In the study of biological structures, pulse dipolar spectroscopy (PDS) is used to elucidate spin–spin distances at nanometre-scale by measuring dipole–dipole interactions between paramagnetic centres. The PDS methods of Double Electron Electron Resonance (DEER) and Relaxation Induced Dipolar Modulation Enhancement (RIDME) are employed, and their results compared, for the measurement of the dipolar coupling between nitroxide spin labels and copper-II (Cu(II)) paramagnetic centres within the copper amine oxidase from Arthrobacter globiformis (AGAO). The distance distribution results obtained indicate that two distinct distances can be measured, with the longer of these at c.a. 5 nm. Conditions for optimising the RIDME experiment such that it may outperform DEER for these long distances are discussed. Modelling methods are used to show that the distances obtained after data analysis are consistent with the structure of AGAO.

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Broadening of the NMR line of 19F in peroxydisulfuryl difluoride, S2O6F2, was investigated between 40 and 100°C. The linewidth contribution from fluorine “exchanging” between the diamagnetic peroxide and paramagnetic SO3F radicals was separated from other linewidth contributions by simultaneously observing stable, inert pyrosulfuryl fluoride, S2O5F2. The concentration of fluorosulfate radicals as a function of temperature was obtained from EPR measurement by comparison of spectral intensity with that of a sample containing a known number of spins. The NMR chemical shift ΔωR of the SO3F radical was estimated from the isotropic hyperfine splitting constant A = 9.2 G, which was determined by EPR from the peroxide at liquid-nitrogen temperature under uv irradiation. The activation energy Ea = 25.0 kcal/mole for the reversible, thermally controlled reaction S2O6F2⇆2SO3F was found from the temperature variation in width of the single fluorine NMR line observed. Equations developed by Swift and Connick and Luz and Meiboom were employed.

The EPR spectrum of the fluorosulfate radical SO3F· in the gaseous state was observed as a single broad line at temperatures between 390° and 480°K. The linewidth between points of maximum slope was found to be 41.5 G, with a g value of 2.010. The change in free energy ΔF of the reversible reaction S2O6F2⇆2SO3F· was found to be 10.6 ± 1.0 kcal/mole from the temperature dependence of the resonance line amplitude, and the energy of dissociation ΔE estimated to be approximately 27 kcal/mole.

Contrary to previous reports, the linewidth of the electron spin resonance spectrum of cupric ion as measured in gauss between points of maximum slope is a sensitive function of ionic strength, chloride ion concentration and cuprous ion concentration. Assuming that electron spin relaxation rates do not change greatly on the addition of cuprous ion, it is possible to derive an electron exchange rate for the reaction: Cu^++Cu^2+↔Cu^2++Cu^+ of k=1.53×109  l/mol/sec. This value is in good agreement with a previous determination of the same rate constant by McConnell and Weaver using the 63Cu nuclear magnetic resonance (0.5×108). The results for the linewidth changes can be rationalized in all cases by assuming that bridge chloride ions between two cupric ions or one cupric and one cuprous ion change drastically in concentration as such factors as ionic strength or chloride ion concentration changes. The g value of the electron (free) decreases as chloride ion is added, indicating delocalization of the unpaired electron onto chloride ligands.