ENDOR and ELDOR studies of X-irradiated polycrystalline dipeptides, myosin, and actomyosin

ENDOR and ELDOR studies have been carried out for nine dipeptide powders as well as powders of myosin and actomyosin X-ray irradiated at 77 K in an attempt to characterize the final radical stable upon annealing between 183 and 260 K. The dipeptides studied were glycylglycine, L-alanylglycine, glycyl-L-alanine, L-alanyl-L-alanine, glycyl-L-aspartic acid, glycyl-L-glutamic acid, glycyl-L-methionine, glycyl-L-serine, and L-lysyl-L-lysine. Nitrogen ENDOR spectra have been observed between 1 and 8 MHz for each powder and the nitrogen hyperfine and quadrupole tensor has been estimated. Analysis of the ENDOR, ELDOR, and ESR spectra indicates at least one of the final radicals in the dipeptide powders (except Gly-Gly, and possibly Gly-Glu, Gly-Ser) to be the decarboxylation product NH₂CHRCONHĊHR′ rather than just the abstraction type (NH₃⁺-CHRONHĊR′COO^-) previously identified in irradiated dipeptide ices. A decarboxylation type radical is also present as a final radical in the irradiated myosin and actomyosin.