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Amino Acids Conjugated Gold Clusters: Interaction of Alanine and Tryptophan with Au and Au 8

20

Marwa H Abdalmoneam, Kevin Waters, Nabanita Saikia, and Ravindra Pandey J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09108 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 19, 2017

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The Journal of Physical Chemistry

Amino Acids Conjugated Gold Clusters: Interaction of Alanine and Tryptophan with Au8 and Au20

Marwa H. Abdalmoneam,1* Kevin Waters,2 Nabanita Saikia,2* and Ravindra Pandey2

1

California Polytechnic State University San Luis Obispo, CA 93407, United States

2

Department of Physics, Michigan Technological University, Houghton, MI 49931, United

States

(Oct 16, 2017)

1 ACS Paragon Plus Environment


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Abstract The stability and electronic properties of gold (Au) clusters interacting with Alanine (Ala) and Tryptophan (Trp) amino acids in their canonical and zwitterionic configurations are investigated using first-principles density functional theory (DFT). The geometrical structures of the Au clusters along with the polarity of the amino acids determine the nature of the interactions in the gas and solvent phases. In the gas phase, the Au8 (D4h) and Au20 (Td) clusters prefer single-site interaction through the amine group for the canonical amino acids, while in the solvent phase, the carboxylic site is preferred for the zwitterionic amino acids. A limited screening of the intermolecular interactions introduced by the solvent medium for the canonical forms of Ala and Trp conjugated with the Aun complexes, suggest the bonding to be primarily covalent in nature. The screening is significantly more pronounced for the zwitterionic complexes where the electrostatic interactions dominate. The cluster-size along with the configurations defines the extent of the interactions and overall stability of the complexes. The structures of Aun clusters govern the charge distribution and electrostatic potential directing the selectivity towards the preferential binding sites with the Ala and Trp amino acids.


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1. Introduction Noble metal clusters and nanoparticles of Gold (Au), Platinum (Pt), and Silver (Ag), have served as versatile intermediaries in the design and fabrication of nanoscale devices with potential applications as diagnostic assays in health-care and medical therapy.1-3 Over the years, Au clusters have drawn considerable research interest, rendered from its size-dependent structural, electronic, optical properties and selectivity at the sub-nanometer scales.4-6 The tunable optical properties like fluorescence7 and biocompatibility with biological molecules like amino acids, small peptides, and nucleic acids have led to novel applications in catalysis,8 biosensors,9 bioimaging,10 targeted drug and gene delivery.11-14 Some of the enzymes and human proteins retain their functionality in the presence of gold nanoparticles13,15,16 and the capping protein layer can modulate the surface properties like molecular charge and hydrophobicity providing the means for intracellular interaction and imaging.7,17 A detailed understanding of the functionality of proteins and amino acids with Au nanoparticles is essential for elucidating the in vivo transferability and intracellular activities. The structures and electronic properties of canonical and ionic Aun clusters have been investigated at the experimental and theoretical levels.18-21 The Aun clusters up to 20 atoms exhibit a range of structural motifs; 2D planar (up to n = 12-14),22 hollow cage (n = 14-18),23 to bulk-like compact structures.24 Wang et al. reported the electronic properties of Aun (n = 2-20) clusters using DFT with the local density approximation (LDA).25 The gold clusters were found to be planar and the transition from 2D to 3D geometries was observed for n > 7. The configurations of medium-sized clusters was determined to be flat cage (for n >10-14) and compact (for n >15).26 Walker determined the stable geometries of canonical and cationic gold clusters at various levels of theory.27 For canonical clusters, the 2D to 3D transition was observed at larger cluster sizes while for cationic clusters, the planar structures are favored up to n = 8. The relativistic effect of the core s-electrons, spherical symmetry and directionality of Au bonds contributes to the unique size-dependent properties of gold clusters.28-30 Clusters up to the size of n = 20 exhibit diverse structural transitions and potential applications. In recent years, there has been a growing interest in elucidating the nature of the interaction of Au clusters with different amino acids.31,32 It has been reported that the conjugates of Aun clusters (for n = 3, 4) with cysteine and glycine are stabilized by an Au-NH2 anchoring bond with charge transfer from the amino acid to the Au clusters.33 The interaction of glycine and cysteine 3 ACS Paragon Plus Environment


