Journey toward the Surface: How Glycine Adsorbs on Titania in

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Journey Towards the Surface: How Glycine Adsorbs on Titania in Water Solution Cui Li, Susanna Monti, and Vincenzo Carravetta J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3060729 • Publication Date (Web): 02 Aug 2012 Downloaded from http://pubs.acs.org on August 6, 2012

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The Journal of Physical Chemistry

Journey Towards the Surface: How Glycine Adsorbs on Titania in Water Solution Cui Li,† Susanna Monti,‡ and Vincenzo Carravetta∗,† CNR - Institute of Chemical and Physical Processes, Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy, and CNR - Institute of Chemistry of Organometallic Compounds, Area della Ricerca, via G. Moruzzi 1, I-56124 Pisa, Italy E-mail: [email protected]

∗ To

whom correspondence should be addressed UOS di Pisa ‡ CNR-ICCOM, UOS di Pisa † CNR-IPCF,

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Abstract The adsorption of glycine (Gly) on titania in water solution and its preferred binding modes are studied by means of classical reactive (ReaxFF) and non-reactive molecular dynamics simulations. A small cluster made of a few glycine molecules, surrounded by water molecules, is placed close to the TiO2 (110) surface and its initial motion towards the substrate is described through classical reactive dynamics. Glycine appears to be less easily and strongly adsorbed on the surface in solution than in the gas phase due to its competition with the surrounding water molecules. Indeed, in line with the experimental observations, the preferential binding mode of glycine in solution is found to be a monodentate coordination of one of its carboxyl oxygens to a Ti surface site. The potential of mean constraint force (PMF) method, combined with the classical non-reactive molecular dynamics simulations, was used to calculate the change in free energy upon glycine adsorption and the results obtained through exhaustive sampling confirm the reactive dynamics view.

Keywords: glycine, titanium dioxide, surface adsorption, molecular dynamics, reactive force fields.

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Introduction Surface decoration techniques which improve the conformational characteristics, mechanical and physicochemical properties of inorganic materials are the subject of extensive experimental and theoretical investigations in different areas including optoelectronics, microfluidics, mechanical engineering, pharmacokinetics, medicinal chemistry, surgical implant design, etc. 1–7 From a biomedical point of view the adsorption of biomolecules such as proteins, peptides and their constituent units on metal and metal oxide substrates has shown to be an effective way of increasing the biocompatibility of the implanted materials and controlling the subsequent biological reactions. 8–27 Titanium-based compounds are especially promising and are one of the most stable, non toxic and easily available polyfunctional substances known today. Together with their advantageous bulk and surface properties, they have a relatively low modulus of elasticity, high strength-to-weight ratio and an excellent resistance to corrosion. As far as their distinctive biocompatibility is concerned, it has been demonstrated that it is mainly ascribable to the strongly adherent oxide film which passivates the metal in presence of water and this could be further improved through surface functionalization procedures. Indeed, bio-nanopatterning has allowed the building of supramolecular self-organized structures with specific properties and the formation of preadsorbed protective layers on highly reactive interfaces. 28–32 In order to interpret and predict the properties of these composite materials, suggest appropriate design strategies, define well-balanced molecular combinations and effective synthetic routes to produce systems with outstanding performance, it is fundamental to understand and unravel the complex interactions and reaction mechanisms between the macromolecules and the substrates. The limited ability of experimental techniques to provide sufficient detailed characterization at the atomic level of such interfaces makes theoretical methods, and in particular molecular modeling and simulations, a complementary approach specifically suited to investigate the complex molecular mechanisms occurring during systems formation and response. All-atom molecular dynamics (MD) methods are able to capture specific aspects of group interactions and to elucidate the role played by each component of the system, solvent medium and external perturbing sources included. As the building blocks of biomolecules, amino acids are well suited to be interacting 3



