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

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 230 (2020) 118059

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Pressure-induced phase transitions in DL-glutamic acid monohydrate crystal F.M.S. Victor a, F.S.C. Rêgo b, F.M. de Paiva b,d, A.O. dos Santos a, A. Polian c, P.T.C. Freire b, J.A. Lima Jr b, P.F. Façanha Filho a,⁎ a

Universidade Federal do Maranhão, CCSST, Imperatriz, MA 65900-000, Brazil Departamento de Física, Universidade Federal do Ceará, Campus do Pici, Fortaleza, CE 60455-760, Brazil Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Université, UMR CNRS 7590, F-75005 Paris, France d Faculdade de Educação, Ciências e Letras de Iguatu, FECLI, Universidade Estadual do Ceará, Iguatu, CE 63502-253, Brazil b c

a r t i c l e

i n f o

Article history: Received 21 June 2019 Received in revised form 6 January 2020 Accepted 9 January 2020 Available online 10 January 2020 Keywords: Raman spectroscopy High pressure Phase transition Amino acid DL-glutamic acid monohydrate

a b s t r a c t 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 CO− 2 , 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 DLglutamic 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. © 2020 Elsevier B.V. All rights reserved.

1. 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 [1–5]. 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 [6–10]. 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 photoelectronic materials. Such a compound has been studied under pressure [11] and the contribution of both organic and inorganic components in ⁎ Corresponding author. E-mail address: [email protected] (P.F.F. Filho).

https://doi.org/10.1016/j.saa.2020.118059 1386-1425/© 2020 Elsevier B.V. All rights reserved.

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 highpressure experiments [15–17]. 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 nonambient conditions. They are the smallest elemental units of proteins,

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corresponding to the molecular bases of living organisms. These com+ pounds have the general formula HCCO− 2 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 αform 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. 2. Experimental 2.1. Crystal Growth DLAGM crystals were obtained from an aqueous solution by a slow evaporation technique using DL-glutamic acid monohydrate powder (Sigma-Aldrich 99%). The temperature was maintained around 25 °C, and the hydrogenic potential (pH) of the solution was 2.0. The solution was sealed with punched plastic wrap and then placed in the crystal growth chamber with temperature of 25 °C. The formation of the first crystals occurred after 6 weeks. 2.2. X ray diffraction measurements To confirm the crystal structure, X-ray diffraction patterns of the DLAGM powder were collected using a Bruker advanced X-ray diffractometer employing CoKα (λ = 1.78896 Å) radiation. The scanning angle 2θ was performed from 10 to 55° with a pitch of 0.02o and with a count time of 5 s/step. The structural characterization of DLAGM acid was obtained by a Rietveld refinement using the GSAS program [28]. 2.3. Raman spectroscopy and high-pressure measurements Raman spectroscopy experiment was performed using a Jobin-Yvon Horiba HR460 single monochromator spectrometer in the backscattering geometry. The excitation source was the 514.5 nm line of an argon

Fig. 1. X-ray diffraction pattern of DLAGM crystal recorded at ambient conditions and refined by the Rietveld method with factor Rwp = 9.98 and S = 1.34 for refinement quality. The inset shows two molecules of glutamic acid with one water molecule.

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laser. High-pressure experiment was performed between 0.0 and 14.3 GPa using a membrane diamond-anvil cell (DAC) [29] and stainless steel gaskets 200 μm initial thickness indented down to 50 μm. The culet of the diamond anvils was 400 μm in diameter and the experimental volume drilled in the gasket at the center of the imprint was 200 μm in diameter. Neon was used as a pressure transmitting medium and the pressure was monitored using the ruby fluorescence lines [30]. The orientation of the crystal in the DAC was not determined. The experiment presented a good scattering signal. It is worth mentioning that during the experiment the crystal did not change its orientation and did not crack. 3. Results and discussion Based on the pattern indicated by the X-ray diffraction, DLAGM crystal crystallizes in the expected orthorhombic system belonging to the space group Pbca (D2h) with eight molecules per unit cell (Z = 8) and lattice parameters a = 9.124 (2) Å, b = 15.505 (7) Å and c = 10.629 (4) Å. The parameters of the unit cell and the space group are in good agreement with those reported by Ciunik and Glowiak [31]. Fig. 1 shows the X-ray diffraction pattern of DLAGM crystal recorded at ambient conditions and refined by the Rietveld method with factor Rwp = 9.98 and S = 1.34 for refinement quality. In this structure, the glutamic acid molecule is in the zwitterionic form and the crystal network is stabilized mainly by N\\H⋯O and O\\H⋯O interactions between the molecules of the amino acids themselves and water molecules. Selection rules based on group theory show that the optical modes of DL-glutamic acid monohydrate are distributed among the irreducible

