D Alanine and Carbon Nanotubes in

1, 394006,Voronezh, Russia. 6. * Corresponding author email address: 7 [email protected] (Elena Butyrskaya). 8. Phone: +7(473)220-89-32. 9. 10. 11. 12...
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Sorption Interactions between L/D‑Alanine and Carbon Nanotubes in Aqueous Solutions Elena Butyrskaya,*,† Sergey Zapryagaev,‡ Ekaterina Izmailova,† and Ludmila Nechaeva† †

Department of Chemistry, Voronezh State University, Universitetskaya pl. 1, 394006, Voronezh, Russia Department of Computer Science, Voronezh State University, Universitetskaya pl. 1, 394006, Voronezh, Russia

‡

ABSTRACT: Sorption isotherms for L- and D-alanine from aqueous solutions on carbon nanotubes MKN-SWCNT-S1 were constructed. It was found that the sorption of D-alanine on the investigated carbon nanotubes (CNTs) is stronger than the sorption of L-alanine. The difference in sorption can serve as the basis for their separation. The quantum chemical calculation carried out in the frame of density functional theory with dispersion corrections showed a difference in the orientation of the L- and D-alanines relative to CNTs in the adsorption process. This difference leads to a greater number of D amino acid molecules adsorbed on CNT than L-alanine on the same nanotube surface.

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INTRODUCTION Carbon nanotubes are relatively new chiral sorbents with unique sorption properties. In the literature, there are no papers containing a comparative analysis of the sorptivity of carbon nanoparticles with respect to L and D amino acids (enantiomers). The experimental and quantum chemical studies of interactions between amino acids and carbon nanotubes (CNTs) are presented in the literature only for L isomers.1−11 In the majority of the papers, the calculations of the amino acid adsorption energies on CNTs are made with quantum chemical models without taking into account the dispersion corrections. Such an approximation is not always correct due to high carbon nanoparticle polarizability. The development of the methods for separating chiral amino acids is one of the urgent problems in modern pharmacology. This is due to the requirement of chiral purity of amino acid pharmaceuticals. Out of tens of thousands of organic substances produced, only 30% of pharmaceutical compounds and 25% of the compounds used in agrochemistry are homochiral.12 Synthesis of amino acids in the laboratory conditions makes it possible to obtain only a mixture of L and D isomers. Therefore, many drugs and vitamins, synthetically prepared, do not possess a chiral purity. However, only one of the enantiomers can produce a pharmacological effect, while the other can have a negative effect on the body. This fact determines the urgency of the problem of producing homochiral drugs closely connected with the task of separating L and D isomers of amino acids. High performance liquid chromatography (HPLC) is used for the quantitative separation of enantiomers, while the ligandexchange HPLC13 is the most selective means of separating the optical isomers.13 The separation of enantiomers is based on using either chiral stationary phases or chiral eluents.14 In an achiral medium, enantiomers have similar chemical and physical © 2017 American Chemical Society

properties apart from the ability to rotate the plane of light polarization by the same angle value but in opposite directions. The main problem of enantiomers separating is to choose an appropriate selector. Nowadays, a large number of chiral stationary phases for liquid chromatography are well-known. Amino acids and amino acid derivatives, proteins, cyclodextrins and cyclodextrin derivatives, polysaccharides and their derivatives, and macrocyclic antibiotics are most commonly used as selectors.15−18 In this paper, we construct the adsorption isotherms of L- and D-alanine from aqueous solutions on carbon nanotubes MKNSWCNT-S1. Using quantum chemistry methods, we study the adsorption of zwitterions of L- and D-alanine on CNTs of 8.14 Å diameter in the neutral aqueous media with dispersion corrections.

