Pressure-induced phase transitions in DL-glutamic acid monohydrate crystal

Highlights

• Glutamic acid monohydrate crystal presents three reversible phase transition at high pressure.

• Raman spectroscopy proven to be an important tool in the detection of phase transition.

• Water molecules play important role on the structural stability of the crystal.

Abstract

DL-glutamic acid monohydrate crystal was synthesized from an aqueous solution by slow evaporation technique. The crystal was submitted to high-pressure (1 atm–14.3 GPa) to investigate its vibrational behavior and the occurrence of phase transitions. We performed Raman spectroscopy as probe and through the analysis of the spectra we discovered three structural phase transitions. The first one occurs around 0.9 GPa. In this phase transition, glutamic acid molecules suffer modifications in their conformations while water molecules are less affected. The second phase transition at 4.8 GPa involves conformational changes related to CO₂⁻, NH₃⁺ units and the water molecules, while the third one, between 10.9 and 12.4 GPa, involves motions of several parts of the glutamic acid as well as the water molecules. Considering the dynamic of high pressure, the second phase of DL-glutamic acid monohydrate crystal presented a better stability compared with the second phase of its polymorphs α and β L-glutamic acid. In addition, water molecules seem to play important role on this structural stability. All changes are reversible.

Introduction

High-pressure is a tool that enabled important contributions to the science and the industry, benefiting to many areas such as physics, Earth sciences, biology, food sciences, pharmaceutical research and biotechnology [[1], [2], [3], [4], [5]]. For instance, the study of particular substances submitted to high pressure and high temperature can help to the understanding of the Earth's interior composition [1,2]. The effect of compression and decompression on inactivation of microorganisms has also been studied [3]. In particular, concerning the topic of the present paper, the discovery of new amino acid polymorphs (including the racemic ones) obtained under high-pressure has attracted attention in recent years [[6], [7], [8], [9], [10]].

High pressure has naturally a large effect on the low dimensional compounds, like two-dimensional (2D), 1D and 0D (i.e. molecular) crystals, where the interactions between the building units are weak (van der Waals or hydrogen bonds). For example, 2D hybrid halide perovskite is an emerging compound for the next generation of photo-electronic materials. Such a compound has been studied under pressure [11] and the contribution of both organic and inorganic components in the observed pressure transformation have been elucidated. In another work, an organic-inorganic perovskite-like hybrid multiferroic also presented different pressure induced modification related to its components [12]. Raman spectroscopy proved to be a powerful tool in the interpretation of the phase transitions. This technique made it possible to probe each part of the molecule and detect some distortion, or even extract information about structural changes.

As application of pressure sensor, a pressure-induced emission (PIE) and pressure-induced emission enhancement (PIEE) have been obtained from low-dimension halide perovskites and triphenylethylene, respectively [13,14]. These studies demonstrated the effect of high pressures on photoluminescence caused by changes in intermolecular interactions.

Chemical pressure has also become another promising field, which could give us insights about the hydrostatic pressure effects on various materials. The effect of chemical substitution (change of atoms, for instance) may cause distortions equivalent to a pressure applied at one crystallographic site, and is then similar to those observed in high-pressure experiments [[15], [16], [17]].

Amino acids are fundamental molecules to be studied in relation with life sciences, and high pressure should be utilized to determine their stability range and the possible damages they suffer under non-ambient conditions. They are the smallest elemental units of proteins, corresponding to the molecular bases of living organisms. These compounds have the general formula HCCO₂⁻NH₃⁺R, where R is a characteristic side chain of each molecule. Among the 20 protein-forming amino acids, glutamic acid (or glutamate) is considered one of the most abundant in the nature. It is classified as non-essential, which means that it can be synthesized by the body through other amino acids. It acts as an excitatory neurotransmitter in the brain and changes in this process has been associated with neurodegenerative diseases [18].

Hydrogen bonds and van der Waals interactions play an important role in the packaging process of the molecules in the crystalline lattice and their structural stability [19,20]. Distortions in the hydrogen bonds can be induced by the action of external forces. Pressure varying studies may help to understand the nature of these bonds (including to estimate binding energy and force constants) as well as to better understand the role of hydrogen bonds in determining the structures and properties of systems [21].

Two crystallographic forms of glutamic acid are known, both orthorhombic. The α-form has been described as early as 1931 [22], belongs to the P2₁2₁2₁ space group with a = 7.06, b = 10.3 and c = 8.75 Ǻ and 4 molecules in the unit cell; the β-form belongs to the same space group with a = 5.17, b = 17.34 and c-6.95 Ǻ and Z = 4 [23].

The β-form has been studied up to 21.5 GPa, and the modifications observed in the Raman spectrum were associated with four phase transitions in the pressure range 0.0–1.3 GPa; 1.9–3.0 GPa; 5.4–6.4 GP and 13.9–15.2 GPa [24]. The α-form has been studied up to 7.5 GPa and two phase transitions were observed. The first one between 1.9 and 2.3 GPa and the second one between 3.3 and 3.7 GPa. Both phase transition are characterized by changes in lattice modes [25]. Thus, the α-form appears as slightly more stable at high pressures.

Additionally, the effect of pressure on glutamic acid hydrochloride was investigated in the range of 0–10 GPa; a structural phase transition was observed at about 2.1 GPa, characterized by a large reduction of the intensity of the bands assigned as lattice modes [26].

Racemic amino acids have been studied under high pressure. These studies were performed on DL-cysteine, DL-alanine, DL-alaninium semi-oxalate mono-hydrate and DL-leucine; all these amino acid crystals undergo phase transitions [6,8,9,27]. However, to date, no high-pressure study has been performed on the DL-glutamic acid monohydrate crystal, an interesting system where hydrogen bonds play special role on the crystal structure.

The objective of the present work is to study the behavior of the vibrational modes of DL-glutamic acid monohydrate (DLAGM) crystal under high pressures up to 14 GPa using Raman scattering and discuss the modifications observed in the Raman spectra and present possible interpretations.