EPR and ENDOR Studies of Single Crystals of α-Glycine X-ray Irradiated at 295 K

Single crystals of the amino acid α-glycine N−CH 2 −COO - were X-ray irradiated at 280 K and studied at 295 K and at 100 K using EPR, ENDOR, and EIE techniques. Five radicals were detected and characterized. Two of these are well known from previous studies, the oxidation product N−ĊH−COO - (radical I) and the reduction product ĊH 2 −COO - (radical II). It is shown that at 295 K the ENDOR and EIE spectra from radical I are characteristic of efficient W 1 x cross relaxation induced by rapid rotation of the amino group. This allows for the determination of the absolute signs of the hyperfine coupling constants of radical I. The other three radicals are two geometrical conformations of the product H 2 N−ĊH−COOH (radicals III and IV) and a species suggested to be the dimer product N−CH 2 −CO−ĊH−COO - (radical V). Radical III exhibits spectral parameters related to those previously reported by M. Brustolon et al. ( J. Phys. Chem. 1997, 101, 4887). However, on the basis of our analysis of ENDOR and EIE spectra from normal as well as from partially deuterated crystals, the structural assignment for radical III is different from that suggested by these authors. A comparison with a new radical species recently observed and characterized in irradiated crystals of the amino acid l -α-alanine indicates that the new alanine radical and radical III presented here have similar structures. A review of the mechanistic aspects of radical formation in glycine is given, and the newly detected species (radicals III−V) are tentatively fitted into this scheme.