Structures and Bonding Properties of Gold–Arg-Cys Complexes: DFT

Article pubs.acs.org/JPCC

Structures and Bonding Properties of Gold−Arg-Cys Complexes: DFT Study of Simple Peptide-Coated Metal Sung-Sik Lee,† Bongsoo Kim,*,‡ and Sungyul Lee*,† †

Department of Applied Chemistry, Kyung Hee University, Deokyoungdaero 1732, Gyeoggi-do 446-701, Republic of Korea Department of Chemistry, KAIST, Daehak-ro 291, Yusung-ku, Daejeon 305-701, Republic of Korea

‡

S Supporting Information *

ABSTRACT: We present the structures, bonding characteristics, and infrared spectra of the gold surface (111)−Arg-Cys (Arg-Cys@Au(111)) complex calculated by a periodic plane wave DFT technique. We examine the detailed features of bonding between the gold surface and dipeptide. The dipeptide is revealed to form a covalent bond via the −SH group with 2−3 gold atoms, and also weak noncovalent interactions via the carboxyl and guanidine side chain lying more or less parallel to the gold surface. The S−H bond dissociates as a result of the S−−(Au)n bond formation, with the hydrogen atom binding to the guanidine moiety. The acidic proton stays at the carboxyl group in the most stable structure of Arg-Cys@Au(111). The calculated infrared spectra are compared with experimental observations reported by Petoral and Uvdal (Colloids Surf., B 2002, 25, 335).

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INTRODUCTION Bottom-up self-assembly of hybrid materials produces novel materials for electronics and optical applications.1−8 Nanocomposite materials could also be useful as biological probes for diagnosis and treatment of human diseases. Understanding and controlling the metal−biomolecule hybrid systems offers new paradigms in materials science, chemistry, biology, and medicine.9 Among the various candidates, peptide-coated metal surfaces10,11 exhibit appealing properties for biomaterial science and biosensor technology. Protein−surface interactions12−14 may also provide a means to control the activity of active sites in enzymes. The gold surface has received a lot of attention as an instrumental template for immobilizing biomolecules such as DNA, peptides, and enzymes for diverse purposes.15−30 Adsorption of proteins on gold electrode is of great importance in biosensor research. Because the selectivity and sensitivity of biosensors are highly dependent on the binding characteristic between protein and gold surface, it is very important to understand the nature of their interactions. Protein−surface recognition is also a powerful tool for cellular signal transduction, DNA transcription, and protein antigen/antibody recognition.12−14,31−36 A considerable amount of computational work has also been reported on the metal−biomolecule complexes at various levels of theory. Self-assembly of sulfide-containing amino acids and related compounds (cysteine, cysteamine, cystamine, etc.) on metal surfaces37−44 is a simplistic model system that was investigated by quantum chemical methods in combination with molecular dynamic technique to unravel the structures and bonding properties of metal−protein complexes. Adsorption of large proteins on metal surfaces has been studied largely by © 2014 American Chemical Society

classical molecular dynamics simulation based on force fields.45,46 Here we study the gold surface−arginine-cysteine complex (Arg-Cys@Au(111)) as a model for a functionalized metal surface. Arginine (Arg) is chosen to examine the binding to gold surface via its very basic side chain guanidine. Cysteine (Cys) is a good model to probe the role of the thiol group. We calculate their structures to examine what part(s) of the dipeptide interacts with the gold surface through what kinds of interactions (covalent or not). We also calculate the infrared spectra to discuss in detail the normal modes in relation to the structure of the complex. The results are compared with experimental observations reported by Petoral and Uvdal,15 especially focusing on elucidating the similarity and difference in calculated and experimental infrared spectra in terms of the structures of the Arg-Cys moiety and the positions of H atoms resulting from the formation of S−Aun (n = 2−3) bond.