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representations of the D2h factor group as Γop = 66 Ag + 66 B1g + 65 B1u + 66 B2g + 65 B2u + 66 B3g + 65 B3u. 66 Ag + 66 B1g + 66 B2g + 66 B3g are the 264 Raman active modes, 65 B1u + 65 B2u + 65 B3u are the 195 infrared active modes and 1 B1u + 1 B2u + 1 B3u are acoustic modes [32]. As will be shown, DLAGM crystal presents three phase transitions, from ambient pressure up to 14.3 GPa. We will define the crystal phases as phase I (1 atm), phase II (0.9–3.8 GPa), phase III (4.8–11.8 GPa) and phase IV (12.4–14.3 GPa). Fig. 2(a–d) presents Raman spectra of DLAGM crystal in the spectral range 45–3650 cm−1 at ambient pressure and 0.9 GPa. The broad band around 100 cm−1 was fitted with 4 modes in the spectrum at ambient pressure. In Fig. 2a these four bands (marked with an arrow) become two very strong bands at 0.9 GPa. Similarly, the three weak bands around 1100 cm−1 become two strong ones at 0.9 GPa. On the other hand, the intensity of the bands marked with squares decreases at high pressure. Fig. 2b shows great differences of relative intensities for almost all bands. In the spectral range attributed to CH stretching ν(CH) (~2800–3100 cm−1), we observe strong changes of the relative intensities of the bands, as shown in Fig. 2c. On the contrary, Fig. 2d evidences no crucial modification related to the water molecules, since the O\\H stretching bands just undergo redshift and a little change in relative intensity. From the above results, we can infer that in the 1 atm–0.9 GPa pressure interval, a phase transition occurs involving glutamic acid molecules conformation, however, no significant change in the O\\H stretching was observed through the bands associated with the water molecules. Fig. 3a presents Raman spectra of DLAGM crystal in the spectral range 45–290 cm−1, corresponding to the lattice modes region, and

Fig. 2. Raman spectra of DLAGM crystal in the spectral region 45–3650 cm−1 at ambient pressure and 0.9 GPa. (a) 45–1185 cm−1. (b) 1350–1800 cm−1. (c) 2800–3100 cm−1. (d) 3350–3650 cm−1.

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Fig. 3. (a) Raman spectra of DLGAM crystal in the spectral region 45–290 cm−1 and pressure range 1 atm–14.3 GPa. (b) Wavenumber vs. pressure plots of DLGAM crystal for spectral region 45–290 cm−1.

pressure range 1 atm–14.3 GPa. Significant changes can be clearly distinguished in this region, particularly for the three lowest wavenumber bands, recorded at 69, 97 and 122 cm−1 (bands A, B and C at 0.9 GPa). The intensity of the A and B bands decreases between 3.0 and 3.8 GPa, besides splitting of the A band in this same pressure range. At 4.8 GPa, four new bands appear in the spectrum (indicated by F, G, * and H). The intensity of G and H bands increases from 4.8 to 11.8 GPa, that of X decreases, while the C band remains up to the highest pressure reached in the experiment. In fact, the spectra show modifications regarding the relative intensity of the bands when the pressure is increased from 4.8 to 11.8 GPa. When 12.4 GPa is reached, a new Raman pattern is observed, in particular with the appearance of peaks of relatively high intensity at ~141 and 180 cm−1. This pattern is maintained up to the highest pressure reached in this experiment. The above modifications occur in the spectral region associated with lattice modes and therefore are markers for three structural phase transitions undergone by DLGAM crystal. The correlation with internal modes will be discussed below. Fig. 3b presents another important indication of phase transition, the slope of the fitted curve to experimental data (dω/dP). In many cases, the modification of dω/dP is associated with the occurrence of a phase transition [6–10]. In the interval between 4.8 and 12.4 GPa the wavenumber of some modes presents a nonlinear behavior, indicating the contribution of hydrogen bonds for these modes. The behavior of DLGAM crystal presented previously is enough to allow us summarize the following main modifications: (a) From 1 atm to 0.9 GPa, a transition from phase I to phase II is observed. (b) From 3.0 to 3.8 GPa, little changes (involving the weakening of A and B bands) are observed, probably related to the hydrogen bond network, before the occurrence of the second phase transition. These

