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Neutral vs Zwitterionic Glycine Forms at the Water/Silica Interface: Structure, Energies, and Vibrational Features from B3LYP Periodic Simulations Albert Rimola, Bartolomeo Civalleri, and Piero Ugliengo* Dipartimento di Chimica IFM and NIS (Nanostructured Interfaces and Surfaces), Centre of Excellence and INSTM (Materials Science and Technology) National Consortium, UniVersita` di Torino, Via P. Giuria 7, 10125 Torino, Italy ReceiVed September 6, 2008. ReVised Manuscript ReceiVed September 30, 2008 B3LYP periodic calculations with a triple-ζ-polarized Gaussian basis set have been used to study adsorption of glycine on a hydroxylated silica surface (2.2 OH/nm2) model derived from the (001) surface of edingtonite. The simulation envisages glycine adsorbed either as a gas-phase molecule or when microsolvated by up to five H2O molecules. Both neutral and zwitterionic forms of glycine have been considered and their structural, energetic, and spectroscopic vibrational features compared internally and with experiments. As a gas phase glycine sticks in its neutral form at the silica surface, the zwitterion being highly unstable and with transition-state character. When glycine is microsolvated at the silica interface, two H2O molecules render the zwitterion population comparable to that of the neutral form whereas with four H2O molecules the neutral glycine population is wiped out in favor of the zwitterion. With four H2O molecules the most stable structure shows no direct contact between glycine and the silica surface, H2O acting as a mediator via H-bond interactions. The B3LYP energies and structural data were also supported by comparing the scaled harmonic vibrational features with literature FTIR data of glycine adsorbed on an amorphous silica surface either from the gas phase or in water solution.
Introduction Understanding the contact between relevant biochemical molecules and inorganic materials is important to make progress in several fields such as bionanotechnology,1-3 proteomics,4-8 and prebiotic chemistry.9,10 An open and a very important subject is the interaction of amino acids, peptides, and proteins with hydrophilic/hydrophobic surfaces. In this context and in order to understand the behavior of large biomolecules interacting with solid surfaces it is of interest to simulate the specific contribution of the various constituting components. Among the most important biocompatible materials, the silicabased ones are good candidates to study interactions with biochemical molecules because of their large abundance in nature. Calcium apatites also enjoy a central position since they constitute the major inorganic component in bone and teeth enamel. Accordingly, different studies related to the aforementioned fields are reported in the literature: biomineralization processes involving the presence of amino acids and peptides,11-13 * To whom correspondence should be addressed. E-mail: piero.ugliengo@ unito.it. (1) Brown, S. Nat. Biotechnol. 1997, 15, 269. (2) Gray, J. J. Curr. Opin. Struct. Biol. 2004, 14, 110. (3) Sarikaya, M.; Tamerler, C.; Jen, A. K.-Y.; Schulten, K.; Baneyx, F. Nat. Mater. 2003, 2, 577. (4) Zhang, Y.; Wang, X.; Shan, W.; Wu, B.; Fan, H.; Yu, X.; Tang, Y.; Yang, P. Angew. Chem., Int. Ed. 2005, 44, 615. (5) Castellana, E. T.; Russell, D. H. Nano Lett. 2007, 7, 3023. (6) McLean, J. A.; Stumpo, K. A.; Russell, D. H. J. Am. Chem. Soc. 2005, 127, 5304. (7) Xia, N.; May, C. J.; McArthur, S. L.; Castner, D. G. Langmuir 2002, 18, 4090. (8) Wagner, M. S.; Horbett, T. A.; Castner, D. G. Langmuir 2003, 19, 1708. (9) Hazen, R. M. Am. Mineral. 2006, 91, 1715. (10) Orgel, L. E. Origins Life EVol. Biosphere 1998, 28, 227. (11) Hench, L. L.; Splinter, R. J.; Allen, W. C.; Greenlee, T. K. J. Biomed. Mater. Res. Symp. 1971, 2, 117. (12) Belton, D.; Paine, G.; Patwardhan, S. V.; Perry, C. C. J. Mater. Chem. 2004, 14, 2231. (13) Patwardhan, S. V.; Patwardhan, G.; Perry, C. C. J. Mater. Chem. 2007, 17, 2875.
