Biomolecule−Biomaterial Interaction: A DFT-D Study of Glycine

ARTICLE pubs.acs.org/Langmuir

Biomolecule-Biomaterial Interaction: A DFT-D Study of Glycine Adsorption and Self-Assembly on Hydroxylated Cr2O3 Surfaces D. Costa,* P.-A. Garrain, B. Diawara, and P. Marcus Laboratoire de Physico-Chimie des Surfaces, CNRS-ENSCP (UMR 7045), Ecole Nationale Sup
erieure de Chimie de Paris, Chimie-Paristech, 11 rue Pierre et Marie Curie, 75005 France

bS Supporting Information ABSTRACT: The adsorption of glycine, the building block of amino acids, on hydroxylated (0001)-Cr2O3 model surfaces, representing the stainless steel passive film surface, was modeled by means of the GGA þ U method. The roles of glycine coverage and surface termination (hydroxylated Cr- and O-terminated surfaces) on the adsorption mode and self-assembly properties were explored. The hydroxylated Cr-terminated Cr2O3 surface, which presents two types of (H)OH groups exhibiting different acidic character, is more reactive than the hydroxylated O-terminated surface, where one single type of OH group is present, for all adsorption modes and coverages considered. Outer sphere adsorption occurs in the zwitterion form, stabilized at low coverage through H-bond formation with coadsorbed water molecules, and at the monolayer coverage by glycine self-assembling. The OH substitution by glycinate is favored on the hydroxylated Cr-terminated surface and not on the O-terminated one. The inclusion of dispersion forces does not change the observed tendencies. An atomistic thermodynamics approach suggests that outer sphere adsorption is thermodynamically favored over inner sphere adsorption in the whole domain of glycine concentration. The obtained SAM’s free energies of formation are rationalized in a model considering the balance between sublimation and solvation free energies, and extrapolated to other amino acids, to predict the SAMs formation above hydroxylated surfaces. It is found that hydrophobic AA tend to selfassemble at the surface, whereas hydrophilic ones do not.

’ INTRODUCTION Understanding the interactions between biological molecules and inorganic surfaces is nowadays a challenge1,2 in several fields such as fouling,3 biomaterials,4 prebiotic conditions and origin of life studies,5 or nanodevices.6 In this respect, amino acids (AA), which are the building blocks of biological molecules, represent interesting species to be studied. Their structures are simple enough to serve as a model for the chemisorption of biofunctional molecules. In addition, AA adsorption on metal and/or oxide surfaces has applications in surface functionalization, enantiospecific catalysis, or emerging fields such as “green” corrosion inhibition or nanoparticle shape control, as recently reviewed.7-9 In order to better understand the interactions between biological molecules and stainless steel surfaces, we performed, in the first part of this work, a DFT study of the interaction of glycine with the dry Cr2O3 surface,10 a model for passive films formed on stainless steels. We showed that glycine dissociates on the surface into glycinate by forming Cr-COO and Cr-NH2 iono-covalent bonds. To maximize bonds with the surface, glycine adsorbs in a parallel orientation to the surface and forms a pluri-nuclear adduct. However, in ambient conditions, as well as in a large range of (T, P) values, the passive film formed on Cr and stainless steels is hydroxylated.11-13 Therefore, glycine is expected to interact with a fully hydroxylated surface. Two scenarios may be envisaged r 2011 American Chemical Society

when considering adsorption on a hydroxylated surface: adsorption of glycine above hydroxyl groups (also called outer sphere adsorption), and substitution of an OH group by the glycinate anion (referred to as inner sphere adsorption).5 Several works have shown that the adsorption of amino acids may occur through inner sphere and/or outer sphere mode7,14-17 on oxide surfaces. This is an important question because it is usually believed that outer sphere adsorption is reversible whereas inner sphere adsorption is irreversible.2,18,19 Irreversible adsorption causes fouling and hygiene problems in biomaterials, drug manufacturing processes, food industry, and hospital structures.20 Conventional cleaning methods using surfactant detergents do not succeed at removing proteins adsorbed as a monolayer.21,22 Works on protein adsorption on Cr and stainless steel have evidenced irreversible adsorption.23-32 In contrast, according to Imamura et al.,33 glycine does adsorb reversibly on the stainless steel surface. Theoretical studies are increasingly used to understand biomolecule/surface interactions at the atomistic scale. Numerous works have been devoted to amino acid/metal interactions, as the natural propensity of amino acids to self-assemble at surfaces is used to generate chiral surfaces (for recent reviews, see Received: October 28, 2010 Revised: January 12, 2011 Published: February 21, 2011 2747

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silica40,44,53-55 and hydroxylated R-quartz59,60 or edingtonite;63 it may form an inner sphere complex on the hydroxylated RAl2O3 surface.57 Glycine and water coadsorption is also favored on a hydrophilic, hydroxylated silica surface.56 In the present paper, we study the interaction of glycine with two models of hydroxylated-Cr2O3 surfaces, the hydroxylated Cr2O3 Cr- and O-terminated surfaces. The hydroxylated Cr2O3 surfaces represent models of passive films formed on pure Cr and stainless steel.11,12,64,65 To elaborate such a model, the interaction of the R-(0001)-Cr-Cr2O3 with water was studied by GGA þ U.66,67 Several water coverage levels were studied and the termination of the surface as a function of the (T, P) conditions was calculated. We showed that, in ambient conditions, the surface is fully hydroxylated. The obtained models for the hydroxylated surfaces will now be used to study the adsorption of glycine on hydroxylated surfaces. The inner and outer sphere adsorptions of glycine are studied at several coverages, from the limit of the isolated molecule to the formation of self-assembled monolayers at the surface. Finally, to bridge the gap between 0 K calculations and experimental data at room temperature and the solid/liquid interface, an atomistic thermodynamics approach is used to evaluate the free energy of adsorption of glycine from the liquid phase to the surface. The extrapolation to other amino acids is then proposed through a simple model.

