Kinetics of radical decay in crystalline amino acids. II. High-temperature study

Abstract

Radical decay in x-irradiated L-leucine, and DL-valine, L- and DL- alanine, and L-arginine hydrochloride was studied between 300 and 4300K. Free radical decay was observed to take place with two distinct rates: an initial short duration process having an activation energy of 18 kcal/mole and a second, slower process. In most cases, the fraction of radicals decaying by the faster process is the same as the very slow room-temperature decay previously observed. The major process has a 3--6-kcal/mole higher activation energy. There is no evidence of radical conversion in either the initial or the slower process. However, in at least two cases residual peroxy radicals were observed at the termination of the slower process. Both decays obey second-order rate laws. The peroxy radical in at least one case decayed by a second-order law also. Vacancy controlled bulk diffusion is proposed for the major process, and the initial process is thought to be a conformationally aided bulk diffusion. It was observed that at any given temperature the spin concentration reaches a constant value. Increasing the temperature results in a second decay to a new constant value in a stepwise fashion. In the case of hydrated L-arginine hydrochloride and its partially deuteratedmore » derivative, the decay was found to be related to the dehydration process. A rapidly propagating radical transfer mechanism involving hydrogen abstraction is postulated for this decay process.« less