Tuning the Surface Chemistry of Chiral Cu(531)S for Enhanced

Article pubs.acs.org/JPCC

Tuning the Surface Chemistry of Chiral Cu(531)S for Enhanced Enantiospecific Adsorption of Amino Acids Ho Seong Song and Jeong Woo Han* Department of Chemical Engineering, University of Seoul, Seoul 130-743, Republic of Korea S Supporting Information *

ABSTRACT: Amino acids are important bioorganic compounds composed of amine and carboxylic acid because they are the main building blocks of many biomolecules. All of them are chiral except glycine. Thus, they have two enantiomers which provide dramatically different biological effects, thereby requiring their separation. High Miller index metal surfaces often define intrinsically chiral structures. A number of previous studies have proved the enantiospecific adsorption difference of chiral molecules on those surfaces. To further enhance the enantiospecificity, step decoration, which is doping the kink site of chiral metal surface with a second metal, can be one route. It may induce one enantiomer adsorbed on the surface to become more stable than the other, inducing the larger enantiospecific energy difference. In this study, we performed density functional theory (DFT) calculations to systemically examine the adsorption geometries and energetics of each enantiomer of alanine, serine, and cysteine, and their enantiospecific energy differences on pure, Pd-, Pt-, and Au-decorated Cu(531)S, respectively. By decorating the kinked site with an Au atom, the enantiospecificity of adsorbed cysteine was meaningfully enhanced by 0.08 eV, in the case when the side chain has a high affinity with the surface. Our results provide useful insight of how to tune chiral metal surfaces to enlarge the enantiospecificity of chiral molecules.

1. INTRODUCTION The enantiomers of chiral molecules have very similar physical and chemical properties, but they have often shown dramatically different biological effects. One enantiomer of pharmaceutical compounds such as antibiotic, cardiovascular, chemotherapeutic, psychotropic, pulmonary, and rheumatic drugs may have a good efficacy, whereas the other enantiomer may have an undesired or inactive effect.1,2 A large number of chiral characteristics have been used as the promising applications in a wide range of scientific areas.3 The demand for separating either enantiomers is therefore rapidly increasing in chemical and pharmaceutical industries.4 Single crystalline materials can create chiral surfaces by cutting along the high Miller index directions.5 These surfaces have particular surface configurations composed of terrace, step, and kink sites.6 Especially, the kink sites on the chiral surfaces provide a potential for adsorbates to have their enantiospecificity on the surfaces. The intrinsically chiral surfaces have been considered as one of the novel applications for chiral process to manufacturing of enantiopure material.7−12 Thus, a large number of studies have been performed to investigate the enantioselectivity of chiral organic molecules on those surfaces.13−29 Among the chiral Cu surfaces, Cu{531} has the smallest surface unit cell, which consists of (111), (100), and (110) microfacets. Multiple research has been performed to understand the enantioselective properties of intrinsically chiral Cu{531}.30−38 Rampulla et al. reported that the difference in the kink structures and densities of Cu{531} has an effect on the © 2015 American Chemical Society

enantioselectivity of desorption and debromination of R-2bromobutane.30 Gladys et al. explored the enantiospecific difference for the adsorption of alanine on Cu{531} using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy.31 They showed both enantiomers of alaninate preferentially adsorb on {110} and {311} microfacets of the Cu{531}. Huang et al. reported that enantiospecific difference for desorption energies of R-3-methylcyclohexanone on Cu(531)S is 0.5 ± 0.2 kJ/mol.32 Using XPS and NEXAFS, Thomsen et al. examined that L-cysteine adsorbs in a quadrangular footprint, whereas L-methionine adsorbs in a triangular footprint with the adsorption sites on {110} and {311} microfacets of Cu{531}.34 Clegg et al. showed from scanning tunneling microscopy (STM) and LEED that the real Cu{531} has a high degree of atomic scale roughness, demonstrating that its net chirality is preserved.35 Eralp et al. experimentally observed that glycine molecules adsorb on the {110} and {311} microfacets of Cu{531} in a triangular footprint.36 They also reported that both enantiomers of serine adsorb on Cu{531} in μ3 configuration at saturation coverage but in μ4 configuration at low coverage.38 Tuning the chemistry of chiral surfaces can be a way to improve the enantiospecificity of these surfaces. The decoration of the kink site with the other metal may induce the larger Received: March 20, 2015 Revised: June 3, 2015 Published: June 8, 2015 15195

DOI: 10.1021/acs.jpcc.5b02695 J. Phys. Chem. C 2015, 119, 15195−15203

Article

The Journal of Physical Chemistry C

Figure 1. Top views of (a) the most, (b) the second most ({110} site), and (c) the third most ({311} site) favorable adsorption configurations of glycine on pure Cu(531)S. Cu atoms are shown as red-yellow spheres, O atoms as red spheres, H atoms as white spheres, C atoms as gray spheres, N atoms as blue spheres, respectively. The white dashed lines indicate the surface unit cell used in our DFT calculations. The full sets of coordinates for (a)−(c) are shown in Tables S4−S6 of Supporting Information.

enantiomeric differences.37,39,40 Eralp et al. reported that the angle between the [112̅] direction of Cu{531} and the surface projection of the normal of the carboxylate O−C−O triangle of serine is significantly changed upon Au-modification of Cu{531}.37 Our previous works also showed that decorating the kink site can provide a potential to elevate the enantiospecificity of amino acids on intrinsically chiral surfaces.39,40 It is still unclear, however, how the step decoration can enhance the enantiospecificity of chiral metal surfaces. Amino acids, the building blocks of proteins, play an important role on both the constituents of the human body and the derivatives in the industries. Glycine, alanine, serine, and cysteine, which are relatively simple among the amino acids, can provide basic information for studying the enantiospecificity of chiral molecules on Cu{531}. These molecules are therefore good starting points to study their chiral interaction with the intrinsically chiral surfaces. The aim of this work is to enhance the enantioselectivity of amino acids on Cu(531)S by tuning the surface chemistry. Our quantitative calculations of the detailed configurations and the adsorption energies of amino acids on the surface are performed by density functional theory (DFT) calculations, leading to take an advantage to reduce time and cost for achieving this purpose. The organization of the rest of this paper is as follows. Section 2 describes the computational approach and details. The detailed adsorption configurations and energetics of amino acids on pure and decorated Cu(531)S in their most stable states are discussed in Section 3, followed by the analysis of trend in the adsorption energies of amino acids and their enantiospecificity on those surfaces. Section 4 summarizes our major observations in this paper.

