Electron–nuclear double resonance on copper (II) tetraimidazole

Abstract

We have investigated the electron-nuclear double resonance (ENDOR) from frozen aqueous solutions of •'Cu++(imidazolel" •'Cu++ (imidazole--1'NI" and "Cu++(imidazole--D, I" where n = I, 2, 3, and 4 for selectively deuterated imidazole. We have observed ENDOR associated with the imidazole protons and the two imidazole nitrogens. The selective deuteration has allowed us to attempt identification of the weakly coupled protons responsible for the ENDOR spectrum, and a comparison of the overall line shape of that spectrum taken at two extreme points of the EPR spectrum suggests that some of the imidazole planes are tilted with respect to the plane of the complex. The ENDOR arising from the nitrogen nearest the copper is primarily isotropic with A (g 1) = 41.6 ± 1.5 MHz and A (gIl) = 39.8 ± 1.5 MHz. The resonance shows little structure and seems consistent with a picture that requires some inequivalence among the various imidazoles. The remote nitrogen ENDOR reveals both hyperfine and quadrupole effects with approximately isotropic Ae'N) = 1.79 MHz, Q",,~0.360 MHz, and Qx'X'Y'x,~0.349 MHz. These values are in agreement with the results of the nuclear modulation effect [J. Chern. Phys. 69, 4921 (1978)]. The values for the quadrupole constants are thought to be accurate within 10% and are the same as are found in free imidazole. It is also demonstrated that, in this instance, ENDOR and the nuclear modulation effect are complementary in that they have each provided different parts of the same hyperfine spectrum.

An examination of the cyanide derivative of bovine superoxide dismutase with electron-nuclear double resonance

“…We attribute these differences to the presence of the cyanide nitrogen. The spectra in the low-frequency region resemble those already reported [17] for the model compound, tetraimidazole-Cu(II). Since the analysis of the observations on the cyano complex will rely heavily on that already made for the model compound [17], we have reproduced in Fig.…”

“…The spectra in the low-frequency region resemble those already reported [17] for the model compound, tetraimidazole-Cu(II). Since the analysis of the observations on the cyano complex will rely heavily on that already made for the model compound [17], we have reproduced in Fig. 8 region was markedly reduced compared to the high frequency region, and the intensity in the low-frequency range was so low at the g, extreme (where the EPR intensity is also low) that it discouraged study.…”

“…These traces were best obtained with a field modulation amplitude of 6 Gp_p and with the aid of frequency modulation superimposed upon the radio frequency oscillation (modulation frequency = 1 kHz and frequency deviation = 150 kHz). We have reported [ 17] that tile latter seems to improve the signal-to-noise ratio, to these spectra seem to enhance the S/N and clarity of the peaks above 18 MHz while suppressing the proton region (8-13 MHz). Other conditions will bring about the reverse situation, but we will not concentrate on the protons in this work.…”

“…The ENDOR spectrometer has also been described elsewhere [17,18], but we mention some features of it that are important for this work. The spectrometer employs field modulation at 100 kHz, homodyne detection, low-power radio frequency (RF) radiation (on the order of 1 Gauss at 10 MHz), and a microwave reference arm for discrimination between absorption and dispersion.…”

“…A lack of structure in a strongly coupled nitrogen has been seen previously. For instance, the near nitrogen of the model compound tetraimidazoleCu(II) [17] displayed a nitrogen ENDOR resonance devoid of structure and with a suspected small quadrupole effect. The proteins cytochrome c oxidase [23] and stellacyanin [21] also seem to have poorly resolved quadrupole structure in their nitrogen ENDOR spectra.…”