Low-Temperature Raman Spectra of L-Histidine Crystals

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 NH3 (+) and CO2 (-) motifs indicative of a conformational phase transition that changes the intermolecular bonds. Additional evidence of such a phase transition is provided by differential scanning calorimetry measurements, which identified an enthalpic anomaly at similar to 165 K.