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EXPERIMENTAL AND THEORETICAL METHODS Materials. In order to construct the adsorption isotherms, we used single-walled carbon nanotubes MKN-SWCNT-S1 (Canada). L- and D-Alanine produced by Sigma-Aldrich (Russia) were used as amino acids. Construction of Adsorption Isotherms. Carbon nanotubes were immersed in the aqueous solution of amino acids (pH ∼ 7). The resulting suspensions were sonicated for 3 min using an ultrasonic device MEF91.19 The suspensions were continuously stirred for 4 h in a Shaker-Incubator ES-20 and then kept for 20 h under static conditions at a temperature of 25 °C to establish equilibrium. This preparation mode has been identified as a result of kinetic studies. After 24 h, the suspension was centrifuged and the determination of amino Received: July 12, 2017 Revised: August 24, 2017 Published: August 28, 2017 20524

DOI: 10.1021/acs.jpcc.7b06849 J. Phys. Chem. C 2017, 121, 20524−20531

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The Journal of Physical Chemistry C

carbon nanoparticle (CNP), and Esorbate+CNP is the energy of the system CNP−sorbed particle.

acid concentration in supernatant by spectrophotometry was carried out. Computer Experiment. The quantum chemical calculation of adsorption energies of L- and D-alanine in the form of zwitterions was performed on carbon nanotubes of diameter 8.14 Å (chirality (6.6)) with open ends with a length of 8.61 Å. The tested amino acids exist in the form of a zwitterion in neutral aqueous solutions; the calculations were performed using the PCM solvation model.20 In the starting structures sorbate+CNT, the amino acids were positioned in three ways: on the outer side wall, at the open end, and inside CNT. In the first and second cases, the amino acids were oriented relatively CNT by carboxyl or by amino group. In cases of adsorption on the lateral surface or inside the CNT, the initial structure of D amino acid was constructed from the structure of L amino acid by exchanging the amino group and the α hydrogen atom with the same position of the other atoms. Optimization of the structures was carried out using the Gaussian 09 program21,22 by B3LYP/6-31G (d,p) method with dispersion correction GD3.23 The total DFT-D3 energy is EDFT‑D3 = EDFT − Edisp, where EDFT is the DFT energy and Edisp is the dispersion correction. Last one is defined as a sum of two terms, Edisp = E(2) + E(3). The most important two-body term E(2) is given by23 E(2) =

∑ ∑ AB n = 6,8,10,...

sn

CnAB n fd, n (rAB) rAB

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RESULTS AND DISCUSSION Adsorption Isotherms of L- and D-Alanine on Carbon Nanoparticles. Sorption isotherms of L- and D-alanine on the investigated CNTs are shown in Figure 1.

Figure 1. Sorption isotherms of D-alanine (curve 1) and L-alanine (curve 2) on CNTs MKN-SWCNT-S1.

The isotherms have inflections and two plateaus. From Figure 1, it follows that the sorption of D-alanine by carbon nanotubes is stronger than that of L-alanine. The difference in the affinity of CNTs to L and D amino acids, which is caused by the difference in the interactions of chiral isomers of amino acids with chiral sorbents in the form of CNTs, can serve as a basis for the technology of their separation. Quantum Chemical Calculation of the Sorption Energy of L and D Amino Acids on Carbon Nanoparticles. The optimized structures of L- (D-) alanine are shown in Figure 2a (the additional dashed line passes through the C1−N atoms and is used to indicate the orientation of the molecule in the space). The dummy atoms X1 and X2 are denoting the nanotube axis and are shown in Figure 2b. The notation of atoms in Figure 2 is used to interpret the results below. Five different cases of the optimized structures and the adsorption energies of zwitterions of L and D amino acids on CNTs in aqueous solution are shown in Figures 3−7. For brevity, the atoms of amino acids are not indicated in these figures. The relative position of sorbent (Figure 2b)−sorbate (Figure 2a) structures is characterized (Figures 3−7) by the mutual position of an auxiliary axis via C1N (Figure 2a) of the amino acid and the axis of the nanotube via dummy atoms X1X2. It is assumed in Figures 3−7 that the dummy atom X1 is located closer to the N atom of the amino acid. Case 1. In the starting structures, L and D isomers were turned to the side surface of the CNT by the carbonyl group. During the optimization, the amino acids turned and the amino groups of the isomers moved closer to the CNT (Figure 3). In the final optimized structures, the amino group for the D isomer is located slightly further from the CNT than for the L isomer, and the carbonyl group is slightly closer. In Figures 3−7, dO(1)C and dNC are the distances between the atom of oxygen (nitrogen) of the amino acid and the nearest carbon atom of CNT. A is the angle between the crossed lines C1N and X1X2. Dummy atom X1 is located closer to the N atom of the amino acid.

