Resolved Electron-Electron Spin-Spin Splittings in EPR Spectra

Abstract

Interactions between unpaired electrons may be examined via a variety of physical techniques. Of particular interest in biological systems are the interactions observable by electron paramagnetic resonance (EPR) between various pairs or clusters of paramagnetic transition metals and/or organic free radicals. These paramagnetic centers may be natural components of the biological system or they may be species added as probes or labels. Interactions between these centers may affect the EPR spectra enough to invalidate the interpretations that would be valid in the absence of interaction. For example, if a paramagnetic metal is present, a spin-labeled biomolecule may yield an EPR spectrum that is not validly interpreted in terms of changes in the motion of the label. If, however, the effect of the metal on the nitroxyl EPR spectrum were understood in detail, additional information could be obtained from the EPR spectrum. An assessment was made several years ago of the effect of the dipolar interaction between a metal and a nitroxyl radical on the EPR line shape of the nitroxyl in a rigid matrix (Leigh, 1970). This approach has been used by many investigators to obtain metrical information on spin-labeled biomolecules. Others have studied the metal-organic radical interactions in vitamin B12-dependent enzyme reactions also seeking metrical information (Pilbrow, 1982). Examples are considered in Section 10.