other dispersion force corrected methods, we employed DFT-D3 method50 with Becke−Johnson (BJ) rational damping.51 A plane wave expansion with a cutoff of 400 eV was used with a 3 × 3 × 1 Monkhorst−Pack k-point52 sampling of the Brillouin zone for the total energy calculations. Total energy calculations were conducted using the residual minimization method for electronic relaxation, accelerated using Methfessel− Paxton Fermi-level smearing with a width of 0.2 eV. The periodicity of the material in the plane of the surface was defined using the DFT optimized lattice parameter of Cu, 3.64 Å. The computational supercell contained a (2 × 2) surface unit cell of Cu(531)S with a vacuum spacing of 15 Å in the direction of the surface normal. The surface unit cell is shown as white dashed lines in Figure 1a. Our calculations were performed using slabs equivalent in thickness to six (111)-oriented layers. The geometries of amino acids in the gas phase were optimized in a supercell of 20 Å × 20 Å × 20 Å. The calculations for adsorbed amino acids were performed for the coverage corresponding to one molecule per a (2 × 2) surface unit cell of Cu(531)S. This gives an area of 38.7 Å2/molecule where the adsorbed molecules are well separated. When examining adsorption, molecules were placed on only one side of the slab. All degrees of freedom of all the metal atoms and the molecule were allowed to relax in all energy minimization calculations. To compare experimentally observable configurations of both D- and L-amino acids adsorbed on the surface, scanning tunneling microscope (STM) is simulated by using the Tersoff and Hamann scheme.53,54 From our test calculations, it is found that the effect of spin polarization in this system is negligible. The effect has not also been considered on chiral Cu surfaces in the previous reports.27,40,55−57 Thus, we did not take into account the effect of spin-polarization in this study. In addition, we considered the coverage dependence of our results. As test cases, we expanded our surface unit cell to (4 × 4) for both enantiomers of adsorbed μ4-serine and μ4-cysteine, providing an area of 154.8 Å2/molecule. The adsorption strengths of both amino acids are slightly increased compared to ones at the coverage we originally used, but their enantiospecific adsorption energy differences are not changed, which holds our key results (see Table S1 in the Supporting Information.) On the other hand, at the higher coverages, hydrogen bonding between the adsorbates may be formed, which would induce the adsorption to be more stabilized with higher adsorption energy.27,36,38,58 Because the adsorption geometries and the enantiospecificities can be correspondingly changed by the hydrogen bonding, we will separately consider in detail the coverages that allow hydrogen bonding in our follow-up study.

2. COMPUTATIONAL DETAILS We used the Vienna Ab initio simulation package (VASP) for our plane wave DFT calculations.41−43 The Perdew−Burke− Ernzerhof (PBE) generalized gradient functional44 along with the projector augmented wave (PAW) method was employed to describe ionic cores.45,46 DFT-D2 was employed to all our calculations to take into account the van der Waals interaction. For H, C, N, O, and Cu, the dispersion coefficients (C6) and van der Waals radii (R0) defined by Grimme47 were used. For Pt and Au whose parameters are not listed in the original paper of Grimme, 42.44 J nm6/mol and 1.75 Å for Pt48 and 40.62 J nm6/mol and 1.772 Å for Au49 were taken as C6 and R0, respectively. To check if the DFT-D2 results are qualitatively changed under 15196


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Figure 2. Top views of the most favorable adsorption configurations and the corresponding simulated STM images (bias voltage corresponds to 2 eV below the Fermi level) of (a) D-alanine, (b) L-alanine, (c) D-serine in μ3, (d) L-serine in μ3, (e) D-serine in μ4, (f) L-serine in μ4, (g) D-cysteine in μ3, (h) L-cysteine in μ3, (i) D-cysteine in μ4, and (j) L-cysteine in μ4 on pure Cu(531)S. The numbers are the corresponding adsorption energies and their enantiospecific adsorption energy differences in eV. The full sets of coordinates for (a)−(j) are shown in Tables S7−S16.

To characterize the enantiospecificity of adsorption, we used the enantiospecific difference of adsorption energies, ΔEenantio, defined as the total energy of the most stable structure of the adsorbed L enantiomer minus the total energy of the most stable structure of the adsorbed D enantiomer. By our definition, a positive value means that the D enantiomer is more strongly adsorbed on the surface than the L enantiomer.