(1)

Here, the first sum in eq 1 is over all atom pairs in the system, CAB n is the average value of the nth order dispersion coefficient (orders n = 6, 8, 10, ...) for atom pair AB, and rAB is their internuclear distance; sn are scaling factors. The damping function fd,n is given by23 −1 ⎡ ⎛ r ⎞−an ⎤ AB ⎢ ⎥ ⎟ fd, n (rAB) = 1 + 6⎜⎜ AB ⎟ ⎢⎣ ⎝ sr, nR 0 ⎠ ⎥⎦

(2)

where sr,n is the order-dependent scaling factor of the cutoff radii RAB 0 and parameters an are also not fitted but adjusted manually such that the dispersion correction is 0, blue for C < 0).

CNT are due to the very high polarizability of the CNT,25 that is a result of a high mobility of the π-electron cloud of the tube. This leads to the appearance of significant amino acid dipole− CNT induced dipole interactions in the CNT−sorbate system. The analysis of Figure 3 shows that the orientations of the isomers relative to CNTs, defined by the angle A, are different. Thus, in the case of the L isomer, the angle A is equal to 3.3°, while in the case of the D isomer the angle A is equal to ∼34.8°. Thus, the line C1N in the case of the L isomer is almost parallel to the CNT axis, and in the case of the D isomer, this line is located at an angle to the CNT axis. Therefore, with dense packing, more molecules of the D isomer can settle on the outer side wall of the CNT than the L isomer molecules. Besides the analysis of Figure 4a, Figure 4b shows that the D isomer may attach itself to the outer side wall of the CNT by an amino group, which does not occur for the L isomer. Herewith, the angle A is equal to 97.6°. Therefore, more molecules of the D isomer can settle on the outer side wall of the CNT than for the L isomer. Interpretation of Adsorption Isotherms. The isotherms (presented in Figure 1) have inflections and two plateaus. The presence of a second plateau may be due to many layer sorption or to a change in the adsorption mechanism, i.e., the existence of unequal energy adsorption centers. Multilayer sorption on a homogeneous surface from a solution can be described by the BET theory for liquid media26

Figure 6. Optimized structures of systems “the alanine zwitterion − CNT”: the adsorption by the CNT open end from the aqueous solution: (a) L isomer, (b) D isomer, case 4.

Figure 7. Optimized structures of the systems “L- and D-alanine zwitterion − CNT”: the encapsulution from the aqueous solution inside the CNT: (a) L isomer, (b) D isomer, case 5.

A=

energy occurs when an amino acid is attached to the outer side in side wall of the CNT (Eend ads > Eads > Eads ). In all cases, the interaction energies of D-alanine and L amino acid with CNT in similar structures are close but not equal to each other. It is due to adsorption of enantiomers on the CNT curved surface. When alanine zwitterion reacts with the CNT end, the dissociative chemisorption of the amino acid takes place. The adsorption energy of the amino acid on the CNT has the greatest value (Figure 6). This is because at the open ends and near the CNT their electron density is greater than the electron density near the tube side wall due to the presence of the breaking bonds of the carbon atom at the CNT ends. This conclusion is based on the analysis of isosurfaces of the molecular orbitals (Figure 8) constructed in the present study. From Figure 8, it is clear that the probability density of the presence of the electron near the ends of the nanotube is greater than that near the center of the side wall. The amino acid reacts with the CNT side wall by van der Waals interactions; the ionic form of alanine as a result of such interactions with CNTs does not change. The sufficiently strong van der Waals interactions of chemical compounds with

A∞KSCe (1 − KLCe)(1 − KLCe + KSCe)

where A is adsorption; A∞ is the monolayer adsorption capacity of the adsorbent; Ce is the liquid phase equilibrium concentration; KL is the equilibrium constant of upper layers, a parameter related to the binding intensity for all layers; and KS is the equilibrium constant of adsorption for the first layer.25 Calculated parameters for BET isotherm models applied for adsorption of L (D) amino acid on the carbon nanotube are shown in Table 1. However, the quantum chemical calculation showed that unequal energy adsorption centers of CNTs are possible. For defect-free tubes with open ends, the interaction with the end and side wall of the tube, encapsulation inside the CNT is possible. All of these processes are characterized by different sorption energies of L and D isomers. This effect is due to the curved surface of the CNTs. For a more correct interpretation of the isotherm, we may analyze the available literature data on the structural and sorption properties of CNTs. In the synthesized CNTs without dopant atoms, the Stone−Wales defects (nonhexagonal rings) and vacancies, on which the probability of adsorption of chemical compounds is higher than on the side wall, are possible.27,28 For the vacancies, this is due 20527