nanocomposites as adequate devices for bioseparation of nucleic acids,14,15 nanostructured materials as both drug delivery systems and bone tissue regenerators,16 and polymerization of amino acids to form peptides enhanced by the presence of mineral clays.17-22 Focusing specifically on the protein/solid surface interactions it seems that proteins denature due to contact with the inorganic solid substrate. Accordingly, the ultimate goal is to know how and why proteins change their conformation as a result of adsorption and what amino acids are the preferred anchoring points at the surfaces. Experimental techniques have been used to address this point: ATR-FTIR for adsorption of immunoglobulin G on different silica surfaces,23 IR spectroscopy for adsorption of bovine serum albumin on silica,24 CD spectroscopy to measure the changes in the secondary structure of BSA along the adsorption-desorption cycle of the BSAsilica system,25 and solid-state NMR to investigate the terminal helix of statherin when adsorbed on hydroxyapatite surface.26-29 (14) Sen, T.; Sebastianelli, A.; Bruce, I. J. J. Am. Chem. Soc. 2006, 128, 7130. (15) Bruce, I. J.; Sen, T. Langmuir 2005, 21, 7029. (16) Vallet-Regı´, M. Chem. Eur. J. 2006, 12, 5934. (17) Rao, M.; Odom, D. G.; Oro´, J. J. Mol. EVol. 1980, 15, 317. (18) Ferris, J. P.; Hill, A. R., Jr.; Liu, R.; Orgel, L. E. Nature 1996, 381, 59. (19) Bujdak, J.; Rode, B. M. J. Mol. EVol. 1997, 45, 457. (20) Bujdak, J.; Rode, B. M. React. Kinet. Catal. Lett. 1997, 62, 281. (21) Parson, I.; Lee, M. R.; Smith, J. V. Proc. Natl. Acad. Sci. U.S.A. 1998, 95, 15173–15176. (22) Smith, J. V. Proc. Natl. Acad. Sci. U.S.A. 1998, 953370-3375. (23) Giacomelli, C. E.; Bremer, M. G. E. G.; Norde, W. J. Colloid Interface Sci. 1999, 220, 13. (24) Tarasevich, Y. I.; Monakhova, L. I. Colloid J. 2002, 64, 482. (25) Giacomelli, C. E.; Norde, W. J. Colloid Interface Sci. 2001, 233, 234. (26) Shaw, W. J.; Long, J. R.; Dindot, J. L.; Campbell, A. A.; Stayton, P. S.; Drobny, G. P. J. Am. Chem. Soc. 2000, 122, 1709. (27) Long, J. R.; Shaw, W. J.; Stayton, P. S.; Drobny, G. P. Biochemistry 2001, 40, 15451. (28) Gibson, J. M.; Popham, J. M.; Raghunathan, V.; Stayton, P. S.; Drobny, G. P. J. Am. Chem. Soc. 2006, 128, 5364. (29) Goobes, R.; Goobes, G.; Shaw, W. J.; Drobny, G. P.; Campbell, C. T.; Stayton, P. S. Biochemistry 2007, 46, 4725.
10.1021/la8029352 CCC: $40.75 2008 American Chemical Society Published on Web 11/12/2008
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Far less has been done by means of molecular modeling techniques. Basically, semiempirical30 and molecular dynamicbased studies31-35 have appeared in the literature (silica-based materials usually being the inorganic phase), whereas ab initio studies are less prevalent due to the enormous complexity of the considered systems. Consequently, to apply ab initio techniques one needs to derive simplified systems, envisaging single amino acids or small peptides in interaction with mineral surfaces.36-41 Among amino acids, much work has been focused on glycine, because (i) its functionalities are not influenced by the lateral chain and (ii) it can easily sublimate before decomposing, so that chemical vapor deposition on silica surfaces can be attained to study adsorption without the presence of water.37-39 Whereas it is easy to simulate gas-phase experiments by computer experiments, it is also clear that the copresence of water is essential to better mimic biological body fluids in interaction with inorganic surfaces. In this respect, Lambert and co-workers reported a very detailed and accurate analysis of glycine adsorption on silica surface from aqueous solution using several spectroscopy techniques42 as well as computer modeling.36 The present work is an extension of our own previous one38 in which the same silica surface model was adopted to study adsorption of glycine from the gas phase. Here we extend that study by addressing the role of water on the interaction of glycine molecule on a well-defined silica model surface in aqueous solution. Water molecules have been added to the dry glycine/ silica system in a step by step fashion, reaching a loading of five H2O molecules per glycine molecule. Some key questions will be addressed in the paper about the role that H2O microsolvation and silica surface may play to stabilize the zwitterionic form of glycine (known to be the most stable form in solution) at the silica surface and assess if the glycine/silica contact is direct or mediated through the H2O molecules.