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radicals with Au3 was favored through the amine site, while for the anionic cysteine, the Au-S bond facilitated the most stable conjugated complex. Gottschalk and co-workers34 investigated the free-energy of adsorption of 20 different amino acids on Au (111) surface using molecular dynamics (MD) simulation to address the relative interaction strengths between the amino acids and Au surface. The free-energy of interaction followed the order: aromatic < S- < cationic < polar < aliphatic ~ negative, which was a collective contribution of electrostatic, dispersion, metal polarization, chemical, and hydrophobic interactions. To design asymmetric heterogeneous nanocatalysts, the enantiospecific interaction of cysteine with chiral Au34 and Au55 clusters were also investigated.35-37 The enantioselectivity was related to the size and facet of Aun cluster and the chiral Au55 cluster results in the enantiospecific adsorption with an energy difference of ~100 meV between D- and L- cysteine.36 Likewise, a higher adsorption energy was predicted for the sulfur atom of cysteine bonded to an asymmetrical bridge site at the facet of Au55 cluster along the lowest coordination site.37 Motivated by the recent advances and the requirement for an in-depth understanding on the interaction of small Au clusters with amino acids varying in structure and polarity, herein we report the interaction of Aun (for n = 8 and 20) clusters with L-Alanine (Ala) L-Tryptophan (Trp) amino acids in their canonical and zwitterionic (zw) configurations. The Au8 cluster with D4h symmetry,38,39 and Au20 cluster with Td symmetry40,41 are considered to study the influence of geometrical configuration, symmetry and dimensionality towards the interaction with Ala and Trp. Planar geometries of Au8 cluster yield more stable isomers compared to the nonplanar geometries (relative energy difference of 15.3 kcal/mol) calculated at the B3LYP level of theory.42-44 An Au8 cluster supported on MgO (001) surface is the smallest cluster active for CO oxidation,45 and has been reviewed in detail by Tyo and Vajda46 from a catalysis perspective. Likewise, the Au20 cluster with Td symmetry is one of the most widely investigated clusters due to its high symmetry and wide HOMO-LUMO gap of 1.77 eV, which provides structural stability compared to other small Au clusters.47 Since both the clusters have been synthesized experimentally,15,48 we adopted the two cluster in our study with the objectives to interpret how small gold clusters interact with amino acids and/or biological molecules, and how a larger cluster differ. The choice for Ala and Trp are based on the following: Ala is the smallest naturally occurring chiral, nonpolar amino acid with a methyl functional group,49 while Trp is a nonpolar 4 ACS Paragon Plus Environment

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amino acid, a derivative of Ala with an aromatic indole group.50 The indole functional group plays a vital role in terms of high quantum yield for fluorescence.51 Trp absorbs strongly in the ultraviolet region of the spectrum and is highly sensitive to the variations in local environment.52 In the gas phase, both amino acids are in their canonical forms, while in the solvent phase, Trp and Ala exist in their zw configurations with pK1 = 2.38 and 2.34 and pK2 = 9.39 and 9.69, respectively.53,54 The inherent polarity of the amino acids drive the site-specific interaction with Aun clusters, especially for the zw configuration, which has not been reported earlier. The predicted trends in the energetics of interaction in the solvent phase contrast previous theoretical reports on the gas phase Aun clusters interacting with different amino acids.55 Calculations involving Trp can serve as benchmarks for experimental and theoretical studies on Trp conjugated metallic nanoparticles56,57 and define the role of indole functional group on the overall stability of the conjugated complexes compared to Ala.

2. Computational Method First-Principles DFT calculations on Aun clusters, amino acids and conjugated complexes in the gas and solvent phases were performed using the Gaussian09 program.58 The Becke’s three parameter Lee-Yang-Parr hybrid exchange-correlation (B3LYP) functional along with the Los Alamos effective-core potential LANL2DZ basis set59 for Au atoms and the 6-31G (d, p) basis set60,61 for hydrogen, carbon, nitrogen, and oxygen atoms of the amino acids was employed. The previous reports on Au clusters employed the B3LYP functional form which satisfactorily described the interaction of gold clusters with biomolecules. The B3LYP functional provides a reliable description of the electronic structure of biomolecules interacting with Au clusters,62 and consistent results for gold and other transition metal complexes.63-65 Pakiari et al.,31 investigated the binding of Au3, Ag3 clusters with glycine and cysteine at the DFT-B3LYP level of theory. Further, Xie et al.,33 considered the DFT-B3LYP level for Aun (for n=3, 4) clusters interacting with cysteine and glycine amino acids. The B3LYP functional was also considered for investigating the electronic properties and stability of Aun-amino acid complexes.66,67 An extensive conformational search for the most stable (minimum energy) Aun-amino acid complex in canonical and zw configurations were performed in the gas and solvent phases. For each of the minimum energy configurations, we predicted the most favorable interaction site by considering all possible modes of interaction within the amino acids and Aun clusters, i.e. single5 ACS Paragon Plus Environment