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probes. Gly, which is the simplest amino acid residue, is the most appropriate choice to start these investigations. A large amount of experimental and theoretical data on the structure, adsorption and self-assembly of amino acids on titanium oxide surfaces is available in the literature. In the majority of the cases it has been observed that, in the gas phase, the binding of the amino acids to the substrates is realized through a mono- or bidentate coordination of the deprotonated carboxyl group to a Ti cation of the top layer. 4,33 On the contrary, in water solution the approaching path towards the interface seems not so straightforward and it is often hindered by the presence of the hydration layers surrounding the molecule and covering the substrate. Experimental investigations have revealed that the adsorption in solution is weaker and an acidic side chain is often necessary in order to create a stable and ordered adsorbate layer. 33 Roddick-Lanzilotta and McQuillan 34 observed that the coordination of the carboxyl group was not established when the molecules had a single carboxylate group and thus suggested that both glycine and glutamine did not adsorb directly onto TiO2 from aqueous solution. The present investigation is an extension of an earlier work 35 where the adsorption of glycine molecules in gas phase on rutile TiO2 (110) was studied by standard classical all-atom MD simulations and reactive dynamics. The simulations revealed that the adsorbed molecules have the tendency to interact with each other and with above layers but, on the whole, are stably adsorbed and quite still at the interface. Outer amino acid layers were, instead, more mobile but unable to reach the surface. The settled molecules had a reasonable propensity to preserve their orientations with the carboxyl group providing the main anchoring point to the substrate. This adsorption mode was sometimes reinforced by other strong coordinations and weaker hydrogen bonding interactions. Here, the methodology developed in the previous study 35 is used to explore dynamics, reactivity and the conformational property of glycine along its approaching path towards the TiO2 (110) surface in water solution. Gly-TiO2 binding is estimated by means of the potential of mean constraint force (PMF) approach and the results are compared with the in vacuo adsorption profile. PMF calculations together with other methodologies have been already used to estimate the adsorption free energy of different types of molecules on various substrates. 36–43 However, the PMF

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approach has proven to give a more efficient sampling of the configurations corresponding to the high-energy region of the NVT ensemble (when the molecule and the surface are held very close together) and, as a consequence, it should provide a more accurate estimate of the binding energy at distances that are not well-sampled during standard unconstrained simulations. A procedure similar to the one adopted in Refs., 40,42,43 has been followed in order to obtain the binding trend of glycine as a function of the distance from the rutile surface. As far as reactivity is concerned, a new ReaxFF 44 development, aimed at combining the description of solid inorganic supports with biomolecules, is used in this study to simulate glycine adsorption, investigate possible inter-glycine proton exchange mechanisms and analyze glycine-surface interactions. The reactive dynamics is applied to a small cluster made of sixteen molecules placed in water solution near the titanium oxide surface.

Computational Details Reactive Dynamics: System Setup and Simulation Procedure The starting configuration of the reactive dynamics simulations was extracted from a classical non-reactive trajectory in the gas phase where a nano-droplet of glycine molecules was placed close to the surface in order to simulate the adsorption process. 35 The original droplet consisted of 182 glycine molecules but only those in proximity to the interface were extracted and used in the present reactive dynamics runs. The selected cluster was made of 16 molecules with mixed protonation states (neutral terminus regions and zwitterionic forms). The titanium dioxide surface was simulated by a slab of rutile(110), which consisted of five layers with the frozen bulk geometry, whose size was about 37 and 35 Å along the x and y directions, respectively. Three dimensional periodic boundary conditions were applied with the z axis lying along a direction normal to the slab. The height of the simulation box was fixed at 40 Å. The system was solvated with 653 water molecules and the time step was set to 0.25 fs. 45 ReaxFF was parametrized against data obtained by quantum mechanical calculations (namely equilibrium geometries, reaction energies, bulk properties, etc.) on a training set including a huge 5



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variety of chemical species related to the system under study. The electronegativity equalization method (EEM) 46,47 was used to calculate partial atomic charges and all the reactive simulations were performed at T=310 K in the NV T ensemble using the Berendsen thermostat 48 with a relaxation constant of 0.1 ps. In order to remove bad contacts and to equilibrate the systems, molecular dynamics simulations at T=10 and 310 were carried out in the NV T ensemble for 25 and 310 ps, respectively. Then, the production phase was carried out starting from the last equilibrated configuration and system geometries were sampled every 0.1 ps during the subsequent 615 ps. Short MD runs of 10 ps at 400 K were periodically (once every 100 ps) included in the production phase in order to randomize the position of the solute and explore different regions of the potential energy surface (PES). A new version of ReaxFF incorporated into the Amsterdam Density Functional (ADF) 49 program was used for all the reactive simulations.