’ COMPUTATIONAL DETAILS

Figure 1. (a) Optimized geometry of the fully hydroxylated Crterminated-(0001)-Cr2O3 surface, noted (HY-Cr-term). (b) Detail of the H-bond network in the extreme surface (H)OH plane. (c-f) Configurations considered for the NH3 adsorbed on the HY-Cr-term surface: (c) H-bond acceptor from a μ1-OH group, (d) H-bond acceptor from an undissociated water molecule, (e) H-bond acceptor from an undissociated water molecule and a μ1-OH, and (f) bridging adsorption, H-bond acceptor from water and H-bond donor to μ1-OH. Cr atoms are in gray, O atoms are in red, H atoms are in yellow, and N atom is dark blue.

refs 7,9, and 34-36 and references therein). In a recent paper, the “soft epitaxy” concept (i.e., the interaction of lone electron pair containing atoms of the biomolecule with metal surface sites) was proposed to describe the adsorption of peptides on metal surfaces in water.37 The small peptides organize in a manner dependent on the metal crystalline lattice structure, atoms with lone pair electrons of the peptide stabilizing on top of surface metal atoms, and the resulting interaction is relatively strong, typical of chemisorption rather than physisorption. Theoretical works on the adsorption of amino acids on oxide surfaces are less numerous than on metals38-51 (see also refs 7-9 and references therein), due to the lack of experimental structural information. Indeed, to our knowledge, there is one unique paper reporting the STM study of glycine adsorption on an oxide surface, TiO2.52 There are also a few theoretical works considering adsorption of amino acids on hydroxylated oxide surfaces.53-62 Glycine has been shown theoretically and experimentally to form outer sphere complexes with amorphous

Methods. Total energy calculations were performed within the density functional theory (DFT) and the generalized gradient approximation (GGA) of Perdew and Wang.68 Despite the lack of a correct treatment of dispersion forces, pure functionals have been shown to correctly account for structural and spectroscopic properties of different polymorphs of glycine, in the form of a molecular crystal.69 To solve the Kohn-Sham equations, we use the Vienna ab initio simulation package (VASP).70,71 VASP performs an iterative diagonalization of the KohnSham Hamiltonian via unconstrained band-by-band minimization of the norm of the residual vector to each eigenstate and via optimized charge density mixing routines. The (GGA þ U) approach was used as in ref 72, in order 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. Spin polarized calculations were performed as explained in refs 72 and 66. It was checked that the adsorption of glycine had no effect on the total magnetic moment, as the system remained antiferromagnetic. A K-point grid of (2  2  1) was used for the (2  2) cell used (vide infra). 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.73 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 Born Openheimer molecular dynamics (MD) in order to explore the potential energy surface (PES) and reach the absolute minimum in the PES. The MD runs were performed in the NVE microcanonical ensemble at 300 K. The time step was set to 1.5 fs. We considered several starting conformations (but no statistic was performed on the starting conformations) and the run was stopped after several hundreds of femtoseconds and even several picoseconds once thermalisation was reached. To avoid fluctuations due to the large time step chosen, the mass of the hydrogen atom was set to 3. The results of the MD simulations are shown in the Supporting Information sections III and V, while in the body of the article, only the most stable structures are described. 2748

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Figure 2. (a) Optimized geometry of the hydroxylated O-terminated-(0001)-Cr2O3 surface, noted (HY-O-term). (b) Detail of the H-bond network in the extreme surface (H)OH plane. (c-e) Configurations considered for the NH3 adsorbed on the HY-O-term surface: (c) H-bond acceptor from a μ2OH group, (d) H-bond acceptor from two μ2-OH group, and (e) H-bond acceptor from three μ2-OH group.

Models. The models for the fully hydroxylated surfaces were described in our previous papers.66,67 Briefly, for the bulk corundum Cr2O3 structure (R-3c), we obtained cell dimensions of a = 5.07 Å and c = 13.84 Å, thus a c/a ratio of 2.73 in agreement with the experimental values of a = 4.95 Å, c = 13.57 Å, and c/a = 2.74. To build the (0001) surface, a (1  1) slab along the (x, y) direction, with four Cr2O3 layers in the z direction was built, which is composed of 20 atoms (8 Cr and 12 O atoms). This slab is symmetric, exhibiting two Cr-terminated surfaces. We considered the fully hydroxylated Cr-Cr2O3 termination (noted HY-Cr-term, Figure 1a) and the H-terminated O-Cr2O3 surface (HYO-term, Figure 2a). In order 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. Briefly, the Cr surface atoms have their coordination saturated to a hexagonal symmetry and the OH density is that of an oxygen plane in Cr2O3, i.e., 14.1 OH/nm2. The two hydroxylated surfaces differ by the coordination of the OH groups, the HY-Cr-term being covered with μ1-OH groups and the HY-O-term with μ2-OH groups, as will be detailed later. The glycine molecule was optimized in the neutral state in the same conditions (cell size, cutoff, and K points grid) as the slab. Different conformers may be stabilized on the surface as the neutral (NGly, COOH-CH2-NH2), zwitterionic (ZGly, -OOC-CH2-NH3þ), and anionic (AGly, -OOC-CH2-NH2) species. The cationic species (HOOC-CH2-NH3þ) was not considered here as will be justified later. Different coverages, from low coverage (0.25 ML) to the monolayer, were investigated. The monolayer corresponds to one adsorbed molecule per Cr atom. To this end, a (2  2) cell was used, allowing introduction of 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). Energetics. The calculation of the energies of adsorption of glycine on the surface was described in our preceding paper.10 In this work, we also introduced the calculation of the contribution of dispersion forces. Indeed, recent works take into account dispersion forces in a DFT approach of the adsorption of organic molecules on inorganic surfaces.40,74-77 The reader may be interested in a review of the present advantages and limits of each method, available in refs 77 and 78. Here, we used the dispersion contribution calculations recently proposed by Grimme et al. in the DFT-D scheme.78 The possibility of both glycine outer sphere adsorption and OH substitution by a glycinate anion was considered, following the reaction ðHY-X-termÞ þ ðn þ mÞNGLY gas f ððHY-mÞ-term-ðn þ mÞGlyÞ þ mH2 Ogas

ð1Þ

where (HY-X-term) is the hydroxylated X-terminated surface, with X = Cr or O, (HY-m)-X-term-(n þ m)Gly is the HY-X-terminated surface where m OH groups have been substituted by glycinate anions and n glycine are adsorbed in outer sphere mode. Then, m water molecules desorb from the surface. The energy of adsorption/substitution is ΔEads=subst ðn, mÞ ¼ f½EðHY-mÞ-X-term-ðn þ mÞGlyÞ þ mEðH2 OÞ - ½ðm þ nÞEðNGLY gas Þ þ EðHY-X-termÞg=ðn þ mÞ 2749

