engleski

Dynamic disorder in γ -irradiated L-alanine crystal studied by ENDOR spectroscopy

L-alanine as the smallest naturally occuring amino acid, as well as radiation induced stable L-alanine radicals, SAR1 and SAR2, have been already extensively studied over many years. The first stable alanine radical, SAR1, has been examined by various electron paramagnetic resonance techniques (ESR, FT ESR, ENDOR, ELDOR, ESEEM, HYSCORE). Despite of these investigations, molecular environment of the radical regarding to the unusual large shift of the central carbon atom and the dynamic processes associated with the radical motion are still not fully understood. Understanding structure and thermally driven motions in crystalline amino acids are of great interest for their biological applications, because of their contribution to dynamic properties of the proteins. Thus, for complete descriptions of the SAR1 centre one expects that additional improvement on more accurate estimation of the local position of the centre and on more detailed evaluation of dynamical properties are essentials. In order to examine motional dynamics of the SAR1 centre as a whole, the study of temperature dependence of the α -proton hyperfine coupling have been undertaken in the temperature interval from 160 K to 330 K. The present ENDOR study indicates averaging of the α -proton hyperfine coupling between two different conformational structures of SAR1 and presents evidence of the local dynamics. Comparison of the data of the ENDOR frequency shift of the α -proton hyperfine coupling for the cyrstal measured immediately after irradiation and two years old the suports the model in which SAR1 centre shows local disorder (changes in the orientation of molecular fragments). The experimentally obtained energy difference between these structures is Δ E~700 K is in the good agreement with results extracted by Raman spectroscopy and SAR1 centre at room temperature represent an statistical average between two conformations of SAR1 centres.

alfa-proton; ENDOR; L_alanine

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