Non-Reactive Molecular Dynamics Simulations Classical molecular dynamics simulations of the adsorption of glycine on TiO2 (110) were performed adopting the same model of the slab already used for the reactive simulations. Only one glycine molecule was inserted in the simulation box, with height 40 Å, which was filled with 1134 water molecules. Glycine was described by means of the AMBER force field 50,51 whereas the titania surface was depicted through the model developed by P˘redota and co-workers, 52 which has been already employed in previous works. 16,26,40 The whole slab was frozen during all the MD (non-reactive) runs, which were carried out in the NV T ensemble at T=310 K. This temperature was maintained by means of the Nose-Hoover thermostat 53,54 and long-ranged electrostatic interactions were handled through the particle-mesh Ewald method 55 with a real space cutoff of 12 Å. The equations of motion were integrated using a time step of 1 fs, and the system configurations were saved every 500 steps (0.5 ps). Water was described through the modified TIP3P force-field 26,40,56–58 and its density was adjusted to the bulk value in a series of NV T runs followed by energy minimizations. 26,40 All simulations were performed with the GROMACS 4.5 code. 59

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The free energy adsorption profile for glycine was calculated using the PMF approach. The distance from the center of mass of the two oxygen atoms of the carboxyl group and the first layer of Ti atoms 36,40,43 was chosen as the reaction coordinate. Notwithstanding the constraint, the molecule could move freely in the plane parallel to the interface, and explore different regions of the rutile (110) layer. The bulk region of the solvent, identified by a distance of 16 Å from the top layer of the slab (containing the five-fold titanium atoms), was chosen as the zero for the integration and the error due to the constraint force was calculated by means of the block-averaging method available in GROMACS. 60 Around 80 distances were considered in each case and the whole configurational space was sampled, through constrained MD simulations, for about 280 ns (3.5 ns per position with the first 2 ns used for equilibration). The 80 distances were selected by means of a steered molecular dynamic (SMD) approach. SMD is an extended molecular dynamics simulation performed by adding an external force on part of the system to mimic, in a relatively short time, an adsorption/desorption process. It is used, for instance, for exploring biological processes, such as unbinding of ligands and conformational changes in biomacromolecules, which are currently difficult to be reached by conventional MD simulations. Starting from an adsorbate with both oxygens on top of two Ti atoms an external force with a force constant of 500 kJ·mol−1 · nm−2 was applied to the molecule to accelerate its desorption from the substrate. The pulling force rate and the time step were set to 0.01 Å·ps−1 and 1 fs, respectively, in order to have a relatively slow desorption and thus a trajectory with a rather large number of distances for the following umbrella sampling.

Simulation analysis The minimum distance of oxygen atoms of the carboxyl group and water oxygen, hydrogen and nitrogen atoms from the surface was monitored during the whole production phase analyzing both its evolution and distribution. Water and glycine distributions and positions were mapped by means of density profiles as a function of the distance (z) from the plane of Ti atoms of the first layer and through two-dimensional (2D) contour plots in the xy planes. In order to obtain the 2D contour plots average number density maps (NDMs) were computed on slices through the simulation box considering a displacement of each horizontal layer by 0.5 Å in z direction. Then, these data were

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projected from the entire simulation volume onto the xy plane. The violet, blue, light blue, cyan and green areas in Figure 1 indicate regions with increasing density (from 5 to 45 times the bulk value). The different components of the solute, for which the NDMs are computed separately, include the terminal carboxyl oxygens group, the nitrogen atom, and water oxygen and hydrogen atoms. The atomic density profiles in the direction perpendicular to the surface were calculated by averaging over 6000 configurations (corresponding to the last 600 ps of the production run).