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COMPUTATIONAL DETAILS A periodic DFT method (GGA-PBE)47 is employed with a PAW-PBE48 norm conserving pseudopotential with 400 eV kinetic energy cutoff, a plane-wave basis as implemented in the Vienna ab initio simulation package (VASP)49,50 suite of programs. In order to include the weak van der Waals interactions, we used the DFT-D2 Grimme method,51 which we find to be of better economy than other similar methods. Cut-off radius for pair interactions is taken to be small (7.0 Å) Received: December 19, 2013 Revised: August 12, 2014 Published: August 14, 2014 20840

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Scheme 1. (a) Building Process of Four Layers of Au(111) Surface and (b) Definition of Positions, Angles, and Dihedral Angles in Arg-Cys@Au(111)

Table 1. Energies, Some Bond Lengths, and Torsional Angles of Isomers of Arg-Cys@Au(111) at Various Positions of the Protons (C-1) Emol (eV) Eform (eV) positions of protons Φ Ψ χ1 θs RS···Au (Å)

RH(η2)···Au (Å) (RN(η2)···Au)

(C-2)

−218.24632 −1.97515 guanidinium, COOH (Cys) −147.7° 130.5° 177.1° 148.4° 2.97 2.59 2.49 2.91 2.88

(C-3)

−216.46851 −3.18764 guanidinium, CO+−H (amide bond) −140.2° 161.4° 173.0° 164.4° 2.51 2.46 2.46 3.11 3.10

(C-4)

(C-5)

(C-6)

−214.16743

−213.79214

−213.80060

guanidinium, Au surface −148.3° 127.7° −176.6° 164.8° 2.47 2.44 2.42 3.14 3.06

COOH, Au surface −142.5° 134.6° 170.9° 159.3° 2.81 2.58 2.45 2.46 (2.25)

−217.12289 −1.72233 CO+−H, COOH −155.3° 163.9° 165.4° 135.3° 3.19 2.77 2.52 2.53 (2.37)

CO+−H, Au surface −143.1° 134.6° 171.1° 160.4° 2.78 2.56 2.45 2.46 (2.26)

calculated by using the linear response perturbation theory53−56 by reoptimizing the structures obtained at Gamma points with the gold atoms frozen at equilibrium positions. Electron localization function (ELF)57 was calculated at 1 × 1 × 1 kpoints sampling conditions at geometries reoptimized for infrared spectra, employing the same criteria for optimization. All wrap around errors were neglected except for the IR spectra and ELF calculations.

in order to reduce the interactions with the neighboring images. The van der Waals radii R0 for C, H, N, O, and S are chosen as 1.452, 1.001, 1.397, 1.342, and 1.683 Å, respectively, and the C6 parameters used for the DFT-D2 method are 1.75, 0.14, 1.23, 0.70, 5.57 J nm6 mol−1, respectively. The values of R0 and C6 for Au are 1.772 Å and 40.62 J nm6 mol−1, respectively.52 In order to construct the gold (111) surface, we constructed an FCC gold unit cell consisting of 4 layers of Au atoms (Scheme 1), obtaining the equilibrium lattice constant of 4.11 with lowest electronic energy by varying the lattice constant at the interval of 0.01 at 9 × 9 × 9 k-points sampling conditions. This lattice constant is slightly larger than the experimental ones (4.08). A hexagonal supercell is built by duplicating seven and eight (111) unit cells along a⃗ and b⃗, and by reconstructing this as 4 layers of Au(111) surface of rectangular symmetry, which may be represented as 74 80 in matrix notation. The size of the supercell is 20.34 and 20.13 Å along x and y, respectively, and 32.20 Å along z. The distance between the gold layers along z is 24.77 Å. A Gaussian smearing of halfwidth 0.1 eV is applied. Ionic relaxation is considered to be converged when Hellman−Feynman forces become smaller than 0.01 eV/Å. with 3 × 3 × 1 k-points sampling, and the electronic threshold of 10−7 eV was adopted. IR spectra were

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RESULTS AND DISCUSSION We carried out the search for the most stable (lowest energy) structure of Arg-Cys@Au(111) in two steps: First, we optimized the structures of Arg-Cys@Au(111) with protons at various positions around the backbone on the basis of the experimental observations by Petoral and Uvdal, in order to examine what positions of the protons would stabilize the ArgCys@Au system, specifically to see the relative stability of canonical versus zwittrionic forms of the dipeptide. The results are listed in Table 1. We define a number of energies to compare the stability of the obtained structures. Esurface (=− 699.15144 eV) is the energy of the Au surface with no adsorbate, and Emol is the single point energy of Arg-Cys. The interface energy Eint, listed in Supporting Information, is the total energy of Arg-Cys@Au, whereas the formation energy

( )

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Figure 1. Structures and relative energies in eV (kcal/mol) of Arg-Cys@Au(111) at various positions of protons.