ð1'Þ

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Table 1. Conformer Stabilized, Adsorption Modes, Adsorption Energies (eV), and Free Energy of the Interface for an Infinite Dilution of Glycine (J/m2), for Glycine Adsorbed on the Hydroxylated Cr-Terminated and O-Terminated Surfaces surface

coverage

hydroxylated Cr-terminated Cr2O3 surface

0.25 ML

1 ML hydroxylated O-terminated Cr2O3 surface

N perp COOH N perp NH2

3a 3b

-0.65 -0.20

N para

3c

-0.27

Z para-water coadsorption

6

-0.72

N perp COOH

7a

-0.93

Z para

7b

-1.33

N perp COOH

4a

-0.29

4b 4c

-0.44 -0.19

N NH2 perp

4d

-0.08

N para

4f

-0.10

Z para

8a

-1.00

ΔEads ¼ ½EðHY-term-X-nGlyÞ-nEðNGLY gas Þ-EðHY-X-termÞ=n ðcase m ¼ 0Þ ΔEsubst ¼ ½EðHY-mÞ-X-term-mGlyÞ þ mEðH2 OÞ - ½mEðNGLYgas þ EðHY-X-termÞ=m ðcase n ¼ 0Þ Thermochemistry. To compare the ab initio results with available experimental data and predict the behavior of the system studied under different thermodynamic conditions, we take into account the free energies of the species (glycine and water) in solution. The treatment of solvent effects on the thermochemistry of systems in condensed phases has been studied theoretically in a number of fields. There are often ambiguities in the literature with regard to standard states and reference states. The reader is invited to refer to R. Ashcrafts' work78 for a clarification in this domain. In the present work, to estimate the free energies of adsorption/substitution of OH groups in the presence of solvent we consider the following steps. If (n þ m) GLY molecules are adsorbed on the surface, from which m substitute an OH group (practically, on the 2  2 cell used, n = 1-4 and m = 0-4), we consider, in addition to eq 1, the following steps:

ZGly gas f ZGly¥ solv

ð0 K, isolated moleculeÞ

ð2Þ

ð0 K, isolated molecule to 300 K, infinite dilutionÞ

ð3Þ H2 Ogas f H2 O¥ solv

ð0 K, isolated molecule to 300 K, infinite dilutionÞ

ð4Þ The free energy of glycine adsorption on the surface from the liquid phase under standard conditions, and at infinite dilution, ΔG¥ is then calculated considering reaction (5) = (1) - (3) - (2) þ (4) ðn þ mÞZGly solv þ HY-X-term f ðHY-mÞ-X-term-ðn þ mÞGly þ mH2 Osolv ð5Þ ,with ΔG = ΔEads/subst - (n þ m)ΔGsolv¥(Gly) þ mΔGsolv¥(H2O), where ΔGsolv(Gly)¥ = ΔGsolv¥(ZGly) þ E(ZGlygas)(0 K) E(NGLYgas)(0 K). The free energy for zwitterionic glycine solvation ΔGsolv¥(ZGly) was calculated with the Gaussian03 code,79 using the B3LYP functional and ¥

adsorption energy (eV) ΔEads

ΔΓ¥ (J/m2) -0.08

-0.41 -1.10

0.25 ML

1 ML

NGlygas f ZGly gas

fig

conformer and adsorption mode

the 6-311þþG** basis set, and the PCM approximation.80 We calculated free energies of solvation of ΔGsolv¥(Gly) = -0.41 eV (respectively, ΔGsolv¥(H2O) = -0.45 eV) from neutral glycine (respectively, water) isolated in the gas phase at 0 K to zwitterionic glycine (respectively, water) in infinite dilution at 300 K, P = 1 atm. However, thermochemistry in solution is often described at a solute concentration of 1 mol/L. Gaussian does not provide a corrective term for the nonideal conditions, in particular, the mixing entropy contribution when shifting from infinite dilution to standard conditions. If the species of interest is in the dilute limit, this term may be neglected.79 In some works, the computed free energy under the gas phase is converted from the 1 atm standard state into the standard state of molar concentration (ideal mixture at 1 mol/L and 1 atm), and then the solvation energies added to the obtained term.81 This approach is applied here and leads to a free energy of reaction in standard conditions ΔG1MOL/L expressed as ΔG1MOL=L ¼ ΔEn-ads=m-subst - ðn þ mÞðΔGgas st ðNGlyÞ þ RT Ln Pð1 mol=LÞ þ ΔGsolv ¥ ðGlyÞÞ þ mððΔGgas st ðH2 OÞ þ RT Ln Pð1 mol=LÞ þ ΔGsolv ¥ ðH2 OÞ where ΔGgasst(NGly) (respectively, ΔGgasst(H2O)) is the difference of free energy between glycine (respectively, water) in the standard conditions (298.15 K, 1 atm) and at 0 K, P(1 mol/L) is the pressure (in atm) of a ideal gas at 298.15 K and 1 mol/L (P = 24.8 atm). These assumptions neglect a number of thermodynamic data, as detailed in ref 79 for the free energy of solvation and in ref 82 for the solid-liquid interfaces. Then, the correction for activity, RT Ln(c), where c is the glycine concentration in mol/L, can be considered. Finally, we express, as explained in more detail in refs 82,83, the interfacial free energy difference ΔΓ = θ(ΔG), using the appropriate units (the free interface energy is expressed in J/m2; in this case, the energy values are in J/ molecule and θ in molecule/m2).

