band (95 GHz) pulsed EPR and electron—nuclear double resonance (ENDOR) spectroscopic techniques were used to determine the hyperfine couplings of different protons of Cu(II)—histidine complexes in frozen solutions.The results were then used to obtain the coordination mode of the tridentate histidine molecule and to serve as a reference for Cu(II)—histidine complexation in other, more complex systems. Cu(II) complexes with L-histidine and DL-histidine-ct-d,/3-d~ were prepared in H20 and in D20, and orientation-selective W-band ‘H and 2H pulsed ENDOR spectra of these complexes were recorded at 4.5 K. These measurements lead to the unambiguous assignment of the signals of the Ha, Hfl, imidazole HE, and the exchangeable amino, Ham, protons. The “N superhyperfine splitting observed in the X-band EPR spectrum and the presence of only one type of H~ and Hfi protons in the W-band ENDOR spectra show that the complex is a symmetric bis complex. Its gii is along the molecular symmetry axis, perpendicular to the equatorial plane that consists of four coordinated nitrogens in histamine-like coordinations (NNNN). Simulations of orientation-selective ENDOR spectra provided the principal components of the protons’ hyperfine intel-action and the orientation of their principal axes with respect to gil. From the anisotropic part of the hyperfine interaction of Ha and Hfl and applying the point-dipole approximation, a structural model was derived. An unexpectedly large isotropic hyperfine coupling. 10.9 MHz. was found forHa. In contrast, Ha of the Cu(II)-l-methyl-histidine complex, where only the amino nitrogen is coordinated, showed a much smaller coupling. Thus, the hyperfine coupling of H~ can serve as a signature for a histamine we coordination where both the amino and imino nitrogens of the same molecule bind to the Cudi). formine a of six-membered chelating ring. Unlike H~ the hyperfine coupling of HE is not as sensitive to the presence of a he coordinated amino nitrogen of the same histidine molecule.