Eform is defined as Eform = Eint − (Emol + Esurface). These energies were calculated at 3 × 3 × 1 k-points sampling conditions. The results are (C-1)−(C-6) presented in Table 1 and Figure 1. In our model peptide-coated metal surface Arg-Cys@Au(111), there exist two protons (from −SH and −COOH in dipeptide) which may transfer to the electron-rich sites (Au surface, COO− in Cys, NH2 in Arg, guanidine, and amide CO). The positions of these two protons are very important in determining the structure of the Arg-Cys@Au(111) system. H-transfer between −SH and the metal surface would mean that the thiol would form a covalent bond with the metal surface, as observed in many metal−peptide or metal−protein complexes. On the other hand, transfer of a proton between the carboxylate and the amino group constitutes the transformation between the canonical and zwitterionic forms of amino acids and peptides extensively studied recently.58 Table 1 and Figure 1 present the calculated isomers of Arg-Cys@Au(111) obtained by binding the protons at electron-rich sites, based on the ArgCys backbone described in the experimental work of Petoral and Uvdal.15 The terminal amino group in Arg was found not to be protonated, because the protons in all initial structures with the ammonium group NH3+ were found to relocate to amide carbonyl to form CO+H in optimization processes. In (C-1)−(C-3), the guanidine moiety is protonated at the η2 position to form −NH2+, whereas it is not protonated in (C4)−(C-6). The low energy structure (C-1) is similar to the structure of Arg-Cys@Au(111) described by Petoral and Uvdal,15 but the dipeptide Arg-Cys is in canonical form rather than in zwitterionic form. The hydroxyl group of COOH acts as a H-bond donor to the S atom with RH···S = 2.00 Å, and the distance to the nearest Au atom is a bit large (RH···Au = 3.23 Å). The O atom of the carboxyl is in contact both with the amine and with the ε −NH of the neighboring guanidinium moiety, with the distances of 2.24 and 1.83 Å, respectively. The sulfur atom is located at the gold surface such that RS···Au = 2.97, 2.59, 2.49 Å on the bridge site, with θs (defined in Scheme1b as the angle between z-plane (gold surface) and the S−C bond) of 148.4°. Guanidinium is positioned obliquely above the gold surface, and the distances between the H atoms of the η2 amine

groups and the Au atom is 2.91 and 2.88 Å, indicating that the amine interacting with the carboxyl is closer to the gold surface. In the structure (C-2), the carboxyl proton is transferred to amide carbonyl to form −CO2− and −CO+−H, the latter of which interacts with the terminal amino. The hydroxyl −OH bends toward the nitrogen N under the influence of Arg −NH2 (N-terminus) with RH···N = 1.79 Å. The H atoms at η2 and ε positions interact with the two O atoms of the two carboxylates, with the same RH···O distances of 1.83 Å. The sulfur atom is located at the FCC hollow site (RS···Au = 2.45, 2.45, 2.51 Å). In the zwitterionic (C-3), the H atom is positioned at a FCC site of gold surface, with the distances to the neighboring gold atoms 1.86, 1.89, and 1.94 Å. The guanidine moiety exhibits a pattern similar to that of (C-1), with the H atoms at η2 and ε positions interacting with a single O atom of the carboxylate. On the other hand, the distances (1.93 and 1.72 Å) of RH···O indicate that the guanidinium moiety is attracted to carboxylate in this zwitterionic structure more strongly than in (C-1). The sulfur atom in Cys also binds to the FCC hollow site with the S−Au distances of 2.42, 2.44, 2.47 Å. The angle between the guanidine [carboxylate] group and gold surface is 59.2° [55.5°], and the hydrogen atoms at η2 positions are closest to gold surface with RAu···H = 3.05, 3.14 Å, and RAu···O = 2.43 Å. The torsional angle χ1 is −176.6°. This structure is characterized by Au atom between the sulfur and H atoms protruding from the gold surface by ∼0.75 Å along z. The structures (C-4)−(C-6) possess deprotonated guanidinium (that is, neutral guanidine). In (C-4) and (C-6), a proton attaches to the gold surface, whereas it binds to the amide CO in (C-5). They possess either −COOH ((C-4), (C-5)) or −CO2− (C-6). Their interface energies are, however, much higher than (C-1) by >1.3 eV. These results suggest that protons would be located at COOH and guanidinium in low energy structures of Arg-Cys@Au(111). This is in line with the observations in our previous work59 on the stability of zwitterionic conformer of Arg, which demonstrated that Arg would be in canonical form in the absence of solvent (water). Next, we carried out optimizations with various Arg-Cys with two protons at COOH and guanidinium, in order to obtain the 20842

