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Crystalline Films of L-Threonine Complexed with Copper (II) Dispersed in a Galactomannan Solution: A Structural, Vibrational, and Thermal Study.

INTRODUCTION

The demand for complexed materials with metallic synthesized ions and a focus on biotechnological applications has increased considerably lately. It is frequently necessary to immediately treat certain diseases caused by knife cuts, car accidents, insect bites, among others. The contact of this type of wounds with air facilitate microorganism's proliferation that can cause infections. Depending on time of contact, there may be life-threatening cases [1-3].

Currently, the pharmaceutical industry offers several types of bactericidal agents, which have specific functions according to material composition. These compounds that combat microorganisms have, as pharmacological principle, induced bacterial death. Data show that this bacterial death can reach 99.99%. This has motivated researchers to develop a new material that exhibits bactericidal properties to solve contamination problems that occur every day. However, biocompatibility, in many cases, is a drawback [3-7].

Today, there are numerous compounds in nature which, when used in combination with other material types, are biocompatible. Among many materials, the polymers can be highlighted, once they are non-toxic and biodegradable. This class of materials is characterized by having macromolecules formed by monomeric units that remain throughout the structural lattice. Due to these characteristics, several materials are being currently developed based on polymers, promoting the formation of composites that provide varied biological and technological properties [8-12].

Composites formed of polymers and crystalline systems, such as semi organic crystals, may have bactericidal properties depending on the metallic ion used. Copper, for example, when complexed to organic molecules such as amino acids and polymers, has antifungal, antibiotic, antibacterial, and antitumor properties, for example [13-16].

There is abundant published research on the use of composites formed by metals and organic molecules for diseases treatment. The main advantages of these systems are the fact that they are easy to obtain, low cost, and efficient as pharmacological agents [17-19].

According to documents found at the National Industrial Property Institute (IPNI), in Brazil, there are numerous materials (films, blends, microspheres, drugs, and complexes) that are developed based on polymers and metal ions, which have pharmacological, healing, bacteriostatic, and bactericidal properties, among others [20].

Galactomannans (Gals) are neutral polysaccharides isolated from seed endosperm of some leguminous. They are catabolized to provide energy and carbon skeletons to the plant during germination. It presents a linear skeleton consisting of D- mannopyranose joined by [beta]-glycoside bonds (1 [right arrow] 4). The main difference of Gals is the mannose/galactose ratio, as well as the distribution along the mannopyranosyl skeleton that depends on the species [21].

Within this context, the objective of this study was to produce a new crystalline film based on a single crystal of L-threonine complexed with copper (II) dispersed in galactomannan (Gal) solution, as well as to study its structural, vibrational, and thermal properties for possible applications in bactericide agents and respective molecular models.

MATERIALS AND METHODS

Crystal Synthesis

The crystal was synthesized using slow evaporation technique. Two compounds were used: L-threonine and copper (II) chloride dihydrate (Sigma Aldrich), in a molar ratio of 2:1, respectively. The materials were solubilized in 15 mL of deionized water under constant stirring at 240 RPM on a magnetic stirrer until complete solubilization of the solutes. The two solutions were then mixed, and the pH was adjusted to 5.8, starting from sodium hydroxide (NaOH) solution 1 M. Then, the new solution formed under a magnetic stirring period of 5 h until the total homogenization. Afterward, the solution remained at constant temperature (25 [degrees]C), until the formation of single crystals.

Galactomannan Extraction from Adenanthera pavonina L. Seeds

To obtain the Gal, seeds of Adenanthera pavonina L. were collected, and then selected and cleaned. Later, the seeds were deposited in a beaker for the heating process (at 100[degrees]C for 30 min) and swelling. Thus, the endosperm was separated manually. Thereafter, the vials with the material were freeze dried.

Preparation of Crystalline Films

The films were prepared by means of a 2%-Gal solution in distilled water. L-threonine with a copper single crystal in powder form was added to the saccharide medium, at concentrations of 0.25, 0.50, and 0.75. The solution was stirred for 1 h with magnetic rotation at room temperature to achieve homogenization of the material. The solution was placed in Petri dishes to form crystalline films and submitted to low temperature (~ 4[degrees]C) in a cooler.

Characterizations

X-Ray Diffraction. The X-ray diffraction (XRD) measurements were carried out in an X-ray diffractometer (Bruker), model D8 Advance. The Cu [K.sub.[alpha]] radiation ([lambda] = 1.5418[??]) was used to obtain diffiactograms, with a voltage of 40 kV and a cuirent of 40 mA. The analyses were performed at room temperature (27[degrees]C), in the 20 angular range ranging from 5 to 50[degrees], using a step size of 0.02 and time of 2 s/step.