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vs. multiple-sites, and apical vs. surface atoms for the Aun clusters. The conformational search or ‘global energy search’ on the high energy configurations reverted to the minimum energy geometries, irrespective of the starting configuration, thereby confirming a preferential selectivity in interaction for the high-symmetry Au8 and Au20 clusters. The minimum energy geometries of Au clusters and the conjugated complexes were relaxed until the convergence criteria for the atomic forces (i.e. maximum force of ~ 10-2 eV/Å, root mean square (RMS) force of ~ 10-2 eV/Å), displacement (i.e. maximum displacement of ~10-4 Å, and RMS displacement of ~ 10-4 Å) and energy (~ 10-5 eV) was achieved. For the solvent phase, the polarizable continuum model (PCM)68,69 was implemented within the Gaussian09 using the dielectric constant of 78.35 to mimic the water solvent media. The binding energy (∆ ) is calculated using Eq. (1) given by:

∆ = ( ) − ( + )

(1)

where, ( ) , and are the total energies of the complexes and individual constituent units in the gas and solvent phases, respectively.

3. Results and Discussion 3.1. Au8 and Au20 clusters Figure 1 depicts the equilibrium geometries and electrostatic potential (ESP) of Au8 and Au20 clusters. In the Au8 cluster, a detailed conformational search predicts the planar D4h symmetry to be the minimum energy configuration with bond lengths of 2.7 and 2.9 Å along the apical and inner rings atoms (see Figure S1, Supporting Information).38 The Au20 (Td) cluster has bond lengths of 2.8, 2.9, and 3.1 Å, which are in agreement with reported values.40,41 For Au8, the Td capped tetrahedron was initially predicted to be the minimum energy configuration,22 though recent theoretical studies proposed the planar structure with D4h symmetry to be the minimum energy configuration.38,39,70 In the solvent phase, both Au8 and Au20 clusters demonstrate minor variations in structural parameters and frontier molecular orbitals relative to the gas phase is depicted in the Supporting Information, Table S1 and Figure S2. In Figure 1b and d, the ESP of Au8 and Au20 clusters reveal the inner Au atoms as regions with high (negative) electron density while the apical atoms are associated with intermediate to low (positive) electron density. This is reflective of the coordination number of Au atoms within the two clusters; in Au8 the four inner ring atoms have four-fold coordination and the four apical 6 ACS Paragon Plus Environment

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atoms have two-fold coordination. The Au20 cluster has four atoms with six-fold coordination (at the center of each face), twelve atoms with four-coordination (along the edges) and four atoms with three-fold coordination (apical atoms). Within the two clusters, the apical atoms with the lowest coordination number facilitates/directs the interaction with Ala and Trp (Figure S3, Supporting Information). A comparison of the Mulliken charge distribution (Figure S3b and d, Supporting Information) shows comparable results as the apical atoms exhibit intermediate to negative partial charges within the cluster.

Figure 1. (a) The gas phase equilibrium configuration of Au8 (D4h) cluster, (b) ESP of Au8 (D4h), (c) gas phase equilibrium configuration of Au20 (Td) cluster, and (d) ESP of Au20 (Td). It is to be noted that the ‘basin-hopping’ or ‘Monte Carlo-minimization’ has been the method of choice for the global optimization of medium-sized to large Aun clusters.71-73 In this paper, Au8 (D4h) and Au20 (Td) are predicted to be the high symmetry, stable clusters which specifically favor single-site interaction through the low coordination (apical) sites with Ala and Trp. We therefore relied on the extensive conformational search for the most plausible (highest binding energy) interaction site in predicting the most stable Aun-amino acid complexes. 3.2. Trp and Ala amino acids: canonical and zw configurations The gas phase equilibrium configurations of the canonical Ala and Trp along with the ESP are depicted in Figure 2. The structural properties of Ala and Trp are in good agreement with prior theoretical studies.74-76 In the equilibrium configuration, the -OH group is offset from planarity by a torsional angle of 12.5º (see Table S2, Supporting Information), and restricting the -OH group to remain in-plane yields a higher energy configuration by ~ 0.70 eV. To ascertain the minimum energy configuration of canonical Trp and Ala obtained in the gas phase, we also considered the different isomers as shown in Scheme S1 of Supporting Information. These 7 ACS Paragon Plus Environment