Results and Discussion Reactive Dynamics Adsorption of Water: molecular and partially dissociated species Considering that surface solvation can affect the adsorption and reaction mechanisms of the other species present in solution it is interesting, as a first step, to analyze the behavior of the first and second water layer throughout the course of the reactive dynamics runs. During the past few years, the adsorption of water on rutile(110) was investigated exhaustively 4 and the majority of the latest studies concluded that a partially dissociated structure is favored over a completely intact adsorption. 61–64 The picture portrayed in this work is in line with these findings. Indeed, in the present simulations, water adsorbed to and desorbed from the surface continuously but remained close to the slab forming a quite stable solvation network on the (110) face. The adsorbed water molecules were found to adopt a geometry with the molecular plane perpendicular to the surface and with the O atom sitting on top of a five-fold Ti atom, at an approximate distance of 2 Å. This is evident in both the minimum distance plot shown in Figure 2, where water oxygens close to Ti are represented by red filled circles, and in the distribution histogram of 3b (cyan peaks). Such arrangement seems to correspond to the strongest bound state whereas other weaker adsorption positions are those with water molecules hydrogen bonded to the bridging oxygens of the (110) surface. In this case, the observed long water stripes have an improved binding orientation due to the additional hydrogen bonding interactions with the in plane oxygens of the layer ( 2 - blue filled triangles). 8


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The complexity of the water adsorption picture is further enriched by the presence of dissociated molecules with OH− groups adsorbed on top of Ti atoms and the corresponding protons (H+ ) attached to the closer bridging oxygens. This situation is clearly depicted in Figure 2 where the water-hydrogen (Hw) atoms closest to the bridging oxygen (Oout ) atoms (green filled circles) have a distance which oscillates around 0.88 Å, whereas the proton transfer to the in plane oxygens appears less probable, as confirmed by the Hw· · ·Osur f minimum distance, whose distribution is centered at about 1.8 Å (see 3). Examination of the present reactive simulation results reveals that a partial fragmentation of the adsorbed water molecules disrupts the local order observed in earlier investigations where nonreactive simulations were carried out and favors a water clustering which is characterized by a higher degree of mobility. Even though the exact value of the OH− /H2 O ratio has not been determined, it has been found 65 that partially ordered solvation layers can be formed for quite a range of OH− /H2 O ratios. The most stable structure, identified in the present simulations, turned out to be the one with a coverage of about 54% not dissociated waters and around 40 % fragmentation groups. The almost complete hydrogen bonding network between adsorbed species and their interaction with the bridging oxygens determine the formation of water rows which prevent access to outer waters. As a consequence the degree of protonation of the in plane oxygens is low (16%). The distribution of solvent on the slab was also characterized through atomic density maps in the planes parallel to the interface ( 1a,b). The map displayed in Figure 1a shows that water oxygens are distributed all over the surface but appear preferentially located above the vacant Ti sites and near the bridging oxygens. On the one hand, this depiction is compatible with molecular adsorption and formation of a hydrogen bonding network which keeps together the molecularly adsorbed waters and connect them to the bridging oxygens. On the other hand, the preferential position of the water oxygens is also in line with a dissociative adsorption, that is the positioning of OH− groups on top of the 5-fold Ti atoms and the transfer of the hydrogen to the nearby bridging oxygens. Indeed, both phenomena have been observed in the present reactive MD simulations and surface solvation turns out to be a balanced mixture of molecular and dissociative adsorption. Passing to the water hydrogen location and dynamics, it can be noticed that the other two-dimensional

