BiomaterialâBiomolecule Interaction: DFT-D Study ... - ACS Publications

J. Phys. Chem. C 2011, 115, 719–727

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Biomaterial-Biomolecule Interaction: DFT-D Study of Glycine Adsorption on Cr2O3 P-A. Garrain, D. Costa,* and P. Marcus Laboratoire de Physico-Chimie des Surfaces, CNRS-ENSCP (UMR 7045), Ecole Nationale Supe´rieure de Chimie de Paris, 11 rue Pierre et Marie Curie, 75005 France ReceiVed: October 10, 2010; ReVised Manuscript ReceiVed: NoVember 21, 2010

The adsorption of glycine, the smallest amino acid and building block of proteins, on a Cr2O3 surface, taken as a model of stainless steel surfaces, was studied by means of GGA + U (on site Coulomb repulsion). The role of coverage on the adsorption mode and the self-assembly properties were explored. The dispersion contribution to the adsorption energies was also calculated. Glycine adsorbs at low and high coverage in the anionic form. The adsorption is driven by the formation of iono-covalent bonds with the Cr surface atoms. The most stable configurations correspond to maximization of the bonds (Cr-O and Cr-N), where the O or N atoms replace O atoms from the missing anionic plane above the terminal Cr plane. To maximize the bonds formation and minimize lateral repulsion, glycine lies parallel to the surface at low coverage and has a bent orientation at high coverage. The inclusion of dispersion forces strengthens this trend. At mean coverage under the ML, clusterization of high density domains may coexist with domains of lower coverage at the surface. At saturation, a glycinate monolayer with a 1:1 Gly:Cr ratio is formed. Introduction A solid surface in contact with natural (soils, seawater) and industrial environments (marine structures, medical devices, food industry, biocompatible materials) is readily covered with natural occurring matter (NOM) such as biological and organic matter.1-3 Therefore, the adsorption of biomolecules and organic molecules on solid surfaces requires considerable interest for understanding and solving questions such as fouling, geochemical processes, corrosion, food hygiene, health hygiene, and biocompatibility. Stainless steel and Co-Cr alloys are biocompatible alloys, which, due to their good resistance to corrosion and hardness, are used as implants.4 The adsorption of proteins on those alloys has been extensively studied with surface analysis tools.5-19 Among others, Immamura et al.,20-27 to identify the domains of proteins responsible for adsorption, have realized a series of studies of adsorption of proteins, protein fragments, peptides and amino acids on stainless steel surfaces. Indeed, amino acids (AA), which are the building blocks of biological molecules, represent interesting species to be studied.28,29 The authors found by difference between the concentrations before and after adsorption that amino acids adsorb reversibly on stainless steel surfaces. Most of the carboxylic groups of the amino acids are dissociated in the adsorbed form as evidenced by FT-IR. Despite the great number of experimental studies available, including model experimental studies, and due to the huge complexity and differing natures of inorganic surfaces and biological entities, the biomolecule/surface interactions are not yet understood at the molecular level.28-30 To this respect, theoretical studies (and especially ab initio studies) may provide insight into the chemical events at the molecular scale leading to adsorption, to the preferential mode of adsorption (inner sphere/outer sphere), and to the adsorption geometry and surface coverage. Since some years, we have undertaken ab initio theoretical studies about the mechanisms of adsorption of * Corresponding author. Phone: 33 1 44 27 25 25Fax: 33 1 46 34 07 53. E-mail: [email protected].

biomolecules and small size organic molecules on several inorganic surfaces, metallic or oxidized.4,31-43 The works have shown that the complexity and diversity of possible scenario requires that, even for the smallest amino acid, glycine, each case is a unique one combining several aspects (surface structure, hydroxylation rate and local OH density, hydroxyls acid-base character, cation Lewis acidity, local topology) and requires an accurate study. To our knowledge, such a study has not been performed yet for the adsorption of amino acids on stainless steel surfaces. Stainless steels and Co-Cr alloys are covered with a Crrich thin passive film (some atomic layers thick) at their surface.44-50 Experimental studies performed on pure Cr and Fe-Cr alloys have evidenced the enrichment in Cr2O3 in the passive film.51,52 Structural studies on monocrystalline Fe-22Cr alloys have evidenced the formation of an inner R-Cr2O3 (0001) oxide film covered with hydroxyls or a hydroxide layer.53 The reactivity of the stainless alloys toward adsorption of molecules is governed by the composition, chemical state, and structure of the first atomic layers at the surface. The strong enrichment of chromium oxide, forming a continuous layer, in the passive film of stainless steels, suggests that the reactivity is governed by this Cr2O3 layer. In fact, the adsorption of inorganic ions or biomolecules on pure Cr and stainless alloys was compared and was found to be similar, or to exhibit only subtle differences.11,12,16,51,52,54 Pure Cr represents thus a model for stainless steels passive films. The Cr2O3 surface and surface reactivity have been recently modeled with the use of ab initio methods.55-69 We recently undertook the theoretical study of water adsorption on the basal plane of Cr2O3, R-(0001)-Cr2O3 by means of GGA +U (onsite Coulomb repulsion, see Computational Details).68,69 Several water coverages were studied, and the termination of the surface as a function of the (T, P) conditions was calculated. In addition, observable properties as vibration frequencies and electronic core level shifts were determined and compared to experiment.68,69 We are thus able to propose a model for passive films formed