Ultraviolet-Visible Spectroscopy. The spectra were obtained in a UV-Vis spectrophotometer (Thermo Scientific) in the region 200 to 1,100 nm, with a dual beam, deuterium lamp, model Evolution, using quartz cuvettes with 0.1 cm optical path.

Raman Spectroscopy. Raman spectra were measured on a triple-spectrometer, Jobin-Yvon model T64000 (HOR1BA Scientific) with air-cooled CCD detector. Using solid-state laser, with [lambda] = 514.5 nm, power of 10 mW and spectral resolution of 2 [cm.sup.-1].

Thermogravimetry--Differential Thermal Analysis. The thermogravimetry (TG) and differential thermal analysis (DTA) measurements were performed simultaneously in a DTG-60 thermogravimetric (Shimadzu) in open [alpha]-alumina pan under a nitrogen atmosphere (100 mL/min), in a range from 25 to 900 [degrees]C and heating rate 10[degrees]C/min.

RESULTS AND DISCUSSION

Crystal and Films Synthesis

Copper (II) L-threoninate crystal (Fig. 1a) was obtained by slow evaporation solvent method after a period of 30 days. After that period, it presented violet-blue coloration. This suggests the possible complexation of the amino acid molecules to [Cu.sup.2+] ions.

The reaction between L-threonine and the metal occurred in aqueous solution due to the high-water solubility of L-threonine (90 g/L), in a ratio of 2:1, favored at zwitterionic form under pH 5.8 5.8 [22]. The reaction that occurs is represented in the following equation:

2([C.sub.4][H.sub.9]N[O.sub.3]) + (Cu[Cl.sub.2]).2[H.sub.2]O + 2(Na0H) [right arrow] Cu[([C.sub.4][H.sub.8]N[O.sub.3]).sub.2].[H.sub.2]O + 2NaCl + 3[H.sub.2]O

After the synthesis of L-threonine crystals complexed with copper (II), they were pulverized and dispersed in a 1%-Gal solution. Posteriorly, they were placed in Petri dishes to form crystalline films. Figure 1a-d is representative of the films prepared after evaporation of the solvent at controlled low temperature.

The crystalline films acquired violet-blue color as well as the LTCu crystal, being this an indication of the presence of [Cu.sup.2+] ion in the organic samples. According to the increase of the concentration of LTCu in the Gal solution, the sample obtained its intensified color.

X-Ray Diffraction

The crystal structure was confirmed by XRD measurements. The Rietveld method was applied to the diffraction pattern of LTCu crystal. At a temperature of 25 [degrees]C, the sample crystallized in the monoclinic structure with [P2.sub.1]-space group, containing two molecules per unit cell. The lattice parameters are a = 11.356(5) [Angstrom], b = 4.954(8) [Angstrom], c = 11.287 (6) [Angstrom], [beta] = 94.195 (1)[degrees] and V = 633.444(4) [[Angstrom].sup.3] (Fig. 2a). The estimated values between observed and calculated intensities were: Rwp: 6.74% and Rp: 4.73%, which indicate a good quality regarding the refinement of the used profile. The data obtained from this study resembles the values reported by Subhashini et al., who performed some characterizations on bis(L-threonine) copper (II) crystal [23].

Figure 2b shows the pure Gal film and the films of LTCu crystals dispersed in Gal at different concentrations. We can observe on the diffractogram of the Gal film that there have been significant structural modifications. The main one was the change of amorphous system for crystalline system, once the crystals dispersed in galactomannan on XRD revealed the presence of several peaks, characteristic of LTCu crystal.

The peak at 2[theta] = 31.8[degrees] refers to a preferred orientation of the (311) plane from the Cu(II)(L Threonine) complex crystal in samples. That fact can be confirmed with the increase of LTCu crystal concentration dispersed in Gal.

The crystallinity degree of the polymeric films was calculated based on the diffractograms by following mathematical relation:

Crystallinity = Area of crystallinity peaks/Area of all peaks (crystalline + Amorphous) (1)

The films crystallinity increases with the LTCu crystal amount deposited on the polymer, the obtained values were Gal(LTCu0.2s) -48.1%; Gal(LT[Cu.sub.0.50]) - 48.8% and Gal(LT[Cu.sub.0.75]) - 49.1%.