Because it has been known that amino acids deprotonate to form H2NCHRCOO on Cu surface,59−65 all of our DFT calculations in this study have performed for the amino acids adsorbed in the deprotonated form on the surface. We will refer to them simply as glycine, alanine, serine, and cysteine for convenience. The adsorption energy, Eads, for the deprotonated molecules is defined as ⎛ ⎞ 1 Eads = E H2NCHROO(ads) − ECu(S) − ⎜E H2NCHRCOOH(g) − E H2(g)⎟ ⎝ ⎠ 2

3. RESULT AND DISCUSSION 3.1. Pure Cu(531)S. Glycine is the simplest amino acid and may provide basic information to further investigate the adsorption configurations of more complex amino acids on Cu(531)S. The preferential binding of the N atom in the amino group onto the kink atom has been previously reported in the other chiral molecules that include an amino group adsorbed on chiral Cu surfaces.22,27,31,37 The initial configurations were therefore generated by rotating glycine relative to the amino group on kinked step by increments of 30°. We also examined the adsorption configurations of the mirror images of the glycine generated in the same way. Thus, 24 initial structures

(1)

where EH2NCHRCOO(ads) is the total energy of the system containing the adsorbed deprotonated amino acid, ECu(S) the total energy for the optimized solid Cu surface, EH2NCHRCOOH(g) the total energy for the amino acid in the gas phase, and EH2(g) the total energy for the hydrogen molecule in the gas phase, respectively.20,40,66 For the amino acids that contain deprotonated β−OH or β-SH, we used the following definition for the calculation of Eads, Eads = E H2NCHROO(ads) − ECu(S) − (E H2NCHRCOOH(g) − E H2(g))

(2)

By our definition, a negative value is favorable to the adsorption. 15197


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in Figure 2c−f. The adsorption energies of the most stable μ3 configurations for D- and L-serine are −2.31 and −2.37 eV (Figure 2c,d), whereas those of the most stable μ4 configurations are −2.23 and −2.09 eV, respectively (Figure 2e,f). Eralp et al. previously reported the adsorption energies of serine on chiral Cu{531}.38 In their results, the adsorption energies of the most stable μ3 geometries for D- and L-serine are −2.00 and −2.00 eV, whereas those of the most stable μ4 geometries are −1.98 and −2.50 eV, respectively. The difference from our results is mostly attributed to the fact that they did not take into account the van der Waals interaction. Cysteine is the amino acid with a CH2SH group attached to α-carbon. From the results of serine, an oxygen atom attached to the β−OH in serine was substituted with a sulfur atom. The optimized adsorption geometries and the corresponding simulated STM images of both enantiomers of cysteine are shown in Figure 2g−j. The adsorption energies of the most stable μ3 configurations for D- and L-cysteine are −2.64 and −2.70 eV (Figure 2g,h), and those of the most stable μ4 configurations are −3.19 and −3.31 eV, respectively (Figure 2i,j). Interestingly, μ3 adsorption geometry is more stable for serine than μ4, whereas cysteine in μ4 configuration adsorbs more strongly on the surfaces than one in μ3. To unravel the origin of the difference, we examined the adsorption energies of O, OH, S, and SH on Cu(111), as shown in Table 1. For these calculations, 1/2O2,

were calculated. The most (Figure 1a), the second most (Figure 1b), and the third most (Figure 1c) stable adsorption configurations are shown in Figure 1, respectively. A tridentate configuration is preferred with an N atom on top of the kink and two O atoms in the carboxylate group at the step edge. Similarly to the results in DFT-D2, we also observed from DFT-D3 that these three body interactions play a key role in the adsorption of amino acids on Cu(531)S. The adsorption energy of the most stable configuration of glycine (Figure 1a) is −2.01 eV. Eralp et al. previously reported that glycine prefers to adsorb on {110} and {311} microfacets of Cu{531}, forming the bonds with Cu atoms through two O atoms and an N atom by LEED, NEXAFS, and XPS experiments.36 Our DFT calculations also confirm that the adsorption structures are preferentially observed on {110} and {311} sites but with 0.02 and 0.04 eV lower adsorption energies than the most stable one, respectively (Figure 1b, c). Alanine is the simplest chiral amino acid which has a methyl group as a side chain. Initial adsorption configurations of alanine on Cu(531)S were constructed from 24 structures of glycine adsorption. From the results of glycine, each hydrogen atom attached to the α-carbon in glycine was substituted with a methyl group. Two different configurations were generated from a single glycine. Consequently, we considered 48 initial configurations. After DFT optimization, the 48 structures were converged into 34 configurations. The most stable configurations and the simulated STM images of D- and L-alanine on pure Cu(531)S are shown in Figure 2a,b. The adsorption energies are −2.11 and −2.14 eV, respectively. The STM images clearly show the difference between the adsorption configurations of D- and S L-alanine on Cu(531) . Gladys et al. reported from LEED, XPS, and NEXAFS that alanine prefers to adsorb on (110) and (311) microfacets of Cu{531}.31 L-Alanine has the most stable adsorption configuration on the (311) microfacet as in Gladys et al.’s report. Our DFT results, however, show that D-alanine on the (110) and (311) facets are 0.03 and 0.07 eV less stable than the most stable one (Figure 2a), respectively. Most of amino acids commonly have the adsorption configurations of forming bonds with Cu surfaces through the two O atoms in the carboxylate group and the N atom in the amino group.56,57,67−72 This geometry is referred to as μ3 configuration. However, if a side group attached to the chiral center can form one more bond to the surface than in μ3 configuration, it is referred to as the μ4 configuration. Eralp et al. observed that both enantiomers of serine adsorb on Cu{531} in μ3 configuration at saturated coverage and in μ4 configuration at lower coverage.38 Because the transition coverage from μ4 to μ3 has not been clearly identified, we considered both geometries for the adsorption of serine and cysteine. We first optimized μ3 configuration, then μ4-adsorbed geometry was calculated by removing one hydrogen atom attached to the β-OH or β-SH in the μ3 geometry. Serine has a CH2OH group as a side chain. Initial adsorption configurations of serine on Cu(531)S were built from the converged 34 structures of alanine adsorption on the surface. Each hydrogen atom subjected to the methyl group in alanine was substituted with a hydroxyl group. Three different configurations were thus generated from a single alanine structure. As a result, we considered 102 initial structures in μ3 geometries. Initial configurations of μ4 geometries are constructed from the DFT-optimized 34 most stable structures in μ3 geometries. The optimized adsorption geometries and the corresponding simulated STM images of both enantiomers of serine are shown

Table 1. Adsorption Energies (in eV) of O, OH, S, and SH on Cu(111), Pd(111), Pt(111), and Au(111)a Cu(111) Pd(111) Pt(111) Au(111)

O

OH

S

SH

−1.97 −1.43 −1.33 −0.57

−3.45 −2.76 −2.58 −2.49

−5.00 −5.35 −5.51 −4.32

−3.10 −3.55 −3.24 −2.62

a

The adsorption energies of O, OH and S were taken from our previous report,73 and that of SH was calculated in this study.

