Formation of Hierarchical Structures of L-Glutamic acid with L-Arginine

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Formation of Hierarchical Structures of LGlutamic acid with L-Arginine Additive Irena Nemtsov, Yitzhak Mastai, and Michal Ejgenberg Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00429 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Crystal Growth & Design

Formation of Hierarchical Structures of Lglutamic acid with an L-Arginine Additive Irena Nemtsov, Yitzhak Mastai*, Michal Ejgenberg* Department of Chemistry and Bar-Ilan Institute for Nanotechnology and Advanced Materials (BINA), Bar-Ilan University, Ramat-Gan 5290002, Israel

* Corresponding authors

[email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT Formation of hierarchical structures of organic crystals has great potential for many applications in various areas including pharmaceuticals, food, pigments, and organic catalysis. In this paper, we investigate the formation of hierarchical structures of glutamic acid using arginine as a charged additive to control the formation of the hierarchical structures. We used a reprecipitation method as a route to crystallized hierarchical structures of glutamic acid. The crystallization of hierarchical structures of glutamic acid under various experimental parameters such as solvent compositions, arginine concentrations, and the chirality of the additive were thoroughly studied using several techniques including X-ray diffraction, scanning electron microscopy, and polarized microscopy. It was found that the formation of the hierarchical structures and their size, morphology and porosity depend strongly on the arginine concentration. Based on our results, we propose a possible mechanism for the formation of hierarchical structures of glutamic acid based on the selfassembly principle. Overall, in this work we used the crystallization of L-glutamic acid hierarchical crystals in the presence of arginine as additive as a model system to better understand the effects of polyelectrolyte additives on the crystallization of organic hierarchical crystals.

Keywords. hierarchical structures, glutamic acid, arginine, reprecipitation method


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INTRODUCTION Mesocrystals1–8 are oriented crystalline superstructures composed of highly ordered nanocrystals. They are formed by non-classical crystallization processes, which rather than proceeding via ion-by-ion crystal growth mechanisms, are based on flexible mechanisms of nano- or micro-sized crystal organization. Cölfen and Antonietti9 reported formation of mesocrystals from various inorganic crystal systems including CaCO3,10 BaCrO4,11 BaSO4,12 Fe2O3,13 and CoC2O4·2H2O.14 The first works reporting the formation of mesocrystals were mostly on inorganic crystals.15–19 However, organic mesocrystals20–22 have also been reported. For example, Colfen and Antonietti23 crystallized DL-alanine mesocrystals in the absence of any additives and showed that the formation of mesocrystals with highly porous structure and rough surfaces can be achieved by controlling the pH and the temperature. In addition, the effect of a miscible anti-solvent on the growth morphology of DL-alanine was studied.

24,25

In recent years, organic hierarchical structures have also been studied. In most

cases, amino acid crystallization was used for the formation of hierarchical structures. For example, our group26 demonstrated that the crystallization of cystine under biomimetic conditions results in unique spherical, flower-like, or hexagonal crystalline superstructures We proposed a mechanism for the formation of these hierarchical crystalline superstructures based on the three-dimensional assembly of micro-sized cystine crystals. Another work on organic mesocrystals was reported by Li et al.27 who devised a facile reprecipitation method to achieve the hierarchical peony-like flower structure with the diphenylalanine peptide in an organic solvent. In addition, Cölfen et al.28 provided a systematic investigation for the preparation

of

DL-glutamic

acid

macroporous

microspheres

with

branched

polyethyleneimine as an additive to promote the polymer-induced liquid-precursor process of crystallization. A series of parameters, including the pH value, were tuned to optimize the procedure for the preparation of pure microspheres. The formation of the spheres was