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Table 2. Energies, Some Bond Lengths, and Torsional Angles of Arg-Cys@Au(111) Conformational Isomers Emol (eV) Eform (eV) Φ Ψ χ1 θs RS···Au (Å)

RH(η2)···Au (Å)

(C-1)

(C-7)

(C-8)

(C-9)

(C-10)

−218.24632 −1.97515 −147.7° 130.5° 177.1° 148.4° 2.97 2.59 2.49 2.91 2.88

−217.81985 −2.35949 −168.1° −28.6° −141.4° 173.0° 2.51 2.49 2.49 2.88 2.65

−217.01182 −3.24859 −145.1° 8.81° −122.1° 116.4° 2.49 2.51 3.30 3.18 2.85

−216.69172 −3.13623 −133.8° −16.96° −140.5° 172.9° 2.45 2.45 2.46 3.29 3.19

−216.57914 −2.66645 174.0° 157.9° −48.29° 143.9° 2.50 2.52 3.03 too far

Figure 2. Structures and relative energies in eV (kcal/mol) of Arg-Cys@Au(111) conformational isomers, with top and side views of the global minimum energy structure (C-8).

these bond lengths as the results of interactions between −NH2 and −COOH with the gold surface in (C-8) are minimal. It is also useful to compare the binding energy (which is of the same magnitude as the formation energy but with the opposite sign) of (C-8) (3.249 eV), with that (2.666 eV) of (C-10). Because only the S atom binds to the gold surface in (C-10), if we approximate the latter (2.666 eV) as the binding energy of sulfur to the gold surface, then the difference (0.58 eV) between this and the binding energy of (C-10) may be a good estimate of the magnitude of interactions between −NH2 and −COOH with the gold surface. Although this interaction energy (0.58 eV) is larger than weak interactions (for example, hydrogen bonding is ∼0.3 eV), it is still too small to say that the bonding between −NH2 and −COOH with the gold surface is covalent. The structure (C-8) is somewhat different from (C-1) which is more similar to the structure reported by Petoral and Uvdal. The θs angle (116.4°) in (C-8) is very different from that (148.4°) in (C-1). It seems that in both structures the S−C bonds deviates from 180° by the influence of the lone pair at the S atom. The plane of the peptide bond is almost perpendicular to the gold surface in (C-1), whereas it is nearly parallel in (C-8). The ε-H of guanidinium is facing the carboxyl O and peptide O atom in (C1) and (C-8), respectively. The two structures (C1) and (C-8) are, however, nearly of identical

global minimum energy structure. We first obtained 15 structures with three layers of gold atoms and upgraded the structures with four layers of gold. Motivated by the experimental observations by Petoral and Uvdal, we mainly focused on the structures in which both the guanidinium moiety and the sulfur atom interact with the gold surface. We tried a large number of angles and dihedral angles in Arg-Cys beginning from (C-1). The results are given in Table 2 and Figure 2. The structure (C-8) exhibits the lowest Eint and Eform. In this conformer the S atom is located at the nearly bridging site with the distances of 2.49, 2.51, 3.30 Å from the Au atoms. The angle χ1 (−122.1°) is very different from those (165° to −176.6°) in (C-1)−(C-6). The distances between the two η2 H atoms in guanidine and the nearest Au atom are 2.85 and 3.18 Å, whereas the other two η2 H atoms are located with the distances of 3.83 and 1.67 Å from the Au atom and the O atom of the peptide CO. These Au−H distances are much larger than that (∼1.5 Å) of the Au−H covalent bond.60 Compared with the structure (C-10), in which the CO and guanidinium groups are far from the gold surface and only the sulfur atom binds to the gold surface, the bond distances of CO and η2 NH in (C-8) only slightly increased from 1.24 to 1.25 Å, and 1.01 to 1.02, 1.03 Å, respectively, indicating that the changes in 20843

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energy with the difference 0.03894 eV. Their thermodynamic stabilities are essentially the same, and therefore, the analysis of their infrared spectra will be very useful especially with regard to Petoral and Uvdal’s experimental observations. Isosurfaces obtained by the ELF57 (a function that is useful for analyzing the bonding character topologically, of which the value of 0.5 and 1.0 corresponds to electron gas and perfect localization, respectively) of (C-8) are presented in Figure 3.