’ RESULTS Hydroxylated Surfaces Topology and Bronsted Acidity. The hydroxylated Cr-Cr2O3 surface (noted HY-Cr-term) and the H-covered O-terminated Cr2O3 surface (HY-O-term) are the possible terminations of the hydroxylated (0001)-Cr2O3 2750

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Langmuir surface (Figures 1a,b and 2a,b). Those two hydroxylated surfaces have the same OH density (14.1 OH/nm2), but differ by their OH coordination number, being monocoordinated (μ1) on the Cr-terminated surface and dicoordinated (μ2) on the O-terminated one, as described in ref 67. In addition to μ1-OH groups, undissociated water molecules are also present on the hydroxylated Cr-Cr2O3 surface, with the ratio of undissociated/ dissociated water being 2:1. Thus, this surface exhibits two different terminal groups, (μ1-OH) and (μ1-HOH) groups. In contrast, the HY-O-term surface presents only one type of OH groups, namely, μ2-OH groups. The μ1-OH groups at the HYCr-term surface are expected to exhibit a more basic character than the μ2-OH groups at the HY-O-term one, as shown by a previous electronic analysis.67 In a first step, we checked the Bronstedt acidity of the three types of OH groups by adsorbing the NH3 molecule probe (Figures 1 and 2). NH3 has been indeed shown to adsorb on hydroxylated Cr2O3 surfaces and form an NH4þ adduct distinguished by XPS.84 Two types of runs were performed: in a first set of runs, the NH3 was placed in the initial geometry on top of each OH species (i.e., the N atom was aligned with the O and H atoms) and the system was relaxed, imposing the N atom to relax along the z axis only (axis perpendicular to the surface). Then, the molecule was completely relaxed. In a second set of runs, the NH3 molecule was let totally free to relax above the OH groups. These different calculations converged to the same results. When the NH3 molecule is placed above the μ1-OH group, it stabilizes at 2.01 Å from this group, with an adsorption energy of -0.13 eV (Figure 1c). When adsorbed above the undissociated water molecule, the NH3 group makes a stronger H bond, with a N-H distance of 1.95 Å and an energy of adsorption of 0.39 eV (Figure 1d). If NH3 is the H bond acceptor from both μ1-OH and undissociated water, the H bonds are weak (N—H = 2.25 and 2.36 Å), and consequently, the adsorption energy is low, -0.16 eV (Figure 1e). Finally, the most stable configuration is found when NH3 is the H-bond acceptor from the HOH molecule (1.80 Å) and H bond donor to the μ1-OH group (2.16 Å), with an energy of adsorption of -0.45 eV (Figure 1f). These results suggest that the HOH group has a higher Bronsted acidity than the μ1-OH group, which in contrast exhibits rather a basic character toward NH3. For the study of the adsorption above the μ2-OH groups of the HY-O-term surface, the NH3 molecule was placed either on top of one OH group or above the mass center of two or three OH groups, and allowed to relax along the z axis. The configurations obtained are nearly isoenergetic, with adsorption energies between -0.16 and -0.20 eV (Table 1 and Figure 2c-e). The most stable structure (-0.20 eV) corresponds to the NH3 molecule above one OH group with a H bond of 1.95 Å. However, the situation where it is H-bond acceptor three times is nearly isoenergetic (-0.19 eV). The weaker energy of adsorption calculated for NH3 above the μ2-OH groups than above HOH (-0.20 eV versus -0.39 eV) suggests that the μ2-OH groups are less acidic than undissociated water. They are, however, slightly more acidic than the μ1-OH groups, as the adsorption energy is somewhat higher (-0.20 and -0.13 eV, respectively). Finally, the Bronsted acidity ranking is μ1-HOH > μ2-OH > μ1-OH. This means that the undissociated water molecule is more acidic than the OH groups, in agreement with the fact that water adsorbed on a Cr surface atom may easily be deprotonated.66 Not surprisingly, we also find that the acidity of OH groups increases with their coordination number. Finally, we may conclude that the

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Figure 3. Configurations and energies of adsorption obtained for the adsorption of neutral glycine above the HY-Cr-term surface (Crterminated hydroxylated (0001) Cr2O3 surface) (outer sphere adsorption mode). (a,b) The glycine backbone is perpendicular to the surface: (a) adsorption through the COOH end; (b) adsorption through the NH2 end. (c) The glycine backbone is parallel to the surface. Color code: gray, Cr atoms; red, O atoms; yellow, H atoms; dark blue, N atom; light blue, C atoms.

HY-Cr-term surface presents (H)OH groups of different Bronsted acidity. It is also interesting to notice that the two studied surfaces (HY-Cr-term and HY-O-term) do not present the same H-bond network between OH groups. The H-bond network in the HYCr-term surface plane has a density of two H-bonds per unit cell (Figures 1b and 2b). Note that one H of the molecular water points out of the surface without being involved in H-bonds. The HY-O-term surface consists of OH groups involved in triangular H-bonds networking, thus with a density of three H-bonds per cell, that is, all OH groups are involved in the H-bond networking. Outer Sphere Adsorption. In the following paragraph, we present the results of adsorption of glycine at low coverage on the two types of terminations for the hydroxylated (0001)-Cr2O3 surface. At this low coverage, the coadsorption of glycine and water molecules is also considered. Then, the formation of a selfassembled glycine monolayer above the hydroxylated surfaces is considered. All results are summarized in Table 1. Adsorption at Low Coverage (0.25 ML). Figures 3 and 4 show the most stable configurations obtained for the adsorption of glycine in the neutral state (HOOC-CH2-NH2) with the hydroxylated surfaces at low coverage. Additionally, less stable configurations are shown in the Supporting Information (sections I and II). It is worth mentioning that in any case no stabilization of the zwitterionic state was observed, suggesting that the hydroxylated surfaces do not solvate the glycine 2751

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Figure 4. Configurations obtained for the adsorption of neutral glycine above the HY-O-term surface (O-terminated hydroxylated (0001) Cr2O3 surface) (outer sphere adsorption mode). (a-d) The glycine backbone is perpendicular to the surface: (a-c) adsorption through the COOH end; (d,e) adsorption through the NH2 end; (f) the glycine backbone is parallel to the surface.

