Vibrational dynamics of crystalline L-alanine

The authors report a new, complete vibrational analysis of L-alanine and L-alanine-d{sub 4} which utilizes IINS intensities in addition to frequency information. The use of both isotopomers resulted in a self-consistent force field for and assignment of the molecular vibrations in L-alanine. Some details of the calculation as well as a comparison of calculated and observed IINS spectra are presented. The study clarifies a number of important issues on the vibrational dynamics of this molecule and presents a self-consistent force field for the molecular vibrations in crystalline L-alanine

Low-wavenumber dynamics of L-alanine

L-Alanine is one of the simplest amino acids, the building block for many polypeptides and proteins. A previous Raman scattering study showed an anomalous temperature dependence of the intensity of the two lowest Raman modes, attributed to dynamic localization of vibrational energy. The present Raman data, collected in the b(cc)b scattering geometry, were aimed at understanding the dynamics of the low-wavenumber modes in connection with the reported anomalies in other physical properties. Our study shows that the energy and intensity of the observed external modes show unusual behaviour in the temperature range similar to 150-250 K. The anomalous temperature dependence of the peak of the NH3+ torsional mode is confirmed. Its wavenumber has a singularity around similar to 220 K and its full width at half-maximum does not increase linearly with temperature. The CH3 torsion and CO2- rock undergo similar discontinuities below similar to 250 K. The intensity of the two lowest wavenumbers librons at 42 and 48 cm(-1) shows an anomalous temperature dependence and vary in an opposite way from that reported for the c(bb)a scattering geometry. The nature of the subtle symmetry breaking occuring below similar to 250 K is discussed. Copyright (c) 2005 John Wiley & Sons, Ltd