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density map displayed in Figure 1b provides further evidence of a high degree of protonation of the inorganic substrate and at the same time shows the tendency of the dissociated water hydrogens to bind the bridging oxygens and then explore different orientations in response to the solute/solvent perturbation. The light color of the areas spanned by Hw support the view that these atoms are quite strongly adsorbed but in a few cases they can be exchanged between the neighboring sites as suggested by the cyan extended areas which connect the green patches. The presence of glycine on the surface is manifested by the violet depressed area visible in Figure 1a but also present in Figure 1b as a wider blue patch. This region is characterized by a lower water content which is evidently replaced by the various glycine moieties. The discussion regarding glycine adsorption is postponed to the next section. To summarize the results of this first preliminary analysis, it seems apparent that during the reactive dynamics the water dissociation process is favored by the perturbation of the lateral interactions between the molecules, as already found in our previous quantum MD simulations for the water/TiO2 (110) system, 64 and that the reaction mechanism is mainly characterized by the direct proton transfer between water and bridging oxygens. This is responsible for the formation of terminal hydroxyls which are then stabilized by neighboring water molecules. The coexistence of differently organized self-assembled water molecules in close contact with the surface appears to promote the establishment of a network of hydrogen bonds which connects waters and surface atoms. This determines a significant reduction in the orientational mobility of the first adsorbed water layer relative to bulk water and the interface appears well covered by the solvent. Adsorption of the Glycine Cluster Before commenting the results regarding glycine adsorption onto the surface, it should be reminded that amino acids exist in two different dominant forms: canonical or neutral in the gas phase and zwitterionic (GZW) in solution. 66–69 Theoretical studies found that these two protonation states are nearly isoenergetic when the two species are surrounded by water molecules. This implies that a large number of different low-energy structures can be identified and a statistical treatment is necessary to obtain a meaningful description of the system. The initial composition of the glycine

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cluster, defined for these simulations, consisted of a mixture of canonical and zwitterionic conformations with a net prevalence of the neutral form (88%). As already stated in a previous work, 70 where the development of the reactive force field for glycine was described, the intermolecular proton-transfer mechanism may occur through mediating water molecules which capture the COOH proton and then relay it to the NH2 moiety. These transfers are difficult to realize because of the proper conditions which need to be met (for example, the intervening water molecules should be precisely aligned to the transferable protons, to the amine and the carboxyl groups) and could not necessarily occur within a relatively short simulation time. It is not the aim of the present work to investigate the exact equilibrium concentration of the various species but only to provide a diversity of structures which could differently interact with each other and with the substrate. Notwithstanding this, the employed reactive force field was parametrized 70 to reproduce both glycine −→ GZW and GZW −→ glycine conversion events in the gas phase as well as in solution and should be able, in principle, to simulate them in other complex environments such as the one studied in this work. Indeed, this was the case: at the end of the present reactive simulations the initial content of canonical glycine (88%) is reduced to half its value (44%) and the proton exchange mechanisms induce the formation of glycine zwitterions (from 12% to 56%). Even though the sampling time was relatively short and not all the molecules were able to reach the inner portion of the surface, that is the plane containing Ti atoms, and penetrate the strong structured water layer assembled on top of the bridging oxygens, it was sufficiently long to give a realistic picture of the glycine adsorption process in agreement with experiment. 34 From the inspection of the z-density profiles of Figure 4 it is evident that glycine hardly adsorbs very close to the interface. Indeed, the weak density peaks for both carboxyl oxygen and nitrogen atoms in the region close to the bridging oxygens suggest that high energy barriers should be overcome to push the molecule directly onto the interface displacing the already adsorbed water molecules. On the contrary, glycine layers far from the substrate are more well-defined than those observed for pure water, suggesting that the glycine molecules have the tendency to self-interact and form cluster structures in the bulk. This, according to other studies, 15 weakens glycine-surface interactions and reduces the adhesion of the molecular layer. However, the minimum distance evo-