10.1021/jp109704b  2011 American Chemical Society Published on Web 12/16/2010

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J. Phys. Chem. C, Vol. 115, No. 3, 2011

on stainless steels and stainless alloys, which we now use for further study of the adsorption of biomolecules. In the present work, we describe the adsorption of glycine on the (0001)-Cr2O3 surface, taken as a model for passive films on stainless steels. The mode of adsorption is studied at several coverages, from the limit of the isolated molecule to the formation of self-assembled monolayers at the surface. The bond between glycine and the surface is described on the energetic and electronic sides. In this paper, we present the interaction of glycine with the anhydrous Cr2O3 surface. In the most relevant cases we performed an evaluation of the contribution of dispersion energy to the energy of interaction between the glycine molecule and the Cr2O3 surface. Computational Details Methods. Total energy calculations were performed within the density functional theory (DFT) and the generalized gradient approximation (GGA) of Perdew and Wang.70 To solve the Kohn-Sham equations, we use the Vienna ab initio simulation package (VASP).71,72 VASP performs an iterative diagonalization of the Kohn-Sham Hamiltonian via unconstrained bandby-band minimization of the norm of the residual vector to each eigenstate and via optimized charge density mixing routines. For correlated systems, i.e., a number of transition metals (TM), TM sulfides and oxides, it has been shown that the problems intrinsic to DFT studies can be for the most part overcome by adopting a DFT + U approach, where the effects of strong intraatomic electronic correlations are taken into account by adding an on-site Coulomb repulsion, namely a Hubbard term (U) to the DFT Hamiltonian.73,74 The (GGA + U) approach was used here as in ref 73 to describe the strong correlation effects of the oxide. A value of U ) 5 with J ) 1 was used. The cutoff energy was set to 520 eV. A K-point grid of (2 × 2 × 1) was used for the (2 × 2) cell used (vide infra). For electronic analysis, an increased K-points mesh of (3 × 3 × 1) was used and smearing was eliminated. The radii of the elements were evaluated to 1.275 (Cr), 1.43 (O), 0.37 (H), 1.1 (C), and 1.0 (N) based on the analysis of bulk Cr2O3, of the hydroxylated Cr2O3 surface and of isolated glycine. The eigenstates of the electron wave functions are expanded on a plane-wave basis set using pseudopotentials to describe the electron-ion interactions within the projector augmented waves (PAW) approach.75 The optimization of the atomic geometry at 0 K is performed by determining the exact Hellmann-Feynman forces acting on the ions for each optimization step and by using a conjugate gradient algorithm until the geometric convergence criterion on the energy (1 meV/cell) is reached. In addition, in some cases we performed preliminary ab initio molecular dynamics to explore the potential energy surface (PES) and reach the absolute minimum in the PES. Although standard density functional theory (DFT) is useful in predicting structures and electronic properties of systems characterized by strong chemical bonds, and to a certain extent, to H-bonded molecular crystals with a remarkable agreement with spectroscopic data,76 pure DFT (in the local (LDA) or generalized gradient (GGA) approximations, cannot capture the long-range dispersion forces. Recent works take into account dispersion forces in a DFT approach of the adsorption of organic molecules on inorganic surfaces.77-81 The reader may be interested in a review of the nowadays advantages and limits of each method, available in refs 80 and 81. In the present work, we used the dispersion contribution calculations recently proposed by Grimme et al in the DFT-D scheme.81 The total DFT-D3 energy is given by

Garrain et al.

E(DFT-D3) ) EDFT-D ) EKS-DFT + Edisp

(1)

where EKS-DFT is the usual self-consistent Kohn-Sham energy as obtained from the chosen density functional (DF) and Edisp is the dispersion correction calculated considering the most important two-body term given by

Edisp ) ΣABΣn)6,8Sn

CAB n n rAB

fd,n[rAB]

