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W-band EPR study of stable radicals in gamma irradiated L- and D-alanine

L-alanine radicals have been explored extensively using EPR spectroscopy by numerous experimental groups for the long period of time since 1950s. It is well known that at least three different radicals induced by radiation are formed in single crystal of L-alanine at room temperature. As a result of extensive EPR, ENDOR (Electron Nuclear Double Resonance), EIE (ENDOR induced EPR) and HYSCORE (Hyperfine Sub- level Correlation) studies on irradiated L- alanine, a substantial amount of experimental data concerning electronic g-values and hyperfine coupling constants of magnetic nuclei in alanine-derived radicals are available. In order to reproduce obtained experimental data, various detailed microscopic models were suggested to describe the first stable L- alanine radical. The other two radicals have not been explored in such details. Therefore, open questions are tempting today. Moreover, studies on L-alanine radicals, in contrast to D-alanine, were in the spotlight, despite of EPR results on D-alanine which show different substructure and radical yield. Most of the EPR spectroscopic investigations that have been performed were limited to low frequency (mainly X band i.e., ~ 9.5 GHz), and only a few studies have been performed in Q band (~ 35 GHz) and W band (~ 95 GHz). This limitation in spectral and time resolution restricts the wealth of information which could be accessed from the magnetic properties of these amino acid radicals. In this report, EPR measurements at high microwave frequency (W band) and corresponding high magnetic field were performed to enhance the resolution of the differences in the g values in order to get insight into subtle differences in the substructure of both enantiomers of alanine, L- and D-alanine. In addition, comparison of the temperature dependent relaxation data, focused on dynamics of the different parts of molecules of both enantiomers of alanine, will be reported.

alanine; chirality; EPR; relaxation

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