d-Alanine and Carbon Nanotubes in

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Sorption Interactions Between L/D Alanine and Carbon Nanotubes in Aqueous Solutions Elena Vasil'evna Butyrskaya, Sergey Aleksandrovich Zapryagaev, Ekaterina Izmailova, and Ludmila Stanislavovna Nechaeva J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06849 • Publication Date (Web): 28 Aug 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

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Sorption Interactions Between L/D Alanine and Carbon Nanotubes in Aqueous Solutions

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Elena Butyrskaya†*, Sergey Zapryagaev††, Ekaterina Izmailova†, Ludmila Nechaeva†

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† Department of chemistry, Voronezh State University,

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Universitetskaya pl. 1, 394006,Voronezh, Russia

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†† Department of Computer Science, Voronezh State University,

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Universitetskaya pl. 1, 394006,Voronezh, Russia

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* Corresponding author email address:

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[email protected] (Elena Butyrskaya)

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Phone: +7(473)220-89-32

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Abstract

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Sorption isotherms for L and D alanine from aqueous solutions on carbon nanotubes

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MKN-SWCNT-S1 were constructed. It was found that the sorption of D alanine on the

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investigated carbon nanotubes (CNT) is stronger than the sorption of L alanine. The difference

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in sorption can serve as the basis for their separation. The quantum chemical calculation carried

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out in frame of density functional theory with dispersion corrections showed a difference in the

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orientation of the L and D alanines relative to CNTs in adsorption process. This difference leads

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to a greater number of D amino acid molecules adsorbed on CNT than L alanine on the same

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nanotube surface.

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Introduction

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Carbon nanotubes are relatively new chiral sorbents with unique sorption properties. In

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the literature there are no papers containing a comparative analysis of the sorptivity of carbon

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nanoparticles with respect to L and D amino acids (enantiomers). The experimental and quantum

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chemical studies of interactions between amino acid and carbon nanotubes (CNTs) are presented

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in the literature only for L isomers1-11. In the majority of the papers the calculations of the amino

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acids adsorption energies on CNT are made with in quantum chemical models without taking

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into account the dispersion corrections. Such approximation is not always correct due to high

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carbon nanoparticle polarizability.

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The development of the methods for separating chiral amino acids is one of the urgent

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problems in modern pharmacology. This is due to the requirement of chiral purity of amino acid

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pharmaceuticals. Out of tens of thousands of organic substances produced only 30% of

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pharmaceutical compounds and 25% of the compounds used in agrochemistry are homochiral12.

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Synthesis of amino acids in the laboratory conditions makes it possible to obtain only a mixture

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of L and D isomers. Therefore, many drugs and vitamins, synthetically prepared, do not possess

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a chiral purity. However, only one of enantiomers can produce a pharmacological effect, while

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the other can have a negative effect on the body. This fact determines the urgency of the problem

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of producing homochiral drugs closely connected with the task of separating L and D isomers of

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amino acids.

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High performance liquid chromatography (HPLC) is used for the quantitative separation

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of enantiomers, while the ligand-exchange HPLC13 is the most selective means of separating the

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optical isomers13 . The separation of enantiomers is based on using either a chiral stationary

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phases or chiral eluents14 . In an achiral medium enantiomers have similar chemical and physical

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properties apart from the ability to rotate the plane of light polarization by the same angle value

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but in opposite directions. The main problem of enantiomers separating is to choose an

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appropriate selector. Nowadays a large number of chiral stationary phases for liquid 3 ACS Paragon Plus Environment


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chromatography are well-known. Amino acids and amino acid derivatives, proteins,

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cyclodextrinsand cyclodextrin derivatives, polysaccharides and their derivatives, macrocyclic

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antibiotics are most commonly used as selectors15-18 .

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In this paper we construct the adsorption isotherms of L and D alanine from aqueous

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solutions on carbon nanotubes MKN-SWCNT-S1. Using quantum chemistry methods we study

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the adsorption of zwitterions of L and D alanine on CNTs of diameter 8,14Å in the neutral

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aqueous media with dispersion corrections.