(2)

where the sum is over all A and B atom pairs in the system, CnAB denotes the averaged isotropic sixth- and eighth-order dispersion coefficients for atom pair AB, rAB is their internuclear distance, and sn are global DF dependent scaling factors. More explanations are developed in ref 81. It is worthwhile to mention that the dispersion contribution is calculated at optimized geometries obtained with pure GGA, but not at each step of the geometry optimization. In other words, this represents an a posteriori correction to the total energy. Models. The model for the anhydrous surface was described in our previous papers.68,69 We studied the Cr-terminated (0001)Cr2O3 surface (Figure 1a). To study several surface coverages of the adsorbed molecules, we built a (2 × 2) cell of dimensions (10.14 × 10.14 Å), which exhibits 4 Cr at the surface. As shown in the available literature on (0001)-Cr-Cr2O3 reactivity,62,63,68 the adsorption of electron donating molecules may occur either on top of a surface Cr atom or in a hollow site, as shown in Figure 1b,c. The glycine molecule was optimized in the same conditions (cell size, cutoff and Kpoints grid) as the slab. Different conformers may be stabilized on the surface as the neutral (N), the zwitterionic (Z), and the anionic (A) forms, which have been considered here. Different coverages, from 0.25 ML to the monolayer, were investigated. To this end, a (2 × 2) cell was used, allowing us to introduce n ) 1-4 molecules at the surface. The corresponding coverage was 0.25 (n ) 1, 1.2 molecule/nm2), 0.5 (n ) 2, 2.4 molecule/nm2), and 1 monolayer (ML) (n ) 4, 4.9 molecule/ nm2). We notice here that the term monolayer corresponds to one adsorbed molecule per Cr atom. All configurations were constructed and analyzed by means of the Modelview software.82 Nomenclature. To describe the different adsorption modes of glycine on the surface, the following nomenclature was adopted: The overall orientation of glycine (defined here as the orientation of the C-C bond) to the surface/binding mode (glycine moiety in interaction with the surface-orientation of this moiety toward the surface)/(adsorption site); for example, perp/unid(COOH-para)/top refers to glycine perpendicular to the surface (i.e., the glycine C-C bond is perpendicular to the surface), adsorbed through the COOH moiety in a unidentate mode, the COO being parallel to the surface and the O atom in the CrOCO bond being in a on-top position. Para/bridge(COOHpara,NH2)/hollow refers to glycine parallel to the surface (the glycine C-C bond is parallel to the surface), adsorbed through the COOH and the NH2 moieties in an bridging mode, the COO being parallel to the surface and the O and N atoms in the CrOCO and CrN bonds being in hollow positions. Energetics. The energies of adsorption of glycine on the surface (Cr2O3) were calculated considering the reaction 3,

nNGLYgas + (Cr2O3) f (nGLY; Cr2O3)

(3)

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J. Phys. Chem. C, Vol. 115, No. 3, 2011 721

Figure 1. Optimized geometry of (a)-(c) the Cr-terminated (0001)-Cr2O3 surface, (noted Cr-Cr2O3): (a) dry surface; (b), (c) possible adsorption sites for glycine, in a hollow site (b) and on top (c).

where n glycine molecules adsorb on the Cr2O3 surface; NGLYgas refers to the neutral glycine (NGLY) conformer in the gas phase. The glycine molecule may adsorb in the neutral (NGLY), anionic (AGLY), or zwitterionic (ZGLY) forms. The energy of adsorption (or reaction when deprotonation occurs) is given by formulas 4 and 5,

∆Etotal ) E(nGLY; Cr2O3) - n(E(NGLYgas)) - E(Cr2O3) (4) and

∆Eads ) ∆Etotal /n

(5)

where E(GLYgas) and E(Cr2O3) are the total electronic energies of glycine (neutral) and the Cr2O3 surface, n is the number of glycine molecules in the unit cell, and E(nGLY;Cr2O3) is the total electronic energy of glycine molecules in interaction with the (2 × 2) surface. Formula 4 allows us to calculate the energy of adsorption per cell, thus per surface area, whereas formula 5 allows us to calculate the energy of adsorption per adsorbed molecule. The dispersion forces contribution to the energy of adsorption was calculated by subtracting dispersion energies of the separate reactants (surface and molecule) to the dispersion energy calculated for the whole system (molecule adsorbed on the surface). Combining eqs 1 and 5 gives

∆Eads(DFT-D3) ) ∆Eads(KS-DFT) + ∆Edisp

(6)

To get an insight on the “binding” energy, defined here as the strength of the iono-covalent bonds formed, ∆Eb, between glycine and the surface, after each optimization, we calculated the energies of deformation of the glycine molecule and of the slab. It consisted in performing a single point calculation of (a) the amino acid and (b) the slab in the geometries obtained at the end of the geometry optimization of the whole system (slab + amino acid). To perform calculations on neutral systems, the slab was considered without the adsorbed proton, and the glycine molecule was reneutralized in the geometry fixed for the glycine skeleton (only the position of the added proton was relaxed). We thus obtain

∆Eb ) ∆Eads - ∆Edeformation(GLYgas) ∆Edeformation(Cr2O3) (7)