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Table 1. Calculated Parameters for BET Isotherm Models Applied for Adsorption of L (D) Amino Acid on the Carbon Nanotube system L-alanine D-alanine

+ CNT MKN-SWCNT-S1 + CNT MKN-SWCNT-S1

KL (Dm3/mmol)

A∞ (mmol/g)

KS (Dm3/mmol)

R2

0.0145 0.0145

21.52 44.82

0.397 0.620

0.858 0.979

Table 2. Parameters of D- and L-Alanine Sorption from the Aqueous Solution by Carbon Nanotubes mkNANO MKN-MWCNT S1 Calculated Using the Langmuir and Freundlich Equations parameters

system L-alanine

+ CNT mkNANO MKN-MWCNT S1

D-alanine

+ CNT mkNANO MKN-MWCNT S1

Langmuir isotherm KLC A(C) = A∞ 1 1 + KLC

Freundlich isotherm

plateau 1 A∞(1) = 16.95 mmol/g KL(1) = 0.16 mL/mmol R(1)2 = 0.996 plateau 2 A∞(2) = 27.03 mmol/g KL(2) = 0.17 mL/mmol R(2)2 = 0.987 plateau 1 A∞(1) = 52.63 mmol/g KL(1) = 0.37 mL/mmol R(1)2 = 0.999 plateau 2 A∞(2) = 27.03 mmol/g KL(2) = 0.31 mL/mmol R(2)2 = 0.680

plateau 1 KF(1) = 3.90 (L1/n·mol−1/n)/g n(1) = 2.39 R(1)2 = 0.999 plateau 2 KF(2) = 5.70 (L1/n·mol−1/n)/g n(2) = 2.36 R(2)2 = 0.931 plateau 1 KF(1) = 16.29 (L1/n·mol−1/n)/g n(1) = 2.46 R(1)2 = 0.976 plateau 2 KF(2) = 8.13 (L1/n·mol−1/n)/g n(2) = 2.63 R(2)2 = 0.676

A(C) = KFC1/ n

method shows that if the geometrical factor is such that a molecule can enter the carbon nanotube then the number of atomic contacts of an adsorbed molecule inside the nanotube is bigger than at the surface and thus the adsorption energy greater.31 Water molecules and small organic molecules can form a molecular wire inside CNT; the polyalanine chain forms the bent helix in CNT confinement according to research by MD.32 Therefore, in our opinion, the presence of two plateaus on the isotherms is due to the presence of two sorption mechanisms: 1. the adsorption of the amino acid by the CNT outer side wall 2. the encapsulution of the sorbate inside the CNT The following schemes for implementing of these mechanisms are possible: I. After the completion of the first sorption mechanism is implemented a second one. II. The adsorption by both mechanisms begins and finishes simultaneously. III. Adsorption by both mechanisms starts simultaneously, but one of them finishes earlier than the other. IV. Adsorption by one of the mechanisms begins earlier; then, sorption according to the second mechanism begins, when the first mechanism is not completed. Both mechanisms end simultaneously. In the case of scheme I, the first plateau corresponds to outer side wall adsorption mechanism, which realized first, and the second plateau corresponds to encapsulation mechanism, which realized next. In this case, each plateau can be described by the Langmuir and Freundlich equations, and so on. The parameters