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isomers were found to be higher in energy relative to the minimum energy configuration (Figure 2a and c). The ESP (Figure 2b and d) displays regions with high (negative) electron density along the amine and carboxylic groups corresponding to the NH2-CH(R)-COOH backbone unit. The indole group of Trp also exhibits a region with high (negative) electron density, which is contributed from the delocalized aromatic ring. The Mulliken charge distribution is consistent with the ESP, (see Figure S4 of supporting Information) with negative charges residing on the electronegative N and O atoms.

Figure 2. (a) The gas phase equilibrium configuration of Ala, (b) ESP of Ala (ESP range: -0.07 to 0.07), (c) gas phase equilibrium configuration of Trp, and (d) ESP of Trp (ESP range: -0.025 to 0.045). Although the calculated structural parameters of Ala and Trp in the gas and solvent phases are similar, there exists a difference in the zw configuration as shown in Table S2 of Supporting Information. The structural parameters of Trp(zw) in the solvent phase was compared with the crystal structure of D,L-Trp which crystallizes to monoclinic form at 173 K.77 Likewise, the structural parameters of Ala(zw) are in good agreement with the previously reported values calculated at the B3LYP/cc-pVDZ level of theory.78 In the zw configurations of Ala and Trp (Figure 3b and d), the ESP depicts distinct domains in the electronic charge distribution; the NH3+ region has a positive electron density and COO– has a negative electron density. Thus, the sites with negative electron density either in the canonical or zw configurations are expected to determine the extent of interaction with the Aun clusters.


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Figure 3. (a) The equilibrium configuration of Ala(zw), (b) ESP of Ala(zw) (ESP range: -0.07 to 0.07), (c) equilibrium configuration of Trp(zw), and (d) ESP of Trp(zw) (ESP range: -0.025 to 0.045).

3.3. Ala and Trp conjugated Aun clusters 3.3.1. Aun-Ala clusters in the canonical configuration The equilibrium configurations and corresponding ESP of Au8-Ala and Au20-Ala complexes in the gas phase are depicted in Figure 4 and Figure S5 of Supporting Information. The Au8 cluster prefers a single-site interaction with the amine and carboxylic groups as shown in Figure 4a and c, with the carboxylic site favored over the amine site by 0.39 eV (Table 1). Likewise, the Au20 cluster prefers a single-site interaction with the amine group via the apical Au atom as shown in Figure 4e. Interaction with the Au20 cluster via the carboxylic site forms two stable complexes; (i) apical Au atom interacting with -COOH (Figure 4g) and (ii) edge Au atoms interacting with COOH (see Figure S6a of Supporting Information). The former configuration is 0.12 eV lower in energy suggesting the preference for a single-site interaction with Ala.



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Figure 4. The gas phase equilibrium configurations and ESP of Aun-Ala complexes: (a)-(b) Au8Ala (amine site), (c)-(d) Au8-Ala (carboxylic site), (e)-(f) Au20-Ala (amine site), and (g)-(h) Au20-Ala (carboxylic site).

We considered additional configurations where both the amine and carboxylic groups were oriented along the cluster. In all cases, the amino acid oriented towards one of the terminal group (amine or carboxylic) discarding the possibility of coordination through multiple sites. The nature of interaction in the Aun-Ala complexes was found to be in good agreement with prior theoretical studies.31,33 For the carboxylic group, only one of the oxygen atoms is coordinated to the Au clusters via a single-site interaction as depicted in Figure 4c and g. The presence of lone pair of electrons on the oxygen atom of C=O facilitates charge transfer with the apical Au atom while the other oxygen is passivated by hydrogen. A similar rationale holds for Trp interacting through the carboxylic site as discussed in the following sections.