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Experimental and Theoretical Methods

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Materials

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In order to construct the adsorption isotherms we used single-walled carbon nanotubes

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MKN-SWCNT-S1 (Canada). L and D alanine produced by Sigma Aldrich (Russia) were used as

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amino acids.

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Construction of adsorption isotherms

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Carbon nanotubes were immersed in the aqueous solution of amino acids (pH ~ 7). The

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resulting suspensions were sonicated for 3 minutes using an ultrasonic device MEF9119. The

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suspensions were continuously stirred for 4 hours in a Shaker-Incubator ES-20 and then kept for

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20 hours under static conditions at a temperature of 250С to establish equilibrium. This

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preparation mode has been identified as a result of kinetic studies. After 24 hours, the suspension

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was centrifuged and the determination of amino acid concentration in supernatant by

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spectrophotometry was carried out.

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Computer Experiment

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The quantum chemical calculation of adsorption energies of L and D alanine in the forms

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of zwitterions was performed on carbon nanotubes of diameter 8,14Å (chirality (6.6)) with open

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ends with the length of 8.61 Å. The tested amino acids exist in the form of a zwitterion in

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neutral aqueous solutions, the calculations were performed using PCM solvation model20.

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In the starting structures sorbate+CNT the amino acids were positioned in three ways: on

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the outer side wall, at the open end and inside CNT. In first and second cases the amino acids

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were oriented relatively CNT by carboxyl or by amino group. In cases of adsorption on the

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lateral surface or inside the CNT, the initial structure of D amino acid was constructed from the

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structure of L amino acid by exchanging the amino group and the α hydrogen atom with the

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same position of the other atoms.

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Optimization of the structures was carried out using Gaussian 09 program21-22 by B3LYP

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/ 6-31G (d, p) with dispersion correction GD323. The total DFT-D3 energy is =

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− ,

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as a sum of two terms = () + () . The most important two-body term () is given

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by23

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() = ∑ ∑

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Here, the first sum in (1) is over all atom pairs in the system, CnAB is the average value of n-th

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order dispersion coefficient (orders n=6,8,10,...) for atom pair AB, and rAB is their internuclear

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distance, sn are scaling factors. The damping functions fd,n is given by23

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, ( ) = &1 + 6 )

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where sr,n is the order-dependent scaling factor of the cut off radii R0AB, parameters an are also

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not fitted but adjusted manually such that the dispersion correction is < 1% of max( | Edisp | ) for

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typical covalent bond distances. The expression for three-body energy () is23

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() = ∑ ,() (̅ ) ,

where EDFT is DFT energy, Edisp is dispersion correction. One is defined

!,",#$… , ( ).

(1)

*+

,,

−0, #

/ -*+ 0

1 ,

(2)

(3)



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where the sum is over all atom triples ABC in the system and in Eq. (2) an = 16, sr,n=4/3, and

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averaged radii ̅ is used as a damping function23.

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The calculations were carried out at the Siberian Supercomputer Center of the Institute

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for Computational Mathematics and Mathematical Geophysics, Siberian Branch of the Russian

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Academy of Sciences24. Since CNTs have a high polarizability, the dispersion correction

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provides more accurate values of the sorption energy on the CNTs of chemical compounds. Sorption energies were calculated by the formula:

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E s= E sorbate + ECNP- E sorbate+CNP ,

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where E s is sorption energy, E sorbate is the energy of the particle whose sorption is studied, ECNP

(4)

sorbate + CNP

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is the energy of a carbon nanoparticle (CNP), E

is the energy of the system CNP -

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sorbed particle.

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Results and discussion

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Adsorption isotherms of L and D alanine on carbon nanoparticles

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Sorption isotherms of L and D alanine on the investigated CNTs are shown in Figure 1.

