**Acknowledgements**

of the crystals under extreme conditions, low temperature or high pressure. However, a complete understanding involving connections, for example, between the hydrogen bonds and the physical properties of the crystal is still lacking. Obviously, some preliminary attempts are already known, such as a possible connection between the dimensions of hydrogen bonds

+

and taurine [17]). A fundamental question in biochemistry is to realize why the proteins of all living beings are formed by the l-form of amino acids (the d-form is found only isolated in the plasma of certain cells). Some glimpses were given by Abdus Salam who speculates the occurrence of a phase transition explained through BCS theory, gauge field theory, and Higgs mechanism [44]. There is also suggestion that ultraviolet radiation should be able to select one of the chiral forms of the amino acid, but, in fact, all these suggestions are suppositions waiting for confirmation. This problem deserves future investigations. But, is the behavior of d-amino acid crystals the same of l-amino acids under extreme conditions? At first, the answer to this question should be positive because both l- and d-forms of the amino acids are equivalent from an energetic point of view. However, some preliminary results point to different behavior for the two forms in some special cases, but we do not have space to discuss such intriguing point in this chapter. Maybe, surprising information is waiting for us in the coming years.

The success obtained by the investigation of amino acids has incentivized the study of other simple organic molecules of living beings. After furnishing a more or less closed picture about amino acids, the next natural step is the study of peptides, but we will not discuss them in this chapter. We prefer to analyze another natural choose, molecules involved in the DNA structure. One example we will explore in this chapter is thymidine, a nucleoside constituted of a deoxyribose and the pyrimidine base thymine. It is found in the DNA of all living organisms. The Raman spectrum presents a very intense set of bands in the low wavenumber region that are associated with the lattice modes (**Figure 12**). This is very interesting because in future analysis of the crystal under extreme conditions, the behavior of the lattice modes should be a pivotal point in order to understand eventual structural modification. A strong band observed at 1665 cm−1 is assigned as in-plane vibration involving C = O and C = C and a band at 1690 cm−1 is assigned as stretching C = O, ν(C=O). Bending of CH3, δ(CH3), is identified as the band at 1438, 1457, and 1480 cm−1. The band observed at 1031 cm−1 is associated with bending of CNH, δ(CNH), and the band at 1000 cm−1 is associated with bending OCH, δ(OCH). An out-of-plane vibration involving CH is observed at 972 cm−1 and a pyrimidine ring breathing is observed at 773 cm−1. Additionally, out-of-plane vibration involving CCH3 group is observed at 396 and 378 cm−1 and in plane vibration involving the same group is observed at 276 and 306 cm−1. In the high wavenumber region of the Raman spectrum is possible to observe a series of bands, among them one observed at 3298 cm−1 that was assigned as stretching of OH, ν(OH). A series of bands is observed at 2952, 2965, 2973, and 2991 cm−1 and they are classified as stretching of CH, CH2, and CH3 units. Finally, let us single out an important point related to the study of thymidine, its behavior as a function of temperature.

under high pressure (for l-alanine, l-threonine,

and the behavior of torsional vibration of NH3

218 Raman Spectroscopy and Applications

**7. Beyond amino acids**

The authors acknowledge financial support from CNPq and FUNCAP through PRONEX program.
