Amino acids are molecules that form the proteins of all living beings and have the general formula HCCO2-NH3+R, where R is a lateral chain characteristic of each molecule. In crystalline state amino acid are mainly linked by hydrogen bonds, giving rise to a series of structures.
High-pressure measurements can cause modifications in the hydrogen bonds that stabilizes the unit cell of amino acid crystal and many interesting results are observed. For instance, Glycine is the simplest amino acid (its side chain has only the H atom) have three polymorphs at ambient conditions (α, β, and γ). β-glycine undergoes a phase transition at 0.76 GPa (01), this modification is accompanied by many changes in the Raman spectrum of the crystal, including jumps and kinks at the curves of wavenumber band positions vs. pressure. γ-glycine presents a very long phase transition beginning at 2.7 GPa, when changes in the volume of the unit cell extending at least up to 7.8 GPa are verified. On decompression, part of the new phase – the δ-phase – remains stable down to atmospheric pressure; additionally, another phase, assigned as ζ-phase, was observed for pressures below 0.62 GPa (02-05). Surprisingly, the structure of the α-form is stable for pressures up to 23 GPa (06). A reversible crystal to amorphous phase transition was observed in L-alanine crystal at about 15.0 GPa (07). L-asparagina monohydrated undergone some phase transitions but remains crystalline for pressures up to 30 GPa (08). Other facet of amino acid that attracts attention of researchers is the comparison between responses of L and DL-enantiomers to high pressure. DL-valine was recently studied under pressure up to 20 GPa and phase transitions around 1.4 and 8.8 GPa were observed (09). Additionally, two phase transitions were also reported for L-valine (10) at different pressures.
Combining Raman spectroscopy and high-pressure technique it is a useful manner to identify phase transitions, amorphisation and compare the behavior of different polymorphs. In this work it will be shown some results on Raman spectroscopy applied to amino acids crystals published in the last 20 years.
References
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