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A, mmol/g

1

70.00 60.00 50.00

2

40.00 30.00 20.00 10.00

C, equit, mmol/dm 3

0.00 0.00

10.00

20.00

30.00

40.00

50.00

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

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The isotherms have inflections and 2 plateaus. From Figure 1 it follows that the sorption of D

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alanine by carbon nanotubes is stronger than of L alanine. The difference in the affinity of CNTs

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to L and D amino acids, which is caused by the difference in the interactions of chiral isomers of

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amino acids with chiral sorbents in the form of CNTs, can serve as a basis for the technology of

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their separation.

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Quantum chemical calculation of the sorption energy of L and D amino acids on carbon

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nanoparticles

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The optimized structures of L (D) alanine are shown in Figure 2a (the additional dashed

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line passes through the C1-N atoms and is used to indicate the orientation of the molecule in the

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space).The dummy atoms X1 and X2, are denoting the nanotube axis and are shown in Figure 2b.

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The notation of atoms in Figure 2, is used for interpret the results bellow.

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a)

b)

Figure 2. The optimized structure of L- & D- alanine (a) and CNT (b)

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Five different cases of the optimized structures and the adsorption energies of zwitter-

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ions of L and D amino acids on CNTs in aqueous solution are shown in Figure 3-7. For brevity, 7 ACS Paragon Plus Environment


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the atoms of amino acids are not indicated at these figures. The relative position of sorbent

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(Figure 2b) - sorbat (Figure 2a ) structures is characterized (at Figure 3-7) by the mutual position

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of an auxiliary axis via C1N (Figure 2a) of the amino acid and the axis of the nanotube via

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dummy atoms X1X2. It is assumed at Figure 3 - 7 that the dummy atom X1 is located closer to the

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N atom of the amino acid.

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Case 1 . In the starting structures, L and D isomers were turned to the side surface of the

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CNT by the carbonyl group. During the optimization, the amino acids turned and the amino

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groups of the isomers moved closer to the CNT (Figure 3a, 3b). In the final optimized structures,

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the amino group for the D isomer is located slightly further from the CNT than for the L isomer,

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and the carbonyl group is slightly closer.

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Eads = 8.76 kcal/mol; dO(1)С = 3.3 Å; dNC = 3.5 Å; А = 3.30.


a) L isomer

b) D isomer

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Figure 3. Optimized structures of systems " the alanine zwitterion – CNT": the adsorption by the CNT side wall from the aqueous solution: a) L isomer, b) D isomer, case 1.

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At Figure 3-7 dO(1)С and dNC are the distances between the atom of oxygen (nitrogen) of

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the amino acid and the nearest carbon atom of CNT . А - is the angle between the crossed lines

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С1N and Х1Х2. Dummy atom X1 is located closer to the N atom of the amino acid.

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The angle A determines the orientation of the amino acid relative to CNTs. Comparison

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of structures of Figure3a, Figure 3b shows that for the L isomer, the line C1N is almost parallel 8 ACS Paragon Plus Environment

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to the axis of the nanotube (angle A = 3.30). But for the D isomer, this line is inclined at an angle

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to the CNT axis (angle A = 34.80). Because of this, the contact surface area by each molecule of

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D isomer is less than for L isomer.

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Case 2. In the starting structures the L and D isomers were oriented relatively the outer

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side wall of CNT by the aminogroup. A rotation of the L isomer have occurred during the

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optimization and the carbonyl group have approached to the surface of the CNT. The final

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structure for L isomer is at (Figure 4a). This structure is identical to the previous one in Fig 3a.

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For the D isomer amino acid rotation did not take place, the amino group in the optimized

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structure is located closer to the CNT than the carbonyl group (Figure 4b).

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a) L isomer

b) D isomer

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Figure 4. Optimized structures of systems " the alanine zwitterion – CNT": the adsorption by the CNT side wall from the aqueous solution: a) L isomer, b) D isomer, case 2.

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Fig 4a and 4b display that in these cases the contact surface area by D isomer is less than

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for L isomer too, the angle A = 97.60 for d and А = 3.30 for l-isomer.