Computational Details All calculations have been carried out using the latest version CRYSTAL06 periodic code.43 In contrast to plane wave-based periodic codes, CRYSTAL uses a local Gaussian basis set, which allows defining a true bidimensional system using a slab of finite thickness.44 This ensures exponentially decaying to zero of the wave function at infinite distance above and below the slab, also avoiding the problem of separating by a large distance the artificial replicas along the direction perpendicular to the slab plane. The calculations were run on standard Linux boxes using the parallel version of the code, whereas for structure manipulations and visual analysis of the (30) Latour Jr, R. A.; Hench, L. L. Biomaterials 2002, 23, 4633. (31) Raffaini, G.; Ganazzoli, F. Langmuir 2004, 20, 3371. (32) Raut, V. P.; Agashe, M. A.; Stuart, S. J.; Latour, R. A. Langmuir 2005, 21, 1629. (33) Carravetta, V.; Monti, S. J. Phys. Chem. B 2006, 110, 6160. (34) Gambino, G. L.; Grassi, A.; Marletta, G. J. Phys. Chem. B 2006, 110, 4836. (35) Gambino, G. L.; Lombardo, G. M.; Grassi, A.; Marletta, G. J. Phys. Chem. B 2004, 108, 2600. (36) Costa, D.; Lomenech, C.; Meng, M.; Stievano, L.; Lambert, J.-F. J. Mol. Struct.: THEOCHEM 2007, 806, 253–259. (37) Lomenech, C.; Bery, G.; Costa, D.; Stievano, L.; Lambert, J. F. ChemPhysChem 2005, 6, 1061. (38) Rimola, A.; Sodupe, M.; Tosoni, S.; Civalleri, B.; Ugliengo, P. Langmuir 2006, 22, 6593. (39) Rimola, A.; Tosoni, S.; Sodupe, M.; Ugliengo, P. ChemPhysChem 2006, 7, 157. (40) Rimola, A.; Sodupe, M.; Ugliengo, P. J. Am. Chem. Soc. 2007, 129, 8333. (41) Nonella, M.; Seeger, S. ChemPhysChem 2008, 9, 414. (42) Meng, M.; Stievano, L.; Lambert, J. F. Langmuir 2004, 20, 914. (43) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; Zicovich-Wilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco, P.; Llunell, M. CRYSTAL2006 User’s Manual; University of Torino: Torino, 2006; http://www.crystal.unito.it. (44) Dovesi, R.; Civalleri, B.; Orlando, R.; Roetti, C.; Saunders, V. R. ReV. Comput. Chem. 2005, 21, 1.