to the increased electron density near these defects, which is a consequence of the redistribution of the electron density originally located on carbon−carbon bonds that break at the formation of a vacancy. Besides, usually, in addition to the defects mentioned above, the ends of most of these nanotubes are closed with fullerene caps. The calculation shows that the energy of interaction of amino acids with C60 fullerene in aqueous medium is much lower than with the CNT open end. It is close to the interaction energy of amino acids with the side surface of CNT (the adsorption energies of zwitterions of L- and D-alanine on the fullerene C60 are 6.78 and 6.97 kcal/mol, respectively). Due to this, the adsorption probability on the CNT ends closed with fullerene caps is lower than that on the open CNT ends and it is close to ones by the side surface of CNT. In the construction of sorption isotherms, the procedure of opening the CNT ends was not carried out. Therefore, the quantum chemical calculation describes the real sorption process only approximately and is given for qualitative demonstration of the difference in sorption energies of L and D isomers on CNTs and the existence of various mechanisms of monolayer sorption. The calculation results and the literature data allow us to conclude that in the CNTs there are sections with different reactivity with respect to the sorbed chemical compounds. A very insignificant concentration of monovacancies, open ends, and the Stone−Wales defects29 suggest that a small part of the sorbate will be sorbed by these sections. As a result, their influence will not be reflected on the shape of the isotherm, despite the fact that their sorption energies are most significant. However, as follows from the literature, a significant amount of the amino acid can penetrate inside the CNTs through the existing defects.30 For example, the molecular dynamics (MD) 20528

DOI: 10.1021/acs.jpcc.7b06849 J. Phys. Chem. C 2017, 121, 20524−20531

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The Journal of Physical Chemistry C for each plateau are determined by linearizing the corresponding equation (Table 2). The results are shown in Table 2. In the case of scheme II, the part of the isotherm with the first plateau reflects the summary adsorption according to both mechanisms, and the part with the second plateau is the multilayer adsorption. In the case of scheme III, the part of the isotherm with the first plateau reflects the summary adsorption by both mechanisms, and the part with the second plateau corresponds to the adsorption according to the second mechanism, when the first mechanism completed. In the case of scheme IV, the part of the isotherm with the first plateau means the sorption by the mechanism that starts at first and the part with the second plateau means the sorption by both mechanisms. It is known that, in the case of several simultaneously realized adsorption mechanisms, the isotherm is a sum of the isotherms, each of which corresponds to a separate mechanism. Thus, in the case of two simultaneously realized Langmuir mechanisms, the resulting isotherm is the so-called bi-Langmuir isotherm. The equation of the bi-Langmuir isotherm has the form33 A(Ce) = A∞ 1

K1LCe K 2LCe + A∞ 2 1 + K1LCe 1 + K 2LCe

Figure 9. Densities of aqueous solutions of L-alanine as a function of concentration at the temperature 25.0 ± 0.1 °C.

solutions. This is indicative of the monolayer sorption of the alanine on CNTs. On the basis of the arguments outlined above, it is possible to conclude that the part of isotherms with the first plateau corresponds to the summary adsorption of alanine by the outer and inner side surface of the CNT, and the part with the second plateau reflects the encapsulation of the amino acid inside the CNT, when adsorption by the outer side surface was finished. At simultaneous sorption on energetically unequal sorption centers, the parameters of the isotherm equation (for example, eq 5 under the Langmuir mechanism) cannot be determined.

(5)

where K1L and K2L are adsorption−desorption equilibrium constants for the sample distributions between the bulk phase and adsorption sites of types 1 and 2, respectively, A∞1 and A∞2 are the saturation capacities for adsorption sites of types 1 and 2, respectively, and Ce is the liquid phase equilibrium concentration. The amino acid molecules are uniformly distributed around the CNT. Therefore, it is most probably that adsorption of amino acids by the outer side wall of CNT and encapsulation of the sorbate inside the CNT begin simultaneously, despite the difference in adsorption energies. Therefore, in our opinion, the part of the isotherm with the first plateau reflects the summary adsorption caused to the adsorption centers on the outer and inner side surfaces of the CNT. However, adsorption on the outer surface, in our opinion, occurs faster than the encapsulation of the amino acid inside the CNT. It is due to the higher availability of adsorption centers on the outer side wall of the CNT. The formation of the second layer during the adsorption of an amino acid on the outer side wall of CNT is realized, if an alanine concentration in an external solution is equal to (or greater than) the concentration, at which the amino acid dimerization begins. To answer the question of the possible dimerization of alanine in aqueous solution, the density of aqueous solutions of L-alanine of various concentrations by the pycnometric method was measured. The results are shown in Figure 9. The dependence of the density of aqueous solutions of Lalanine on the concentration is a linear function without kinks, which corresponds to the literature.34 Therefore, there is no reason to believe that dimerization of alanine begins at a certain concentration in the investigated interval C, since the inflection in Figure 9 should correspond to the dimerization beginning. This conclusion is confirmed by studies using the IR spectroscopy method,35 which supports the dimerization of alanine only in acidic aqueous solutions (pH ∼ 1) at a concentration of 0.1 M and its absence in neutral aqueous

