  Two special dynamical transitions of universal character have been recently
observed in macromolecules at $T_{D}\sim 180 - 220$ K and $T^{*}\sim 100$ K.
Despite their relevance, a complete understanding of the nature of these
transitions and their consequences for the bio-activity of the macromolecule is
still lacking. Our results and analysis concerning the temperature dependence
of structural, vibrational and thermodynamical properties of the orthorhombic
polymorph of the amino acid L-cysteine (at a hydration level of 3.5%) indicated
that the two referred temperatures define the triggering of very simple and
specific events that govern all the biochemical interactions of the
biomolecule: activation of rigid rotors ($T<T^{*}$), phonon-phonon interactions
with phonons of water dimer ($T^{*}<T<T_{D}$), and water rotational barriers
surpassing ($T>T_{D}$).