modifications present correlation with few internal modes, as will be shown in the next paragraphs. (c) From 4.8 to 11.8 GPa, the spectrum at 4.8 GPa shows the pattern of the new phase (phase III), with small changes related to intensity of some bands up to 11.8 GPa. Therefore, we can state the occurrence of a phase transition around 4.8 GPa. (d) From 11.8 to 12.4 GPa, an undoubted structural phase transition is demonstrated based on great spectral differences between the corresponding Raman spectra. (e) From 12.4 to 14.3 GPa, this new phase (phase IV), with characteristic spectra remains up to 14.3 GPa. Fig. 4a shows Raman spectra of DLAGM crystal between 1 atm and 14.3 GPa in the spectral region 230–610 cm−1. Vibrations associated – with skeletal of the molecules and NH+ 3 , COOH and CO2 units, characterize this spectral region [25,26]. The modifications of the spectra at 4.8 and 12.4 GPa are important, in agreement with the behavior of the lattice modes. Also, one notes that (i) the spectra at 3.0 and 3.8 GPa have different aspects when compared with spectra at 2.1 and 4.8 GPa (the intensity of the bands at ~500 cm−1) and (ii) the spectrum at 11.8 GPa is different from the spectra at 10.9 and 12.4 GPa, indicating an intermediate phase between well characterized phases. The strongest band around 270 cm−1 (E band), attributed to skeletal vibration, undergoes a blueshift of ~35 cm−1 from 3.8 to 4.8 GPa and splits and broadens at 12.4 GPa. The band at about 497 cm−1, assigned as τ(NH+ 3 ) [33], changes intensity between 2.1 and 3.8 GPa, as already mentioned. This τ(NH+ 3 ) is a very sensitive mode, directly involved in the hydrogen bonds. At 12.4 GPa, the band marked with a square, splits and remains up to 14.3 GPa. The band at 553 cm−1 (at 0.9 GPa) assigned to δ(CO− 2 ) also undergoes modifications both at 4.8 and 12.3 GPa, in particular an impressive redshift jump between 3.8 and 4.8 GPa. Then, while the band associated with τ(NH+ 3 ) is blue shifted with increasing

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Fig. 4. (a) Raman spectra of DLAGM crystal for pressure range 1 atm–14.3 GPa in the 230–609 cm−1 spectral region. (b) Wavenumber vs. pressure plots of DLGAM crystal for spectral region 230–609 cm−1.

pressure, the band assigned to δ(CO− 2 ) suffers redshift. This behavior is associated with the configuration of hydrogen bonds network. It will be shown that other bands related to CO–2 and OH units also present a similar behavior. Therefore, a more detailed discussion is found below. Finally, for this spectral range, the appearance of the band marked with a circle at 12.4 GPa can be attributed to the vibration of NH+ 3 or eventually to CO− 2 . All these results show changes related to units participating of hydrogen bonds for both transitions. Fig. 4b presents the dependence of wavenumber as a function of pressure. The discontinuities at 0.9, 4.8 and 11.8 GPa also helps us to have confidence in the occurrence of the three phase transitions. Raman spectra of DLGAM crystal in the spectral region 600–1300 cm−1 and pressure range 1 atm–14.3 GPa is presented in the Fig. 5a. In this spectral region, we find modes related to units as CO2, NH+ \C stretching. The strong band around 853 cm−1, 3 and C\ assigned to the C\\C stretching, loses intensity but remains well visible up to 14.3 GPa. The bands, marked with circles, also involving stretching vibration C\\C, ν(C\\C) [34], suffer few changes except a redshift when 4.8 GPa is reached (for the band at 917 cm−1 at ambient conditions). On the other hand, the band at 1161 cm−1 (marked with a square) assigned + as rocking of the NH+ 3 units, r(NH3 ), splits in two weak bands at 4.8 GPa and gain intensity when 12.4 GPa is reached. The phase transitions effects also can be seen by discontinuities in the wavenumber versus pressure plot in Fig. 5b, confirming the main results established through the previous analysis of the lattice modes spectral region. It is also important to mention that the intensity of modes δ(CO2), δ(CO2) and r (CH2), at 640, 671 and 770 cm−1, respectively, change substantially with increasing pressure (better observed in Fig. 6a). In the spectrum recorded at 0.9 GPa the mode at 770 cm−1 becomes the most intense