OH, S, and SH are used as the isolated gas phase species. O and OH are representative of serine in μ3 and μ4 geometries, whereas S and SH stand for cysteine in μ3 and μ4 configurations, respectively. The adsorption energy of OH is ∼1.5 eV stronger than that of atomic O, and the adsorption energy of atomic S is ∼2 eV stronger than that of SH. This elucidates the reason why the preferential adsorption between μ3 and μ4 geometries is different for serine and cysteine. On the basis of these results, it is thought that the binding strength between the functional group of side chain and the metal surface plays a very critical role in determining the adsorption strength of amino acids. 3.2. Step Decorated Cu(531)S. The change in the adsorption configuration induced by step decoration may induce the change in the enantiospecific adsorption energy difference.37,39,40 We examined the adsorption energies of amino acids on Pd, Pt, and Au-decorated Cu(531)S, compared to those on the pure surface. To understand a principle that potentially enhances the enantiospecificity by the step decoration, we investigated the size effect of decorated kink atom upon the adsorption strength of amino acids, thereby modifying the enantiomeric features of those surfaces. To elucidate the effect, we chose Pd, Pt, and Au as step decorators which have different Wigner-Seitz radii from Cu (Cu: 1.41, Pd: 1.52, Pt: 1.54, and Au: 1.59 Å).74 Additionally, we studied the effect of dopant concentration by decorating one more atom at the Cu kink site. The configuration of dopant 15198


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weakening the interaction of the molecule’s contacting points with the surface. Table 1 supports the argument, which lists the adsorption energies of the small constituents in the side chain of amino acids on the closest packed (111) surfaces composed of the step decorators (Pd, Pt, and Au) and Cu. Because the adsorption strengths of small species on Cu(111) are mostly the strongest, the larger dopant reduces more the space exposed of Cu, thereby making overall adsorption strength of amino acids weakened. In most cases, therefore, the adsorption strengths are weakened with increasing atomic size. We also found that increasing dopant concentration lowers the adsorption strength of amino acids on Cu(531)S (Figure 4). Because Cu(531)S has the smallest unit cell among the chiral Cu surfaces, the step decoration can considerably reduce the space exposed of Cu, making the adsorption strength of amino acids weakened. Using Bader charge analysis,75−77 we calculated the charges transferred during the adsorption of amino acids on pure, Pd-, Pt-, and Au-decorated Cu(531)S (Table 3). By our definition, a positive value is the charge transferred from each surface to the amino acid enantiomers. From this analysis, we identify that the adsorption of amino acids on Cu(531)S occurred via chemisorption. 3.3. Enantiospecific Difference in Adsorption Energy. Magnifying the adsorption energy difference of enantiomers of amino acids enables the easier separation of the two enantiomers. We refer to the absolute value of the enantiospecific energy difference as enantiospecificity, |ΔEenantio|. On the basis of the results in Table 2, we calculated |ΔEenantio| of amino acids on pure, Pd-, Pt-, and Au-decorated Cu(531)S, as can be seen in Figure 5. In Figure 5, the blue dashed line denotes |ΔEenantio| of amino acids on pure surface, and the red dashed line implies the largest enhanced |ΔEenantio| upon the step decoration. The colored area between the two dashed lines implies the enhancement of enantiospecificity, Δ|ΔEenantio|, upon the step decoration. Alanine has |ΔEenantio| of 0.02 eV on pure surface (Figure 2a,b). |ΔEenantio| of alanine is a bit increased to 0.03 eV on 50% Pd-decorated surface (Figure S2b and S3b) and 50% Au-decorated surface (Figure S2b and S3a), respectively. Serine in μ3 geometry has |ΔEenantio| of 0.06 eV on pure surface (Figure 2c,d). It is enhanced up to 0.13 eV on 50% Pd-decorated surface (Figure S4b and S5b). Serine in μ4 configuration has |ΔEenantio| of 0.14 eV on pure (Figure 2e,f), 25% Au- (Figure S6a and S7a), and 50% Pt-decorated surfaces (Figure S6b and S7b), respectively. Cysteine in μ3 geometry has |ΔEenantio| of 0.06 eV on pure surface (Figure 2g,h), which is increased up to 0.08 eV on 25% Au-decorated surface (Figure S8a and S9a). Cysteine in μ4 configuration has |ΔEenantio| of 0.11 eV on pure surface (Figure 2i,j), which is enhanced up to 0.19 eV on 50% Au-decorated surface (Figure S10b and S11b). In this case, the Au-doped kinks inhibit the strong adsorption of D-cysteine. This destabilization changes the adsorption geometry of D-enantiomer from the adsorbed structure in Figure 2i to the one in Figure 3b.

located at the farthest kink site from the already existing decorated atom (Figure 3b) is 0.06 eV more stable than the other two possible configurations (see Figure S1 in the Supporting Information). We will refer to this as 50%-decorated surface. Figure 3 shows an example of the most stable adsorbed amino acids on both 25% and 50%-decorated Cu(531)S; the adsorbed

Figure 3. Top views of the most favorable μ4 adsorption configuration of S D- and L-cysteine on (a) 25% and (b) 50% Au-decorated Cu(531) . The numbers are the corresponding adsorption energies in eV.