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possible at specific pH values, where the DL-glutamic acid and polyethyleneimine were oppositely charged. The mechanism for the formation of mesocrystals is very difficult to analyze due to the large range of parameters and time scales involved in the process. However, in the literature a few mechanisms have been proposed based on nanoparticle self-assembly. In some mechanisms this self-assembly and alignment of nanoparticles is assisted (or templated) by an organic macromolecular matrix. In other mechanisms the alignment of the nanoparticles is achieved by physical fields or spatial constraints. In this paper, we investigate the formation of hierarchical structures of glutamic acid using a charged additive, namely arginine. This was achieved using the reprecipitation method;29,30 the target compound is dissolved in a solvent, e.g. water, at millimolar concentrations, and this dilute solution is injected rapidly into a vigorously stirred poor solvent (anti-solvent) at a constant temperature. In this way, the target compound is reprecipitated, and this typically leads to the formation of nanocrystals. The reprecipitation method is frequently used to obtain nanocrystals dispersed in solutions. The experimental parameters such as arginine additive concentration and chirality, crystallization temperature, solvent composition, and pH values were thoroughly investigated for the L-glutamic acid crystals. The formation of the hierarchical crystals and the investigation of the crystallization process were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD) and polarized optical microscopy (POM). Finally, a mechanism was suggested to explain the formation of the crystals. Overall, in this work we used the crystallization of L-glutamic acid hierarchical crystals in the presence of arginine additive as a model system to better understand the effects of polyelectrolyte additives on the crystallization of organic hierarchical crystals. Studies of the formation of hierarchical organic crystals are significant for a number of industrial applications. For example, the


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performance and efficiency of many pharmaceutical solids and agricultural products depends directly and strongly on crystal morphology and hierarchical structure.

EXPERIMENTAL DETAILS All chemicals were of analytical grade and used as received without any further purification. L-glutamic acid (purity >99%), L-arginine (purity >99.5%) and ethanol (reagent grade) were purchased from Sigma Aldrich. Double distilled water (pH= 5.5) was used for the preparation of aqueous solutions for crystallization. Crystallization Experiments In this work L-glutamic acid was crystallized under different conditions. In these experiments, L-glutamic acid was crystallized with and without an L-arginine additive in pure water by a re-precipitation method using two miscible solvents – water and ethanol. L-glutamic acid crystallization in water: In the crystallization experiments, 88 mM of L-glutamic acid were dissolved in 10 mL of H2O. The solution was stirred and heated at 70°C for half an hour and then left to crystallize at 4°C. Crystals were initially observed after 24 hrs. The duration of the crystallization was ca. 48 hrs. Similar experiments of L-glutamic acid crystallization were conducted with L-arginine as an additive. For crystallization with low concentration of L-arginine additive, only 0.4 mM of arginine were added to the crystallization solution, while for high additive concentration, 8.6 mM of L-arginine were added to the crystallization solution. L-glutamic acid crystallization in solvent mixtures: We also carried out crystallization of L-glutamic acid by the re-precipitation method using two miscible solvents. In this case control crystallization experiments were performed. 52 mM of L-glutamic acid were dissolved in 13 ml of H2O, and the solution was stirred and heated at 50°C for half an hour. Then, 4 mL of this solution were placed in a centrifuge tube, and cold ethanol (at 4°C) was


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added to attain a final volume of 40 mL. The solution was shaken by hand and left to crystallize at 4°C. Crystals were initially observed after a number of hours (ca. 4-5). The duration of the crystallization was ca. 24 hours. In other experiments, low (0.4 mM) and high (6.6 mM) concentrations of L-arginine were added to the L-glutamic acid solution before heating. After the crystallization was completed, the crystals were isolated from the solution by decantation and kept in a centrifuge tube for further examination by various techniques. Several systems of amino acids were also crystallized under similar conditions: 52.2 mM of L-glutamic acid and 57.7 mM of L-aspartic acid were crystallized with charged amino acids (7.9 mM of L-lysine and 7.4 mM of L-histidine) and uncharged amino acids (9.85 mM of Lvaline, 9.68 mM of L-threonine, 12.95 mM of L-alanine and 8.8 mM of L-Leucine) as additives. 13

C NMR of L-glutamic acid with high concentration of L-arginine:

L-glutamic acid:

13

C NMR (D2O, 600 MHz) d 177.93 (C), 174.55 (C), 54.62 (CH), 30.91

(CH2), 26.28 (CH2) L-arginine:

13

C NMR (D2O, 600 MHz) d 174.90 (C), 157.44 (C), 54.92 (CH), 41.13 (CH2),

28.15 (CH2), 24.28 (CH2) Characterization techniques: Scanning electron microscope (SEM) images were obtained with a FEI instrument − Inspect S model at acceleration voltages of 15 kV and 30 kV. Powder X-ray diffraction patterns were acquired with a Bruker AXS D8 Advance diffractometer with Cu Kα (λ = 1.5418 Å) operating at 40 kV/40 mA. Data were collected from 10° to 70° with a step size of 0.01°, Time/Step=0.5 Sec. Polarized optical microscopy (POM) images were taken with a BX51-P Olympus microscope equipped with a U-AN360-3 polarizer. L-glutamic acid morphology calculations were performed by the Materials Studio program and Mercury 3.9 software using the crystallographic information file (CIF) taken


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from the Cambridge crystallographic database (ref code LGLUAC11).