Ultraviolet-Visible (UV-vis) Spectroscopy

Figure 3 represents the optical absorption spectra for Gal precursor solutions with LTCu crystals dispersed at different concentrations. The broad band at 620 [cm.sup.-1] can be attributed by the presence of the [Cu.sup.2+] metal ion in the polymer material, due to an increase in the concentration for the LTCu crystal, which is revealed through a change in its intensity. In fact, we can affirm that there was an interaction between the Gal molecules and the crystalline system [24, 25].

Bands in the range from 200 to 380 [cm.sup.-1] are associated with the solvent used for the material synthesis, which in this case was water. The other low-intensity bands in the spectra refer to the presence of the Gal molecules in the sample. The band shoulder located at 620 [cm.sup.-1], evident in the spectrum of the film solution Gal(LTCu.75%), is characteristic of copper (II) complexes for materials, which are subjected to Jahn Teller effect. This results in the elongation of the molecule along the axes, making the square planar geometry preferable to this complex, as discussed by Persson [24, 26].

Raman Spectroscopy

In Figs. 4 and 5, we can observe the Raman spectra for the crystalline films in the 30-3,600 [cm.sup.-1] region. For a better understanding of the vibrational properties, the spectra were divided into four ranges.

Figure 4a shows the vibrations of low wavenumbers. For this region, the vibrational modes associated with lattice modes (external modes) are known. In this spectral region, there are no external modes for the Gal, since that the sample is amorphous. Some of these modes perform coupling with internal modes [27-31]. For Gal(LTCu) films, it is observed that the external modes are up to 230 [cm.sup.-1]. For wavenumber values above 230 [cm.sup.-1], there is a band centered at 397 [cm.sup.-1], which is characteristic of stretching of the metal ion linked to functional groups of L-threonine, [delta](OCuN) + [delta](CuOC) + [delta](CuNC). These assignments agree with the literature [27], in which they were observed between 290 and 550 [cm.sup.-1] and were associated with stretching modes of nitrogen metal and oxygen metal. Other modes were observed regarding the metallic ion; they are given in Table 1.

For wavenumber region between 700 and 1800 [cm.sup.-1] (Fig. 4b), a set of modes for the three samples was observed. Among them, there was an out-of-phase deformation (twisting) related to hydroxyl group (co(OH)), around 843 [cm.sup.-1]. Since the solvent used for the formation of the crystalline films was water, this assignment seems reasonable. For the Gal film, when compared to the LTCu crystalline films, a band shift was observed. That is due to the strong interaction between the metal complex and the used polymer [29]. According to the literature [30-36], the most vibrational modes in this region are associated with stretching and deformations of C--C, C--H, and [CH.sub.3] units, as can be seen in Table 1.

According to data reported in the literature [36, 37], in the spectrum comprised between the 2,700 and 3,200 [cm.sup.-1] range (Fig. 5a), associated with the CH and NH units for materials involving amino vibrational modes, acids are found. As can be seen for the stretching of C--H bond, v(CH), they were noticed in the four-Gal films with different concentrations of LTCu crystal. However, for the vibration involving the N-H bond, v(NH), they appear only on films that have LTCu crystal, because Gal does not have amino groups in its polymer chain. Other vibrational modes are given in Table 1 [36-38].

In Fig. 5b, the modes in the 3,200-3,600 [cm.sup.-1] range are shown. In this spectra] region, only two vibrational modes were active for all films. The first appeared at around 3,260 [cm.sup.-1], which was assigned to a hydroxyl stretching, v(OH), of the water molecule. The low intensity for this Raman mode occurs due to weak polarizability presented by the O-H bond [23]. The second vibrational mode refers to a stretching of the [v.sub.a]([NH.sub.3]) amino pertain to the amino acid [37, 38].

Thermogravimetry--Differential Thermal Analysis

In order to observe the thermal stability of the synthesized materials, TG-DTA measurements were performed, in Fig. 6, we can observe each film behavior.

At the beginning of the four thermograms of the films, a thermal decomposition event is observed. This fact refers to the existence of water molecules in the system since the films produced were synthesized in water and the L-threonine crystal molecule has also been synthetized in the presence of water. Therefore, it can be inferred that the water molecules are bound to the other compounds by means of secondary bounds, since this demands a low amount of heat for evaporation. The endothermic events in the range of 25 to 100[degrees]C are represented by the DTA curve point to the evaporation of the water molecules in the crystalline films, as well as elucidated by the TG curve.