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Case 3. In the starting structures the L and D isomers were oriented to open end of the

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CNT by the carbonyl group. During of the optimization both zwitterions have dissociated into an

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anion and a hydrogen cation, each of which forms a covalent bond with the carbon atoms at the

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CNT ends (Fig 5a,b).

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Eads = 85.7 kcal/mol; dC-O(1) = 1.4 Å ; dNC = 3.8 Å ;

Eads = 85.2 kcal/mol; dC-O(1) = 1.5 Å ; dNC = 4.4 Å ;

a ) L isomer

b) D isomer

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Figure. 5. 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 3.

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Case 4. In the starting structure the L and D isomers were oriented to open end of CNT

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by the aminogroup. During of the optimization D isomer has dissociated into an anion and a

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hydrogen cation, each of which forms a covalent bond with the carbon atoms at the end of CNT

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(Figure 6b). L isomer has dissociated into an anion of neutral form (which don't observed in

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aqueous solutions without CNT), and hydrogen cation, each of which forms a covalent bond

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with the carbon atoms at the CNT end. (Figure 6a).

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Eads = 95.7 kcal/mol; dO(1)С = 3.3 Å; dNC = 1.4 Å ; (O(1) ∈ OH);

Eads = 82.1 kcal/mol /моль; dO(1)С = 2.6 Å; dNC = 1.5 Å;

a) L isomer

b) D isomer

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. 10 ACS Paragon Plus Environment

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Case 5. In the starting structures the amino acids were positioned inside of the CNT.

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During the optimization the L isomer have remained in the center of the CNT, and the D isomer

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have shifted toward the end of the tube (Figure 7a,b).

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Eads = 22.8 kcal/mol; O(1)C = 4.4 Å; O(2)C=6.2 Å; a) L isomer

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Eads = 21.8 kcal/mol; O(1)C = 3.7 Å ; O(2)C=5.6 Å; b) D isomer

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.

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Analysis of the data presented in Figure 3-7 shows that the adsorption energies of the

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amino acid essentially depend on the site of its attachment to the CNT. The greatest value of Eads

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~ 82 - 96 kcal / mol occurs, when an amino acid is attached to the open end of a CNT. When the

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amino acid is encapsuluted into the nanotube, the adsorption energy has a value of ~21-23 kcal /

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mol. The lowest value (7-10 kcal / mol) of the adsorption energy occurs, when an amino acid is

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5 attached to the outer side wall of the CNT (4 6 4 6 4 ).

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In all cases the interaction energy of D alanine and L amino acid with CNT in similar

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structures are close, but not equal to each other. It is due to adsorption of a enantiomers on the

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CNT curved surface. 11 ACS Paragon Plus Environment


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When alanine zwitterion reacts with the CNT end, the dissociative chemisorption of the

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amino acid takes place. The adsorption energy of the amino acid on the CNT has the greatest

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value (Figure6). This is because at the open ends and near the CNT their electron density is

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greater than the electron density near the tube side wall due to the presence of the breaking bonds

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of the carbon atom at the CNT ends. This conclusion is based on the analysis of isosurfaces of

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the molecular orbitals (Figure 8) constructed in the present study.

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С=±0.001

С=±0.006

С=±0.01

Figure 8. Visualization of isosurfaces φ(x,y,z) = C for HOMO of CNT with the diameter 8,14Å and length of 8.61 Å for different values of C (red for C> 0, blue for C ? ,

Dm3 / mmol

mmol / g

Dm3 / mmol

L alanine + CNT MKN-SWCNT-S1

0.0145

21.52

0.397

0.858

D alanine + CNT MKN-SWCNT-S1

0.0145

44.82

0.620

0.979

Система

R2

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But the quantum chemical calculation showed that unequal energy adsorption centers of

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CNTs are possible. For defect-free tubes with open ends, the interaction with the end and side

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wall of the tube, encapsulution inside the CNT

