Structural Isotopic Effects in the smallest chiral amino acid: Observation of a structural phase transition in fully deuterated alanine.

A first study of possible changes instigated by deuteration in amino acids was carried out using neutron diffraction, inelastic neutron scattering and Raman scattering in L-alanine, C$_{2}$H$_{4}$(NH$_{2})$COOH. Careful analysis of the structural parameters shows that deuteration of L-alanine engenders significant geometric changes as a function of temperature, which can be directly related to the observation of new lattice vibration modes in the Raman spectra. The combination of the experimental data suggests that C$_{2}$D$_{4}$(ND$_{2})$COOD undergoes a structural phase transition (or a structural rearrangement) at about 170 K. Considering that this particular amino acid is a hydrogen-bonded system with short hydrogen bonds (O{\ldots}H $\sim $ 1.8 {\AA}), we evoke the Ubbelohde effect to conclude that substitution of hydrogen for deuterium gives rise to changes in the hydrogen-bonding interactions. The structural differences suggest distinct relative stabilities for the hydrogenous and deuterated L-alanine. De Souza et al. - Journal of Physical Chemistry B (Letters) \textbf{111}, 5034-39 (2007)