For the thermogram shown in Fig. 6a in the range from 25 to 200 [degrees]C, there is a loss of 14.53%, corresponding to the initial mass of the sample (2.773 mg), which indicates the exit of the water molecules from the system (as previously mentioned), as well as the fragmentation of some organic compounds. At 200-600[degrees]C, there is a fairly intense decline in the TG curve. This event refers to the disintegration of the macromolecular chains referring to mannose and galactose groups in the Gal structure, as well as to the total decomposition of the material, resulting in the final loss of 85.47% of initial mass.

As for the TG-DTA curves in Fig. 6b,c,d, they exhibit similarity, due to the dispersion of a crystalline solid phase in the saccharide environment, only varying the crystal concentration. For the three samples containing the LTCu crystal, in 25-150 [degrees]C range, occurred water molecules evaporation present in the system. Subsequently, around 200 [degrees]C, a sharp decay occurs in the TG curve associated to degradation of the polymer structural groups. A discontinuity in this mass loss is observed in TG curves, this fact is explained by L-threonine decomposition that occurs simultaneously with the degradation of polymer. The endothermic events shown in the DTA curves point the organic compounds decomposition in the films and show the mass loss by heat. Table 2 briefly shows the relevant losses of the crystalline samples analyzed.

By means of the data presented in Table 2, it can be noticed that the higher the concentration of the LTCu crystal, greater is the amount of material remaining in the sample, after the system is submitted to temperature increase. Such fact can be explained due to the presence of a copper ion in the films, since it does not undergo decomposition and remains after the heat treatment only having a small mass loss characteristic of partial oxidation of the transition metal.

The thermal measurements showed that after the withdrawal of water molecules from the films, they are in their anhydrous form and can be used for biological applications such as bactericides, fungicides, among others. This happens due to the fact that when in anhydrous form, the material has larger stability when compared to the hydrated form. However, the addition of materials having good thermal stability in the hydrated form also provides these biological properties.

CONCLUSIONS

The crystalline film of LTCu crystal dispersed in Gal solution was successfully obtained by the technique of solvent slow evaporation at low temperature. By means of XRD, we observed that the film crystallinity was proportional to the addition of LTCu crystals. In UV-Vis spectroscopy, we observed that the molecules of the crystalline films have square planar geometry due to the Jahn Teller effect from the [Cu.sup.2+] ion. The Raman spectra showed modes associated with metal ion bound in the organic compounds, as well vibrations related to the water of crystallization in the films. The TG-DTA revealed low thermal stability of the films due to the exit of water molecules with the increase of temperature.

ACKNOWLEDGMENTS

We are thankful to the Brazilian agencies CNPq, CAPES, and FAPEMA for financial support.

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Joao G. Oliveira Neto, (1) Lincoln A. Cavalcante, (2) Eduardo S. Gomes, (2) Adenilson O. Dos Santos (iD), (1) Francisco F Sousa, (1,3) Fernando Mendes, (4) Ana Angelica M Macedo (2)

(1) CCSSr, Universidade Federal do Maranhao, 65900-410, Imperatriz, Maranhao, Brazil

(2) Instituto Federal do Maranhao, 65919-050, Imperatriz, Maranhao, Brazil

(3) ICN, Universidade Federal do Para, 66075-110, Belem, Para, Brazil

(4) Polytechnic Institute of Coimbra, ESTESC-Coimbra Health School, Department Biomedical Laboratory Sciences, Rua

(5) de Outubro, S. Martinho do Bispo, 3046-854, Coimbra, Portugal

Correspondence to: A. O. Dos Santos; e-mail: adenilsonl@gmail.com DOI 10.1002/pen.25260

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. LTCu crystalline films dispersed in gal. (a) LTCu crystal (b) gal; (c) 25% LTCu; (d) 50% LTCu; (e) 75% LTCu. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. X-ray diffraction pattern of (b) films and (a) LTCu crystal refined by Rietveld method. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Optical absorption spectra for the solutions of LTCu crystalline films dispersed in gal. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. Raman spectra of crystalline films in the spectral ranges of (a) 30 to 700 [cm.sup.-1] and (b) 700 to 1800 [cm.sup.-1] [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. Raman spectra of the crystalline films in the spectral ranges of (a) 2,700 to 3,200 [cm.sup.-1] and (b) 3,200 to 3,600 [cm.sup.-1]. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. Thermograms of crystalline films: (a) gal; (b) gal(LTCu0.25); (c) gal(LTCu0.50); (d) gal(LTCu0.75); [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1. Raman-active modes and their respective assignments for the gal
crystalline films and gal films with LTCu crystal in different
concentrations.