Results and Discussion Following a strategy similar to that used in previous works,15,17 we investigated the adsorption of glycine on the Cr2O3 surface testing the orientation of the glycine to the surface (parallel and perpendicular), the orientation of the -COOH group (parallel and perpendicular), the nature of the GLY function adsorbed (-COOH and/or NH2), the site of adsorption (on top or hollow site, see Figure 1b,c), and finally the adsorption mode (unidentate, bidentate, and bridge). We also considered the possibility of a proton transfer from the COOH moiety toward the surface, resulting in the stabilization of the anionic (AGLY) species. The results at different coverage (θ ) 0.25, 0.5, and 1 ML) are summarized in Table 1, Figure 2 (0.25 ML), 5 (0.5 ML), and 6 (1 ML). Glycine Interaction with Chromium Oxide at Low Coverage (θ ) 0.25 ML). Starting from neutral glycine (NGLY), we first investigated which terminal function (-COOH or -NH2) interacts mostly with the Cr2O3 surface. The interaction of NGLY with the Cr2O3 surface through the -NH2 terminal group (Figure 2a) leads to an adsorption energy of ∆Eads ) -0.86 eV. The interaction through the -COOH group (Figure 2b) is more exothermic (∆E ) -1.05 eV). We note that during the geometry optimization, the COOH group dissociates spontaneously on the surface and forms a H-bond with a second neighbor surface oxygen of the Cr ion (oxygen in position 4, see ref 68). The same result was found in the case of water adsorption, for which the stable state is the (1-4) dissociative position. At variance with water, here the dissociation is spontaneous (a very weak activation energy of 0.1 eV was calculated in the case of water). This is likely due to the higher acidic character of the carboxylic acid as compared to water. For the perpendicular configurations, comparing the on top (perp/unid(COOH-para)/top, Figure 2b) and the hollow positions (perp/bid(COOH-para)/hollow, Figure 2c) of the carboxylic oxygen, we observe only a small stabilization in the hollow position (by 0.05 eV). The Cr achieves a tetrahedral symmetry in the top configuration and a bipyramide trigonal symmetry in the hollow configuration. In the later configuration, the mixing of the Cr 3d t2g states with the O 2p ones is favored for geometrical reason and because eg orbitals are stabilized, as detailed below. The most important stabilization is obtained in the parallel configuration when the number of bonds of glycine with the surface increases, through a multianchoring mode (Figure 2d,e). The para/bridge(COOH-para)/hollow configuration is the most stable configuration (∆E ) -1.59 eV), since the two O atoms of the -COOH group are located in a hollow site (Figure 2e), forming Cr-O bonds of 2.13 and 3.00 Å. It is worthwhile to mention that the formation of one additional Cr-O bond has a

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TABLE 1: Geometrical and Energetic Details Calculated for the Configurations Obtained for Glycine on Cr2O3, at Several Coverages from θ ) 0.25 to 1 MLa glycine conformer

orientation and bonding mode

NGly

perp/unid (NH2)/top (Figure 2a)

1

AGly

perp/unid (COOH-para)/top (Figure 2b)

AGly

θ ) 0.25

N(Cr-gly) bonds

θ ) 0.5

θ)1

1

Etot ) -0.86 Eads ) -0.86 (-0.21) EDFT-D ) -1.07 Etot ) -1.05



perp/bid (COOH-para)/hollow (Figure 2c)

2

Eads ) -1.05 Etot ) -1.10

Eads ) -1.06 Etot ) -2.03

Eads ) -1.15 Etot ) -5.63

AGly

para/bridge (COOH-perp, NH2)/ hollow (Figure 2d)

2

AGly

para/bridge (COOH-para, NH2)/ hollow (Figure 2e)

2-3

Eads ) -1.41 Eads ) -1.10 (-0.21) Eads ) -1.01 EDFT-D ) -1.32 Etot ) -1.37 Etot ) -2.60 (a), Etot ) -6.89 -2.70 (b),-3.46 (c) Eads ) -1.72 (-0.47) Eads) -1.37 (-0.32) Eads ) -1.30 (a), -1.35 (b),-1.73 (c) (-0.27) EDFT-D ) -2.00 EDFT-D ) -2.19 EDFT-D ) -1.69 Etot ) -1.59 Etot ) -3.37 Etot ) -5.94 Eads ) -1.59 (-0.33) Eads ) -1.69 (-0.44) EDFT-D ) -1.92 EDFT-D ) -2.13

Eads ) -1.48 (-0.54) EDFT-D ) -2.02

a The energies of adsorption (in eV) are calculated following the equations given in the Calculation section (a) corresponds to the [1 0 0] rows, (b) to the [0 1 0], and (c) to the [1 1 0]). Values in parentheses indicate the dispersion correction. The figures mentioned correspond to the 0.5 ML coverage.