molecule. A similar result was found in the case of glycine adsorption on hydroxylated silica53 and quartz.59 The interaction of neutral glycine with the hydroxylated surfaces was first studied through the COOH moiety. Previous works have evidenced that the interaction of hydroxyls with the COOH moiety is maximized in forming an H-bond cycle in which the carboxyl is H-bond donor to a hydroxyl group and the carbonyl oxygen is H-bond acceptor.53,54 Such an H-bond cycle was constructed on both surfaces; on the HY-Cr-term surface, an undissociated water molecule (the most acidic surface species) is H-bond donor to the carbonyl COOH oxygen, whereas a μ1-OH group (the most basic surface species) accepts an H bond from the COOH moiety (Figure 3a), with an energy of adsorption of -0.65 eV (see Table 1). We notice that no H bond is broken in the surface OH plane. A MD run confirmed that this configuration is stable for more than 0.5 ps. Interestingly, we observed proton exchange from the COOH moiety to a surface OH group

with a frequency of 200-300 fs (see Supporting Information section III), the glycine being alternately in the neutral and anionic states when adsorbed on the surface. The formation of such a planar cycle on the HY-O-term surface (Figure 4a) implies H-bond breaking between the μ2-OH groups. It is consequently less favorable (-0.29 eV, see Table 1) than on the HY-Cr-term surface. A more stable configuration (-0.44 eV, Table 1 and Figure 4b) is obtained for the glycine forming H-bonds with three μ2-OH groups of the O-terminated surface, consisting of two weak H-acceptors bonds with the carbonyl oxygen and one H-donor bond with the carboxyl group (Figure 4b). The formation of a cycle involving two OH groups belonging to different cycles (see Figure 2b) was also investigated (Figure 4c) and was found to be less exothermic (-0.19 eV). The interaction of neutral glycine with the hydroxylated surfaces was then studied through the NH2 moiety. On the HY-Cr-term surface, we considered an H bond from the most 2752

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Figure 5. Water adsorption above the HY-Cr-term surface. (a,b) Starting configuration, after optimization at 0 K: (a) side view and (b) top view of the ice network. (c,d) Most stable configuration of the potential energy surface (PES) obtained after MD at 300 K during 700 fs after thermalization: (c) side view and (d) top view of the water network. The water molecules from the upper water layer are in blue. Oxygen atoms from surface OH groups are in gray. See Supporting Information section IV for details on the proton transfer.

acidic species, undissociated water, to the NH2 end (Figure 3b); the H-N bond is 1.68 Å long, associated with an energy of adsorption of -0.20 eV (Table 1). We do not expect the basic NH2 group to be H donor to a OH group, and indeed, this was not observed. On the HY-O-term surface, the adsorption of glycine through the NH2 moiety with formation of three H-bonds with μ2-OH groups (H—N = 2.12, 2.20, and 2.34 Å; Figure 4d) was found athermic (-0.08 eV), as well as the adsorption on top of a OH group (-0.06 eV, Figure 4e). We note here that NH2 interacting weakly with both hydroxylated surfaces; the stabilization of a cationic species (COOH-CH2NH3þ) on the surface is likely to be excluded. This is in agreement with the known nonacidic character of the hydroxylated Cr2O3 surface. Indeed, the possibility of H transfer to the NH2 end is expected only on acidic surfaces as the hydroxylated silica surface.55 Interestingly, we find that both the acidic species COOH and the basic species NH2 of glycine interact most with the HY-Crterm surface than with the HY-O-term one. As in the case of NH3 adsorption, this is likely due to the presence of hydroxyl groups of different acido-basic character on the HY-Cr-term surface, and to

the possibility of binding new H bonds without breaking the existing H-bond network at the surface. Finally, the adsorption through both functions, COOH and NH2, thus with the glycine backbone in a parallel orientation to the surface, was investigated. In this case, molecular dynamics (MD) preliminary runs were performed for each considered configuration, in order to check the potential energy surface (PES). Interestingly, all MD runs resulted in a reorientation of the glycine molecule perpendicular to the surface, with the COOH moiety facing the OH groups. Indeed, when the COOH moiety lies parallel to the surface, the adsorption is athermic on both surfaces (Supporting Information sections I and II). An exothermic adsorption energy (-0.27 eV) is obtained when the COOH moiety is perpendicular to the HY-Cr-term surface, the carbonyl oxygen being H-bond acceptor from two OH groups (one bond from an undissociated water molecule and one from a terminal OH group) (Figure 3c). On the HY-O-term surface, the adsorption was found to be slightly exothermic by -0.10 eV with the NH2 moiety being H acceptor from a μ2-OH group (Figure 4f). Additionally, less stable configurations are shown in the Supporting Information sections I and II. Again, 2753

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Figure 6. Glycine and water coadsorption above the HY-Cr-term surface. (a) side view and (b) top view of the glycine-water network. The O atoms of the surface OH groups are in gray. Note that, in this figure, glycine has H bonds with water molecules only.

adsorption on the HY-Cr-term surface is favored over adsorption on the HY-O-term one. We attribute the difference of reactivity between the two surfaces to the diversity of sites present on the HY-Cr-term surface (μ1-OH and undissociated water molecules give an amphoteric character to the surface) and to finally the fact that the terminal OH groups may form H-bonds without bond breaking in the hydroxyl network. In a recent work, we have calculated the dispersion force contribution to the glycine adsorption and found it was dependent on the glycine orientation to the surface.10 The dispersion forces accounted for 15-20% of the total energy of adsorption, a significant contribution. Here, we calculated this contribution for the most stable configurations obtained for the perpendicular and parallel orientations. On the HY-Cr-term surface (respectively, HY-O-term surface), the total energy of adsorption EadsDFT-D is -0.83 eV for the perpendicular orientation, adsorbed through the COOH moiety (respectively, -0.69 eV) and -0.54 eV (respectively, -0.43 eV) for the parallel orientation. We thus conclude that, in the present case, the inclusion of dispersion forces does not modify the tendencies obtained with the pure GGA calculations, which are summarized below. Note that this is likely due to the small size of glycine and that this conclusion cannot be extended to larger molecules.