Figure 4. Comparison of calculated infrared spectrum of (C-1), (C-3), and (C-8) with Petoral and Uvdal’s experimental observations for ArgCys@Au(111) (ref 15). Figure 3. (a) Isosurface formed by the sulfur and the two nearest Au atoms in (C-8) shows the sp3 type hybrid bond. (b, c) Isosurface formed between the sulfur and the distant Au atoms is essentially that of the lone pair (isosurface evaluated at 0.8).

examine our calculated spectrum in relation to the experimental observations. We adopted scaling factors of 1.0, 1.015, 1.030 for (C-1), (C-3), (C-8), respectively, so that the most intense experimentally observed band at ∼1688 cm−1 coincides with the guanidinium band in the calculated spectrum. Although some similarities exist between the experimental and calculated spectra, differences are also noticed: The amide I and II modes are observed at 1688 and 1544 cm−1, respectively, whereas they appear at 1700 and 1535 cm−1 in the calculated spectrum. The amide I band overlaps with the η2 NH2 bending and the inplane bending modes of ε-H. The in-plane bending of carboxyl OH at 1131, 1198 cm−1 appears overlapped with the Cys −CH2− twist mode. The amide II band at 1535 cm−1 is closest to the experimentally observed band at 1544 cm−1. The structure (C-1), whose energy is slightly higher (by 0.039 eV) than that of (C-8), is similar to the structure described by Petoral and Uvdal. The amide I and amide II modes are observed at 1694 and 1490 cm−1, respectively, whereas the COOH in-plane bending mode appears at ∼1282 cm−1 along with various wagging and twisting modes of −CH2− in Arg-Cys. The −CH2− twist mode of Cys side chain contributes most to 900−1000 cm−1 bands, whereas the wagging modes of Arg side chain and η2 NH2 are in the 1000− 1100 cm−1 regime. Although the energy of the structure (C-3) is much higher than those of (C-1) and (C-8), we present its infrared spectrum in Figure 4 to discuss the possibility of a zwitterionic Arg-Cys@ Au(111) complex. The calculated symmetric stretch of the carboxylate CO2− at 1365 cm−1 is in close agreement with the experimentally observed band at 1375 cm−1. The amide I and amide II bands appear at 1710 and 1498 cm−1, respectively, and the calculated band at ∼1240 cm−1 corresponds to the −CH2− vibration of Arg-Cys. It is worth noting that a hydrogen atom binds to the gold surface in (C-3), and that the intense band of the Au−H stretch (shown in the inset in Figure 4) whose direction is oblique along the z axis appears at 906 cm−1. Because this calculated intense band is not observed in Petoral and Uvdal’s experimental spectrum, it seems that the H atom is not binding to the gold surface in their structure. Considering that the Arg-Cys moiety in their experiments is in zwitterionic

The isosurfaces formed by the sulfur and the two Au atoms are closer to S, which is similar to the isosurface formed by sp3 type hybrid bond discussed by Andreoni et al.61 On the other hand, no bonding is seen between the sulfur and the distant Au atoms, of which the isosurface corresponds to the lone pair perpendicular to the Au−S axis. Isosurfaces of (C-1) is very similar to that of (C-8) depicted in Figure 3. In structure (C-7), whose interface energy is higher by 0.081 eV than that of (C-8), the guanidinium is far from COOH, much nearer to peptide CO. The distances of the carboxyl H atom from the Au and the sulfur atoms are 2.88 and 2.48 Å, respectively. The sulfur atom is located at a nearly FCC hollow site, with the distances of 2.51, 2.49, and 2.49 Å from the nearest Au atoms. The (C-9) conformer is characterized by the guanidinium moiety that is far from from the COOH and the peptide CO groups. Its Eint is much higher (by 0.43 eV) than (C-8). Guanidinium and COOH are nearly parallel and perpendicular, respectively, to the gold surface. In (C-10) the guanidinium moiety is far from the gold surface, and the η2 H is also very far (12.81 Å) from the closest Au atom along z. Its Eint is also much higher (∼1 eV) than those of (C-8) and (C-1). Figures 4 and 5 present the calculated infrared spectra and the normal modes of some important absorption bands of the nearly degenerate structures (C-1) and (C-8), and the zwitterionic conformer (C-3) in comparison with experimental spectra reported by Petoral and Uvdal. Although the difference in environment between experimental observations and our calculations (first, use of polarized infrared radiation in Petoral and Uvdal’s experiments and the unpolarized infrared radiation in our calculated infrared spectra; second, the Arg-Cys@ Au(111) complex was initially prepared in aqueous solution with almost saturated monolayer of the dipeptide in Petoral and Uvdal’s experiments, whereas in our model the adsorbent is virtually separated from each other in gas phase) may render direct comparison very difficult, it would still be useful to 20844