Figure 2. Optimized geometries obtained for glycine on Cr2O3 at θ ) 0.25 ML (n ) 1): (a) perp/unid(NH2)/top; (b) perp/unid(COOH-para)/top; (c) perp/bid(COOH-para)/hollow; (d) para/bridge(COOH-perp, NH2)/hollow; (e) para/bridge(COOH-para, NH2)/hollow.

significant influence on the adsorption energy (compare -1.59 and -1.37 eV for the para/bridge(COOH-perp,NH2)/hollow and the para/bridge(COOH-para,NH2) hollow configurations, respectively). To conclude, at low coverage, based on geometrical optimization, glycine preferentially interacts in the anionic form with two/three Cr present at the surface. Each functional group binds to a Cr atom in a hollow site, the most stable configuration having three atoms (N, O, O) in a hollow site and three bonds with three Cr at the surface (Figure 2e). Dispersion Forces Contribution. Figure 3 reports on the respective contributions of Kohn-Sham and dispersion to the total energy of adsorption, which are also shown in Table 1. For neutral glycine adsorbed through the NH2 end, the dispersion contribution is -0.21 eV, thus the dispersion energy represents 20%, of the total adsorption energy (∆Eads(DFT-D) ) -1.07

eV). The same value (-0.19 eV) is calculated for another configuration where the glycine C-C axis is perpendicular to the surface, the perp/unid (COOH-para)/hollow on-top configuration, and this time, the dispersion energy represents 17% of the total energy (∆Eads(DFT-D) ) -1.22 eV). The dispersion energy increases (in absolute value) to -0.32-0.33 eV when the glycine C-C axis has a parallel orientation to the surface and now, it represents 19% and 17% of the total adsorption energy for the Para/bridge(COOH-perp, NH2)/hollow, and the respectively (Para/bridge(COOH-para, NH2)/hollow, ((∆Eads(DFT-D) ) -1.69 and -1.92 eV, respectively). The present results evoke two comments: first, the dispersion energy increases in absolute value when the glycine molecule shifts from a perpendicular to a parallel orientation to the surface, because the distances of the glycine atoms to the surface shorten;



Figure 3. Kohn-Sham and dispersion contributions to the energy of adsorption of glycine on the Cr2O3 surface: full bars, DFT energy; dashed bars, dispersion energy, in electronvolts. Color codes: black, 0.25 ML; dark gray: 0.5 ML; light gray: 1 ML.

this trend is obvious and was recently observed for phenol on a Al2O3 corundum surface;77 second, in the present case, the dispersion contribution to the total energy does not vary significantly with the orientation of glycine to the surface; indeed, in the parallel orientation, an additional iono-covalent bond is formed, and both Kohn-Sham and dispersion energies increase (in absolute value). In conclusion, for glycine adsorbed at low coverages, the inclusion of the dispersion energy modifies the absolute values of the adsorption energies calculated, but not the ranking obtained with pure DFT when comparing the different structures (see Figure 3). Slab and Glycine Deformation and Energetics. The glycine adsorption occurs by bond making at the cost of surface and glycine deformation. The surface deformation occurs solely in the z axis perpendicular to the surface. Indeed, at the Cr-Cr2O3 surface, the Cr atom is relaxed to the underlying oxygen plane in a triangular symmetry.68 The adsorption of glycine induces an extraction of the Cr atom from this plane. As an example in the para/bridge(COOH-para,NH2)/hollow configuration (Figure 2e), the Cr atoms bonded to COO and NH2 moieties of glycine are in coordination 4; they are extracted from the O plane along the z-axis by +0.4 Å for the Cr bound to COO (respectively +0.3 Å for the Cr bound to NH2). The glycine skeleton is also

deformed in the 〈xz〉 plane as the CCN angle decreases by -8°. With this distortion, the two O and N atoms are located in hollow sites above the Cr plane. It is interesting to note that at higher coverage, when the coordination of the Cr at the surface increases to 5 and 6, there is a further extraction of Cr to around +0.6 Å along the z-axis. In this position, the Cr atom is distant from the underlying O plane of 0.9 Å, as in the bulk structure (where the Cr-O plane distance in the optimized structure is 0.95 Å). Such a trend of gradual Cr extraction from the surface was already observed for the adsorption of water on the Cr2O3 surface.68 The other configurations exhibit similar deformations. It is interesting to decompose the energy of adsorption in several contributions, that is on the one side, the cost of the deformations of both the surface and adsorbate and, on the other side, the strength of the bonds formed (iono-covalent, H bonds, and dispersive interaction), called here binding energy. To obtain this “binding energy”, we simply subtracted the deformation energy of each reactant from the adsorption energy, as explained in the computational details section. For all adsorption modes considered for glycine, Table 2 lists the nature of the bonds formed, the binding mode and the deformation energies. We first notice that when the number of bonds between glycine and the surface increases, the deformation energies increase (in absolute value) for glycine and for the surface, due to constraints to accommodate the glycine molecule to the surface. The binding energy also increases (in absolute value) with the number of bonds, for both contributions (Kohn-Sham and dispersive energies). We noticed that in one case, the Kohn-Sham binding energy is additive: indeed, the binding energy in the para/bridge(COOHpara,NH2)/hollow configuration (2.94 eV) is the sum of the binding energies of the perp/unid(NH2)/top (0.92), perp/unid(COOH-para)/top (2.02). To further analyze this trend, we performed an electronic analysis of these three configurations. DOS Analysis. Figure 4 shows the projected density of states (DOS) on the surface Cr 3d states, COO(-Cr) and N(-Cr) 2p states of the atoms involved in bonding in perp/unid(NH2)/top, perp/unid(COOH-para)/top, and para/bridge(COOH-para,NH2)/ hollow. The DOS exhibit an overlap between Cr 3d electrons and 2p O and N ones, showing that the O-Cr and N-Cr bonds are iono-covalent in nature. The Cr-O overlap is more

