engleski

Study of stable L-alanine radicals by W-band EPR spectroscopy

It is well known from previous investigations that an EPR spectrum of γ -irradiated L-alanine consists of the three stable paramagnetic centers with two of them dominating at room temperature: SAR1 and SAR2. The SAR1 center has been studied for many years and remains the most frequently studied paramagnetic center of a simple amino acid in the solid form. Despite of various specialized EPR techniques and methods employed, dynamic process associated with the possible motion of the whole SAR1 center is still not fully understood. This deficiency has been also reflected in the recent theoretical modeling of SAR1 center where a good agreement was obtained only for the center itself without any interaction with surrounding atoms in the crystal matrix. Moreover, internal rotations of the CH3 group and unknown exact position and dynamic properties of NH3+ group after break up from an undamaged molecule in crystalline lattice produce even more complex local environment of SAR1 center that challenge theoretical description at room temperature. Thus, for improving description of the center, a more accurate estimation of its position and a more detailed evaluation of dynamical properties are essential. In order to investigate detail internal and local dynamics of SAR1 center the present study employs continuous wave, CW, and pulse EPR at two detection frequencies to investigate the frequency dependent terms of relaxation rate. In the process of analyzing spectral components obtained simultaneously at X-band and W-band frequencies the rotation correlation time of CH3 protons was estimated at room temperature. It was also noted that an excess broadening at W-band involves a term that averages anisotropy of g-matrix. This indicates a motion of the SAR1 center as a whole.

L-alanine; paramagnetic center; relaxation time

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