EPR Studies of the uv-Induced Free Radical Conversion in γ-Irradiated Amino Acid Crystals

Ultraviolet energy absorbed by γ‐irradiated α‐amino acids (glycine, L‐alanine, and methylalanine) at 77°K was found to induce a conversion of the trapped radical to another species, which was identified as $({\mathrm{NH}}_2\mathrm{CRR}\prime {\mathrm{CO}}_2\mathrm{H}\mathrm{)-(}\mathrm{R,R}\mathrm{\prime =}{\mathrm{H,CH}}_3)$ . The latter radical was stable in the temperature range 77–250°K, but converted irreversibly back to the former, on raising the temperature beyond 250°K. At room temperature, uv irradiation induced no particular effect except for a noticeable loss in EPR intensity. It was interpreted that the uv excitation in the trapped radical caused a subsequent electron transfer to an adjacent host molecule. During the thermal process above 250°K, most of the captured electrons are believed to return to the original radicals.