TABLE 2: Nature of the Bonds, Binding Energy (∆Eb), Deformation Energies (Edef) (in eV) and Electronic Populations Calculated for Several Configurations of Glycine Adsorbed at 0.25 ML Coverage glycine conformer NGly AGly AGly AGly AGly AGly AGly

a

nature of bonds

∆Eb (eV)

perp/unid (NH2)/top (Figure 2a) ∆Eb(K-S) ) -0.92 ∆Eb(disp) ) -0.21 perp/unid (COOH-perp)/top ∆Eb(K-S) ) -1.27 (not shown) perp/unid (COOH-para)/top ∆Eb(K-S) ) -2.02 (Figure 2b) ∆Eb(disp) ) -0.21 perp/bid (COOH-para)/hollow ∆Eb(K-S) ) -2.11 (Figure 2c) para/bridge (COOH-perp, NH2)/ ∆Eb(K-S) ) -2.48 hollow (Figure 2d) ∆Eb(disp) ) -0.33 para/bridge (COOH-para, NH2)/ ∆Eb(K-S) ) -2.94 hollow (Figure 2e) ∆Eb(disp) ) -0.33 para/bridge (COOH-perp, NH2)/ ∆Eb(K-S) ) -2.76 hollow, 1 ML ∆Eb(disp) ) -0.37

In the absence of adsorbate.

Cr 3d pop Edef mol (eV) Edef slab (eV) (Cr surf ) 3.95)* COO2p pop NH22p pop 0.06

0.21

3.95

5.20

3.35

0.19

0.61

3.88

5.17

3.33

0.37

0.59

0.33

0.87

0.39

0.73

0.59

0.76

3.87 (Cr-OCO)

5.14

3.34

0.54

1.03

3.90 (Cr-NH2)

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Figure 4. Projected DOS obtained for glycine on Cr2O3 at θ ) 0.25 ML (n ) 1) for the Perp/unid (COOH-para)/top (a) and (b), para/bridge (COOH-para, NH2)/hollow (c)-(f), and perp/unid (NH2)/top (g) and (h) configurations. The atom on which the DPS is projected is underlined. The corresponding structure is recalled on the left upper corner of the figure.

important than the Cr-N overlap, and in agreement, the Cr 3d population slightly decreases in the Cr-OCO bond (from 3.95 in absence of the adsorbate to 3.88 electrons in the Cr-OCO bond). It is interesting to notice that the Cr 3d, O 2p, and N 2p total populations and projected DOS exhibit no significative dependence with the binding mode of glycine. This explains why the binding energies are additive. This single result has been confirmed by the study of larger molecules (other amino acids and peptides) adsorbed on the surface, as will be detailed in a future work. The main result obtained at low coverage is that the glycine molecule adsorbs in the anionic form on the Cr2O3 in a multipoint anchoring mode. This result may be compared to the ones obtained on the Al2O3 corundum surface, which is isostructural to Cr2O3. On Al2O3, the most stable conformation

obtained for glycine at low coverage was a perpendicular, ontop adsorption with the formation of a C-O-Al bond with a tricoordinated Al surface atom. This configuration is more stable than the perpendicular bidentate and bridge ones. This contrasts with the actual results on Cr2O3, for which the bidentate mode is the most stable when the molecule is perpendicular to the surface, and the parallel three points anchoring is the most stable configuration of all configurations. In fact, the reactivity of Cr3+ and Al3+ ions are of very different types, as they form ionocovalent bonds with the carboxyl oxygen through their 3d and 2p valence electrons for Cr3+ and Al3+, respectively. As a consequence, whereas the Al-O bond is unidirectional in the z axis (z is the perpendicular to the surface), the Cr-O bond is preferentially bent in the t2g (xz, yz) direction. So, whereas there is only one single possible position on top of an Al atom, there



Figure 5. Optimized geometries of glycine on the Cr2O3 surface at coverage 0.5 ML (para/bridge(COOH-perp,NH2)/hollow). Several patterns are considered.

