Avinash Joshi and Russell H. Johnsen
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Kinetics of Radical Decay in Crystalline Amino Acids. II. High-Temperature Study Avinash Joshi and Russell H. Johnsen* DepaHment of Chemistry, Florida State University, Tallahassee, Florida 32306 (Received June 20, 1975) Publication costs assisted by the Energy Research and Development Administration
Radical decay in X-irradiated L-leucine, and DL-valine, L- and DL-alanine, and L-arginine hydrochloride was studied between 300 and 430 K. Free radical decay was observed to take place with two distinct rates: an initial short duration process having an activation energy of 18 kcal/mol 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 0 b ~ e r v e d . lThe ~ major process has a 3-6-kcal/mol higher activation energy. There is no evidence of radical conversion in either the initial or the slower process. However, in a t least two cases residual peroxy radicals were observed a t the termination of the slower process. Both decays obey second-order rate laws. The peroxy radical in a t 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 a t 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 deuterated 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.
Scheme I
Introduction The free radicals produced by ionizing radiation in many amino acids have been identified by ESR, ENDOR, and ELDOR techniques.l-s In all this work it has been realized that, in general, the initially produced free radicals are not stable a t room temperature. They undergo series of conversions before a radical, relatively stable at room temperature, is produced. Many of these conversions have been studied. It is well known that organic compounds are ionized, when irradiated, into electrons and positively charged molecules. These electrons are seeng to be trapped in some frozen glasses. Thus the initial process in case of amino acids is thought to be
,coo-
cation
anion
Ia
Ib