but its intensity decreases up to 3.8 GPa. Fig. 6b shows the intensity ratios I671/I640 and I770/I640 as a function of pressure. As can be seen a discontinuity occurs between 0.7 and 0.9 GPa. Such a behavior was reported previously in DL-valine [35] and DL-norvaline [36] and can be associated with conformational modifications in the molecules. Fig. 7a presents spectra of DLGAM crystal in the 1 atm–14.3 GPa pressure range and spectral region 1370–1800 cm−1. Before the second phase transition at 4.8 GPa, the intensity of band I increases, that of J decreases, and K broadens in the pressure range 2.1–3.8 GPa. Again, such a behavior can be correlated with those little changes observed in the lattice modes region related to the hydrogen bonds network. Above 4.8 GPa, two new bands are observed (L and M) and the K band becomes narrow again. From 1500 to 1800 cm−1 all the bands are redshifted at 4.8 GPa (from phase II to Phase III). The appearance of two bands readily observed in the spectrum at 11.8 GPa (marked with P and Q) clearly points to the confirmation of the third phase transition. Besides that, all other bands change intensity or merge (bands J, K and M). Therefore, the third phase transition causes strong modifications in parts of the glutamic molecules. At 12.4 GPa, the spectrum is similar to that at 11.8 GPa, unless for the disappearance of the O band. This pattern remains up to 14.3 GPa. Fig. 7b shows wavenumber versus pressure plot, highlighting discontinuities around the three phase transition points. The above spectral region presents modes associated with CH2, H2O and CO–2 units [33,34]. Considering the second phase transition (at 4.8 GPa), M band at 1447 cm−1, represents probably modifications of CH2 group, since J and K bands are attributed to ν(CH2). We assigned N band to deformation of the water molecule, δ(H2O) and O band to the C_O stretching, ν(C_O). Therefore, the second phase transition seems to generate strong modifications in these functional groups. The

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Fig. 5. (a) Raman spectra of DLGAM crystal in the spectral region 600–1300 cm−1 and pressure range 1 atm–14.3 GPa. (b) Wavenumber vs. pressure plots of DLGAM crystal for spectral region 600–130 cm−1.

disappearance of the O band at 12.4 GPa and the appearance of the Q band, for instance, represent modifications related to the carbonyl group. Here we can see again the redshift associated with CO and OH vibrations when phase III was reached. Since redshift/blueshift are related to lengthening/shortening of corresponding chemical bond, probably for the new phase at 4.8 GPa the length of CO and OH (here related to the water molecule) do increase. According to charge distribution on the molecules, the crystal presents different configurations of hydrogen bond network [10,37]. The stiffness of hydrogen bond depends on its length and angle. With compression, changes related to the angle may cause a weakening of the covalent bond involved in the hydrogen bond. Freire et al. [37] associated the redshift of τ(NH+ 3 ) with deviations of the linearity of hydrogen bond (N\\H⋯O). Our results seem to show that this behavior is not exclusive for NH+ 3 unit. However, it should be clear that redshift occurs only when the new phase is reached. This is different from the pressure dependence of the water molecule stretching, in which a redshift is observed for all pressure ranges (Fig. 9). Fig. 8a presents the Raman spectra of DLGAM crystal in the spectral region 2870–3180 cm−1 for pressures up to 14.3 GPa. We find four bands at 0.9 GPa. However, the T band loses intensity or merges with the S band between 2.1 and 3.8 GPa. At 4.8 GPa only three bands remain, and their relative intensity are completely different from that in the 0.9 GPa spectrum. No significant spectral change is observed between 4.8 and 10.9 GPa. On the other hand, the splitting of the R band and the appearance of two new bands, marked with arrows, are observed at 11.8 GPa. Like in other spectral ranges, the spectrum at 11.8 GPa has a different profile from both the spectrum at 10.9 GPa and the spectrum recorded at 12.4 GPa, suggesting an intermediate phase between the phases III and IV. The band at 3113 cm−1 at 11.8 GPa can be