Rimola et al. vibrational normal modes the molecular graphic program MOLDRAW was used.45 Basis Set. Details of the adopted Gaussian basis set are available on the CRYSTAL web site and also in the Supporting Information.46 Here, the exponents of the outer shells for each atom are explicitly given in Bohr-2: Si, 88-31G* (Rsp) 0.193, Rd ) 0.61); O atoms of the silica surface, 8-411G* (Rsp ) 0.181, Rd ) 0.6); O atoms of the glycine molecule, 6-311G* (Rsp ) 0.2556, Rd ) 1.292); C, 6-311G* (Rsp ) 0.1456, Rd ) 0.626); N, 6-311G* (Rsp ) 0.209, Rd ) 0.913); and H 3-11G* (Rs) 0.1027, Rp ) 0.75). Hamiltonian and Computational Parameters. DFT methods, in particular those based on hybrid functionals, have been proven to provide structures, energies, and vibrational frequencies of hydrogen-bonded systems in better agreement with experiment and high-level correlated methods than both LDA and the popular PW91GGA functional.47-49 On the basis of these results and considering that for the present systems the hydrogen-bond interactions play a dominant role, B3LYP50,51 has been adopted for all calculations. Nevertheless, it is now well established that GGA and hybrid functionals fail to deal with dispersive interactions (London forces),52-54 and for the Becke exchange-based functional the interaction at short range tends to be more repulsive than that resulting from the exact Hartree-Fock exchange.55 These errors are, however, less crucial in the present case because absolute binding energies are not computed and only relative energies among different configurations are reported. The Hamiltonian matrix is diagonalized43,56 in 4k points, corresponding to a shrinking factor of 2. Tolerance values controlling Coulomb and exchange series have been set to (6, 6, 6, 6, 12) for all calculations. The exchange and correlation functional is integrated numerically on a grid of radial and angular coordinates through Gauss-Legendre and Lebedev schemes, respectively. A pruned grid consisting of 75 radial points and 5 subintervals with (86, 194, 350, 974, 350) angular points has been used for all calculations. This grid reduces the error to 3 × 10-5 electrons per unit cell in the total integrated electron density (for the present systems ranging from 700 to 780 electrons). The SCF process was stopped when the energy difference between cycles was smaller than 10-8 (for geometry optimizations) and 10-11 Hartree (for frequency calculations). Geometry Optimization. Full optimization of both the internal coordinates and the unit cell parameters have been performed imposing the P2mm layer symmetry group for both the freeedingtonite surface and the cases envisaging glycine and water adsorption. This means that adsorption on the upmost face has always mirrored (by the symmetry plane m) the lowest surface, ensuring zero dipole moment across the slab. Geometry optimization was performed by means of a quasi-Newton algorithm in which the quadratic step (BFGS Hessian updating scheme) is combined with a linear one (parabolic fit) as proposed by Schlegel.57 The convergence optimization was set up so that thresholds for the maximum force (0.00045 au), rms force (0.00030 au), maximum atomic displacement (0.00180 au), and rms atomic displacement (0.00120) were simultaneously obeyed for a converged optimization.58 (45) Ugliengo, P. MOLDRAW: A molecular graphics program to display and manipulate molecular structures; H1 (32-bit) ed.; Torino, 2005; http://www. moldraw.unito.it. (46) CRYSTAL basis set library; Torino, 2005; http://www.crystal.unito.it/ Basis_Sets/Ptable.html. (47) Tosoni, S.; Pascale, F.; Ugliengo, P.; Orlando, R.; Saunders, V. R.; Dovesi, R. Mol. Phys. 2005, 103, 2549. (48) Ugliengo, P.; Pascale, F.; Me´rawa, M.; Labe´guerie, P.; Tosoni, S.; Dovesi, R. J. Phys. Chem. B. 2004, 108, 13632. (49) Pascale, F.; Tosoni, S.; Zicovich-Wilson, C. M.; Ugliengo, P.; Orlando, R.; Dovesi, R. Chem. Phys. Lett. 2004, 396, 308. (50) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (51) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B. 1988, 37, 785. (52) Walsh, T. Phys. Chem. Chem. Phys. 2005, 7, 443. (53) Wu, X.; Vargas, M. C.; Nayak, S.; Lotrich, V.; Scoles, G. J. Chem. Phys. 2001, 115, 8748. (54) Xu, X.; Goddard, W., III Proc. Nat. Acad. Sci. U.S.A. 2004, 101, 2673. (55) Johnson, E. R.; DiLabio, G. A. Chem. Phys. Lett. 2006, 419, 333. (56) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B. 1976, 8, 5188. (57) Schlegel, H. B. J. Comput. Chem. 1982, 3, 214.
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Figure 1. 2D-slab silica model cut out from the edingtonite bulk. Unit cell in light cyan. Periodicity is along the a and b directions indicated by the arrows. Oxygen atoms from the bulk are colored in red (Obulk), whereas oxygen atoms (Osurf) of the silanol surface groups (responsibles to H-bond with glycine) are colored in blue.
