Exploiting the properties of line-shape singularities in orientationally selected electron spin echo envelope modulation spectra of Cu2+−(15N-imidazole)4 for the determination of hyperfine coupling with the remote imidazole nitrogen

Abstract

Orientationally selected two-, three-, and four-pulse electron spin echo envelope modulation (ESEEM) spectra from remote 15N nuclei in a Cu2+-(15N–imidazole)4 complex have been obtained in the microwave frequency region 9.12–9.82 GHz. This range of frequencies has allowed us to reach the conditions corresponding to S and ν⊥ singularities of the lineform of basic transitions. The development of these lineform singularities in orientationally selected spectra is discussed. The knowledge of the magnetic fields corresponding to the singularities was used to find the hyperfine coupling parameters for the remote nitrogen as a=2.44 MHz and T⊥=−0.39 MHz as well as to determine the angle between the axis of the complex and the direction of the largest principal hyperfine tensor element as 40°. Additional geometrical information about the ligand orientation has been obtained from the analysis of shifts of the sum combination harmonic in proton four-pulse ESEEM spectra.

Comparison of EPR-Visible Cu2+ Sites in pMMO from Methylococcus capsulatus (Bath) and Methylomicrobium album BG8

“…(The two basic 15 N nuclear modulation frequencies are ␣ and ␤ .) The 15 N frequencies are very similar to those reported by Dikanov et al (1994b) for coupling to the distant nitrogens in [Cu( 15 N-imH) 4 ] 2ϩ , where imH is imidazole. The frequencies reported in Table 3 were assigned as described by Dikanov et al (1994b).…”

“…The 15 N frequencies are very similar to those reported by Dikanov et al (1994b) for coupling to the distant nitrogens in [Cu( 15 N-imH) 4 ] 2ϩ , where imH is imidazole. The frequencies reported in Table 3 were assigned as described by Dikanov et al (1994b). For copper-imidazole complexes, the anisotropic contribution, B, to the electron-15 N coupling of the distal nitrogen is smaller than the isotropic coupling, A, and so A can be estimated from the expression A Ϸ 2*( ␤ Ϫ I ), where I is the 15 N resonance frequency at that observing field.…”

“…Based on this approximation, an average value for the distal nitrogen superhyperfine coupling, A, across the spectrum is 2.5-2.6 MHz (Table 3), which is likely to be close to the isotropic value. Dikanov et al (1994b) reported A ϭ 2.44 MHz for the distal nitrogen of coordinated 15 N-imidazole. The similarity between their value and the value of 2.5 MHz for pMMO is consistent with assignment of the 15 N modulation to histidine imidazole bound to Cu 2ϩ in pMMO.…”

Magnetic Field (g-Value) Dependence of Proton Hyperfine Couplings Obtained from ESEEM Measurements: Determination of the Orientation of the Magnetic Axes of Model Heme Complexes in Glassy Media

“…Such differences have been observed previously and have already been discussed. 19 In our case, the major part of this difference is caused by partial excitation of the proton frequencies. Therefore, we have divided the absolute amplitudes of the simulated spectra by 1.8 for comparison to the experimental spectra of Figure 8.…”

Porphyrin and Ligand Protons as Internal Labels for Determination of Ligand Orientation in ESEEMS of Low-Spin d5 Complexes in Glassy Media: ESEEM Studies of the Orientation of the g Tensor with Respect to the Planes of Axial Ligands and Porphyrin Nitrogens of Low-Spin Ferriheme Systems

“…For systems where g tensor anisotropy exceeds the anisotropy of the hyperfine interaction (HFI), the convenient RCF is that in which the axes coincide with the principal axis of the g tensor. In this RCF, with HFI caused by dipolar (in the point−dipole approximation) , and isotropic hyperfine interaction, the other terms of eq 1 are defined as where n i are the direction cosines of the radius-vector r connecting the electron and nuclear spins in the RCF, a is the isotropic hyperfine coupling constant, g i are the principal values, and T = −β e β n g n /ℏr 3 , β e = Bohr magneton, β n = nuclear magneton, r = electron−nuclear distance.…”