Distribution of H^upbeta Hyperfine Couplings in a Tyrosyl Radical Revealed by 263 GHz ENDOR Spectroscopy

Abstract

^1H ENDOR spectra of tyrosyl radicals (Y^bullet) have been the subject of numerous EPR spectroscopic studies due to their importance in biology. Nevertheless, assignment of all internal ^1H hyperfine couplings has been challenging because of substantial spectral overlap. Recently, using 263 GHz ENDOR in conjunction with statistical analysis, we could identify the signature of the H^{upbeta _2} coupling in the essential Y_{122} radical of Escherichia coli ribonucleotide reductase, and modeled it with a distribution of radical conformations. Here, we demonstrate that this analysis can be extended to the full-width ^1H ENDOR spectra that contain the larger H^{upbeta _1} coupling. The H^{upbeta _2} and H^{upbeta _1} couplings are related to each other through the ring dihedral and report on the amino acid conformation. The 263 GHz ENDOR data, acquired in batches instead of averaging, and data processing by a new “drift model” allow reconstructing the ENDOR spectra with statistically meaningful confidence intervals and separating them from baseline distortions. Spectral simulations using a distribution of ring dihedral angles confirm the presence of a conformational distribution, consistent with the previous analysis of the H^{upbeta _2} coupling. The analysis was corroborated by 94 GHz ^2H ENDOR of deuterated Y_{122}^bullet. These studies provide a starting point to investigate low populated states of tyrosyl radicals in greater detail.