Pressure‐induced phase transitions in L‐leucine crystal

Abstract

Raman spectra of a crystal of L-leucine, an essential amino acid, were obtained for pressures between 0 and 6 GPa. The results show anomalies at three pressure values, one between 0 and 0.46 GPa, another between 0.8 and 1.46 GPa, and a third at P ∼ 3.6 GPa. The first two anomalies are characterized by the disappearance of lattice modes (which can indicate occurrence of phase transitions), the appearance of several internal modes, or the splitting of modes of high wavenumbers. The changes of internal modes are related to CH and CH 3 unit motions as well as hydrogen bonds, as can be inferred from the behavior of bands associated with CO 2 − moieties. The third anomaly is a discrete change of the slopes of the wavenumber versus pressure plots for most modes observed. Further, decompression to ambient pressure generates the original Raman spectrum, showing that the pressure-induced anomalies undergone by L-leucine crystals are reversible.

Low‐temperature Raman spectra of racemate DL‐Alanine crystals

“…This implies that, in general, under cooling these systems undergo only small rotations relative to their crystallographic axes. However, when L-methionine, [32] L-leucine, [33] L-cysteine, and DL-cysteine [34] are subjected to high pressure conditions, large changes are visible on the band profile in the higher spectral region. This is related to prominent modifications of the conformation of the respective molecules in the unit cell.…”

Raman study of nonhydrostatic pressure‐induced phase transitions in monoclinic L‐aspartic acid crystals

“…Although the structure of amino acid crystals is significantly different from that of proteins, the study of the hydrogen bonds, layers, and molecular chains in amino acid crystals under pressure is helpful for understanding the compressibility of the structural fragments of peptides and proteins . Furthermore, studying the stability of amino acid crystals under high pressure is needed to increase the understanding of the structure of biological systems, including those involved in protein folding …”

“…Many studies that have been of importance to the investigation of amino acid crystals, such as those containing glycine, asparagine, alanine, leucine, isoleucine, methionine, taurine, threonine, valine, cysteine, and serine, under pressure higher than 2 GPa have been published in the last 20 years . Some unique properties of amino acid crystals under high pressure have been observed by scholars; for example, phase transitions have been discovered in cysteine and leucine crystals at pressures lower than 3.0 GPa . Furthermore, structural changes occurring at much higher pressures were also investigated .…”

High Pressure Raman Spectra of Amino Acid Crystals

“…Differently, the changes observed between 0.8 and 1.46 GPa occurs both in the internal modes and in the lattice modes of the crystal, indicating a structural phase transition undergone by L-leucine. Finally, around 3.6 GPa change of the slopes of the frequency versus pressure plots associated with CO 2 -moieties were observed, pointing to changes associated to hydrogen bonds (Façanha Filho et al, 2009). The Raman spectrum of L-valine (C 5 H 11 NO 2 ) under high pressure conditions, presents several changes between 0.0 and 6.9 GPa (Hermínio da Silva et al, 2009).…”

“…At atmospheric pressure and room temperature, L-leucine (C 6 H 13 NO 2 ) crystallizes in a monoclinic structure (C 2 2 ) (Harding & Howieson, 1976), although there are reports of a second phase for temperatures higher than 353 K (Façanha Filho et al, 2008) and a third polymorph resulting from residues of a solution at 200 Ma (Yamashita et al, 2007). A series of modifications on the Raman spectrum of L-leucine crystal were observed when it was submitted to high pressure conditions (Façanha Filho et al, 2009). The modifications occur in three different pressure ranges: (i) between 0 and 0.46 GPa, (ii) between 0.8 and 1.46 GPa, and (iii) at around 3.6 GPa.…”