are three possible hollow positions above Cr (which correspond to the missing ligands to achieve a hexagonal symmetry). As a consequence, (i) the bidentate bonding mode is favored for Cr2O3 as one oxygen tends to adopt the hollow position, (ii) the on-top mode is in contrast favored for Al2O3, (iii) the Cr2O3 surface allows more possibilities for bond making than the Al2O3 one, resulting in a multipoint anchoring of glycine on the Cr2O3 surface and not on the Al2O3 surface. This study examplifies the role of the electronic structure on the reactivity for two iso-structural surfaces. Glycine Interaction with Chromium Oxide at Coverages θ ) 0.5 and 1 ML. We now present results at increasing glycine coverage up to the monolayer. The influence of the coverage on the energy of adsorption was investigated for all configurations and is reported in Table 1. Figure 5 refers to the 0.5 ML coverage and Figure 6 to the monolayer coverage. We first notice that the total electronic energy per cell, ∆Etot, increases (in absolute value) with increasing coverage, due to the formation of new Cr-Gly bonds and an increase of the Cr coordination number. In the case of the perpendicular unidentate adsorption through the COOH or NH2 groups (shown in Figure 2 at low coverage), there is no significant influence of coverage on the electronic energy of adsorption (per molecule). Indeed, even at full coverage, molecules in perpendicular configurations only exhibit weak H-bond interactions (e.g., weak CH---NH interactions with a H---N distance of 2.25 Å). The contribution of dispersion to the energy of adsorption slightly increases (in absolute value) with the coverage (-0.21, -0.26, and -0.37 eV for 0.25, 0.5, and 1 ML, respectively), leading to a Edisp/E(DFT-D) ratio increasing with coverage from 20 to 26-27%. Indeed, the distances between atoms of neighbor molecules are around 3.2-3.5 Å, for O-O and O-N distances, for example, corresponding well to the minima of van der Waals forces. For the perpendicular bidentate configuration, we notice an increased exothermicity of adsorption at maximum coverage (1 ML). This may be due to the increase of Cr coordination to the 5-fold coordination (as shown in Figure 2c) of each Cr at the surface.

Figure 6. Optimized geometries of (a) and (b) para/bridge(COOHpara,NH2)/hollow configuration at 1 ML, side view and top view, and (c) and (d) para/bridge(COOH-perp,NH2)/hollow configuration at 1 ML, side view and top view.

We present now the results for the glycine adsorbed parallel to the surface (configurations para/bridge(COOH-perp,NH2)/ hollow (Figures 5 and 6a,b) and para/bridge(COOH-para,NH2)/ hollow (Figure 6c,d)). We analyze the COOH-perp configuration, then the COOH-para one. For the coverage 0.5 ML in the case of the COOH-perp configuration, there are different possibilities of organization of glycine molecules on the surface, forming rows along the [0 1 0], [1 1 0], or [1 0 0] directions (Figure 5). In all cases, we observe a slight reorientation of the COO moiety in a tilted orientation (by 8°) to optimize the lateral H-bonds interactions. In the [1 0 0] direction, glycine molecules are side to side with an intermolecular weak H-bond and no significant change of adsorption energy (-1.30 eV) as compared to a lower coverage. In the [0 1 0] direction, glycine molecules are disposed head to tail without noticeable lateral interactions. As a consequence, the energy of adsorption is the same as at low coverage (-1.36

726


( 0.01 eV). In the [1 1 0] direction, glycine molecules are also head to tail, this time with COO-NH2 attractive interactions (COO---H ) 1.76 Å). As a consequence the adsorption is more exothermic than at low coverage (-1.73 and -1.37 eV respectively at 0.5 and 0.25 ML). For this configuration, the dispersion forces are -0.27 eV, slightly lower than at low coverage. Here, it is interesting to notice that the contribution of H bonding to the stabilization (-0.36 eV) is more important than the contribution of dispersion forces. At full coverage (Figure 6a,b), the electronic energy of adsorption is -1.72 eV. As performed at low coverage, we calculated the different contributions to the adsorption energy (reported in Table 2). We found that the Kohn-Sham energy of cohesion inside the glycinate layer was -0.1 eV, whereas the costs of deformations of the glycine backbone and surface were +0.57 and +1.03 eV, respectively. The glycine deformation is similar to that at low coverage, but the surface deformation energy is higher than at 0.25 ML. Indeed, at 1 ML coverage, the Cr atom at the surface being 5-fold coordinated, is extracted by a value of 0.8 Å from the oxygen plane, to be compared to an extraction of 0.3-0.4 Å for a Cr in a 4-fold coordination at low coverage. Finally we found a binding energy of -2.76 eV for glycine on the surface, a value of the same order as the values found at low coverage for the orientation to the surface (-2.48 and -2.94 eV). The dispersion energy is -0.47 eV, a value that can be decomposed in a contribution of cohesion into the glycinate layer, which is -0.1 eV, and the interaction between the glycinate layer and the surface, -0.37 eV. For the COOH-para configurations, one configuration at 0.5 ML (not shown) was studied (in the [0 1 0] direction). In this case, the COO is parallel to the surface and half Cr atoms are 5-fold coordinated. As a consequence, the adsorption is slightly more exothermic than at low coverage (-1.69 compared to -1.59 eV at low coverage). Considering only Kohn-Sham energies, the COO-perp and COO-para configurations are isoenergetics (-1.73 and -1.69 eV, respectively). However, the dispersion energy of the COO-para configuration was calculated to -0.44 eV, and thus the total energy of adsorption is -2.13 eV, thus more exothermic than the COO-perp position. This is an interesting case where dispersion forces allow us to discriminate two configurations that were found to be isoenergetics at the GGA level. At 1 ML (Figure 6c,d), the Kohn-Sham energy of adsorption is -1.48 eV, thus a less exothermic value than at 0.5 ML, due to lateral repulsion between molecules. The dispersion energy is -0.54 eV, 18% of the total adsorption energy. Finally, taking into account the dispersion energy does not modify the energetic trends found with pure GGA, the adsorption in the COO-perp configuration remaining more stable than the COO-para one at high coverage. To summarize, we observe an increase in the dispersion forces contribution to the total energy of adsorption when (i) the glycine molecule is in a parallel orientation to the surface, thus minimizing the distance between glycine atoms and the surface, and (ii) self-assembled layers form on the surface. As concluded for the low coverage situation, the trends observed with pure DFT (increase of adsorption energy (in absolute value) with the coverage) are unchanged (and rather reinforced) with the inclusion of dispersion forces. From the present results, it is possible to describe the adsorption mode of glycine on Cr2O3. The adsorption is driven by two cooperative factors: the strong interaction of the glycine moieties with the Cr2O3 surface, and the absence of steric