attributed to νa(NH+ 3 ) and the bands marked as R, S, T and U are assigned as ν(CH) and ν(CH2) [33]. Above 11.8 GPa these new bands split and the spectrum at 14.3 GPa presents low intensity and broad bands. Fig. 8b displays the behavior of dω/dP. This figure shows the large slope of the modes as well as discontinuities at 4.8 and around 12.4 GPa. Additionally, the number of bands for the phases III and IV can indicate change of symmetry associated with different conformations of D and L glutamic acid molecules. Moreover, the appearance of the band associated with νa (NH+ 3 ) at 11.8 GPa helps to evidence a structural phase transition, in which this mode becomes allowed with a specific energy in phase IV. The spectral evolution of the two bands in the spectral region 3200–3750 cm−1, which are associated with the stretching of water, is shown in Fig. 9a. Bands V (3463 cm−1) and W (3573 cm−1) represent symmetric and antisymmetric stretching of water molecule. We remember that L-asparagine monohydrate crystal presented these corresponding bands around 3401 and 3461 cm−1, while L-lysine hydrochloride dihydrate crystal at 3358 and 3497 cm−1 [38,39]. As one can note, the corresponding wavenumber values have some variations among these crystals. This means that the intensity of the hydrogen bond plays a pivotal role in the frequency of the stretching vibration of the water molecule. Therefore, the shifts undergone by V and W bands under compression are expected, since compression provides changes in the hydrogen bond network. However, some points deserve special attention. The first one concerns the redshift of both bands in the 0.9–3.8, 4.8–10.9 and 11.8–14.3 GPa pressure ranges (see Fig. 9b). The second point deals with the behavior of these modes at the 4.8 GPa phase transition: while V presents a negative discontinuity (red shift), the W mode presents a positive jump (blue shift). The opposite occurs at the 11.8 GPa phase transition for both modes. As will be shown below, this behavior is intrinsic to the molecular interaction.

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Fig. 6. (a) Raman spectra of DLGAM crystal in the spectral region 600–850 cm−1 and pressure range 1 atm–14.3 GPa. (b) Raman intensity ratio I671/I640 and I770/I640 as a function of pressure.

A similar redshift suffered by V and W bands has already been found in nucleosides and amino acids crystals under extreme pressure or temperature [36,38–42]. The contraction of the unit cell, generally, leads to blueshift of most bands, at increasing pressure or decreasing temperature. However, considering the hydrogen bond network, a charge transfer from the molecular bond belonging to the functional groups as CO− 2 , NH+ 3 and/or H2O, causes a weakening of the covalent molecular bond, and hence a redshift of its vibrational wavenumber [36,38–42]. Water molecules of DLGAM crystal interact via hydrogen bonds with COOH, C_O and NH+ 3 units of different glutamic molecules [31]. In fact, the water molecule is acceptor of one and donor of two hydrogen bonds [31] leading to different behaviors associated with H\\O stretching under pressure. This can explain the wavenumber jump and fall shown by the V and W bands discussed in the previous paragraph. Furthermore, considering that the strength of the interactions around the water molecules are not the same, the effect of the pressure can generate different conformations and different vibrational frequencies. The combination of charge transfer and the angular distribution of atoms that form the hydrogen bond network would be a first point of view to understand the behavior of water bands. However, X ray as a function of pressure experiment would help to clarify this point. Other remarkable point of Fig. 9a is the modification of the line width of these modes. In order to study this modification quantitatively we plotted the line width (Γ) as a function of pressure in Fig. 9c. As can be seen Γ presents a discontinuity in the pressure range of the second phase transition. An interesting recent work about pressure-induced proton transfer from oxalic acid to water molecule can help to understand the redshift of ν(H2O). Results of infrared spectroscopy of oxalic acid dihydrated under pressure also show a redshift of the water stretching modes