                            Wavenumber ([cm.sup.-1])

                            Films

Assignment                   Gal    [Gal.sub.0,75]LT   [Gal.sub.0,50]LT
                                     [Cu.sub.0,25]      [Cu.sub.0,50]

Lattice modes                38            39                 39
                              -            50                 51
                              -            71                 74
                              -            95                 94
                              -           107                111
                              -           170                174
                              -           183                186
                              -           216                217
                              -           230                228
[delta](OCuN) + [delta]       -           398                397
  (CuOC) + [delta](CuNC)
[delta](CuN)                  -           459                459
[delta](C-C=0)                -           485                488
[rho](CO[O.sup.-])            -           530                529
[v.sub.as](CuN) +            595          594                594
  [v.sub.as](CuO) + v(CC)
[omega](OH)                  843          849                845
[gamma]([CH.sub.2])           -           872                872
v(CN)                         -          1,330              1,334
[rho]([CH.sub.3])             -          1,401              1,401
v([CH.sub.3])                 -          1,465              1,462
v(NH)                         -          1,580              1,596
                              -          1,648              1,643
v(CH)                         -          2,880              2,883
                              -          2,907              2,908
v(CH) + v([CH.sub.3])       2,935        2,937              2,939
v(NH)                         -          2,983              2,978
v(OH)                       3,257        3,262              3,260
v([NH.sub.3])               3,298        3,301              3,298

                            Wavenumber
                            ([cm.sup.-1])

                            Films

Assignment                  [Gal.sub.0,25]LT    References
                             [Cu.sub.0,75]

Lattice modes                      40            [29-31]
                                   51
                                   73
                                   93
                                  114
                                  174
                                  186
                                  215
                                  226
[delta](OCuN) + [delta]           395              [27]
  (CuOC) + [delta](CuNC)
[delta](CuN)                      459              [32]
[delta](C-C=0)                    488              [23]
[rho](CO[O.sup.-])                529          [31, 33, 34]
[v.sub.as](CuN) +                 594              [32]
  [v.sub.as](CuO) + v(CC)
[omega](OH)                       847              [35]
[gamma]([CH.sub.2])               873              [23]
v(CN)                            1,338             [35]
[rho]([CH.sub.3])                1,403             [23]
v([CH.sub.3])                    1,460             [35]
v(NH)                            1,603             [36]
                                 1,646             [361
v(CH)                            2,882             [36]
                                 2,908             [35]
v(CH) + v([CH.sub.3])            2,938             [35]
v(NH)                            2,977             [37]
v(OH)                            3,261             [23]
v([NH.sub.3])                    3,298             [38]

V = stretching; [delta] = scissoring; [gamma] = asymmetrical
stretching; [omega] = twisting; [rho] = rocking.

TABLE 2. Information regarding the thermal analyzes of TG-DTA
for [Gal.sub.0.75]LT[Cu.sub.0.25], [Gal.sub.0.50]LT[Cu.sub.0.50],
and [Gal.sub.0.25]LT[Cu.sub.0.75] films.

                TG                            DTA

Gal(LTCu)        [T.sub.i]      [T.sub.f]     T([degrees]C)
crystalline     ([degrees]C)   ([degrees]C)
film (%)

0                    25            200             49
                    200            900           292;471
25                   25            250           107;170
                    250            600           413;476
                    600            900             794
50                   25            250        28; 103; 180
                    250            600           285;464
                    600            900             795
75                   25            250        28;101 ; 187
                    250            600           282;469
                    600            900             790

                DTA

Gal(LTCu)             Event         Mass    Molecules
crystalline                          (%)
film (%)

0               Endo.[down arrow]   14.67     Water
                  Exo.[up down]     86.61       CO
25              Endo.[down arrow]   33.48   Water + CO
                  Exo.[up down]     30.72       CO
                Endo.[down arrow]   21.11       CO
50              Endo.[down arrow]   38.35   Water + CO
                  Exo.[up down]     26.95       CO
                Endo.[down arrow]   20.95       CO
75              Endo.[down arrow]   41.45   Water + CO
                  Exo.[up down]     17.43       CO
                Endo.[down arrow]   15.17       CO

Ti = initial temperature; Tf = final temperature; endo =
endothermic; exo = exothermic; CO = organic compounds.
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Author:Neto, Joao G. Oliveira; Cavalcante, Lincoln A.; Gomes, Eduardo S.; Santos, Adenilson O. Dos; Sousa,
Publication:Polymer Engineering and Science
Geographic Code:3BRAZ
Date:Jan 1, 2020
Words:4498
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