ENDOR and EPR measurements of anionic, neutral and diprotonated radical cations of l3C-I -labelled ubiquinone 0 t in solution are presented. The 13C-carbonyl hyperfine coupling constant in protic solvents is roughly half of that in aprotic solvents. Preferred conformers of diprotonated ubiquinone and neutral radicals exist as a result of intramolecular H-bonding interactions. The variation of spin densities on the ubiquinone ring carbon atoms correlates with the calculated out-of-plane twist angles of the methoxy groups.
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.
The hype&e couplings of the remote nitrogens of h&dine ligands are determined for the 6rst time by an X-band ESEEM spectroscopy study of "N-substituted superoxide dismutase (SOD). They show a significant difference between two groups of ligands with different orientation relative to the metal ion. The ESEEM spectra of "N SOD with cyanide as an inhibitor containing 14N and "N are also discussed. They allow some conclusions to be drawn about structural changes upon inhibitor binding and indicate the necessity of further multifrequency investigations.
ENDOR measurements of anion radicals of artificial dimethoxy-or halo-l,4-benzoquinones with an alkyl side chain of differing length in solution yield a number of proton isotropic hyperfine constants. Analysis of the constants obtained gives the orientation of substituents relative to the quinone plane. ESEEM measurements of the ion radicals in frozen solutions of deuteriated solvents indicate that the cation, but not the anion, radicals experience a strong interaction with solvent deuterium, which is hydrogen bonded to the quinone oxygen.
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