cysteine in μ4 geometry on Au-decorated surface. This example shows how each decorated atom is distributed and how the step decoration influences on the adsorption geometries compared to on the pure surface (Figure 2i,j). Other stable adsorption configurations of amino acids on Pd-, Pt-, and Au-decorated surfaces (both 25% and 50%) are described in Figures S2 ∼ S11 of the Supporting Information and their full sets of coordinates are listed in Tables S17 ∼ S76. The corresponding adsorption energies are summarized in Table 2. Our DFT results showed that both adsorption geometries and energetics of amino acids on Cu(531)S are altered by the step decoration. As shown in Figure 4, we observe that as the dopant size is larger, the adsorption strength of amino acids mostly becomes weaker. DFT-D3 results also show that this trend is maintained although the numbers in adsorption energy are slightly changed (Table S2 and Figure S12 in the Supporting Information). This may be attributed to the fact that the larger kink atom causes the binding of amino acid to be destabilized by

Table 2. Adsorption Energies (in eV) of Both Enantiomers of Amino Acids on Each Surface pure Ala Ser (μ3) Ser (μ4) Cys (μ3) Cys (μ4)

25% Pd

25% Pt

25% Au

50% Pd

50% Pt

50% Au

D

L

D

L

D

L

D

L

D

L

D

L

D

L

−2.11 −2.31 −2.23 −2.64 −3.19

−2.14 −2.37 −2.09 −2.70 −3.31

−2.11 −2.42 −2.10 −2.65 −3.16

−2.09 −2.35 −1.98 −2.71 −3.02

−2.19 −2.47 −2.12 −2.77 −3.18

−2.19 −2.46 −2.03 −2.76 −3.08

−1.77 −2.07 −1.90 −2.30 −2.76

−1.78 −1.99 −1.76 −2.37 −2.75

−2.08 −2.43 −1.93 −2.66 −3.05

−2.05 −2.30 −1.82 −2.70 −2.98

−2.13 −2.46 −1.92 −2.70 −3.03

−2.12 −2.36 −1.78 −2.75 −3.02

−1.74 −2.04 −1.51 −2.28 −2.56

−1.77 −1.95 −1.50 −2.33 −2.75

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Figure 4. Adsorption energies of amino acids on (a) 25% Pd-, Pt-, and Au-decorated Cu(531)S and (b) 50% Pd-, Pt-, and Au-decorated Cu(531)S as a function of Wigner-Seitz radius of kink atom.

Table 3. Charges (in e) Transferred during the Adsorption from Each Surface to the Examined Amino Acid Enantiomers pure Ala Ser (μ3) Ser (μ4) Cys (μ3) Cys (μ4)

25% Pd

25% Pt

25% Au

50% Pd

50% Pt

50% Au

D

L

D

L

D

L

D

L

D

L

D

L

D

L

0.61 0.64 1.18 0.56 0.98

0.59 0.62 1.17 0.59 0.98

0.59 0.62 1.14 0.53 0.90

0.56 0.63 1.17 0.59 0.93

0.54 0.57 1.09 0.59 0.83

0.52 0.55 1.12 0.52 0.89

0.57 0.60 1.12 0.52 0.90

0.54 0.62 1.14 0.57 0.93

0.58 0.61 1.12 0.53 0.87

0.57 0.61 1.14 0.56 0.93

0.52 0.56 1.05 0.46 0.78

0.52 0.53 1.07 0.50 0.88

0.55 0.59 1.10 0.51 0.91

0.54 0.56 1.10 0.53 0.92

On the other hand, for μ3-serine and μ4-cystenine, we observed the significant Δ|ΔEenantio| upon the Pd (0.07 eV) and Au (0.08 eV) decoration, respectively. Interestingly, they exhibited the strongest adsorption strengths among the amino acids we examined due to the strong attraction between their side chain and metal surfaces (Table 2). Although step decoration may reduce |ΔEenantio| in some cases, as shown in the outside of the colored area in Figure 5, it can provide great potential to enhance |ΔEenantio| of amino acids, especially for the adsorption with a side chain that has a high affinity with surface material. Despite the slight change in |ΔEenantio|, our qualitative result that among the amino acids we examined, μ3-serine and μ4-cysteine show the largest Δ|ΔEenantio| upon the step decoration is not changed under DFT-D3 (see Figure S13 in the Supporting Information). On the basis of the enantiospecific energy difference listed in Table 2, we calculated the D/L ratio of the population distributions of the adsorbed amino acids as below;

Figure 5. |ΔEenantio| of amino acids on pure and step decorated Cu(531)S. The blue dashed line indicates |ΔEenantio| of amino acids on pure Cu(531)S, and the red dashed line denotes the highest |ΔEenantio| of each amino acid among all surfaces.

L

D

D/L ratio of population distributions = e(Emin − Emin) / kT

Meanwhile, the adsorption configuration of L-cysteine is stably maintained upon the Au-decoration (compare Figure 2j with Figure 3b). As a result, Au-decoration induces the meaningful Δ|ΔEenantio| of μ4 adsorbed cysteine by 0.08 eV. On the basis of our results, it is found that decorating the kink site considerably affects the enantiospecific adsorption of amino acids. Δ|ΔEenantio| are slightly increased for alanine, μ4-serine, and μ3-cystenine upon the step decoration (less than 0.02 eV).

(3)

where EL(D) min is the minimum energy of each enantiomer, k is Boltzmann’s constant, and T is temperature. The D/L ratios of the adsorbed amino acids on each surface at 300 K are listed in Table 4. To obtain more accurate population ratios, the summation of all energy states of the adsorption is required, but here we only take into account the most stable adsorption states of both enantiomers.

