Influence of Hyperfine Coupling Strain on Two-Dimensional ESEEM Spectra fromI =1/2 Nuclei

HYSCORE, a variant of two-dimensional ESEEM spectroscopy, is currently one of the most effective tools in high-resolution solid-state EPR spectroscopy. Our systematic studies of hydrogen bonds with semiquinones in(15)N-labeled proteins have previously provided us with an extensive collection of(15)N HYSCORE spectra produced by N-H horizontal ellipsis O nitrogen donors. Examination of these spectra has indicated in several cases lineshape distortions not described by available theoretical models. We were able to explain all observed artificial phenomena in the(15)N spectra of semiquinones considering a single mechanism-strain of the isotropic hyperfine interaction. In this review article, we introduce the HYSCORE experiment and the theoretical background for spectra ofI = 1/2 nuclei in orientation-disordered samples. We then present a description of the observed lineshape distortions and an explanation of the hyperfine strain's influence on the experimental spectra. In the final part of the article, we discuss (i) similar lineshape distortions found in previously published spectra from(15)N,Si-29,P-31, and(57)Fe nuclei that can be explained using the approach applied for the analysis of the(15)N spectra of semiquinones, and (ii) a recently published analysis of(31)P spectra with a different type of lineshape distortion based on a more sophisticated method, where the strength of anisotropic coupling is correlated with the sign and value of the isotropic constant in different conformations of a Cu(II) complex.