New Radical Detected by HF-EPR, ENDOR, and Pulsed EPR in a Room Temperature Irradiated Single Crystal of Glycine

Single crystals of room temperature γ-irradiated glycine have been studied with electron nuclear double resonance (ENDOR) and X-band and high-frequency continuous wave electron paramagnetic resonance (EPR) spectroscopies and electron spin echo spectroscopy. EchoEPR spectroscopy was used to distinguish the radicals with different phase memory relaxation times. Three radicals have been detected, which have been identified as −CH−COO - (radical A), CH 2 −COO - (radical B), and CH 2 −NH 2 (radical C). Whereas radicals A and B had been observed by other authors in the past, radical C was not detected previously. The hyperfine tensors of the two CH 2 protons and of one of the NH 2 protons were determined by ENDOR. ESEEM (electron spin echo envelope modulation) and HYSCORE (hyperfine sublevel correlation spectroscopy) experiments were performed in order to obtain the relative signs of the hyperfine coupling constants of the three protons. The principal values and the principal directions of the hyperfine tensors are in agreement with the hypothesis of a nonplanar structure of the radical. Unrestricted Hartree−Fock calculations have been carried out for different conformations of radical C. The calculated hyperfine coupling constants are compared with the experimental results.