Single crystals of L-valine, C 5 H 11 O 2 N, have been studied by Raman spectroscopy at temperatures from 17 to 300 K over the spectral range 20-3300 cm −1 . A tentative assignment of the bands is given. Careful analysis of the Raman spectra suggests that a series of changes in the low-wavenumber region may be associated with a phase transition undergone by the material, and this possibility is discussed.

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Pressure-dependent Raman scattering studies in the range 0.0 -32 kbar were carried out in L-alanine in order to investigate its external mode phonon spectra in relation to the phase transitions in the crystal. A careful analysis of the spectra shows that the low-energy Raman modes exhibit variation both in frequency and in intensity and between 26 and 28 kbar it is observed a splitting of a external mode, indicating that the D 2 normal phase undergoes a transition. Pressure coefficients for external modes are also given.

Polarized rst-order Raman scattering studies at 300 K were carried out in L -asparagine monohydrate crystal in order to obtain the general assignment of its phonon spectra. A careful analysis of the vibrational spectra shows that the assignment of the fundamental vibrational modes can be done on the basis of amino, carboxilic and water groups vibrations, and correlations with previous data reported for other amino acids. However, some vibrations were correctly assigned from the study of its temperature -dependence behavior performed in the range 10 -300 K. The assignment proposed con rm the zwitterionic structure of L-asparagine monohydrate crystal.

In this chapter, we investigate the Raman spectra of proteinogenic amino acid crystals. Amino acids are fundamental organic molecules that compose polypeptides a linear chain of amino acids and proteins folded polypeptides with speciic functions found in all living beings. Surprisingly, the number of these basic molecules is not more than 22 20 of them commonly known as the standard amino acids, plus pyrrolysine and selenocysteine . They are deined as a molecule formed by an NH 2 group, a COOH group, a lateral chain the R group , and a hydrogen atom, all of them connected to a single carbon, the -carbon. Interestingly, -amino acids show chirality, i.e., they present diferent distributions of group of atoms around the -carbon, being deined as l-and d-form. For amino acids and proteins found in the living beings, the l-form is the dominant form, although some exceptions have been discovered in the last decades. In this chapter, we present the Raman spectra of all standard amino acids and discuss the diferent kinds of vibrations found, comparing them. As complementary part of the work, we present results on vibrational properties of some amino acids using Raman spectroscopy when subjected to speciic conditions, with variation in temperature or pressure. Finally, we present some perspectives as the investigation of purines, a group of molecules associated with the DNA molecule.