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C NMR spectra of

L-glutamic acid with a high concentration of L-arginine were obtained by an AM-600 spectrometer (Bruker).

RESULTS AND DISCUSSION The first phase of our research was to study the effect of the L-arginine additives on the crystallization of L-glutamic acid. For this purpose L-glutamic acid was crystallized with and without L-arginine by the re-precipitation method using two miscible solvents, water and ethanol, as described in the experimental section. SEM images of L-glutamic acid crystals are shown in Figure 1A. As can be seen, L-glutamic acid crystallizes into micro-sized hexagonal sheets with diameter of 10–15 µm and thickness of about 250 nm, and the crystals form flower-like structures. In contrast, the presence of L-arginine results in the formation of spherical superstructures with a large size distribution of 30-100 µm which display a dramatic change in crystal morphology compared with L-glutamic acid crystallized with no additive (Figures 1B,1D). Figures 1B and 1D display L-glutamic acid spheres crystallized in the presence of low (1 / 100 mg) and high (15 / 100 mg) concentrations of L-arginine. Figures 1C and 1E show the surface structures of the spheres obtained with the high and low concentrations, respectively. As can be seen, the spheres obtained at both concentrations are much more compact and organized than L-glutamic acid crystallized without additive. The high concentration spheres display crystal sizes of about 30 µm and have a very dense wrinkled surface containing porous of ca. 0.7-1.4 µm, while the low concentration spheres have a thin sheet-like morphology with a broad size distribution of 30–100 µm and are very porous with a pore size of ca. 0.7-1.4 µm. It is clearly evident that as the L-arginine concentration increases, the thickness of the L-glutamic acid plates decreases; without L-arginine additive the thickness of


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the plate is 250 nm, while in the presence of L-arginine the thickness of the sphere plate is only 50 nm.

A

B

20 µm

C

10 µm

2 µm

E

D

50 µm

5 µm

Figure 1. SEM images of A) L-glutamic acid crystallized in 10:1 ethanol:water solutions, B) hierarchical spheres of L-glutamic acid crystallized with a high concentration of L-arginine, C) surface morphology of L-glutamic acid sphere crystallized with a high concentration of Larginine, D) hierarchical spheres of L-glutamic acid crystallized with low concentration of Larginine, and E) surface morphology of L-glutamic acid sphere crystallized with a low concentration of L-arginine.

In order to study the crystal structure of the low and high concentration spheres, X-ray diffraction (XRD) measurements were utilized. XRD spectra of L-glutamic acid and low and high concentration spheres crystallized in solvent mixtures are shown in Figure 2A. From these results, it is evident that L-glutamic acid spheres crystallized in the presence of the Larginine additive all portray the same crystal structure. These spectra correspond to β-L-


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glutamic acid, which crystallizes in a primitive unit cell (space group P212121) with the following parameters: a = 5.159, b = 17.30, c = 6.948, and α = β = γ = 90°.31 The XRD of Lglutamic acid crystallized in water also corresponds to β-L-glutamic acid shown in Figure S1.

A

B

Figure 2. A) X-ray diffraction spectra of L-glutamic acid crystallized without and with high (top) and low concentrations of L-arginine. B) Side view of the (0 2 0) L-glutamic acid surface (white line). Gray = carbon, red = oxygen, blue = nitrogen.

When comparing the XRD spectra, it is clear that the intensities of the (020) and (040) peaks are markedly reduced in the low and high concentration spectra compared to the L-glutamic acid spectrum. The decrease in peak intensities is proportionate to the L-arginine additive concentration. In order to understand the reason for the (020) peak decrease, this (020) crystal plane was modeled using Materials Studio 4.4 (Accelrys). The crystal morphology of Lglutamic acid grown in water was also modeled using Mercury 3.9 software of the Cambridge Crystallographic Data Centre Figure S2. As can be seen in Figure 2B, the functional groups protruding from the (020) plane are charged with carboxylic groups. Based on these findings, the (020) peak decrease can be explained by strong electrostatic interactions and hydrogen bonds between the positively charged L-arginine additive and the