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Table 1: Calculated electronic parameters of the Au8-Ala and Au20-Ala complexes in the gas and solvent phases. ETotal is the total energy of complex, ∆ is the binding energy, R is bond length, Q is charge transfer from a biomolecule to Au cluster. Interacting site

Parameter

Au8-Ala (gas)

Au8-Ala (solvent)

Au20-Ala (gas)

Au20-Ala (solvent)

Ala

E Total, eV

-38291.23

-38291.79

-82521.74

-82522.32

(amine)

∆E , eV

-0.84

-0.78

-0.61

-0.54

RAu-N, Å

2.27

2.77

2.33

2.30

Q (Au), e

-0.3

-0.3

-0.3

-0.3

µ, Debye

8.9

9.6

10.6

12.1

Ala

E Total, eV

-38290.87

-38291.33

-82521.42

-82522.00

(carboxylic)

∆E , eV

-0.48

-0.33

-0.28

-0.22

RAu-O1, Å

-

3.31

2.50

2.50

RAu-O2, Å

2.36

2.35

-

-

Q (Au), e

-0.3

-0.3

-0.3

-0.3

µ, Debye

4.7

4.3

6.9

8.6

In the solvent phase, screening of the intermolecular interaction results in lowering of the binding energies by ~0.05 and ~0.07 eV for the Au8 and Au20 complexes, respectively. The Mulliken charge analysis shows a net charge transfer of ~0.3e from Ala to the clusters. The high charge density and lower electronegativity of the amine N-atom over the carboxylic O-atom favors interaction via the amine site with an increase in polarizability of the complexes. In both the gas and solvent phases, the HOMO-LUMO gap of Au8-Ala and Au20-Ala complexes are associated with the cluster as depicted in Figure S7 of Supporting Information. Since the difference in binding energies of Aun-Ala in the gas and solvent phases is minimal with a charge transfer of ~0.3e, the screening of intermolecular interaction in the solvent phase is likewise small, indicating the bonding to be of a covalent nature. The frontier orbitals depict states contributed from the Au cluster with the orbital overlap along the O atom (see Figure S8, Supporting Information). The covalent type interaction within the complexes agrees with previous theoretical studies on proline interacting with Aun (n = 3-13) clusters.79,80 The proline-Au complexes are stabilized with binding energy of -0.84 and -0.39 eV 11 ACS Paragon Plus Environment


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for the amine and carboxylic sites, respectively, suggesting the preferentiality of amine over carboxylic site. Similarly, the complexes of glycine and cysteine with Au3 cluster are predicted to be stable by partial electrostatic and partial covalent type bonds with a binding energy of -0.85 eV.33 3.3.2.

Aun-Ala clusters in the zw configuration

The interaction of Ala(zw) with Au8 clusters is mediated through the carboxylic oxygen atom as depicted in Figure 5a with a binding energy of -0.76 eV (see Table 2). The ESP demonstrates regions with negative charge density localized along the Au8 cluster while Ala(zw) exhibits region with positive charge density as shown in Figure 5b. This suggests an enhanced charge density distribution in the complex compared to the bare Ala(zw) counterpart with high (negative) electron density residing on the cluster. A higher energy configuration stabilized via multiple site interaction exhibits a binding energy of -0.20 eV (see Figure S9 and Table S3, Supporting Information). Similarly, for the Au20-Ala(zw) complex (Figure 5c) the interaction via the COO– group has a binding energy of -0.52 eV (Table 2). The ESP show regions with positive charge density on the Ala(zw) and regions with intermediate charge density on the Au20 cluster (Figure 5d). Although, we do observe regions with negative charge density along the Au-O interacting site, the positive charge density predominates within the amino acid. A significant increase in dipole moment in the solvent phase for the zw complexes are attributed to the anionic and cationic moieties of Ala(zw) thereby inducing an enhanced polarizability in the conjugated complex (Table 2).

Figure 5. Aun-Ala(zw) complexes in the solvent phase: (a) equilibrium configuration of Au8-Ala(zw), (b) ESP of Au8-Ala(zw), (c) equilibrium configuration of Au20-Ala(zw), and (d) ESP of Au20-Ala(zw). 12 ACS Paragon Plus Environment

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Table 2. Calculated electronic parameters of Au8-Ala(zw) and Au20-Ala(zw) complexes in the solvent phase. ETotal is total energy of the complex, R is bond length, Q is charge transfer from a biomolecule to Au cluster.

3.3.3.