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characterized by different sorption energies of L and D isomers. This effect is due to the curved

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surface of the CNTs. For a more correct interpretation of the isotherm, we may analyze the

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available literature data on the structural and sorption properties of CNTs. In the synthesized

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CNTs without dopant atoms the Stone-Wales defects (non-hexagonal rings) and vacancies, on

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which the probability of adsorption of chemical compounds is higher than on the side wall, are

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possible27-28. For the vacancies this is due to the increased electron density near these defects,

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which is a consequence of the redistribution of the electron density originally located on carbon-

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carbon bonds that break at the formation of a vacancy.

are possible. All these processes are

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Besides usually, in addition to the defects mentioned above the ends of most of these

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nanotubes are closed with fullerene caps. The calculation shows that the energy of interaction of

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amino acids with С60 fullerene in aqueous medium is much lower than with the CNT open end. It

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is close to the interaction energy of amino acids with the side surface of CNT (The adsorption

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energies of zwitterions of L and D alanine on the fullerene C60 are 6.78 and 6.97 kcal / mol,

20

respectively). Due to this, the adsorption probability on the CNT ends closed with fullerene caps

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is lower than on the open CNT ends and it is close to ones by side surface of CNT.


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In the construction of sorption isotherms the procedure of opening the CNT ends was not

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carried out. Therefore the quantum chemical calculation describes the real sorption process only

3

approximately and is given for qualitative demonstration of the difference in sorption energies of

4

L and D isomers on CNTs and the existence of various mechanisms of monolayer sorption.

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The calculation results and the literature data allow us to conclude that in the CNTs there are

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sections with different reactivity with respect to the sorbed chemical compounds. A very

7

insignificant concentration of monovacancies, open ends and the Stone-Wales defects29 suggest

8

that a small part of the sorbate will be sorbed by these sections. As a result, their influence will

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not be reflected on the shape of the isotherm, despite the fact that their sorption energies are most

10

significant.

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However, as follows from a literature, a significant amount of the amino acid can

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penetrate inside the CNT sthrough the existing defects30. Eg, the molecular dynamics method

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(MD) shows, that if the geometrical factor is such that a molecule can enter the carbon nanotube

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then the number of atomic contacts of an adsorbed molecule inside the nanotube is bigger than at

15

the surface and thus the adsorption energy greater31. Water molecules and small organic

16

molecules can form a molecular wire inside CNT, the polyalanine chain forms the bent helix in

17

СNT confinement according to research by MD32.

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Therefore, in our opinion, the presence of two plateaus on the isotherms is due to the presence of

19

two sorption mechanisms:

20

1. The adsorption of the amino acid by the CNT outer side wall.

21

2. The encapsulution of the sorbate inside the CNT.

22

The following schemes for implementing of these mechanisms are possible:

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I. After the completion of first sorption mechanism is implemented second one.

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II. The adsorption by both mechanisms begins and finish simultaneously.

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III. Adsorption by both mechanisms starts simultaneously, but one of them finish earlier

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than the other. 15 ACS Paragon Plus Environment


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IV. Adsorption by one of the mechanisms begins earlier, then sorption according the

2

second mechanism begins, when the first mechanism don’t completed. Both mechanisms end

3

simultaneously.

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In the case of scheme I the first plateau corresponds to adsorption by the mechanism,

5

which at first realized, the second one corresponds to adsorption by the mechanism, which by

6

second realized. In this case, each plateau can be described by the Langmuir’s, Freundlich’s

7

equations, and so on. The parameters for each plateau are determined by linearizing the

8

corresponding equation (Table 2).The results are shown in Table 2.