Table 4. D/L Ratio of Population Distributions of the Amino Acids Adsorbed on Cu(531)S at 300 K

Ala Ser (μ3) Ser (μ4) Cys (μ3) Cys (μ4)

pure

25% Pd

25% Pt

25% Au

50% Pd

50% Pt

50% Au

0.43 0.09 263.09 0.09 0.01

1.95 13.42 134.73 0.09 278.70

0.87 2.08 27.18 1.38 46.24

0.69 18.01 235.17 0.05 1.27

3.46 167.39 84.21 0.29 14.05

1.27 45.40 230.13 0.15 1.95

0.34 33.46 1.56 0.14 0.07 × 10−2

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(5) McFadden, C. F.; Cremer, P. S.; Gellman, A. J. Adsorption of Chiral Alcohols on “Chiral” Metal Surfaces. Langmuir 1996, 12, 2483−2487. (6) Van Hove, M.; Somorjai, G. A New Microfacet Notation for HighMiller-Index Surfaces of Cubic Materials with Terrace, Step and Kink Structures. Surf. Sci. 1980, 92, 489−518. (7) Sholl, D. S.; Asthagiri, A.; Power, T. D. Naturally Chiral Metal Surfaces as Enantiospecific Adsorbents. J. Phys. Chem. B 2001, 105, 4771−4782. (8) Hazen, R. M.; Sholl, D. S. Chiral Selection on Inorganic Crystalline Surfaces. Nat. Mater. 2003, 2, 367−374. (9) Held, G.; Gladys, M. J. The Chemistry of Intrinsically Chiral Surfaces. Top. Catal. 2008, 48, 128−136. (10) James, J. N.; Sholl, D. S. Theoretical Studies of Chiral Adsorption on Solid Surfaces. Curr. Opin. Colloid Interface Sci. 2008, 13, 60−64. (11) Sholl, D. S.; Gellman, A. J. Developing Chiral Surfaces for Enantioselective Chemical Processing. AIChE J. 2009, 55, 2484−2490. (12) Gellman, A. J. Chiral Surfaces: Accomplishments and Challenges. ACS Nano 2010, 4, 5−10. (13) Sholl, D. S. Adsorption of Chiral Hydrocarbons on Chiral Platinum Surfaces. Langmuir 1998, 14, 862−867. (14) Ahmadi, A.; Attard, G.; Feliu, J.; Rodes, A. Surface Reactivity at “Chiral” Platinum Surfaces. Langmuir 1999, 15, 2420−2424. (15) Attard, G. A.; Ahmadi, A.; Feliu, J.; Rodes, A.; Herrero, E.; Blais, S.; Jerkiewicz, G. Temperature Effects in the Enantiomeric ElectroOxidation of D-and L-Glucose on Pt {643} S. J. Phys. Chem. B 1999, 103, 1381−1385. (16) Attard, G. A. Electrochemical Studies of Enantioselectivity at Chiral Metal Surfaces. J. Phys. Chem. B 2001, 105, 3158−3167. (17) Gellman, A. J.; Horvath, J. D.; Buelow, M. T. Chiral Single Crystal Surface Chemistry. J. Mol. Catal. A: Chem. 2001, 167, 3−11. (18) Horvath, J. D.; Gellman, A. J. Enantiospecific Desorption of R-and S-Propylene Oxide from a Chiral Cu (643) Surface. J. Am. Chem. Soc. 2001, 123, 7953−7954. (19) Horvath, J. D.; Gellman, A. J. Enantiospecific Desorption of Chiral Compounds from Chiral Cu (643) and Achiral Cu (111) Surfaces. J. Am. Chem. Soc. 2002, 124, 2384−2392. (20) Šljivančanin, Z.; Gothelf, K. V.; Hammer, B. Density Functional Theory Study of Enantiospecific Adsorption at Chiral Surfaces. J. Am. Chem. Soc. 2002, 124, 14789−14794. (21) Horvath, J. D.; Koritnik, A.; Kamakoti, P.; Sholl, D. S.; Gellman, A. J. Enantioselective Separation on a Naturally Chiral Surface. J. Am. Chem. Soc. 2004, 126, 14988−14994. (22) Bhatia, B.; Sholl, D. S. Enantiospecific Chemisorption of Small Molecules on Intrinsically Chiral Cu Surfaces. Angew. Chem. 2005, 117, 7939−7942. (23) Greber, T.; Šljivančanin, Ž .; Schillinger, R.; Wider, J.; Hammer, B. Chiral Recognition of Organic Molecules by Atomic Kinks on Surfaces. Phys. Rev. Lett. 2006, 96, 056103. (24) Kühnle, A.; Linderoth, T. R.; Besenbacher, F. Enantiospecific Adsorption of Cysteine at Chiral Kink Sites on Au (110)-(1× 2). J. Am. Chem. Soc. 2006, 128, 1076−1077. (25) Rampulla, D.; Gellman, A. Enantioselective Decomposition of Chiral Alkyl Bromides on Cu (643)R&S: Effects of Moving the Chiral Center. Surf. Sci. 2006, 600, 2823−2829. (26) Schillinger, R.; Šljivančanin, Ž .; Hammer, B.; Greber, T. Probing Enantioselectivity with X-Ray Photoelectron Spectroscopy and Density Functional Theory. Phys. Rev. Lett. 2007, 98, 136102. (27) Bhatia, B.; Sholl, D. S. Characterization of Enantiospecific Chemisorption on Chiral Cu Surfaces Vicinal to Cu (111) and Cu (100) Using Density Functional Theory. J. Chem. Phys. 2008, 128, 144709. (28) Franke, J.-H.; Kosov, D. Enantioselectivity of (321) Chiral Noble Metal Surfaces: A Density Functional Theory Study of Lactate Adsorption. J. Chem. Phys. 2013, 139, 224709. (29) Franke, J.-H.; Kosov, D. S. Adsorption of Lactic Acid on Chiral Pt Surfacesa Density Functional Theory Study. J. Chem. Phys. 2013, 138, 084705. (30) Rampulla, D.; Francis, A.; Knight, K.; Gellman, A. Enantioselective Surface Chemistry of R-2-Bromobutane on Cu (643)R&S and Cu (531)R&S. J. Phys. Chem. B 2006, 110, 10411−10420.

