G.S. Pinheiro

Low-temperature phase transformation studies in the stearic acid: C form.

This paper reports the temperature-dependent measurements in the C form of stearic acid. Raman scattering, X-ray diffraction, and differential scanning calorimetry measurements were performed at low temperatures. The polarized Raman spectra were measured for temperatures ranging from 8 to 300 K over the spectral range of 30-3000 cm(-1). The spectral changes observed in both the lattice vibrational modes and the internal vibrational modes regions of the Raman spectrum, allowed to identify a phase transition undergone by the stearic acid crystal occurring between 210 and 170 K and a change in the structure continues to be observed down to 8 K. The anharmonicity of some vibrational modes and the possible space groups presented by the crystal at low temperatures were also discussed. Low-temperature X-ray diffraction measurements were performed from 290 to 80 K and the results showed slight changes in the lattice parameters at ∼200 K. Furthermore, the evidence of the phase transformation was provided by the differential scanning calorimetry measurements, which identified an enthalpic anomaly at about 160 K.

High Pressure Raman Spectra of Amino Acid Crystals

Amino acids are molecules with general formula HCCO2-NH3+R, where R is a lateral chain characteristic of each molecule, which form the proteins of all living beings. Due the fact that both hydrogen bonding (HB) interactions play a central role on the secondary structure of proteins and the amino acids in crystal structure present complex networks of HB, they have been studied extensively in the last years (Barthes et al., 2004; Boldyreva et al., 2003a, 2003b, 2004, 2005a, 2007a, 2007b; Dawson et al., 2005; Destro et al., 1988; Facanha Filho et al., 2008, 2009; Freire, 2010; Funnel et al., 2010; Goryainov et al., 2005, 2006; Harding & Howieson, 1976; Herminio da Silva et al., 2009; Lima et al., 2008; Migliori et al., 1988; Murli et al., 2003; Olsen et al., 2008; Sabino et al., 2009; Teixeira et al., 2000; Tumanov et al., 2010; Yamashita et al., 2007). These studies can be seen as important background to understand both static structure and dynamics properties of proteins such as denaturation, renaturation, folding itself, changes of folds, among others (Freire, 2010). The simplest protein amino acid is ┙glycine, which was investigated by Raman spectroscopy under high pressure conditions, being discovered that up to 23 GPa the crystal structure does not undergo any phase transition, although modifications in the Raman spectra indicate changes in the intra-layer HB interactions (Murli et al., 2003). In this chapter we investigate the effect of high hydrostatic pressure on L-histidine hydrochloride monohydrate (HHM) and L-proline monohydrate (PM) crystals, in particular observing the effect of pressure on the vibrations related to hydrogen bonds observed in these amino acid crystals.

The temperature-dependent single-crystal Raman spectroscopy of a model dipeptide: L-Alanyl-L-alanine

A single-crystal of peptide L-alanyl-L-alanine (C6H12N2O3) was studied by Raman spectroscopy at low-temperature, and a tentative assignment of the normal modes was given. Evidence of a second order structural phase transition was found through Raman spectroscopy between the temperatures of 80K and 60K. Group theory considerations suggest that the transition leads the sample from the tetragonal to a monoclinic structure. Additionally, our study suggests that the mechanism for the structural phase transition is governed by the occupation of non-equivalent C1 local symmetry sites by the CH3 molecular groups. Analysis based on group theory suggests L-alanyl-L-alanine presents C2 symmetry at low temperatures.

Temperature-induced isostructural phase transition on NaCe(MoO4)2 system: A Raman scattering study

Temperature-dependent Raman spectroscopic study has been performed on scheelite-type sodium‑cerium molybdate - NaCe(MoO4)2 - in the temperature range 113-873 K. This study provides phonon properties of NaCe(MoO4)2, which are very important to understand the mechanism governing eventual phase transitions undergone by the structure, since phonons are very sensitive to structural changes. The ambient scheelite phase remains stable in a low-temperature range (113-293 K), and no relevant modification is observed in the Raman spectra. However, the experiments reveal the existence of one reversible phase transition at high-temperature. The vibrational spectra of NaCe(MoO4)2 system showed anomalies above 748 K, where overlaps of some bands and the appearance of a band at 458 cm-1 are observed. These modifications were attributed to an isostructural phase transition and a discussion about the possible mechanism of this transformation is furnished.

Phase transformation in the C form of myristic-acid crystals and DFT calculations

In this study, the vibrational frequencies of myristic acid (CH3(CH2)12COOH) were obtained using density functional theory calculations, and the results were compared with experimental Raman and infrared data. Additionally, Raman spectra of crystalline myristic acid were recorded in the 300-20 K range. Raman spectroscopy gives important insights into the effect of low temperatures on its monoclinic phase. X-ray diffraction was performed from 298 to 133 K to provide additional information about the cryogenic behavior of the crystals. These undergo a phase transformation, which was confirmed by differential scanning calorimetry through an enthalpy anomaly observed at low temperatures. Raman spectra and X-ray diffraction refinement of the cell parameters in combination with differential scanning calorimetry at low temperatures revealed slight modifications, confirming a conformational change in the myristic acid molecules involving rearrangement of dimers within the unit cell.