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charged carboxylic groups of L-glutamic acid. The L-arginine adsorbs to the (020) plane, thereby slowing or stopping its growth. In addition, the intensities of the (120), (151), and (160) peaks also decrease with increasing L-arginine concentration. These peaks partially expose carboxylic groups and therefore also contribute to the interaction between L-arginine and L-glutamic acid. However, the electrostatic and hydrogen bonding are strongest through the (020) plane. These findings are in line with previous studies of Colfen et. al.28 In order to verify our assumption that L-arginine interacts with L-glutamic acid, the latter was crystallized in water with and without L-arginine additive. Figure 3 presents SEM images of L-glutamic acid crystallized in H2O without (Figure 3A) and with low (Figure 3B) and high (Figure 3C) concentrations of L-arginine. As can be seen, pure L-glutamic acid and L-glutamic acid crystallized with a low concentration of L-arginine crystallize as hexagonal plates with diameters of ca. 250 µm and 100–600 µm, respectively. However, it is clear that with the high L-arginine concentration the crystal plate thickness becomes thinner than the Lglutamic acid crystal plate thickness. The plate thickness of the L-glutamic acid crystals without additive is about 4 µm, and at a low concentration of L-arginine additive it decreases to about 1.6 µm. When the L-arginine concentration is increased, the morphology changes and becomes sheet-like with crystal thickness of 800 nm. This corresponds to the morphology change which occurs when L-glutamic acid is crystallized in H2O/ethanol mixtures and shows that the L-arginine additive is responsible for the crystal morphology change. The pH values of the crystallizing solutions were also measured: the L-glutamic acid solutions with low and high concentrations of L-arginine have pH = 4. Under these conditions, one of the carboxyl groups (pI = 2.19) and the amino group (pI = 9.67) of L-glutamic acid are charged, while the other carboxyl group is partially charged (pI = 4.25) and the L-arginine amino group (pI = 9.04, 12.48) and carboxyl groups (pI = 2.17) are all charged. Under these conditions, electrostatic interactions between L-glutamic acid and L-arginine are possible.


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A

B

100 µm

C

300 µm

400 µm

Figure 3. SEM images of L-glutamic acid crystallized in H2O: A) alone, B) with low concentration of L-arginine, C) with high concentration of L-Arg.

These findings strengthen the results found by X-ray diffraction. In addition, several systems of amino acids were also crystallized under similar conditions, as described in the experimental section. L-glutamic acid and L-aspartic acid were crystallized with charged amino acids (L-lysine and L-histidine) and uncharged amino acids (L-valine, L-threonine, Lalanine and L-leucine) as additives. In all cases where the additives were charged, crystals with spherical shape were observed (Figure 4A) with broad size distribution of 20–50 µm. On the other hand, uncharged additives produced flowerlike structures (Figure 4B). In the latter case, it is apparent that the flowerlike structures crystallized inside spheres, as in the former case. This can be attributed to the strong hydrogen bonds which occur in both systems. However, when crystallized with charged additives, both L-glutamic acid and L-aspartic acid show a perfect spherical shape which results from the electrostatic interactions that exist in addition to the hydrogen bonds. These results further confirm our assumption that the strong electrostatic interactions and hydrogen bonds between L-arginine and L-glutamic acid are responsible for the different crystallization of L-glutamic acid with and without additive.


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A

B

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C

50 µm

40 µm

50 µm

Figure 4. SEM images of A) L-glutamic acid crystallized with L-histidine as additive, B) Laspartic acid crystallized with L-arginine as additive, and C) L-glutamic acid crystallized with L-threonine as additive.

In order to study the structure of L-glutamic acid with low and high concentration of Larginine, polarized optical microscopy (POM) was utilized. Figures 5A and 5B display POM images of low and high concentration spheres, respectively. As can be seen, these images display a clear Maltese-cross extinction pattern, which is indicative of radial crystal orientation.32 We can thus conclude that the L-glutamic acid spheres are composed of radially oriented microcrystals.

A

B

Figure 5. POM images of L-glutamic acid crystallized with L-arginine at A) low concentration and B) high concentration.


