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We present a Raman spectroscopy investigation of the vibrational properties of L-histidine crystals at low temperatures. The temperature dependence of the spectra show discontinuities at 165 K, which we identify with modifications in the bonds associated to both the NH 3 + and CO 2 -motifs indicative of a conformational phase transition that changes the intermolecular bonds. Additional evidence of such a phase transition was provided by differential scanning calorimetry measurements, which identified an enthalpic anomaly at ~165 K.

Raman spectra of DL-Alanine crystals were investigated in the 50-3200 cm −1 spectral region for temperatures ranging from 15 to 295 K. The crystalline structure of DL-Alanine represents a rare example of an amino acid racemate crystallizing in a non-centrosymmetric space group. From this study, we have observed changes in the wavenumber of modes associated with both rocking of CO 2 − and skeletal vibrations. On the other hand, neither changes in the modes associated to CH or CH 3 vibrations nor substantial modifications of the lattice modes of the crystal were observed. Such result indicates slight changes of the CO 2 group orientation without observation of a solid-solid phase transition in the DL-Alanine crystal.