Central Zone Phonon Frequencies. Vibrational frequencies of the considered systems were computed at the Γ point (point k ) 0 in the first Brillouin zone, called central zone) within the harmonic approximation by obtaining the eigenvalues from diagonalization of the mass-weighted Hessian matrix. This dynamical matrix was obtained by numerical differentiation (central-difference formula) of the analytical first-energy derivatives, calculated at the geometries obtained by incrementing, in turn, each of the 3N equilibrium nuclear coordinates by a small amount u ) 0.001 Å. For more detailed discussion, a recent paper59 reports the computational conditions and other numerical aspects related to calculation of the vibrational frequencies at the Γ point. For the considered systems building up the full mass-weighted Hessian matrix would have been very expensive because N atoms in the unit cell implies performing 6N + 1 energy plus gradient calculations in the central-difference formula. Furthermore, experimental infrared spectroscopy focuses on the vibrational frequencies of OH surface sites and the adsorbed glycine, so that only a portion of the dynamical matrix was computed by considering the displacements of a subset of atoms, i.e., the surface OH group and those of the adsorbed water and glycine molecules. This strategy has been recently validated by some of us47 and adopted already in our previous work on glycine adsorption on the dry edingtonite surface.38
Results and Discussion Surface Model. To perform the full periodic B3LYP simulations we adopted a well-studied silica model proposed by some of us in the past60,61 (see Figure 1). This is derived from the edingtonite all-silica framework cut out along the [001] direction and imposing the P2mm symmetry group (layer group 27). To maximize symmetry, the 2D slab has been hydroxylated on both faces, the silanol groups at the opposite sites being separated by 14 Å. In order to simulate low glycine coverage a large unit cell has been adopted, resulting in four SiOH groups per unit cell (two independent OH groups on each symmetry-related face), which are able to establish H-bond interactions with the adsorbed water and glycine molecules. The average distance between two independent OH groups is 6.5 Å, giving an OH surface density of 2.2 OH nm-2, slightly higher than the experimental estimations of 1 OH nm-2 for amorphous silica outgassed at 800 K.62 Adoption of a slab based on a crystalline system to model the surface of amorphous silica seems contradictory due to the geometrical constraints. However, in our recent work on edingtonite/glycine interaction38 it was shown that the edingtonite framework revealed (58) Civalleri, B.; D’Arco, P.; Orlando, R.; Saunders, V. R.; Dovesi, R. Chem. Phys. Lett. 2001, 348, 131. (59) Pascale, F.; Zicovich-Wilson, C. M.; Gejo, F. L.; Civalleri, B.; Orlando, R.; Dovesi, R. J. Comput. Chem. 2004, 25, 888. (60) Civalleri, B.; Ugliengo, P. J. Phys. Chem. B. 2000, 104, 9491. (61) Civalleri, B.; Casassa, S.; Garrone, E.; Pisani, C.; Ugliengo, P. J. Phys. Chem. B. 1999, 103, 2165. (62) Zhuravlev, L. T. Langmuir 1997, 3, 316.
Figure 2. (a) B3LYP-optimized geometries of glycine interacting with silanol groups at the dry silica surfaces. SG-1, SG-2 (minima, neutral glycine), and SG-3 (transition state, zwitterionic glycine). Relative energies (kJ mol-1) in parentheses. Distances in Ångstroms. (b) Labeling scheme of the H2O location sites for microsolvation of the neutral SG-1 and zwitterionic SG-3 structures.