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CONCLUSIONS The isotherms of L- and D-alanine sorption from aqueous solutions on carbon nanotubes MKN-SWCNT-S1 show different values of sorption affinity of chiral isomers of amino acids (L and D) with respect to carbon nanotubes. This is due to the differences in the interactions of chiral isomers of amino acids with CNTs which are chiral structures. The isotherms have inflections and two plateaus. It is found that the sorption of D-alanine on the investigated carbon nanotubes is stronger than the sorption of L-alanine. This fact may serve as the basis for the technology of their separation. The calculation of the interaction energies between L- (D-) alanine and CNTs of diameter 8.14 Å with a length of 8.61 Å was carried out. The calculation of the sorption energies of the amino acids of the given CNT was performed within density functional theory, taking into account dispersion corrections which significantly increase the accuracy of the calculation due to a high value of CNT polarizability. The calculation showed the difference in interaction energies of amino acids with CNTs in three positions of the amino acid: at the CNT end, on the CNT side wall, and under the capillary suction of the amino acid inside the CNT. Besides, the difference in values of the sorption energies on the CNTs of L and D isomers in all of the investigated cases was demonstrated. The quantum chemical calculation made within density functional theory with dispersion corrections confirms a higher sorptivity of carbon nanotubes with respect to D-alanine. On the basis of the calculation results and available literature data, it can be concluded that the presence of two plateaus on the isotherms indicates a change in the sorption mechanism. Namely, at first, the mechanism of the adsorption by the CTN sidewall is realized and then the mechanism of encapsulution of the animo acid inside the CNT. The analysis of the optimized structures “L- (D-) alanine−CNT” showed a difference in the orientation 20529

