  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 was provided by differential
scanning calorimetry measurements, which identified an enthalpic anomaly at
~165 K.