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lution plots together with their corresponding distributions, displayed in Figure 5 and Figure 3 respectively, support the view that a weak adsorption could take place and that the most probable configuration of the adsorbed glycine is the one where the carboxyl oxygen is coordinated to a five-fold titanium site (in line with the z-density profile in Figure 4). On the contrary, the terminus nitrogen is oriented in such a way that effective hydrogen bonding interactions with the bridging oxygens are established. Results in solution compare fairly well with the data obtained for the adsorption of glycine in gas phase on TiO2 (110). 35 The average distances of carboxyl oxygens and nitrogens from Ti in solution are found to be 5.6±1.5 and 4.9±2.5 Å with minima at 2.2 and 2.5 Å, respectively. The first data (2.2 Å) is in perfect agreement with the in vacuo findings (2.1 Å) and suggests coordination of glycine oxygen, whereas the N· · ·Ti minimum distance (2.5 Å) is slightly longer than the gas phase value (2.3 Å which implies direct coordination of N to Ti) and supports the view of hydrogen bond formation. According to the trend observed in Figure 5 the glycine molecules do not show a propensity to release the carboxyl proton to the surface but just engage hydrogen bonding interactions with each other or with the adsorbed water molecules. The inter- or intra-glycine hydrogen transfer from the carboxyl group to the terminal NH2 moiety is confirmed by the trend of the minimum distances in a short time span centered at about 300 and 600 ps (Figure 5). The two-dimensional density plots, shown in Figure 1c,d, can be used to identify the main location of glycine oxygens and nitrogens in relation to the surface during the dynamics. Indeed, it appears that the glycine molecules, which at the beginning of the simulations are placed relative close to the interface, but are separated from it by the adsorbed thin water layer, have the tendency to remain confined to that region (depressed area in the water maps) surrounded by the solvent for the whole simulation time. As it appears from the extension and color of the areas spanned by this atom in Figure 1c the interaction is quite strong and seems to persist till the end of the simulations. The analysis of the final configuration in Figure 6b, shows that a clustering arrangement is preserved. However, during the dynamics the action of water and the thermal motion of the molecules lead to reduction of the compactness of the cluster which is manifested in the broader distribution of monomers. Indeed, 65% of the intermolecular hydrogen bonds are lost and a few molecules get

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closer to the interface (about 25%). Nonetheless, only one direct contact (corresponding to 6% of the units) is established between a carboxyl oxygen and a Ti surface site in the limited time of the simulation.

Non-Reactive Dynamics Both unconstrained and constrained classical all-atom non-reactive molecular dynamics simulations have been used to explore the adsorption behavior of a single glycine molecule in water solution. In the first case the molecule could move freely inside the simulation box and explore both the interfacial and the bulk solvent regions depending on its thermal motion, whereas in the second case the z coordinate of the center of mass of the carboxyl oxygen atoms was constrained and the motion of the molecules for different z values was explored in a series of long-lasting MD runs. The SMD calculation employed to generate the different initial geometries for the constrained MD shows (see Figure 7), that the distances of the two carboxyl oxygen atoms from the surface undergo, under the effect of the external force, a sudden variation that reveals the presence of high potential barriers for the detachment (attachment) of the adsorbate. The data obtained by the constrained MD runs were then used for building the binding free energy profile of glycine to the substrate. In these last calculations the solvation of the interface consists of molecularly adsorbed waters. However, as already proven, 40 this description produced results in satisfactory agreement with high level calculations and experimental data and, as a consequence, could be used confidently to estimate the free energy of binding of glycine to the rutile slab. The data obtained by means of the unconstrained 20-ns MD runs will not be commented in great detail because they are in perfect agreement with the reactive picture as far as the adsorption behavior of glycine is concerned. However, the solute was found, most of the time, in the bulk solvent region quite far from the surface while positions closer to the surface were characterized by the insertion of glycine in the first adsorbed water layer.