DOI: 10.1021/acs.jpcc.7b06849 J. Phys. Chem. C 2017, 121, 20524−20531

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(13) Davankov, V. A. In Ligand-Exchange Chromatography; Davankov, V. A.; Navratil, T. D.; Walton, H. F., Eds.; CRC Press, New York, Inc.: Boca Raton, Florida, 1988; Chapter 5. (14) Chernobrovkin, M. G.; Anan’eva, I. A.; Shapovalova, E. N.; Shpigun, O. A. Determination of Amino Acid Enantiomers in Pharmaceuticals by Reversed-Phase High-Performance Liquid Chromatography. J. Anal. Chem. 2004, 59, 55−63. (15) Davankov, V. A. Ligand-Exchange Chromatoghraphy is a Breakthrough in Enantioselective Technologies. In 100 Years of Chromatography; Nauka: Moscow, 2003; pp 212−232. (16) Armstrong, D. W. Current Issues in HPLC Technology (Supplement to LC GC) 1997, 5, 46−55. (17) Armstrong, D. W.; Tang, Y.; Chen, S.; Zhou, Y. Macrocyclic Antibiotics as a New Class of Chiral Selectors for Liquid Chromatography. Anal. Chem. 1994, 66, 1473−1484. (18) Anan’eva, I. A.; Shapovalova, E. N.; Shpigun, O. A.; Armstrong, D. W. Separation of Optically Active Amino Acids and Isomers of their Derivatives on Macrocyclic Antibiotic “Tikoplanin”. Vestn. Mosk. Unta. 2001, 2, 42. (19) http://melfiz-uz.promzone.ru/catalog/info/112360.htm. (20) Foresman, J. B.; Keith, T. A.; Wiberg, K. B.; Snoonian, J.; Frisch, M. J. Solvent Effects. Influence of Cavity Shape, Truncation of Electrostatics, and Electron Correlation on ab Initio Reaction Field Calculations. J. Phys. Chem. 1996, 100, 16098−16104. (21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009. (22) Butyrskaya, E. V. Computational Chemistry: Basic Theory and Work with Gaussian and GaussView Programs; Solon-press: Moscow, 2011. (23) Grimme, S.; Antony, J.; Ehrlich, S. A Consistent and Accurate ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104−154119. (24) http://www2.sscc.ru. (25) Nechaeva, L. S.; Butyrskaya, E. V.; Zapryagaev, S. A. Computer Simulation of Size Effects and Adsorption Properties of One-Wall Carbon Nanotubes (6,6). Russ. J. Gen. Chem. 2016, 86 (7), 1684− 1691. (26) Ebadi, A.; Mohammadzade, J. S. S.; Khudiev, A. What is the Correct Form of BET Isotherm for Modeling Liquid Phase Adsorption? Adsorption 2009, 15, 65−73. (27) Zhou, L. G.; Shi, S. Q. Adsorption of Foreign Atoms on Stone− Wales Defects in Carbon Nanotube. Carbon 2003, 41, 613−615. (28) Tsafack, T.; Alred, J. M.; Wise, K. E.; Jensen, B.; Siochi, E.; Yakobson, B. I. Exploring the Interface Between Single-Walled Carbon Nanotubes and Epoxy Resin. Carbon 2016, 105, 600−606. (29) Collins, P. G. Defects and Disorder. In The Oxford Handbook of Nanoscience and Nanotechnology; Oxford University Press: New York, 2010; p 3193. (30) Garalleh, H. A. L.; Thamwattana, N.; Cox, B. J.; James, M. H. Encapsulation of L-Histidine Amino Acid Inside Single-Walled Carbon Nanotubes. J. Biomater. Tissue Eng. 2016, 6, 362−369. (31) Shaitan, K. V. Functional Dynamics of Proteins Based on Conformational Mobility. International Workshop Computational and Theoretical Modeling of Biomolecular Interactions; IUPAB, JINR, RAS, Lomonosov Moscow State University, Division of Physico-Chemical Biology of RAS: Dubna, Russia, 2013; pp 62−68. (32) Xiu, P.; Xia, Z.; Zhou, R. Small Molecules and Peptides Inside Carbon Nanotubes: Impact of Nanoscale Confinement Nanotechnology and Nanomaterials. In Physical and Chemical Properties of Carbon Nanotubes; Suzuki, S., Ed.; IntechOpenScience: Croatia, 2013; Chapter 8, DOI: 10.5772/51453. (33) Gritti, F.; Guiochon, G. Analytical Solution of the Ideal Model of Chromatography for a Bi-Langmuir Adsorption Isotherm. Anal. Chem. 2013, 85, 8552−8558. (34) Ninni, L.; Meirelles, A. J. A. Water Activity, pH and Density of Aqueous Amino Acids Solutions. Biotechnol. Prog. 2001, 17, 703−711.

of L and D amino acids relative to CNTs. From this analysis, it was concluded that more D-alanine molecules can be located on the outer side surface of the CNT than L-alanine, while the contact surface area by each molecule of the D isomer is smaller than that of the L isomer. One way to increase the differences in enantiomeric energies on carbon nanotubes is the functionalization of the latter by the compounds used to separate the enantiomers (L-hydroxyproline, L-Ala, Cu ( L-Pro)2 , Cu ( L-His) 2, etc.).13,15 Such functionalization of CNTs should increase the sorption of Dalanine and reduce the sorption of L-alanine.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +7(473)220-89-32. ORCID

Elena Butyrskaya: 0000-0003-4096-6224 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The partial support of this research by the Federal Target Program, Agreement No. 14.574.21.0112 of 21.10.2014, is appreciatively acknowledged.

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DOI: 10.1021/acs.jpcc.7b06849 J. Phys. Chem. C 2017, 121, 20524−20531

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The Journal of Physical Chemistry C (35) Garcia, A. R.; Brito de Barros, R.; Lourencëo, J. P.; Ilharco, L. M. The Infrared Spectrum of Solid L-Alanine: Influence of pH-Induced Structural Changes. J. Phys. Chem. A 2008, 112, 8280−8287.

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DOI: 10.1021/acs.jpcc.7b06849 J. Phys. Chem. C 2017, 121, 20524−20531