[43]. In fact, cooperativity of hydrogen bonds leads to the softening of C\\O and OH bonds of oxalic acid and water, respectively. Proton transfer between oxalic acid and the water provides the formation of dianionic oxalate and hydronium ions (H3O+) when pressure reaches about 2.0 GPa. The adequate pattern of crystalline system and very short hydrogen bonds O\\H⋯Ow (~2.49 Å) make this a good system to proton transfer (the average length of hydrogen bonds in DLglutamic acid crystal is ~2.83 Å). Taking into account this approach, an increase of length must occur for C\\O and OH bonds due to the cooperativity of hydrogen bonds. Since no band associated with ν(H3O+) is observed in the present spectra, this kind of proton transfer does not occur for DL-glutamic acid crystal, at least up to 14.3 GPa. Considering that DL-glutamic acid monohydrate presents hydrogen bonds with average length larger than that in the oxalic acid dihydrate, some kind of proton transfer might be possible at higher pressure (N15 GPa). However, from our results it is hard to infer about which molecule would be acceptor or donor of proton. Yet, regarding the behavior of vibrational modes of water molecule in hydrated amino acid crystals, we can report the paper in which monohydrated L-asparagine was submitted to pressures up to 30 GPa [44]. A similar behavior to that described above has been observed on water molecule stretching mode during compression. This behavior constitutes an important tool to clarify future pressure studies of hydrated organic crystals and, eventually, to discover a precise law governing the influence of hydrogen bond on the H\\O stretching vibration. Table 1 presents the values of ωexp, ωo, α and β for the fitting ω = ωo + αP + βP2 of the DLGAM crystal in different ranges of pressure and the entire spectral region. Furthermore, the assignments of the modes are presented in the Table 1.

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Fig. 7. (a) Raman spectra of DLGAM crystal in the pressure range 1 atm–14.3 GPa and spectral region 1370–1800 cm−1. (b) Wavenumber vs. pressure plots of DLGAM crystal for spectral region 1370–1800 cm−1.

An aspect of interest in the study of phase transitions in organic crystals is related to its reversibility and the observation of eventual hysteresis. Beyond this, in some cases it is possible to observe a new phase that is reached only during the decompression run [45]. In the present study, decompression spectra of DLGAM crystal for all spectral regions evidenced that all phase transitions are reversible (Fig. 10). A thorough examination of Fig. 10 also evidences hysteresis for the phase transitions. The return to phase III occurs between 10.1 and 8.7 GPa, as evidenced in all spectral regions, and even at 3.8 GPa this same phase is still found. At this point it is interesting to comment about the different crystals of glutamic acid under high pressure. Beyond the DLGAM crystal, there are at least three studies involving crystals of glutamic acid at high pressure [24–26]. From these crystals, only the L-glutamic acid hydrochloride (LGAH) presents its molecules in protonated form [26]. LGAHCl and DLGAM crystals present chlorine ions and water molecules, respectively, in their lattices, while α-L-glutamic acid (α-LGA) and β-form of L-glutamic acid (β-LGA) have only glutamic molecules [31,46–48]. Regarding the structural stability under pressure, β-LGA and α-LGA preserve their atmospheric pressure phase up to ~1.0 and ~2.0 GPa, respectively [24,25]. On the other hand, LGAHCl and DLGAM undergo the first phase transition around 2.1 and 0.9 GPa, respectively. Moreover, the pure glutamic acid crystals (α-LGA and β-LGA) show the second transitions at pressures quite close to the first one (around 3.5 and 2.8 GPa for α-LGA and β-LGA, respectively). Despite LGAHCl and DLGAM undergo the first phase transition in pressure values comparable to those for α-LGA and β-LGA, the second phase transition for both LGAHCl and DLGAM polymorphs occur at 7.5 and 5.0 GPa, respectively. It seems that both chlorine ion and water molecule represent an important factor contributing to change the dynamics of the structural

