A pulse EPR study of longitudinal relaxation of the stable radical in gamma-irradiated L-alanine

The longitudinal relaxation rate of the first stable alanine radical, SAR1, was studied by employing pulse EPR technique over a wide temperature inter val (5-290 K). The complex nonexponential recovery of the longitudinal magn etization in this temperature interval has been described with two characte ristic relaxation times, 1 / T-1a*. as the faster component and 1 / T-1b*, as the slower component, respectively. It was shown that 1 / T-1a*, is stro ngly affected by the CH3 group dynamics of the SAR1 center. The complete te mperature dependence of 1 / T-1a*, was described by invoking several relaxa tion mechanisms that involve hindered motion of the CH3 group from classica l rotational motion to coherent rotational tunneling. It was shown that all relevant relaxation mechanisms are determined by a single correlation time with the potential barrier (DeltaE / k = 1570 K). On the other hand the te mperature dependence of 1 / T-1b* is related to the motional dynamics of th e neighborly NH3 and CH3 groups. We found a larger average potential barrie r for this motion (DeltaE / k = 2150 K) corresponding to smaller tunneling frequencies of the neighbor groups. (C) 2001 Academic Press.