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Figure 5. Normal modes of (C-1), (C-3), and (C-8).

form, the possible location of the H atom in Arg-Cys@Au(111) in their experiments is very intriguing. It may either exist somewhere in the Arg-Cys moiety, or it may have detached from the Arg-Cys@Au(111) complex as a H2 molecule, as suggested by Ulman.62 The H atom binding to the gold surface in (C-3) shown in Figure 1, however, is pretty far (RH−S = 3.38 Å) from the Arg-Cys@Au(111) complex. In the absence of adjacent Arg-Cys, the encounter of two H atoms and subsequent detachment as H2 seems unlikely in the gas phase. Most of the difference between the calculated infrared spectra presented in Figure 4 and the experimental spectra reported by Petoral and Uvdal results from two factors. First, the calculated structures of Arg-Cys dipeptide in (C-1) and (C8) are canonical, whereas Petoral and Uvdal found it in zwitterionic form. Second, the hydrogen atom binding to the gold surface is missing in Petoral and Uvdal’s structures. Proton transfer from the carboxyl −COOH to other sites in the dipeptide is considered to be difficult in the absence of microsolvating water as well. The key to elucidating this difference may be the fact that in Petoral and Uvdal’s experiments the gold surface was fully covered by dipeptides initially in water. In that environment, the zwitterionic Arg-Cys would be overwhelmingly more stable than the canonical form, the formation of the former being facilitated by proton transfer mediated by water molecules. The adjacent adsorbents (dipeptides) on the fully covered gold surface in Petoral and Uvdal’s experiments may also have helped the formation of H2

molecule from two neighboring H atoms (from the two adjacent Arg-Cys dipeptides) binding to the gold surface. Because the two mobile hydrogen atoms are removed rather far from the Arg-Cys@Au(111) system in gas phase, they may combine to form a H2 molecule to detach from the Au surface as observed by Petoral and Uvdal in solution phase.

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CONCLUSION Our system here is a prototypical model that may help to reveal the nature of interactions between proteins and the metal surface that may be relevant to a lot of interesting systems, especially the biosensors that are very useful in biological and medical sciences. Our results illustrated a detailed picture of how a biomolecule (Arg-Cys) interacts with a gold surface, which may not be easily obtained experimentally. The nature of interactions between the various functional groups in dipeptide and the gold surface was elucidated. We find that the S atom forms a covalent S−Aun bond, whereas the guanidine and the carboxyl groups interact weakly with the gold surface. Our calculations also describe the dipeptide in Arg-Cys@Au(111) system as canonical in line with the well-known cases of amino acids and peptides of which canonical form is stable in gas phase. This is in contrast to Petoral and Uvdal’s zwitterionic structure of Arg-Cys@Au(111), which is again in agreement with the observations of zwitterionic amino acids and peptides in aqueous solution. It seems that the solvent (water) and the adjacent adsorbents on the fully covered gold surface are 20845

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influencing the structure of Arg-Cys and the related proton transfer processes. This will be left as a future work.

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ASSOCIATED CONTENT

S Supporting Information *

Additional structual data and fractional coordinates. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +82423502836. *E-mail: [email protected]. Phone: +82312012423. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.L. thanks the National Research Foundation (NRF- 398 2012R1A2A2A02013289, NRF-2011-0021836) for financial 399 support, and KISTI Supercomputing Center (2013). B.K. was supported by the Korean Health Technology R&D Project (A121983) funded by the Ministry of Health & Welfare, Korea.

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REFERENCES

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