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With the intention of confirming the presence of L-arginine in the L-glutamic acid crystal spheres, NMR measurements were utilized (see

13

C results in the experimental

section). As is evident, the NMR spectra contain 13C peaks corresponding to L-glutamic acid and L-arginine, confirming the presence of the latter in the spheres. Finally, another parameter was studied –the effect of the chirality of the additive. For this purpose, L-glutamic acid was crystallized in the presence of D-arginine, and the crystals were examined by SEM and XRD. As can be seen (Figure 6), the D-arginine additive results in L-glutamic acid spheres similar to those obtained with the L-arginine additive. Moreover, the XRD results also correspond with those obtained with L-arginine. From these results we learn that the chirality of the additive does not play a role in the crystallization of L-glutamic acid.

50 µm

Figure 6. SEM image of L-glutamic acid crystallized with D-arginine as additive.

CONCLUSIONS In this paper the effect of L-arginine additive on the crystallization of L-glutamic acid was studied. As revealed by SEM, with low and high concentrations of L-arginine, Lglutamic acid crystallized as hierarchical spheres, in contrast to the flower-like structures obtained without additive. XRD measurements indicated a large change in the intensity of the (020) plane. This was explained by the strong electrostatic interactions and hydrogen bonds between the positively charged L-arginine and negatively charged carboxylic groups of L-


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glutamic acid. The L-arginine adsorbed to the (020) plain, thereby slowing or stopping its growth.

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C-NMR confirmed the presence of L-arginine in the high concentration spheres.

Morphology changes between L-glutamic acid crystallized in water without and with Larginine further strengthened our assumption that the additive is responsible for the change in L-glutamic acid crystal morphology in H2O/ethanol mixtures. On the basis of our measurements, we propose a mechanism for the formation of the hierarchical L-glutamic acid microspheres shown schematically in Figure 7. Our proposed mechanism is based on a multistep process. The first step is the formation of a quasiemulsion. L-glutamic acid and L-arginine are dissolved in H2O, and ethanol which serves as the anti-solvent is then added to the solution. Upon the addition of ethanol, a quasi-emulsion is formed where water droplets containing L-glutamic acid and L-arginine are surrounded by ethanol. These water droplets act as micrometer-sized chemical reactors in which the crystallization process takes place, resulting in the formation of spherical hierarchical structures. We assume that the size of the water droplets depends on the ratio of the two phases. The formation of the quasi-emulsion is accompanied by a rapid crystallization of the Lglutamic acid and L-arginine that occurs in the water droplets, resulting in nano- or microsized particles. The morphology of the particles is determined by the concentration of Larginine additive. Overall, hierarchical microspheres of L-glutamic acid composed of nanoand micro-sized crystals are obtained.


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Figure 7. Mechanism proposed for the synthesis of L-glutamic acid spheres by the reprecipitation method. In summary, in this paper we describe the formation of hierarchical structures of L-glutamic acid by a process of adsorption of L-arginine onto the L-glutamic acid, attributed mainly to electrostatic interactions. This study demonstrated the ability to control and grow hierarchical structures of organic crystals by the principle of selective electrostatic interactions. This can be applicable for many other organic crystals as well. Clearly, a thorough study is still needed in order to understand in detail the formation mechanism of the hierarchical structures, relying mostly on new and advanced analytical techniques. However, a basic understanding of the formation mechanism demonstrated in this study is in line with new non-classical models for nucleation and growth of mesocrystals and hierarchical structures. The selforganization of nanoparticles into hierarchical structures by simple and selective electrostatic interactions of oppositely charged amino acids presents a new approach to crystal morphogenesis control that is significant for many chemical and biological applications. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. The PDF file includes: a XRD data plot of L-Glutamic acid crystallized in water and a model of the crystal morphology of L-glutamic acid grown in water.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected], *E-mail: [email protected]


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ORCID Yitzhak Mastai: 0000-0002-7330-0886 Notes: The authors declare no competing financial interest ACKNOWLEDGMENTS. We thank the Bar-Ilan President’s Ph.D. Scholarship Foundation and the Bar-Ilan Nanocenter Ph.D. Scholarship.


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TOC Unique hierarchical structures of glutamic acid were synthesized by re-precipitation crystallization method using arginine as an additive. The formation of hierarchical structures of glutamic acid was studied under various crystallization parameters. A possible mechanism for the formation of hierarchical structures of glutamic acid based on the self-assembly principle is proposed.


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Formation of Hierarchical Structures of L-Glutamic acid with L-Arginine

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