Anharmonicity in Amino Acids

Two special dynamical transitions of universal character have been recently observed in macromolecules (lysozyme, myoglobin, bacteriorhodopsin, DNA, and RNA) at $T^{*}\sim 100 - 150$ K and $T_{D}\sim 180 - 220$ K. The underlying mechanisms governing these transitions have been subject of debate. In the present work it is reported a survey on the temperature dependence of structural, vibrational and thermodynamical properties of a nearly anhydrous amino acid (orthorhombic polymorph of the amino acids L-cysteine and L-proline at a hydration level of $3.5\%$). The temperature dependence of X-Ray diffraction, Raman spectroscopy, and specific heat were considered. The data were analyzed considering amino acid-amino acid, amino acid-water, and water-water phonon-phonon interactions, and molecular rotors activation. Our results indicated that the two referred temperatures define the triggering of very simple and specific events that govern all the interactions of the biomolecule: activation of CH$_{2}$ rigid rotors ($TT_{D}$).