Parameter

Au8-Ala(zw)

Au20-Ala(zw)

E Total, eV

-38307.17

-82555.59

∆E , eV

-0.76

-0.52

RAu-O1, Å

2.23

-

RAu-O2, Å

-

2.30

Q (Au), e

-0.2

-0.4

µ, Debye

20.5

17.5

Aun-Trp clusters in the canonical configuration

Figures 6 and 7 show the gas phase equilibrium configurations and ESP of Au8-Trp and Au20Trp complexes. The single-site interaction of Trp with Au8 is mediated through the amine, carboxylic, and indole groups as depicted in Figure 6a, c and e, respectively. The interaction through the amine N-atom is energetically favored with a binding energy of -0.71 eV followed by the carboxylic and indole groups. The nearest-neighbor distance between an apical Au atom and Trp varies in the range of ~ 2.24 - 2.53 Å. Similarly, the Au20 cluster prefers a single-site interaction via the amine N-atom as shown in Figure 7a and Table 3. The ESP and Mulliken charge analysis suggests that both Au8 and Au20 clusters gain a net charge of ~ 0.3-0.5 e from the amino acid. A high charge density on the electronegative N-atom facilitates the donation of electrons to the valence orbitals of Au, which leads to an increase in polarizability of the complex with dipole moment values of 12-16 Debye in the gas and solvent phases.



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Figure 6. The gas phase equilibrium configurations and ESP isosurface of the Au8-Trp complex: (a)-(b) amine site, (c)-(d) carboxylic site and (e)-(f) indole site.

Figure 7. The gas phase equilibrium configurations and ESP of the Au20-Trp complex: (a)-(b) amine site, (c)-(d) carboxylic site and (e)-(f) indole site. 14 ACS Paragon Plus Environment

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Table 3. Calculated electronic properties of Au8-Trp and Au20-Trp complexes in the gas and solvent phases. ETotal is total energy of the complex, R is bond length, Q is the Mulliken charge transfer from a biomolecule to Au cluster. Interacting site

Parameter

Au8-Trp

Au8-Trp

Au20-Trp

Au20-Trp

(gas)

(solvent)

(gas)

(solvent)

Trp

E Total, eV

-48154.64

-48155.29

-92385.16

-92385.86

(amine)

∆E , eV

-0.71

-0.61

-0.49

-0.40

RAu-N, Å

2.25

2.24

2.31

2.30

Q (Au), e

-0.3

-0.3

-0.3

-0.4

µ, Debye

11.6

12.7

13.6

15.5

Trp

E Total, eV

-48154.46

-48154.87

-92384.89

-92385.55

(carboxylic)

∆E , eV

-0.53

-0.19

-0.22

-0.10

RAu-O1, Å

2.44

2.35

2.45

2.57

Q (Au), e

-0.5

-0.4

-0.3

-0.3

µ, Debye

11.5

8.0

2.2

2.4

Trp

E Total, eV

-48154.20

-48154.83

-92384.79

-92385.47

(indole)

∆E , eV

-0.27

-0.15

-0.13

-0.01

RAu-C, Å

2.50

2.51

2.45

2.54

Q (Au), e

-0.4

-0.4

-0.4

-0.4

µ, Debye

9.5

9.9

10.7

10.4

In the solvent phase, the equilibrium configurations of Au8-Trp and Au20-Trp complexes are predicted to be similar to the gas phase configurations (see Supporting Information, Figures S10 and S11). The single-site interaction through Au-amine is preferred with binding energies of 0.61 and -0.40 eV for Au8-Trp and Au20-Trp complexes, respectively. The solvent phase calculations predict a lower binding energy (~ 0.10 eV) relative to the gas phase, suggesting that the screening of intermolecular interactions is minimal in presence of the solvent media and the covalent-type bonding stabilizes the Au-Trp complexes. It has been suggested earlier that, a 15 ACS Paragon Plus Environment


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notable change in binding energy due to the solvent effect demonstrates the bonding to be mainly electrostatic in nature.81 In the Aun-Trp complexes, the frontier molecular orbitals corresponding to HOMO and LUMO is associated with the Aun cluster as shown in Figure 8. Except for the Au8-Trp complex (Figure 8c and d), where the HOMO is contributed primarily from Trp and the LUMO from Au8 and partially along the amine group, the electronic states within the HOMO-LUMO gap are dominated predominantly by the Au cluster rather than Trp.