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Glycine-Surface Binding The adsorption profile in Figure 8 shows three distinct minima (numbered sequentially starting from the one closest to the substrate), which suggest definite binding regions comparable to the ones observed in the case of water. 40 Considering that the molecule is flexible and possesses different potential "binding" groups, the reaction coordinate has been chosen as the separation between the center of mass of the carboxyl oxygen atoms and the plane of Ti atoms in the first layer (on the basis of the preferential interactions observed for amino acid binding 40 ). A direct comparison with the adsorption of water cannot be made but similarities and differences could be evinced by the general trend and by the position of the molecule. Indeed, the minima are located at distances of 2.8, 3.9 and 6.6 Å and correspond to binding energies of about 2.9, 4.8 and 0.9 kcal/mol, respectively. Minimum two is the lowest in free energy and corresponds to the coordination of one of the carboxyl oxygens to a Ti atom of the surface. Instead the other oxygen is not involved in surface binding and points toward the bulk region of the solvent. This arrangement of the carboxyl group is accompanied by a reorientation of the NH3 + terminus, which, in order to improve the binding of the adsorbate, engages hydrogen bonding interactions with the neighboring bridging oxygens (Figure 9b). Inspection of the trajectory corresponding to this z-position ( 3.9 Å )reveals that the glycine molecule does not display lateral mobility, but retains some rotational freedom which is used to readjust its moieties for finding the most favorable interaction. By this adsorption mode only one water is displaced by its location, that is the one coordinated to the titanium site, whereas the other waters, hydrogen bonded to the bridging oxygens, remain there and become involved in hydrogen bonding interactions with the NH3 + adsorbed moiety. Minimum one corresponds to a bridging bidentate coordination of the carboxyl group, with the two oxygens coordinated to two 5-fold Ti sites as shown in Figure 9a. The carboxylate plane is aligned along the [001] direction, resulting in displacement of two water molecules belonging to the first solvation layer. This orientation is preserved throughout the whole trajectory, and no lateral mobility of the adsorbate is observed. In line with previous findings 40 it could be speculated that both enthalpic and entropic changes upon glycine adsorption in the first water layer are

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perhaps not favorable, due to the replacement of two coordinated waters with a single carboxyl group and to the rigidity of the system which leads to an overall free energy change close to zero (relative to the glycine molecule in the solvent bulk). The third minimum occurs farther from the surface and thus no water layer is displaced to accommodate the adsorbate. Visual inspection of the trajectories identifies the preferential location of the molecule as the one where glycine is above the surface bridging oxygens, with the hydrogens of the NH3 + group pointing down toward the oxygens in an effort to engage hydrogen bonds with those sites (Figure 4). However, considering the distance, the minimum could be classified as an indirect binding interaction which is mediated by the adsorbed waters. The carboxyl group is vertically aligned, rotated towards the solvent and does not seem engaged in hydrogen bonding interactions with the water molecules hydrogen bonded to the bridging oxygens. The adsorbate is quite mobile, changes frequently its orientation and the interaction with the layer is weak. The location of the minima is in agreement with the position of the peaks in the z-density profile shown in Figure 4 and also with the sudden distance changes observed in Figure 7 . As the molecule moves towards the interface from the bulk region of the solvent, it has to overcome two successive barriers to get close to the top layer of the slab. The first barrier is quite low (about 1.8 kcal/mol) and thus could be easily surmounted at ambient temperature, whereas to pass from the second to the first region thermal activation is required due to the high energy barrier of about 11 kcal/mol. However, the reverse path, that is when the molecule moves from the surface towards the bulk, is not straightforward as well because the encountered barriers, which are about 9 and 6 kcal/mol, respectively, are sufficiently high to keep the glycine molecules dynamically trapped in the two first adsorption zones.

Conclusions Reactive dynamics simulations in water solution have been performed to explore the tendency of glycine to establish direct contacts with a titanium dioxide substrate. Both canonical/neutral and zwitterionic glycine molecules were inserted in the simulation box at the beginning of the calculations; they were arranged in a small ellipsoidal cluster with all the molecules interconnected by a 15



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dense network of intermolecular hydrogen bonds. The evolution of the cluster structure, the nature of its constituent units and their location in relation to the substrate were monitored as a function of the simulation time. The final configuration of the system presents a broader distribution of the glycine molecules: half of the neutral species change to zwitterions and 25% of the molecules are found closer to the interface. However, only 6% of the glycines engage direct interactions with the Ti sites coordinating one of the carboxyl oxygens, whereas the other glycines interact indirectly with the surface through the adherent water layer. Differently from the classical non reactive dynamics, where the surface solvation is described as due to water sitting on top of 5-fold Ti atoms and hydrogen bonding the bridging oxygens, the reactive dynamics depicts a more complex scenario where dissociation takes place and the final surface solvation model results in a balanced mixture of molecular and dissociative water adsorption, in agreement with the most extended and accurate quantum MD simulations. 64 In the gas phase, as carboxylic acids, glycine bound dissociatively in a bridging mode configuration, whereas in solution monodentate coordination accompanied by the binding interaction of the positively charged N-terminus group with the bridging oxygen was preferred. PMF free energy calculations were carried out to quantify the adsorption strength of the molecule. The results indicated that glycine had favorable free energy of binding and stable contacts could be established through the carboxyl group but also mediated adsorptions involving the first two solvent layers at the interface were observed. Even though the classical non reactive description of the substrate was a rigid model, based on previous investigations, it was able to reproduce the characteristics of the interfacial water structuring and the trend of the binding free energy of the molecule in satisfactory agreement with previous theoretical studies and experimental observations. This behavior agreed satisfactorily with the reactive dynamics data.