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To summarize, glycine adsorbs in the neutral form on the HYCr-term and HY-O-term surfaces, through the COOH moiety, in a perpendicular orientation. The adsorption is favored on the HY-Cr-term, because of the possibility to form an H-bonds cycle between the COOH moiety and H-donor and H-acceptor hydroxyls at the surface. The inclusion of the dispersion force contribution does not modify the trend deduced from GGA calculations. Water and Glycine Coadsorption. It is interesting to check if glycine is stabilized at low coverage by water coadsorption. Indeed, we showed in a preceding work56 that glycine and water coadsorption lead to significant stabilization of glycine adsorbed in an outer sphere configuration, as water facilitates the creation of an H-bond network which stabilizes the zwitterion glycine conformer. Similarly, a high hydroxyl density as found in some hydrophilic zones of silica may have a strong stabilization impact.85 Thus, we also investigated if water and glycine coadsorption is favored above the hydroxylated HY-Cr-term surface. In a first step, we built a water layer above the hydroxyl groups. Starting from a water layer of the same density as the OH groups, in an ice-like structure (Figure 5a,b), an MD run was performed at 300 K during 1 ps. We first observed after 300 fs of simulation that proton transfer occurs from an undissociated water molecule adsorbed on a surface Cr, to the underlying O atom, forming a bridging OH group (see Supporting Information section IV); then, the dissociated state was found stable during the time of the dynamics. We also observed that part of the water molecules formed trimers or tetramers above a first water layer, thus forming a partial second layer (Figure 5c,d). Several configurations obtained at local minima were optimized at 0 K, and the resulting most stable configuration is shown in Figure 5c,d. This configuration corresponds to a mean energy of adsorption of -0.55 eV, per water molecule, a value in good agreement with the formation of two H bonds per molecule (Figure 5c,d)). The dispersion contribution to the adsorption energy was calculated as -0.11 eV; thus, the total energy of adsorption is ΔEads(DFT-D) = -0.66 eV. Two water molecules from the initial layer (shown in Figure 5a,b) were substituted by one zwitterionic glycine molecule and a molecular dynamics (MD) run was performed (see Supporting Information section V for details). After thermalization, a 6-ps-long run allowed us to explore the potential energy surface (PES). The presence of coadsorbed water has a significant effect on the adsorption of glycine: first, we note that the zwitterionic glycine conformer is stabilized by H bonds with neighbor water molecules. Second, the parallel orientation of glycine to the surface is stable during the time of the dynamics. Glycine forms six H bonds, three acceptors from the COOmoiety and three donors from the NH3þ moiety. Glycine binds mainly with water molecules and may also be H-bond acceptor from an OH surface group; in fact, we observed that the H bond between glycine and the surface OH group lasted half the simulation time, 3.5 over 7 ps. Finally, the coadsorbed water molecules act as glycine solvation sphere. The most stable conformations were extracted from local minima and optimized at 0 K (see Figure 6 for the most stable conformation and Supporting Information section V-2 for an additional configuration). The energy of water substitution by glycine is -0.72 eV, an exothermic value, to which a dispersion contribution of -1.27 eV has to be added, resulting in a total energy of reaction of -1.97 eV. Finally, we conclude that glycine and water coadsorption at low coverage is an exothermic process. 2754

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Figure 7. Optimized geometries obtained for glycine adsorbed on the HY-Cr-term surface at full coverage, 1 ML: (a) in the perpendicular orientation to the surface; (b-d) in the parallel orientation to the surface, (b) side view (the surface is designed in gray), (c,d) top view without the underlying hydroxylated surface, (d) insight showing the H-bond network; H bond lengths are given in Å.

Glycine Self-Assembling (1 ML). As stated in the Introduction, amino acids are known to self-assemble at surfaces. Therefore, it is interesting to explore the possibility of glycine selfassembly at the hydroxylated Cr2O3 surface. To achieve a monolayer, one glycine molecule was adsorbed above each -(OH)3 termination (Figures 7 and 8). The monolayer was considered in two configurations, glycine adsorbed perpendicular to the surface through the -COOH moiety, and glycine being parallel to the surface. In both orientations, the monolayer of glycine adsorbed is significantly more stable than glycine adsorbed at low coverage, (-0.93 eV (ML coverage) for -0.65 eV (low coverage, lc) in the perpendicular orientation on the HY-Cr-term surface, -1.33 eV (ML) for -0.27 eV (lc) for the parallel orientation on the HY-Cr-term surface, and -1.00 eV (ML) for -0.1 eV (lc) in the parallel orientation on the HY-O-term surface. In the parallel orientation, lateral interactions between glycine molecules become predominant on both the HY-Cr-term (Figure 7b) and the HY-O-term surfaces (Figure 8a). The glycine molecules lie parallel to the surface in a bent position, making one acceptor H bond between an OH group and a carboxylate oxygen, one intramolecular bond between NH3þ and COO-, and four lateral bonds with four first neighbors. The cohesion energy in the glycine monolayer is -0.91 eV, so the resulting energy of adsorption (-1.33 and -1.00 eV for the HY-Crterm and HY-O-term, respectively) is the sum of the energy of cohesion in the layer and the interaction with the hydroxylated surface (-0.44 and -0.01 eV, respectively). It is interesting to note that the organizational mode of glycine in this monolayer is very similar to that observed in a glycine solid plane,69 as shown in

Figure 8c. The dispersion contribution to the energy of adsorption was calculated to -0.51 eV, so the total adsorption energy of the glycine monolayer was -1.84 eV on the HY-Cr-term surfaces. Inner Sphere Adsorption. In the preceding paragraph, we have considered the adsorption of glycine on the fully hydroxylated surfaces by reference to the outer sphere mechanism of adsorption. Now, we consider the reaction of ligand exchange at the surface, i.e., the substitution of an OH group by a glycinate anion. The formation of covalent bonds between the glycine molecule and the Cr2O3 surface is exothermic by 1.5-1.7 eV.10 Nevertheless, in contrast with the adsorption on the dry surface, on the hydroxylated surface the driving force for glycine-surface bonds formation is counterbalanced by the cost of OH group elimination. In fact, we found that only the configurations where one single OH group is substituted may be exothermic (see Table 2). The most stable configuration (-0.25 eV) was obtained on the Cr-terminated surface and consisted of the formation of a COO-Cr ester, with the glycine in a perpendicular orientation (Figure 9a), with the NH2 function directed away from the surface to avoid steric hindrance. In this configuration, the contribution to dispersion energy of the OH substitution was -0.26 eV; thus, the total energy of substitution was -0.51 eV. The substitution energy of one OH group by one glycinate was found to not depend on the surface coverage. The monolayer formed is shown in Figure 9b,c. The energy of substitution can be decomposed into the difference between adsorption of the COOH moiety on the surface, evaluated to -1.05 eV,10 plus attractive H-bonds between adsorbed glycine and (H)OH groups, minus the cost of 2755

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Figure 8. (a,b) Optimized geometries obtained for glycine adsorbed on the HY-O-term, hydroxylated O-terminated surface: (a) side view; (b) top view; (c) insight showing the H-bond network; (d) detail of the H bond network in a glycine R-plane. H bond lengths are given in Å.