Figure 8. The frontier orbitals corresponding to the HOMO and LUMO of; (a)-(b) Au8-Trp (g), (c)-(d) Au8-Trp (aq), (e)-(f) Au20-Trp (g) and (g)-(h) Au20-Trp (aq) complexes with the isovalue of 0.02 (e/Å3).

3.3.4.

Aun-Trp clusters in the zw configuration

The zw complexes of Trp prefer multi-site interactions via the Au-O (COO–) and Au-C (indole) bonds. An induced torsion of 14º within Au8 facilitates the interaction with the carboxylic and indole groups as depicted in Figure 9a. On the other hand, a single-site interaction through the COO– group (Figure 9c) is predicted to be slightly higher in total energy by ~0.03 eV, which is near degenerate (within the kT) in energy. For the Au20-Trp complex (Figure 9e), the interaction through the carboxylic group is preferred and multi-site interactions are found to be higher in 16 ACS Paragon Plus Environment

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energy; e.g. multi-site interactions via Au-O and Au-N bonds (Au edge atoms) is 0.23 eV higher in energy while interacting via the Au-O, Au-N and Au-C sites is 0.28 eV higher relative to the equilibrium configuration (see Figure S12 of Supporting Information). In the Trp(zw) configuration, the aromatic indole group plays an active role in mediating the interaction with Au8, in addition to the electronegative oxygen atoms as shown in Figure 9a. The presence of indole group leads to enhanced charge transfer and a net stabilization of the complex, justifying the functionality of an aromatic ring in the amino acid and our choice for Trp in the study. The Au8-Trp(zw) complex has a higher binding energy by about -0.2 eV and charge transfer by 0.2e compared to Au20-Trp(zw) complex, which demonstrates that geometrical configuration in addition to the size of cluster is a determining parameter in governing the energetics of interaction (see Table 4). A similar argument holds for Ala(zw) complexes with Au8 and Au20 cluster, where Au8-Ala(zw) complex has a higher binding energy by -0.24 eV compared to Au20Ala(zw) (see Table 2).

Figure 9. The equilibrium configurations and ESP of the Au8-Trp(zw) and Au20-Trp(zw) complexes in the solvent phase: (a)-(b) carboxylic and indole sites, (c)-(d) carboxylic site and (e)-(f) carboxylic site.



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The selectivity of binding site is further described from the Mulliken charge analysis, wherein a -COO– group facilitates high charge transfer with the formation of stable complexes. However, in the canonical configuration of Trp, the amine group is predicted to be the preferred binding site. The ESP of Au8-Trp(zw) and Au20-Trp(zw) further supports the observed trend from the Mulliken charge analysis (see Figure 9b, d and f). An increase in the dipole moment values of the zw complexes compared to the canonical counterparts is attributed to the NH3+ and COO– groups in Trp(zw) thereby facilitating an increase in polarizability of the complexes. Table 4. Calculated electronic properties of Au8-Trp(zw), Au20-Trp(zw) complexes in the solvent phase. ETotal is total energy of the complex, R is bond length, Q is Mulliken charge transfer from Trp(zw) to Aun cluster. Au8-Trp(zw)

Au8-Trp(zw)

(Configuration I)

(Configuration II)

E Total, eV

-48155.27

-48155.24

-92385.85

∆E , eV

-0.65

-0.62

-0.45

RAu-O1, Å

2.28

-

2.30

RAu-O2, Å

-

2.24

-

Q (Au), e

-0.7

-0.4

-0.5

µ, Debye

16.7

18.7

20.6

Trp (carboxyl site)

Au20-Trp(zw)

The geometries of the Aun clusters determine the binding with the amino acids. Previous studies on the Au32-Trp complex56 have demonstrated multiple-site interaction between Trp with Au32 via the carboxyl and indole groups and mediated by the π-interactions rendered from the indole group. A transition in the structure of the Au cluster from 2D planar (Au8) to 3D pyramidal (Au20) and 3D hollow cage (Au32) suggests the influence of both cluster size and geometrical configuration towards the preferred modes of interaction. The hollow cage-like Au32 cluster facilitates multiple site interaction with Trp, which is due to the uniform distribution of the low coordinated sites along the cluster. We believe that the increase in cluster size and monolayer coverage of the ligand might influence the binding/adsorption sites and lead to enhanced interaction in the conjugated complexes. 18 ACS Paragon Plus Environment