Acknowledgments S.M. thanks the CNR (Short Term Mobility Program 2011) for granting her visit to the group of Adri C. T. van Duin at the Department of Mechanical and Nuclear Engineering of the Pennsylvania State University. C.L. and V.C. acknowledge the contribution of the " Ministero della Istruzione

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della Universita’ e della Ricerca " of the Republic of Italy.

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a

b

c

d

Figure 1: Two-dimensional density maps from the ReaxFF simulation. Water Oxygen (a), Water Hydrogen (b), glycine Carboxyl Oxygens (c) and glycine Nitrogen (d). Denser regions: green, cyan. Density levels: 5, 15, 25, 35, 45 correspond to violet, blue, light blue, cyan, green. The ACS Paragon Plus Environment atoms of the surface are rendered through gray (Ti) and red circles (O, filled circles represent bridging oxygens).

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Minimum Distance to the Surface (Å)


Ow-Ti Hw-Osurf Hw-Oout

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Residue Number

Figure 2: Minimum distances of the water atoms from the atoms of the surface. ACS Paragon Plus Environment

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400

a

H (NH_Gly) HW N (Gly)

Count

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0.5

1.0

1.5 2.0 2.5 X-Oout distance (Å)

3.0

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b

N (Gly) O (Gly) OW

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200

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2.0

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c

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H (NH_Gly) N (Gly) HW

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2.0

2.5

3.0 3.5 4.0 X-Osurf distance (Å)

4.5

5.0

FigureACS 3: Minimum distance distributions. Paragon Plus Environment


200 Ti Oout Ow Hw O(carboxyl) N

150 3

Density (kg/m )

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100

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0 28

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40

z (Å)

Figure 4: Density profiles asACS a function ofPlus the zEnvironment coordinate (absolute reference system). Paragon

4.5 Minimum Distance From the Surface (Å)

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Nitrogen Carboxyl Oxygens

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Time (ps) 4.5 from the surface from GLY Nitrogen

4.0 Minimum Distance of H(COOH) (Å)

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3.5 3.0 2.5 2.0 1.5 1.0 0.5 0

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Time (ps)

Figure 5: Top: evolution of the minimum distance glycine oxygen and nitrogen atoms from the ACS Paragon PlusofEnvironment interface. Bottom: minimum distance of the hydrogen atom of the carboxyl group from the surface (black dots) and from glycine nitrogen atoms (blue dots).


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a

b

Figure 6: Initial (a) and final (b) configurations extracted from the reactive dynamics. The slab is depicted by means of pink (Ti) and red (O) balls. Glycines are represented trough stick models and water (in its molecular and dissociated forms) by means of thin sticks and dots. In (b) a glycine ACS Paragon monodentate adsorption is highlighted by greenPlus linesEnvironment and the O-Ti distance is reported.

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Figure 7: SMD results: carboxyl oxygen atoms distances from the surface vs simulation time ACS Paragon Plus Environment


8 6

PMF free energy (kcal/mol)

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4 2 0 -2 -4 4

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16

carboxyl CM - surface distance (Å)

Figure 8: Potential ofPlus Mean Constrained Force ACS Paragon Environment

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a

b

c

Figure 9: Minimum Free Energy Configurations ACS Paragon Plus Environment


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TOC graphic


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Journey toward the Surface: How Glycine Adsorbs on Titania in

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