4. CONCLUSION The adsorption geometries and energetics of both enantiomer of alanine, serine, and cysteine on pure, Pd-, Pt-, and Au-decorated Cu(531)S and their enantiospecificity are systemically investigated using DFT calculations. To enlarge the enantiospecific energy difference, we decorated the kinked sites of the surface with Pd, Pt, and Au atoms. On the basis of our results, the interaction between the side chain and the surface strongly affects the adsorption strength of amino acid. The larger decorating atom destabilizes the amino acid adsorption because it has less affinity with the side chain. The largest enhancements in enantiospecificity are observed for serine in μ3 geometry and cysteine in μ4 geometry, whose side chains experience the strongest attractions with chiral metal surfaces. Especially by decorating the kinked sites with Au atoms, the enantiospecificity of cysteine is enhanced by 0.08 eV more than that of pure Cu(531)S. Our results provide evidence that decorating the kink sites with other metals can be a route to enhance the enantiospecificity of chiral molecules on intrinsically chiral metal surfaces.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 shows the comparison in the configurations of step decorators on 50%-decorated surface structures. Table S1 lists the results from the test calculations for the adsorption of both enantiomers of μ4-serine and μ4-cysteine at 4 times lower coverage. DFT-D3 results are shown in Table S2 and Figures S12 and S13. The full sets of the optimized adsorption configurations are shown in Figures S2−S11. Tables S3−S76 list the full sets of coordinates for the adsorption structures shown in this study. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02695.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Global Frontier R&D Program on Center for Multiscale Energy System through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF2014M3A6A7074785) and the Human Resources Development Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) under a grant funded by the Korea Government Ministry of Trade, Industry, and Energy (20124010203260), and the supercomputing resource including technical support from Supercomputing Center/Korea Institute of Science and Technology Information (KSC-2014-C2-049).

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REFERENCES

(1) Smith, S. W. Chiral Toxicology: It’s the Same Thing··· Only Different. Toxicological sciences 2009, kfp097. (2) Lin, G.-Q.; You, Q.-D.; Cheng, J.-F. Chiral Drugs: Chemistry and Biological Action; John Wiley & Sons: Hoboken, NJ, 2011. (3) Zucchi, C.; Caglioti, L.; Palyi, G. Advances in BioChirality; Elsevier: Amsterdam, 1999. (4) Agranat, I.; Caner, H.; Caldwell, J. Putting Chirality to Work: The Strategy of Chiral Switches. Nat. Rev. Drug Discovery 2002, 1, 753−768. 15201


Article

The Journal of Physical Chemistry C (31) Gladys, M. J.; Stevens, A. V.; Scott, N. R.; Jones, G.; Batchelor, D.; Held, G. Enantiospecific Adsorption of Alanine on the Chiral Cu {531} Surface. J. Phys. Chem. C 2007, 111, 8331−8336. (32) Huang, Y.; Gellman, A. J. Enantiospecific Adsorption of (R)-3Methylcyclohexanone on Naturally Chiral Cu (531)R&S Surfaces. Catal. Lett. 2008, 125, 177−182. (33) Jones, G.; Gladys, M.; Ottal, J.; Jenkins, S.; Held, G. Surface Geometry of Cu {531}. Phys. Rev. B 2009, 79, 165420. (34) Thomsen, L.; Wharmby, M.; Riley, D.; Held, G.; Gladys, M. The Adsorption and Stability of Sulfur Containing Amino Acids on Cu {531}. Surf. Sci. 2009, 603, 1253−1261. (35) Clegg, M. L.; Driver, S. M.; Blanco-Rey, M.; King, D. A. Atomic Roughness of an Intrinsically Chiral Surface Orientation of an Fcc Metal: Cu {531}. J. Phys. Chem. C 2010, 114, 4114−4117. (36) Eralp, T.; Shavorskiy, A.; Zheleva, Z. V.; Dhanak, V. R.; Held, G. Hydrogen Bond-Induced Pair Formation of Glycine on the Chiral Cu {531} Surface. Langmuir 2010, 26, 10918−10923. (37) Eralp, T.; Cornish, A.; Shavorskiy, A.; Held, G. The Study of Chiral Adsorption Systems Using Synchrotron-Based Structural and Spectroscopic Techniques: Stereospecific Adsorption of Serine on AuModified Chiral Cu {531} Surfaces. Top. Catal. 2011, 54, 1414−1428. (38) Eralp, T.; Ievins, A.; Shavorskiy, A.; Jenkins, S. J.; Held, G. The Importance of Attractive Three-Point Interaction in Enantioselective Surface Chemistry: Stereospecific Adsorption of Serine on the Intrinsically Chiral Cu {531} Surface. J. Am. Chem. Soc. 2012, 134, 9615−9621. (39) Han, J. W.; Kitchin, J. R.; Sholl, D. S. Step Decoration of Chiral Metal Surfaces. J. Chem. Phys. 2009, 130, 124710. (40) Han, J. W. Enantiospecific Chemisorption of Amino Acids on Step Decorated Chiral Cu Surfaces. Top Catal 2012, 55, 243−259. (41) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid Metals. Phys. Rev. B 1993, 47, 558. (42) Kresse, G.; Hafner, J. Norm-Conserving and Ultrasoft Pseudopotentials for First-Row and Transition Elements. J. Phys.: Condens. Matter 1994, 6, 8245. (43) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (44) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865. (45) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. (46) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758. (47) Grimme, S. Semiempirical Gga-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787−1799. (48) Ramos-Sanchez, G.; Balbuena, P. Interactions of Platinum Clusters with a Graphite Substrate. Phys. Chem. Chem. Phys. 2013, 15, 11950−11959. (49) Amft, M.; Lebègue, S.; Eriksson, O.; Skorodumova, N. V. Adsorption of Cu, Ag, and Au Atoms on Graphene Including Van Der Waals Interactions. J. Phys.: Condens. Matter 2011, 23, 395001. (50) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. 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. (51) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456−1465. (52) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188. (53) Tersoff, J.; Hamann, D. Theory and Application for the Scanning Tunneling Microscope. Phys. Rev. Lett. 1983, 50, 1998. (54) Tersoff, J.; Hamann, D. Theory of the Scanning Tunneling Microscope. In Scanning Tunneling Microscopy; Springer: New York, 1993; pp 59−67.