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It is to be noted that another important aspect along with the polarity of the amino acids and its functional forms (canonical or zw configuration) is the effect of chirality or handedness on the nature of interaction with Au clusters. For the biological function and activity, chirality plays a vital role and that explains the preferentiality for L-amino acid in proteins and D-sugars in DNA, attributed to genetic evolution and natural selection.82 The biological self-assembly at the nanoscale is also controlled by the enantiospecific molecular interactions which can be tailored for applications in enantioselective heterogeneous catalysis, optical activity, chiral separations, and molecular sensing.83,84 In this context, chirality of the amino acids are significant when they interact with other chiral molecules or chiral Aun nanoclusters. The enantiospecific properties of chiral molecules on chiral metal surfaces have been reported earlier, and interestingly, the irreversible adsorption of enantiomeric chiral molecules on achiral metal surfaces can render the surface chiral.84-86 For high-symmetry clusters of Au8 and Au20 considered, we believe that polarity, solvent effects and functional form of the amino acids will drive the site-specific interaction with the Au clusters. It would be noteworthy to consider the role of handedness for the chiral amino acids interacting with medium-sized Aun (achiral and chiral) nanoclusters for catalytic and biological applications.

4. Summary Using first principles DFT calculations we have investigated the structure, stability, and electronic properties of Ala and Trp conjugated Au8 (D4h) and Au20 (Td) clusters in the gas and solvent phases. The site-specific selectivity of Aun clusters and polarity of the amino acids governs the energetics of the interaction; i.e. the amine N-atom is energetically preferred for the canonical Ala and Trp, while the carboxylic O-atom is favored in the zw configuration. The conjugated complexes are stabilized via a single-site interaction accompanied with a charge transfer from the amino acids to the Au clusters. In the solvent phase, screening of the intermolecular interactions between the amino acids and Au cluster results in a lower binding energy compared to the gas phase counterparts. For the two studied clusters, the Au8 cluster yields a higher binding energy compared to the Au20 cluster towards Ala and Trp in the canonical and zw configurations. DFT calculations predict that geometry of the cluster (2D planar vs. 3D pyramidal) controls the nature and energetics of interaction. Although Au20, with a pyramidal geometry, has more available surface 19 ACS Paragon Plus Environment


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sites for interaction with the chiral amino acids, the planar 2D geometry is preferred. The geometrical configuration of a cluster influences the charge distribution and ESP within the cluster and regulates its active binding sites. The ESP illustrates significant perturbation in the charge density distribution within the conjugated complexes compared to the isolated counterparts in canonical and zw amino acids. We have demonstrated that the preferential selectivity of interaction of amino acid conjugated Au complexes is governed by the polarity of amino acids along with the geometrical configuration of the Aun clusters considered. It would be interesting to consider Au clusters of varying geometrical configurations, cluster size, and chirality to further support the observed trends in adsorption which constitutes the near-future extension of our research using van der Waals (vdW) dispersion corrected DFT calculations and MD simulation studies.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available: The geometrical structures and frontier orbitals (HOMO-1 to LUMO+2) of Au8 cluster in the gas and solvent phases. The structural parameters of Au8 (D4h) and Au20 (Td) clusters, ESP and Mulliken charges of Au8, Au20 and Au32 clusters. The structural parameters, dipole moments, Mulliken charges and energy gap of Trp and Ala in the gas and solvent phases. The equilibrium configurations and frontier orbitals of Au8-Ala, Au20-Ala, Au8-Ala(zw), and Au20-Ala(zw) complexes in the gas and solvent phases. The electronic properties of Au8-Ala(zw) complex stabilized via multi-site interaction. The equilibrium configurations and ESP of Au8-Trp, Au20Trp, Au8-Trp(zw), and Au20-Trp(zw) complexes in the gas and solvent phases, respectively.

AUTHOR INFORMATION Corresponding Authors *

[email protected] (M. H. Abdalmoneam)

*

[email protected] (N. Saikia)

Telephone: +1-906-487-2086. Fax: 906-487-2933 ORCID 20 ACS Paragon Plus Environment

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Nabanita Saikia: 0000-0002-8777-4898 Notes The authors declare no competing financial interest.

Acknowledgments Helpful discussions with Prof. Max Seel, Prof. Ranjit Pati, William Slough and Dr. S. Gowtham are kindly acknowledged. RAMA and Superior high-performance computing clusters at Michigan Technological University was utilized in obtaining the results presented in this paper.

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