Garrain et al. hindrance between glycine molecules even at high coverage. It is worthwhile to mention here that glycine/(0001)-Cr2O3 is a particular system in which the molecular structure of glycine (with O---N distance of 2.59 Å) is compatible with the surface crystallographic structure of the (0001)-Cr2O3 surface (where the distance between two neighbor hollow sites is 2.65 Å), allowing a multipoint anchoring. We finally find that ionocovalent, H bonds and dispersion forces all contribute significantly to the stabilization of the adsorption. At coverages (θ ) 0.25 and 0.5 ML), the most favored configuration adopted is the para/bridge(COOH-para,NH2)/ hollow, where the parallel adsorption of glycine maximizes the occupation of the surface and the bonds with Cr. However, clusterization of molecules in domains of high density Gly/Cr 1:1 may also occur because of the increase in energy of adsorption with the coverage. It is thus possible that dense domains coexist with domains of low coverage. At full coverage, 1 ML, the configuration of adsorption is the para/bridge(COOHpara,NH2)/hollow configuration shown in Figure 5b, forming a Cr-glycinate dense layer, in which the glycine density is very near that of a plane of solid glycine.76 Such an adduct was also identified in the study of the adsorption of glycine on ZnO(0001).40 Conclusion Glycine, the smallest amino acids and building block of proteins, interacts with the anhydrous (0001)-Cr-Cr2O3 surface by forming coordinative bonds through the COOH and NH2 moiety, stabilizing the anionic species on the surface at any coverage with a spontaneous COOH deprotonation. The carbonyl oxygen of the adsorbed glycine is located at a hollow site above the Cr terminated-Cr2O3 surface, corresponding to a crystallographic site of the missing O-plane above the surface. The formation of the Cr-OCO bond is the driving force for adsorption, and an additive Cr-N bond may be formed because the COO-NH2 distance in glycine is compatible with the distances between hollow sites at the Cr2O3 surface. In addition to iono-covalent bonds, H bonds and dispersive forces account for the stability of the adsorption in a cooperative manner. The most favorable conformation of glycine adsorbed on the surface was calculated at low and high coverage. At high coverage, a Cr-glycinate (1:1) monolayer is formed on the surface. The dispersion forces were calculated to represent 15-20% of the total adsorption energy. The inclusion of the dispersion forces in the calculation reinforces the trends observed when considering only electronic energies. The present work represents a first step for the understanding of the complex interface of stainless steel with biomolecules. It opens the route for future work. In particular, the role of the presence of water in the adsorption of amino acids and peptides on this surface is currently under study. Acknowledgment. The CCR calculation centre of University Paris 6 and the national CINES center (c2010082217) are acknowledged for computational time. References and Notes (1) Gray, J. J. Curr. Opin. Struct. Biol. 2004, 14, 110–115. (2) Kasprzyk-Hordern, B. AdV. Colloid Interface Sci. 2004, 110, 19– 48. (3) Brown, G. E.; Henrich, V. E.; Casey, W. H. Chem. ReV. 1999, 99, 77. (4) Puleo, D. A.; Nanci, A. Biomaterials 1999, 20, 2311–2321. (5) Bai, Z. J.; Filiaggi, M. J.; Dahn, J. R. Surf. Sci. 2009, 603, 839– 846.

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