transition. The role of water molecule in the stability appears, for example, in the study of L-asparagine monohydrate: a phase transition occurs at about 10.0 GPa and great modifications in the stretching vibrational modes of water were observed [44]. Therefore, specifically to the hydrated organic crystals, water molecules seem to change the structural stability of the hydrated crystal. The conformation of glutamic acid in the crystal structures of α-LGA and β-LGA at ambient temperature and pressure have strong differences regarding the dihedral angle involving the carboxylate group and other carbon atoms from skeleton. Moreover, a α-LGA aqueous solution left only a few hours shows a tendency to convert to the β-LGA one [49]. Moreover, it was evidenced a tendency of α-LGA form at low temperatures (b222 K) and β-LGA at ambient temperatures [49]. Other two works proposed the conversion of α-LGA to β-LGA at temperature above 140 °C as well as the obtaining of different polymorphs (α and β-forms) depending on the crystallization temperature [50,51]. Beyond these findings, the DLGAM form is different from both α-LGA and β-LGA form [31], indicating that the glutamic acid molecules present different modifications depending on the interactions around them. As a consequence, it is not surprising to detect various behaviors of glutamic acid crystal under high-pressure regime. Finally, let us consider a biological context where glutamic acid plays a fundamental role. The γ-aminobutyric acid (GABA) is the main inhibitory neurotransmitter in the mammalian cortex, and the glutamate (glutamic acid) is its metabolic precursor. Several works showed that GABA interacts with three different receptors, GABAA, GABAB and GABAC [52]. This means that, even with the loss of CO2 unit during glutamate transformation into GABA, the conformability of glutamate may contribute to the versatility of GABA in binding to three different

F.M.S. Victor et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 230 (2020) 118059

9

Fig. 8. (a) Raman spectra of DLGAM crystal in the spectral region 2870–3180 cm−1 and pressure range 1 atm–14.3 GPa. (b) Wavenumber vs. pressure plots of DLGAM crystal for spectral region 2930–3140 cm−1.

Fig. 9. (a) Raman spectra of DLGAM crystal in the pressure range 1 atm–14.3 GPa and spectral region 3200–3750 cm−1. (b) Wavenumber vs. pressure plots of DLGAM crystal for spectral region 3310–3585 cm−1. (c) Line width of W and V bands as a function of pressure.

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F.M.S. Victor et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 230 (2020) 118059

Table 1 Values for ωexp, ωo, α and β for the fitting ω = ωo + αP + βP2 of the DLGAM crystal in different ranges of pressure and for entire spectral regions. ωexp

Assignment

ω0

α

ω0

0.0 GPa ≤ P ≤ 0.9 GPa 67 93 98 103 117 126 134 156 192 208 269 315 400 436 497 517 545 640 673 770 853 917 1002 1092 1168 1217 1274 1372 1379 1419 1436 1452 1513 1579 1645 1715 2936 2956 2965 2984 3466 3572

Lattice Lattice Lattice Lattice Lattice Lattice Lattice Lattice τ(CO− 2 ) τ(CH) δ(skel) δ(skel) δ(COOH) τ(NH+ 3 ) r(CO2) δ(CO− 2 ) δ(CO− 2 ) δ(CO− 2 ) r(CH2) ν(CC) ν(CC) ν(CC) ν(CC) r(NH+ 3 ) τ(CH2) δ(CH2) ν(CC) δs(CH3) ν(CO2) δ(CH2) ν(CH2) δ(NH3) δ(H2O) νa(CO− 2 ) ν(C_O) νs(CH2) ν(CH) ν(CH) ν(CH) νs(H2O) νas(H2O)