large structural flexibility which mimics the well-known high pliability of a real amorphous silica material. In practice, local rearrangements in proximity of the adsorption sites caused by interaction with adsorbates are well represented by the present approach. Structures of the Glycine/Water/Silica Complexes. Water MicrosolVated Glycine System. A lot of previous theoretical work has focused on studying the microsolvation of glycine by water molecules. For sake of brevity, reference is done here only to the excellent experimental and computational work of Ramaekers and co-workers63 in which it was shown that at least four water molecules were needed to favor the glycine zwitterionic form over the neutral one. This fact is relevant here, in which it might be expected that, by virtue of the OH surface groups, less than four H2O molecules will be needed to stabilize the zwitterionic form of the adsorbed glycine over the neutral one. If this is true, the silica surface may be regarded as a “solid solvent”. Water-Free Glycine/Silica System. Before focusing on the present results it is useful to briefly recap the structures of our previous work38 on glycine interacting with the water-free silica surface because these serve as a base to rationally define the procedure followed to study water microsolvated structures. The most relevant water-free structures, reoptimized with the present basis set, are shown in Figure 2a. The most stable form SG-1 envisages glycine adsorbed in neutral form engaged in three H bonds, two of them involving the carbonyl group. Because of that, a strong bathochromic shift of the CdO stretching frequency in comparison with the gas-phase value is computed, in very good agreement with the experiment, thereby supporting SG-1 as the reference structure.38 As for the glycine zwitterionic forms, SG-3 was computed to be less stable by 64.2 kJ mol-1 compared to SG-1. Furthermore, (63) Ramaekers, R.; Pajak, J.; Lambie, B.; Maes, G. J. Chem. Phys. 2004, 120, 4182.
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this structure was characterized as a transition state, the imaginary frequency being related to a mode involving the NH3+ group. Removal of the phonon instability drives one of the NH3+ protons back to one carboxylate oxygen atom, resulting in structure SG2, in which glycine is again in the neutral form (see Figure 2a). SG-2 is a minimum lying 26.6 kJ mol-1 above SG-1. In conclusion, our previous results showed that for low glycine loading on a highly dehydroxylated silica surface the glycine zwitterionic form is unstable compared to the neutral one. Water MicrosolVation of the Neutral Glycine/Silica System. In principle, water solvation can be studied by adopting unbiased first-principles molecular dynamics based on the Car-Parrinello and plane-waves method.64 However, these calculations are extremely expensive, and in order to ensure a good exploration of the complex potential-energy surface either a very long time evolution or adoption of metadynamics65 is needed. A more straightforward approach, which is followed here, is to perform a progressive microsolvation, that is, to add water molecules step by step (from one to five H2O molecules in the present case) setting the position of each water molecule in order to (i) maximize the H-bond interactions and (ii) follow the clues of electrostatic complementarity between adsorbate/adsorbent electrostatic potentials. This heuristic approach has the computational advantage of avoiding blind and expensive potential-energy searches at the risk of conditioning the final minima structures by the chosen starting ones (local minima trapping). Following the above ideas, the sites where H2O should be added to the SG-1 structure are shown in Figure 2b. Sites labeled by In, IIn, and IIIn envisage H-bonds with glycine at the lone pair of NH2 group, at lone pairs of the OH group, and at the NH2 protons, respectively. Because these three sites are all in the exterior part of the already adsorbed glycine of the SG-1 structure, it is expected that microsolvation at these sites will not dramatically alter the pristine direct H-bond interaction of glycine with the dry silica surface. On the contrary, when sites IVn and Vn are considered, H2O is forced to behave as a H-bond mediator between the silica surface and the adsorbed glycine. As it seems clear from the present scheme, this approach implies calculation of a large number of cases, each for a given H2O loading. For instance, the number of optimized structures is 5, 5, 4, 4, and 1 for SG-1 structure with 1, 2, 3, 4, and 5 adsorbed H2O molecules, respectively. To lighten the discussion of the results, only the most stable structure for each H2O loading is reported in Figure 3, whereas the whole set of structures is shown in Figures S1-S4, Supporting Information. It is worth mentioning that for all structures shown in Figure 3 glycine is adsorbed on the silica surface, adopting its most stable gas-phase conformation, i.e., exhibiting a bifurcated H bond between the NH2 and the CdO groups. The general trend shown by the structures of Figure 3 is that at all H2O loadings the most stable adducts are those in which H2O occupies the IVn and Vn sites, i.e., interaction of neutral glycine with the silica surface is always mediated by water. For SN1w-1 (n ) 1) the H2O molecule bridges the glycine acidic OH group toward the oxygen of the surface hydroxyl group (site Vn), whereas the CdO group is engaged in a second H-bond with the surface. The other structures are about 8, 9, 16, and 25 kJ mol-1 higher in energy than SN1w-1. SN2w-1 (n ) 2) structure can be interpreted as SN1-w1 in which the second H2O molecule has been added to bridge the surface OH group with the NH2 lone pair (IVn site). Examination of the resulting structure shows a large deformation of the silica (64) Marx, D.; Hutter, J. Ab Initio Molecular Dynamics: Theory and Implementation. In Modern Methods and Algorithms of Quantum Chemistry; Grotendorst, J., Ed.; NIC, FZ: Ju¨lich, 2000; p 301. (65) Laio, A.; Parrinello, M. Proc Natl. Acad. Sci. U.S.A. 2002, 99, 12562.