9 10

Table 2. Parameters of D and L alanine sorption from the aqueous solution by carbon nanotubes mkNANO MKN-MWCNT S1 calculated using the Langmuir’s and Freundlich’s equations. System Parameters Langmuir isotherm * (@) = *A# L alanine + CNT mkNANO MKN-MWCNT S1

>B @ 1 + >B @

* (@ ) = > @ #/

Plateau 1

Plateau 1

А∞

KF(1) = 3.90 (l1/n · mol-1/n )/g

(1) =

16.95 mmol/g

КL (1) = 0.16 ml/mmol

D alanine + CNT mkNANO MKN-MWCNT S1

Freundlich isotherm

n(1) = 2.39

R(1)2 =0.996

R(1)2 = 0.999

Plateau 2

Plateau 2

А∞

KF(2) = 5.70 (l1/n · mol-1/n )/g

(2) =

27.03 mmol/g

КL (2) = 0.17 ml/mmol

n(2) = 2.36

R(2)2 =0.987

R(2)2 = 0.931

Plateau 1

Plateau 1

А∞

KF(1) = 16.29 (l1/n · mol-1/n )/g

(1) =

52.63 mmol/g

КL (1) = 0.37 ml/mmol

n(1) = 2.46

R(1)2 =0.999

R(1)2 = 0.976

Plateau 2

Plateau 2

А∞

KF(2) = 8.13 (l1/n · mol-1/n )/g

(2) =

27.03 mmol/g

КL (2) = 0.31 ml/mmol

n(2) = 2.63

R(2)2 =0.680

R(2)2 = 0.676 16

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1

In the case of scheme II, the part of the isotherm with the first plateau reflects the

2

summary adsorption according to the both mechanisms, and the part with the second plateau is

3

the multi layer adsorption.

4

In the case of scheme III, the part of the isotherm with the first plateau reflects the

5

summary adsorption by the both mechanisms, and the part with the second plateau corresponds

6

the adsorption according to second mechanism, when the first mechanism completed.

7

In the case of scheme IV, the part of the isotherm with the first plateau means the

8

sorption by the mechanism that starts at first, the part with the second plateau means the sorption

9

by both mechanisms.

10

It is known that in the case of several simultaneously realized adsorption mechanisms, the

11

isotherm is the a sum of the isotherms, each of which corresponds to a separate mechanism.

12

Thus, in the case of two simultaneously realized Langmuir mechanisms, the resulting isotherm is

13

the so-called Bi-Langmuir isotherm. The equation of the Bi-Langmuir’s isotherm has the form33 :

14

* (@5 ) = *A#

9D< ;

#=9D< ;

+ *A

9E< ;

(5)

#=9E< ;

15

where K1L , K2L are adsorption−desorption equilibrium constants for the sample distributions

16

between the bulk phase and adsorption sites of types 1 and 2, respectively, *A# , *A are the

17

saturation capacities for adsorption sites of types 1 and 2, respectively, Ce is the liquid phase

18

equilibrium concentration.

19

The amino acid molecules are uniformly distributed around the CNT. Therefore, it is

20

most probably that adsorption of amino acids by the outer side wall of CNT and encapsulution of

21

the sorbate inside the CNT begin simultaneously, despite the difference in adsorption energies.

22

Therefore, in our opinion, the part of the isotherm with the first plateau reflects the summary

23

adsorption caused to the adsorption centers on the outer and inner side surfaces of the CNT.

24

However, adsorption on the outer surface, in our opinion, occurs faster than the encapsulation of

25

the amino acid inside the CNT. It is due to the higher availability of adsorption centers on the

26

outer side wall of CNT. 17 ACS Paragon Plus Environment


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Page 18 of 23

1

The formation of the second layer during the adsorption of an amino acid on the outer

2

side wall of CNT is realized, if an alanine concentration in an external solution is equal (or

3

greater), than the concentration, at which the amino acid dimerization begins. To answer the

4

question of the possible dimerization of alanine in aqueous solution the density of aqueous

5

solutions of l alanine of various concentrations by pycnometric method was measured. The

6

results are shown at Figure 9. 1.04

D, g/cm3 y = 0.2139x + 0.9968 R² = 0.9993

1.03

1.02

1.01

1

С, mmol/dm 3 0.99 0

0.05

0.1

0.15

0.2

7 8

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

9 10

The dependence of the density of aqueous solutions of L-аlanine on the concentration is