66.5 93.0

117.0

3.6 4.3

6.2

α

β

0.9 GPa ≤ P ≤ 3.8 GPa 69.9 91.2

117.0

−1.95 6.2

6.2

156

14.9

207.9

13.9

315.1 400.0 435.8 394.3

7.2 14.2 14.6 3.62

405.8 443.6 394.3

8.3 4.2 3.62

545.0 640.1 672.9 769.5 852.0 916.8 1002.0 1094.2 1168.3 1217.9 1279.7 1373.0

7.38 9.0 4.5 3.1 7.4 9.4 5.1 5.9 3.9 4.2 4.7 2.1

545.0 643.0 672.9 770.7 856.2 920.2 1007.0 1094.2 1162.4 1217.9 1276.0 1368.9

7.4 3.3 4.5 1.8 4.1 6.5 4.4 5.9 5.0 4.2 2.4 7.8

187.9

6.4

260.0

12.7

1420.5

−0.9

1420.5

−0.9

1453.4

1.6

1453.4

1.6

1579.5 1646.4 1715.2 2936.0 2957.0 2965.7 2985.4 3465.8 3572.4

−0.2 −0.3 0.9 13.6 12.3 16.8 9.4 −23.5 −10.5

1579.5 1647.0 1715.2 2940.5 2956.4 2966.6 2984.3 3560.7 3572.8

−0.2 −2.4 0.9 7.3 11.8 13.8 9.6 −15.5 −7.1

ω0

α

β

3.8 ≤ P ≤ 11.8 GPa 1.22

82.4 91.4 92.3 86.7 92.6 130.6 195.8 197.8 226.4 310.0

0.6 1.5 2.6 12.0 8.5 2.9 12.2 −3.8 3.9 6.2

502.84 538.91 627.4 680.1 791.8 858.4 916.7 1030.4 1095.7 1193.2

2.95 2.47 1.56 1.4 0.71 3.7 2.58 3.39 5.66 2.1

1370.9 1420.0 1424.7 1453.0

2.02 1.6 2.70 1.27

1570.7 1619.8 1689.8

−0.07 1.96 2.32

2940.5 2967.4 2991.1 3380.0 3578.9

7.3 8.3 7.6 −5.8 −2.1

ω0

α

β

11.8 GPa ≤ P ≤ 14.3 GPa

−0.5 −0.3

43.8

5.4

112.46

5.4

331.6

3.3

514.01 509.48 650.0 674.5 769.7 855.4 930.5 1030.4 1103.5 1157.8

2.30 5.68 −0.2 1.8 0.2 3.8 1.4 3.4 4.7 3.3

1425.8 1455.9 1495.9 1576.5 1715.8

1.9 0.6 1.6 0.3 1.7

2958.3 2995.7

5.1 5.6

2994.6 3102.9 3920.1

6.7 62.2 −56.3

−0.5 0.4

−2.6 2.1

Nomenclature: τ = torsion; δ = bending; sc = scissoring; tw = twisting; ν = stretching; r = rocking; νas = asymmetric stretching; νs = symmetric stretching.

Fig. 10. Decompression spectra of DLGAM crystal for all spectral regions evidenced that all phase transitions were reversible. The evidence of hysteresis for the phase transitions also was observed. (a) 50–1190 cm−1. (b) 1380–1850 cm−1. (c) 2750–3300 cm−1. (d) 3250–3800 cm−1.

F.M.S. Victor et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 230 (2020) 118059

receptors. A recent work evidenced three phase transitions undergone by GABA crystal under pressure [53]. A series of phase transitions at ~0.1, 2.0 and 4.0 GPa were observed during increasing pressure experiment. This shows that the pressure studies of organic crystal can contribute to the knowledge of some important biological process.

[10]

[11]

4. Conclusions Our results of high-pressure experiment via Raman spectroscopy for the DLGAM crystals evidences three structural phase transitions around 0.9, 4.8 and 12.4 GPa. Little changes associated with a rearrangement of hydrogen bonds were also observed in the 2.1–3.8 GPa pressure range. For the first phase transition, beyond the modifications in the hydrogen bond network, glutamic acid molecules suffer enough modifications to provide the new phase. For the other two structural transitions, both glutamic acid and water molecules proven flexible under compression. Moreover, DLGAM crystal demonstrates a similar stability compared with its polymorphs α-LGA and β-LGA (for the first phase transition). However, phase II of DLGAM demonstrated the highest structural stability. Finally, all changes appear as reversible.

[12]

[13]

[14]

[15]

[16]

CRediT authorship contribution statement F.M.S. Victor: Investigation, Writing - original draft. F.S.C. Rêgo: Investigation. F.M. de Paiva: Resources. A.O. dos Santos: Investigation. A. Polian: Writing - review & editing. P.T.C. Freire: Writing - review & editing. J.A. Lima: Writing - review & editing. P.F. Façanha Filho: Writing - review & editing, Project administration. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[17]

[18] [19]

[20]

[21]

[22]

Acknowledgments Authors thank FAPEMA (Universal-40/2015 and Universal-002/ 2018) and CNPq (Universal 454941/2014-5), FUNCAP (Pronem 4520937/2016), FUNCAP/CNPq (PRONEX PR2-0101-00006.01.00/15) and CAPES. We also thank Keevin Beneut from the “Plateforme Spectroscopie”, IMPMC for helping in the execution of the experiments.

[23] [24]

[25]

[26]

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