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C.A.A.S. Santos, R.J.C. Lima, W. Paraguassu et al.
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 263 (2021) 120142
have the purpose of meeting this need. New materials with high
optical non-linearity are important due to their practical applica-
tion in information technology and industrial devices [1–3]. This
is due to the possibility of generating a highly efficient second har-
monic, enabling the application in photonics, therefore, amino acid
crystals can perform such applications because many of them have
an asymmetric carbon atom and crystallize in non-
centrosymmetric spatial groups [4].
transitions. We also show calculations using functional density
theory (DFT), providing accurate assignment of the normal modes
of vibration. In addition, the role of the amino acid side chain in the
configuration of the hydrogen bond network and the stability of
the structure are discussed.
2. Material and methods
Pressure is a thermodynamic variable that, like temperature,
can be used as a parameter to study the energy of a system [5].
The main effect resulting from the application of hydrostatic pres-
sure in solids, crystalline or not, is the reduction of the interatomic
and intermolecular distances of the material and, consequently, its
volume [6,7]. This decrease in volume implies an increase in the
energy of the system. The effects of hydrostatic pressure are more
pronounced for intermolecular bonds than for interatomic bonds
[8].
The hydrogen-bond (H-bond) is the interaction responsible for
the stability of a large number of complex biological molecular sys-
tems. Changes in the architecture of the H-bond can be induced by
high hydrostatic pressures and can have an effect on system stabi-
lization. This effect is seen in many aspects of high hydrostatic
pressure in protein biochemistry [9].
2.1. Material synthesis
L-tyrosine hydrobromide (LTHBr) single crystals were crystal-
ized from an aqueous solution by the slow evaporation technique
using L-tyrosine (98%; Sigma-Aldrich) and hydrobromic acid
(48%; Sigma-Aldrich) with stoichiometric ratio 1:1. The hydrogenic
potential (pH) was measured at a value of 2.1, and temperature
control was made without exceed the value of 323 K. The solution
was sealed with plastic wrap which was then punched and placed
in the crystal growth chamber maintained at room temperature of
298 K. The crystals were obtained typically after 3 weeks.
2.2. X-ray diffraction
The amino acids in their entirety behave like monovalent
anions. Aspartic and glutamic acid, as well as tyrosine and cysteine,
have the ability to form monovalent and divalent anions. This class
of materials can form crystalline structures through various combi-
nations, due to the large availability of cations [10]. Amino acid
crystals are simpler models of organic molecules representative
architecture of H-bond. The H-bond network is composed of an
amino group and a carboxylic acid group linked by N-HꢂꢂꢂO bonds.
Thus, the study of hydrogen bonds under high pressure can provide
valuable information to probe the stability of the biological molec-
ular systems. For example, L-alanine crystal is a stable molecular
system [11]. On the other hand, some amino acid compounds have
been investigated in the search for structural phase transition and
conformational phase transition in crystals, in particular, by Raman
spectroscopy techniques [12–19].
L-tyrosine hydrobromide (LTHBr) is a semi-organic non-linear
optical material (NLO) and is shown to be suitable for photonic,
optoelectronic device fabrication, and laser related applications
[20,21]. In a previous study with L-tyrosine hydrochloride crystal
(LTHCl) [22] the high-pressure Raman spectra were obtained from
1.0 atm to 7.0 GPa, indicating, that the crystal remained in the
same monoclinic structure of the ambient pressure. LTHCl at a
pressure of 1 atm has an intense band at 125 cmꢁ1 (A1) associated
with the twisting of the L-tyrosine molecule; between 0.5 and 1.0
GPa it shows a shoulder that at 3.0 GPa has an intensity inversion
with the mode initially recorded at 125 cmꢁ1. In addition, this A1
The LTHBr powder sample was subjected to X-ray diffraction
and data were collected with a powder diffractometer Rigaku
Mini-flex II using Cu K
a (k = 1.5418 Å) radiation. The diffraction
patterns were carried out in the 2h angular range 5–50° with a step
size of 0.02° and with a counting time of 2 s/step. The structural
characterization of LTHBr was obtained by Rietveld refinement
using the GSAS program [26].
2.3. Raman spectroscopy and high-pressure measurements
High pressure Raman spectra were recorded in back scattering
geometry using a microscope attached to a triple-grating spec-
trometer Jobin Yvon T64000. For Raman measurements, the
514.5 nm line of an Ar-Kr ion laser was used as excitation and
the spectral resolution was set to 2 cmꢁ1. To achieve high pres-
sures, a DiacellÒ lScopeDAC-RT(G) diamond anvil cell from Almax
EasyLab with 0.4 mm culets diamonds was used. The sample was
loaded into a 100 mm hole drilled in a stainless-steel gasket with
a thickness of 200 mm using an Almax EasyLab electric discharge
machine. Nujol served as the pressure transmitting medium
(PTM) and the main purpose of the compressor medium is to keep
the sample under hydrostatic conditions; this mineral oil is rela-
tively hydrostatic up to 10 GPa [27–29]. The pressures were mea-
sured based on the shifts of the R1 and R2 ruby fluorescence lines.
mode also showed a variation in the d
x
/dP slope between 1.0
2.4. Computational details
and 1.5 GPa. These changes characterized a conformational phase
transition undergone by the LTHCl crystal. In the external modes
region, there were no major changes around 3.0 GPa and in the
decompression process, it was observed that the conformational
phase transition was reversible, without hysteresis. Therefore,
Raman spectroscopy is a powerful experimental technique for
obtaining information on changes in H bonds, studying the vibra-
tional behavior of amino acid crystals [23]. The interpretation of
Raman spectra of amino acids can provide an important contribu-
tion to the study of changes in hydrogen bonds under pressure
variation and the stability of biological molecular systems [24,25].
In this research, we describe the effect of high pressure on the
crystal of L-tyrosine hydrobromide (LTHBr) in the range between
1 atm and 8.2 GPa, with special attention to the bands in the spec-
tral region of the external modes (below 200 cmꢁ1) where it is pos-
sible to collect evidence of structural and conformational phase
The structural and vibrational properties of the monoclinic
structure of L-tyrosine hydrobromide crystal were investigated
with the QUANTUM-ESPRESSO plane-wave code [30], which uses
density functional theory [31,32]. The initial structure was based
on the experimental crystallographic data reported for the L-
tyrosine hydrochloride crystal [33]. For the exchange–correlation
potential, the local density approximation (LDA) [34] was adopted,
considering the Perdew-Zunger [35] functional with 4 ✕ 4 ✕ 4
MonkHorst-Pack [36] K-points and a plane waves cut off of 250
Ry. For the dispersion correction, Grimme-D2 method was used
[37]. The structure was completely relaxed, including the cell
parameters, until the forces became less than 1 ✕ 10-4 Ry/Bohr,
stress less than 0.01 kbar and the energy threshold set to 1 ꢃ 10-
12
Ry. The relaxed lattice parameters were found to be a = 11.153
Å, b = 8.707 Å, c = 4.942 Å and b = 90.032°.
2