Alanine Radicals: Structure Determination by EPR and ENDOR of Single Crystals X-Irradiated at 295 K

Single crystals of the amino acid l-α-alanine have been X-irradiated at room temperature, and the free radical formation has been studied using X-band and K-band EPR, ENDOR, and EIE (ENDOR-induced EPR) spectroscopy in the temperature interval 220−295 K. Aided by the results from EIE, as well as ENDOR from selected magnetic field positions, nine hyperfine coupling tensors were obtained and assigned to three different radicals. Room-temperature relaxation behaviors characterized by efficient W1x and W1e and by slow W1n relaxation rates allowed for determination of the signs of the various hyperfine couplings from the ENDOR spectra obtained at room temperature. The temperature dependence of the W1x relaxation is qualitatively discussed. The EPR spectra from alanine are dominated by the well-known resonance of the “stable alanine radical”, SAR, formed by a net deamination of the protonated alanine anion. Precise hyperfine coupling tensors due to the α-proton coupling, the methyl group coupling, and a dipolar ...