Table 2. Coverage, Number of Substituted OH Groups, Energies (eV), and Interface Free Energies at Infinite Glycine Dilution (J/m2) for the Substitution of OH Groups by Glycinate, Same Units as Table 1 surface hydroxylated Cr-terminated Cr2O3 surface

hydroxylated O-terminated Cr2O3 surface

coverage

number of OH substituted

0.25 ML

substitution energy (eV)

ΔΓ¥ (J/m2)

-0.25

-0.10

1

9a

3

9b

1 ML

1

1 ML þ SAM

1

0.25 ML

1

energy for water desorption (-1.1 eV).66 A water-glycinate network is formed at the surface as illustrated in Figure 9c for the full coverage. In this network, a glycinate anion replaces one-third of the surface OH groups in the ice-like hexagonal network formed on Cr2O3.66 In contrast to the Cr-terminated surface, the substitution of one twofold OH group at the O-terminated hydroxylated surface is endothermic by 0.5 eV (Figure 10). This is due to the high energy cost to eliminate a μ2-OH group. It is often considered that physically bonded adsorbate multilayers may grow over a first, strongly chimisorbed layer. In other words, outer sphere adsorption may occur above a first layer composed of inner sphere adsorbed molecules. In order to

figure

10

0.24 -0.25

-0.41

-1.08

-0.93

0.49

explore if this phenomenon of inner and outer sphere coadsorption may occur on the same surface sites, we considered the SAM formation over a hydroxylated surface where one-third of the OH groups are substituted by a glycinate ion. We found that the energy of SAM formation above the substituted OH layer is -0.88 eV, a slightly lower energy than on the hydroxylated surface. The mean energy of reaction (substitution and adsorption) is thus -0.55 eV (per mole of glycine). The inclusion of the dispersion energy contribution leads to a total reaction energy of -0.78 eV. To summarize, the adsorption of glycine is favored on the hydroxylated Cr-terminated surface over the O-terminated one, in both the outer and inner sphere modes. The results suggest 2756

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Figure 9. Glycinate adsorbed in substitution to OH groups of the HY-Cr-term surface at (a) 0.25 ML, (b,c) 1 ML. The network formed by glycinate ions and OH groups is shown in (c).

Figure 10. Glycinate adsorbed in substitution to OH groups at the HY-O-term surface at 0.25 ML.

that the inner sphere adsorption may occur specifically on the Crterminated surface, where a glycinate ion substitutes a μ1-OH group, and that it is not coverage dependent. In strong contrast, outer sphere adsorption is not site-specific and is highly stabilized at high coverage through SAM formation on both surface terminations. Extrapolations to the Solid-liquid Interface. The calculations reported in the preceding paragraphs have been performed at the solid-vacuum interface at 0 K. We now try to examine how those results can be put in relationship with adsorption at the solid-liquid interface. The HY-Cr-term surface only is considered here, as for the HY-O-term surface, outer sphere (SAM) formation exclusively is predicted to occur. We first mention that, with the pZC of Cr2O3 being 7-8,86 the surface is expected to be uncharged when exposed to water, and at neutral pH, the glycine molecule is in the zwitterionic state. Therefore, even in the absence of water, the adsorbate and the surface can be considered in the same chemical state as exposed to pure water. Now, in order to bridge the gap with the adsorption at the solidliquid interface, the free energy of solvation of glycine in the zwitterionic state and of water was considered, as detailed in the Computational Details. Shortly, when glycine adsorbs in an outer sphere mode at the surface from the liquid phase, there is a loss in free energy of solvation. When glycine substitutes an OH group, the free energies of glycine transfer from the solvated state to the

Figure 11. Free interface energy (in J/m2), referred to the hydroxylated surface, of glycine adsorbed in outer sphere and inner sphere, as a function of the glycine concentration in the solution (mol/L).

adsorbed state, and water transfer from the surface to the liquid state have to be considered. The ab initio calculations show that outer sphere adsorption through SAM formation is much more exothermic than inner sphere adsorption. The question now is to evaluate whether this trend is conserved when considering the solid-liquid interface. Note that we use a rough model which neglects the solvent relaxation around the solute, mixing entropy, entropic variations of the surface before and after adsorption, as well as nonideality of the solution. Let us first consider the ΔΓ¥ calculated at infinite dilution (Tables 1 and 2). The major and very important gain in interface 2757

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Langmuir energy (-1.1 and -0.93 J/m2) is found in the case of the outer sphere SAM formation on the hydroxylated surface or on the glycinate substituted surface. The glycine-water coadsorption stabilizes the surface energy by -0.41 J/m2. The formation of one inner sphere layer and the formation of two layers (inner and outer sphere adsorption) induce a free energy lowering of 0.41 J/m2. Thus, at infinite dilution, the SAM outer sphere formation remains the most exothermic event. Again, we present here a very simple, thermodynamic model where electrostatic interaction between locally charged species that may be present in a real interface is not considered, and where no kinetics approach is performed to evaluate diffusion of glycine in water. It is also feasible to extrapolate the obtained data as a function of the glycine concentration in solution. The main difference with the previous approach is that now the standard states of glycine in the gas and liquid phases (1 atm, 298.15 K and 1 mol/L, 298.15 K, respectively) are introduced in the equations in order to gain insight into the dependence of the interface free energy with glycine concentration in the solution (see the Computational Details section and refs 79,82,83 and for explanations on the different approaches and their limits). Figure 11 reports the free interface energy (referred to the HY-Cr-term surface) of the surface with adsorbed glycine as a function of the glycine concentration in the solution. Again, outer sphere adsorption is the most stable mode in the very wide range of glycine concentration. At extremely low concentration, with the interfacial energy difference being positive, no adsorption is expected. Glycine outer sphere adsorption at low coverage, stabilized by water molecules, is then predicted to occur, and at relatively high glycine concentration, the 1 ML SAM is the most stable mode of adsorption. Caution has to be taken with the exact values of glycine concentration, as they are highly dependent on the reference states chosen. However, the semiquantitative trend obtained should be conserved. It is interesting to note that this result is in agreement with experimental data on adsorption of amino acids on stainless steel, where it is found that glycine adsorbs reversibly (thus, in an outer sphere mode) on those surfaces.33 The general tendency of self-assembly observed for glycine may be rationalized in the following way: Glycine (as other amino acids, with the exception of proline) has a positive solubility enthalpy (when the solid state is taken as the reference) and is soluble for entropic reasons.87 In contrast, the formation of a glycine plane has a negative enthalpy.69 Finally, the formation of SAM depends on the balance between loss of entropy of solvation and gain in enthalpy in self-assembly. This balance may be approximated by the difference between cohesive energy in the solid phase and free energy of solvation. To get an estimation of this difference, we used available data in literature, which are experimental ΔH values of sublimation88 and calculated free energy of solvation89 of amino acids.90 We obtained the following range for the free energy of outer sphere SAM formation: MET < ALA < GLY < HIS < LEU < CYS < VAL ≈ PHE < GLU < TYR ≈ THR < SER < ARG ≈ Asp ≈ 0 < LYS. We notice that the hydrophobic amino acids, methionine and alanine, are those for which the balance is the most favorable for SAM formation. For those, the free energy of water solvation is the less negative. At the opposite of the ranking, we find the hydrophilic amino acids arginine, aspartic acid, and lysine, which gain more entropic energy in solvating than enthalpic energy in self-assembly.