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Figure 3. B3LYP-optimized geometries of neutral glycine interacting with silanol groups at the silica surfaces as a function of H2O loading (n ) 1-5). The label 2W identifies two water molecules related by the symmetry plane and resulting eclipsed in the picture. Distances in Ångstroms.
framework which also brings two former isolated surface OH groups in mutual H-bond contact. The other four structures examined were 13, 19, 27, and 30 kJ mol-1 less stable than SN2w-1 (see Figure S2, Supporting Information). For the case of three H2O molecules, the resulting SN3w-1 most stable structure looks rather different from the previous n ) 1 and 2 cases. Here, two H2O molecule act as H-bond mediators with the silica surface by bridging the NH2 proton toward the framework oxygen atoms of siloxane bonds. Because the basicity of the siloxane bridge is known to be rather small, these H bonds are generally weak (see the bond distances in Figure 3), so that two other structures (SN3w-2 and SN3w-3 of Figure S3, Supporting Information) become close in stability to SN3w-1, respectively, only 5 and 6 kJ mol-1 higher in energy. At least four H2O molecules are needed to fill both IVn and Vn sites, giving rise to structure SN4w-1, the first one in which glycine does not form any direct H-bond with the silica surface. Any change in the optimal positions of the H2O molecules (namely, leading to direct glycine interaction with the surface) brings about a large energy penalty, as shown by structures SN4w2, SN4w-3, and SN4w-4 (see Figure S4, Supporting Information) which are 32, 35, and 61 kJ mol-1 higher in energy than the reference SN4w-1 one. Structure SN5w-1 with the highest H2O loading (n ) 5) does not show unexpected features (see Figure 3) because it can be regarded as SN4w-1 with addition of the fifth H2O molecule to fill in the last possible H-bond site at the outside glycine OH
Glycine Forms at the Water/Silica Interface
group. In short, SN5w-1 can be regarded as microsolvated glycine (complete first solvation layer) properly docked to H-bond interact with the silica OH surface groups. It is, however, worth noting that SN5w-1 does not exactly adopt the same structural features of glycine surrounded by the five water molecules as in the work of Ramaekers et al.63 This is due to the symmetry constraints imposed in our calculations in order to save computer time. Despite that, for both cases water interacts with the same relevant points of binding of neutral glycine. Water MicrosolVation of the Zwitterionic Glycine/Silica System. Earlier studies, in particular Ramaekers and co-workers,63 have showed that addition of at least four water molecules will stabilize the zwitterionic over the neutral form of glycine. In agreement, it has been reported by some of us38 that the glycine zwitterion does not exist as such upon adsorption on silica from the gas phase. The key question is to establish if four H2O molecules are also needed to stabilize the zwitterionic over neutral form when glycine is adsorbed on the silica surface considering that surface OH groups may contribute to stabilize the zwitterionic form. To address this point the SG-3 structure (see Figure 2a,b) has been considered as a starting point for microsolvation following the same approach already described for the SG-1 case. Here, the IIIz solvation site is at the exterior of the adsorbed zwitterions, whereas sites Iz, IIz, and IVz all envisage positions in which H2O mediates between the adsorbed zwitterions and the silica surface. As done for the neutral case, only the most stable structures are shown in Figure 4, whereas the whole set of structures is shown in Figures S5-S8, Supporting Information. For the n ) 1 case structure SZ1w-1 shows the carboxylate group in direct contact with the silica surface whereas H2O bridges the NH3+ toward the OH surface group by a rather strong H-bond with one of the three NH bonds (see Figure 4). Remarkably, SZ1w-1 is a true minimum, at