(55) Fajín, J. L.; Illas, F.; Gomes, J. R. Effect of the ExchangeCorrelation Potential and of Surface Relaxation on the Description of the H2o Dissociation on Cu (111). J. Chem. Phys. 2009, 130, 224702. (56) James, J. N.; Sholl, D. S. Density Functional Theory Studies of Dehydrogenated and Zwitterionic Glycine and Alanine on Pd and Cu Surfaces. J. Mol. Catal. A: Chem. 2008, 281, 44−48. (57) Rankin, R. B.; Sholl, D. S. Structures of Glycine, Enantiopure Alanine, and Racemic Alanine Adlayers on Cu (110) and Cu (100) Surfaces. J. Phys. Chem. B 2005, 109, 16764−16773. (58) Jones, G.; Jenkins, S.; King, D. Hydrogen Bonds at Metal Surfaces: Universal Scaling and Quantification of Substrate Effects. Surf. Sci. 2006, 600, 224−228. (59) Barlow, S.; Kitching, K.; Haq, S.; Richardson, N. A Study of Glycine Adsorption on a Cu {110} Surface Using Reflection Absorption Infrared Spectroscopy. Surf. Sci. 1998, 401, 322−335. (60) Booth, N.; Woodruff, D.; Schaff, O.; Giessel, T.; Lindsay, R.; Baumgärtel, P.; Bradshaw, A. Determination of the Local Structure of Glycine Adsorbed on Cu (110). Surf. Sci. 1998, 397, 258−269. (61) Hasselström, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Pettersson, L. G.; Samant, M. G.; Stöhr, J. The Adsorption Structure of Glycine Adsorbed on Cu (110); Comparison with Formate and Acetatecu (110). Surf. Sci. 1998, 407, 221−236. (62) Zhao, X.; Gai, Z.; Zhao, R.; Yang, W.; Sakurai, T. Adsorption of Glycine on Cu (001) and Related Step Faceting and Bunching. Surf. Sci. 1999, 424, L347−L351. (63) Nyberg, M.; Hasselström, J.; Karis, O.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Pettersson, L. G. The Electronic Structure and Surface Chemistry of Glycine Adsorbed on Cu (110). J. Chem. Phys. 2000, 112, 5420−5427. (64) Chen, Q.; Frankel, D.; Richardson, N. Chemisorption Induced Chirality: Glycine on Cu {110}. Surf. Sci. 2002, 497, 37−46. (65) Kang, J.-H.; Toomes, R.; Polcik, M.; Kittel, M.; Hoeft, J.-T.; Efstathiou, V.; Woodruff, D.; Bradshaw, A. Structural Investigation of Glycine on Cu (100) and Comparison to Glycine on Cu (110). J. Chem. Phys. 2003, 118, 6059−6071. (66) Blankenburg, S.; Schmidt, W. Steric Effects and Chirality in the Adsorption of Glycine and Phenylglycine on Cu (110). Nanotechnology 2007, 18, 424030. (67) Rankin, R. B.; Sholl, D. S. Assessment of Heterochiral and Homochiral Glycine Adlayers on Cu (110) Using Density Functional Theory. Surf. Sci. 2004, 548, 301−308. (68) Barlow, S.; Louafi, S.; Le Roux, D.; Williams, J.; Muryn, C.; Haq, S.; Raval, R. Polymorphism in Supramolecular Chiral Structures of Rand S-Alanine on Cu (110). Surf. Sci. 2005, 590, 243−263. (69) Rankin, R. B.; Sholl, D. S. Structure of Enantiopure and Racemic Alanine Adlayers on Cu (110). Surf. Sci. 2005, 574, L1−L8. (70) Haq, S.; Massey, A.; Moslemzadeh, N.; Robin, A.; Barlow, S. M.; Raval, R. Racemic Versus Enantiopure Alanine on Cu (110): An Experimental Study. Langmuir 2007, 23, 10694−10700. (71) Barlow, S.; Raval, R. Nanoscale Insights in the Creation and Transfer of Chirality in Amino Acid Monolayers at Defined Metal Surfaces. Curr. Opin. Colloid Interface Sci. 2008, 13, 65−73. (72) Han, J. W.; James, J. N.; Sholl, D. S. Chemical Speciation of Adsorbed Glycine on Metal Surfaces. J. Chem. Phys. 2011, 135, 034703. (73) Ko, J.; Kwon, H.; Kang, H.; Kim, B.-K.; Han, J. W. Universality in Surface Mixing Rule of Adsorption Strength for Small Adsorbates on Binary Transition Metal Alloys. Phys. Chem. Chem. Phys. 2015, 17, 3123−3130. (74) Ruban, A.; Hammer, B.; Stoltze, P.; Skriver, H. L.; Nørskov, J. K. Surface Electronic Structure and Reactivity of Transition and Noble Metals. J. Mol. Catal. A: Chem. 1997, 115, 421−429. (75) Tang, W.; Sanville, E.; Henkelman, G. A Grid-Based Bader Analysis Algorithm without Lattice Bias. J. Phys.: Condens. Matter 2009, 21, 084204. (76) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. Improved Grid-Based Algorithm for Bader Charge Allocation. J. Comput. Chem. 2007, 28, 899−908. 15202


Article

The Journal of Physical Chemistry C (77) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354−360.

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