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’ CONCLUSION The adsorption of glycine in the outer sphere and inner sphere modes on two types of hydroxylated Cr2O3 surfaces was considered. The hydroxylated Cr-terminated surface, which presents a diversity of monocoordinated (H)OH groups of different acidic character to the surface, is more reactive toward glycine than the hydroxylated O-terminated surface covered by one type of OH groups. The formation of a monolayer of zwitterionic glycine above the surface leads to a significant stabilization due to the formation of H bonds between glycine molecules in the SAM. It is calculated to occur on both hydroxylated surfaces, thus being non-site-selective. It was also found that glycine and water coadsorption significantly stabilizes the zwitterion conformer at the hydroxylated surface. The substitution of an OH group by a glycinate anion leading to an inner sphere adduct was also considered. This type of adsorption appears to be site-selective, as it is thermodynamically favored on the Cr-terminated surface and not on the O-terminated one. Extrapolation to the solid-liquid interface confirms that outer sphere is favored over inner sphere adsorption, in the form of zwitterionic glycine coadsorbed with water at low glycine concentration and SAMs formation at increasing glycine concentration in solution. Finally, an extrapolation to other amino acids was performed and suggests that hydrophobic AA have a propensity to self-assemble on the surface, whereas hydrophilic AA tend to be solubilized. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Corresponding author. 33 1 44 27 25 25; 33 1 46 34 07 53; [email protected].

’ ACKNOWLEDGMENT The national calculation centers IDRIS and CINES, and the local mesocentre CCRE are acknowledged for computational time attribution. ’ REFERENCES (1) Gray, J. J. Curr. Opin. Struct. Biol. 2004, 14, 110. (2) Brown, G. E.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gratzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F..; Zachara, J. M. Chem. Rev. 1999, 99, 77. (3) Compere, C.; Bellon-Fontaine, M. N.; Bertrand, P.; Costa, D.; Marcus, P.; Poleunis, P.; Pradier, C. M.; Rondot, B.; Walls, M. G. Biofouling 2001, 17, 129. (4) Lin, H. Y.; Bumgardner, J. D. Biomaterials 2004, 25, 1233. (5) Hazen, R. M.; Sverjensky, D. A. Cold Spring Harbour Perspect. Biol. 2010, 2, a002162. (6) Heinz, H.; Farmer, B. L.; Pandey, R. B.; Slocik, J. M.; Patnaik, S. S.; Pachter, R.; Naik, R. R. J. Am. Chem. Soc. 2009, 131, 9704. (7) Ernst, K. H. Top. Curr. Chem. 2006, 265, 209. (8) Lambert, J. F. Orig. Life Evol. Biosph. 2008, 38, 211. 2758

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Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B. Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian Inc.: Wallingford, CT, 2004. (80) Stefanovich, E. V.; Truong, T. N. Chem. Phys. Lett. 1995, 244, 65. (81) Freccero, M.; Di Valentin, C.; Sarzi-Amade, M. J. Am. Chem. Soc. 2003, 125, 3544. (82) Chiche, D.; Chizallet, C.; Durupthy, O.; Chaneac, C.; Revel, R.; Raybaud, P.; Jolivet, J. P. Phys. Chem. Chem. Phys. 2009, 11, 11310. (83) Bouzoubaa, A.; Costa, D.; Diawara, B.; Audiffren, N.; Marcus, P. Corros. Sci. 2010. (84) Ma, H.; Berthier, Y.; Marcus, P. Corr. Sci. 2002, 44, 171. (85) Folliet, N.; et al. , in preparation (86) Kittaka, S. J. Colloid Interface Sci. 1974, 48, 327. (87) Palecz, B.; Piekarski, H.; Romanowskib, S. J. Mol. Liq. 2000, 84, 279. (88) Gaffney, J. S.; Pierce, R. C.; Friedman, L. J. Am. Chem. Soc. 1977, 99, 4293. (89) Wolfenden, R.; Andersson, L.; Cullis, P. M.; Southgate, C. C. B. Biochemistry 1981, 20, 849. (90) To this end, we considered the following equation: Zliq þ Surf f Surf-ZSA. The free energy of reaction is ΔGSA = GSA - GZliqGsurf = GSA - GGgas(300K)-Gsurf þ GGgas(300K) - GZliq. We note that GSA - GGgas(300K)-Gsurf = ΔHSA-gas-TSSAþ TSgas (300K) þ TSsurf. We now consider that the variations of entropy from the surfaces of the solid phase are negligible, so ΔGSA = ΔHSA-gas þ TΔSgas (300K)-ΔGsolv (300K), where ΔGsolv (300K) = GZliqGGgas(300K). We now approximate that ΔHSA-gas = -ΔHSubl (at sublimation temperature), so ΔGSA = -ΔHSubl-ΔGsolv (300K) þ TΔSgas (300K) = -ΔHSubl-ΔGsolv, where ΔGsolv is the free energy of solvation from the gas phase at 0K to the liquid phase in standard conditions (see section computational details). The ΔGsolv values were calculated in adding to the calculated value for glycine solvation the solubilities of side chains given in ref 89.

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dx.doi.org/10.1021/la104317j |Langmuir 2011, 27, 2747–2760