11

a linear function without kinks, what corresponds to the literature34. Therefore, there is no reason

12

to believe that dimerization of alanine begins at a certain concentration in the investigated

13

interval C, since the inflection in Figure 9 should correspond to the dimerization beginning. This

14

conclusion is confirmed by studies using the IR spectroscopy method35, which supports the

15

dimerization of alanine only in acidic aqueous solutions (pH ~ 1) at a concentration of 0.1 M and

16

its absence in neutral aqueous solutions. This is indicative of the monolayer sorption of the

17

alanine on CNTs.

18

Based on the arguments outlined above, it is possible to conclude that the part of

19

isotherms with the first plateau corresponds to the summary adsorption of alanine by the outer

20

and inner side surface of the CNT, and the part with the second plateau reflect the encapsulation 18 ACS Paragon Plus Environment

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1

of the amino acid inside the CNT, when adsorption by the outer side surface was finished. At

2

simultaneous sorption on energetically unequal sorption centers, the parameters of the isotherm

3

equation (for example, (5) under the Langmuir’s mechanism) can not be determined.

4

Conclusions

5

The isotherms of L and D alanine sorption from aqueous solutions on carbon nanotubes

6

MKN-SWCNT-S1 show different values of sorption affinity of chiral isomers of amino acids (L

7

and D) with respect to carbon nanotubes. This is due to the differences in the interactions of

8

chiral isomers of amino acids with CNTs which are chiral structures. The isotherms have

9

inflections and two plateaus. It is found that the sorption of D alanine on the investigated carbon

10

nanotubes is stronger than the sorption of L alanine. This fact may serve as the basis for the

11

technology of their separation.

12

The calculation of the interaction energies between L (D) alanine and CNTs of diameter

13

8,14 Å with the length of 8.61 Å was carried out. The calculation of the sorption energies of the

14

amino acids of the given CNT was performed within density functional theory, taking into

15

account dispersion corrections which fact significantly increases the accuracy of the calculation

16

due to a high value of CNT polarizability. The calculation showed the difference in interaction

17

energies of amino acids with CNTs in three positions of the amino acid: at the CNT end, on the

18

CNT side wall and under the capillary suction of the amino acid inside the CNT. Besides, the

19

difference in values of the sorption energies on the CNTs of L and D isomers in all the

20

investigated cases was demonstrated. The quantum chemical calculation made within density

21

functional theory with dispersion corrections confirms a higher sorptivity of carbon nanotubes

22

with respect to D alanine. On the basis of the calculation results and available literature data it

23

can be concluded that the presence of two plateaus on the isotherms indicate a change in the

24

sorption mechanism. Namely, at first the mechanism of the adsorption by the CTN sidewall is

25

realized, then the mechanism of encapsulution of the animo acid inside the CNT. The analysis of

26

the optimized structures "L (D) alanine-CNT" showed a difference in the orientation of L and D 19 ACS Paragon Plus Environment


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Page 20 of 23

1

amino acids relative to CNTs. From this analysis, it was concluded that more D alanine

2

molecules can be located on the outer side surface of the CNT than L alanine, while contact

3

surface area by each molecules of D isomer is smaller then of L isomer.

4

One way to increase the differences in enantiomeric energies on carbon nanotubes is the

5

functionalization of the latter by the compounds used to separate the enantiomers (L-

6

hydroxyproline, L-Ala, Cu (L-Pro)2, Cu (L-His)2, etc.)13,15. Such functionalization of CNTs

7

should increase the sorption of D alanine and reduce the sorption of L alanine.

8

ACKNOWLEDGMENT

9

The partial support of this research by the

10

Federal

Target Program,

the

Agreement

№ 14.574.21.0112 of 21.10.2014 is appreciatively acknowledged.

11

12 13 14 15 16 17 18 19 20 21 22 23

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TOC Graphic

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d-Alanine and Carbon Nanotubes in

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