Micro-conformational transitions in L-alanine single crystals revisited by low wavenumber Raman spectroscopy

The properties of the crystalline L-alanine have been attracting attention for a very long time, and there is much controversy in the results already reported. One of the most interesting problems is that of the localization of vibrational energy and the micro-transitions (named 'breathers' by Barthes et al. [M. Barthes, H.N. Bordallo, F. Denoyer, J.-E. Lorenzo, J. Zaccaro, A. Robert, F. Zontone, Eur. Phys. J. 2004, B37, 375]. In the present study, we have addressed this problem once again, having measured the high-resolution polarized Raman spectra in the temperature range from 4 to 300 K. The wavenumber, bandwidth and intensity of all the modes polarized along the c-axis (the axis of the head-to-tail chains of zwitterion in the structure) show discontinuities at 30-50, 100-120 and 180-220 K. We interpret the observed anomalies of spectral characteristics in terms of micro-conformational transitions accompanied by the rearrangement of the hydrogen-bond network and relate them to previously published work. Copyright (C) 2010 John Wiley & Sons, Ltd.