variance with the transition state resulting for the zwitterion glycine adsorbed on the dry silica surface SG-3. Swapping the H2O position from the NH3+ to the COO- groups causes an energy penalty of about 12 kJ mol-1 (structure SZ1w-3 of Figure S5, Supporting Information). A common feature of all the most stable structures for n ) 2-5 cases are two H2O molecules, each one making a H-bond ring between the NH3+ and the COO- groups plus a weak contact with the oxygen atom of the framework siloxane bridge. This water bridging feature was already reported by Costa et al. using silica-based minimal clusters.36 For n ) 2, altering this bonding scheme raises the corresponding energy by 20, 25, 35, and 37 kJ mol-1 for SZ2w-2, SZ2w-3, SZ2w-4, and SZ2w-5, respectively (see Figure S6, Supporting Information, for details). Interestingly, addition of the third H2O molecule occurs equivalently at bridging either the NH3+ (SZ3w-1) or the COO(SZ3w-2) group toward the OH surface groups (see Figure 4). Again, any change of these patterns increases the instability dramatically (see Figure S7, Supporting Information). As for the neutral case (see structure SN4w-1 of Figure 3) four H2O molecules are needed to avoid any direct H-bond between glycine and the silica surface, as shown by structure SZ4w-1, and similarly to structure SN5w-1, the fifth H2O molecule is H-bonded to the exterior part of the carboxyl group COO- (see structure SZ5w-1 of Figure 4). It is, however, worth noting that SZ5w-1 does not exactly adopt the same structural features of glycine zwitterions surrounded by the five water molecules as in the work of Ramaekers et al.63 This is due to the symmetry constraints imposed in our calculations in order to save computer time. Despite that, for both cases water interacts with the same relevant points of binding of glycine zwitterion.
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Figure 4. B3LYP-optimized geometries of zwitterionic glycine interacting with silanol groups at the silica surfaces as a function of H2O loading (n ) 1-5). Energy difference between SZ3w-1 and SZ3w-2 (kJ mol-1) in parentheses. The label 2W identifies two water molecules related by the symmetry plane and resulting eclipsed in the picture. Distances in Ångstroms.
The dichotomy of direct/indirect contact between Gly and the silica surface in the presence of water molecules is probably dependent on the adopted surface model. For moderate hydroxylated silica surfaces as in the present case (2.2 OH nm-2), results indicate that water lays between glycine and the surface, so that this behavior will probably be followed by more hydrophobic surfaces (for instance, 1.2 OH nm-2). However, for silica surfaces with higher silanol population (for instance, 4.5 OH nm-2) direct contact might indeed occur since there are more surface OH available for interaction. Furthermore, the presence of geminal sites, known to be more acidic than the isolated ones,66 or even chains67 and rings68 exhibiting much higher acidity than both isolated and geminal sites will probably enhance the affinity of the silica surface for glycine. Much work is needed to assess the above points along the line suggested by very recent work from the Lambert group.69 Relative Stability of Neutral vs Zwitterionic Structures. As for the analysis carried out by Ramaekers and co-workers63 to assess the relative stability of the neutral vs the zwitterionic (66) Ferrari, A.; Ugliengo, P.; Garrone, E. J. Phys. Chem. 1993, 97, 2671. (67) Fubini, B.; Bolis, V.; Cavenago, A.; Garrone, E.; Ugliengo, P. Langmuir 1993, 9, 2712. (68) Bordiga, S.; Ugliengo, P.; Damin, A.; Lamberti, C.; Spoto, G.; Zecchina, A.; Spano′, G.; Buzzoni, R.; Dalloro, L.; Rivetti, F. Top. Catal. 2001, 15, 43. (69) Costa, D.; Tougerti, A.; Tielens, F.; Gervais, C.; Stievano, L.; Lambert, J. F. Phys. Chem. Chem. Phys. 2008, 10, 6360.
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Figure 5. Relative electronic energies (∆E) of the zwitterionic systems with respect to the neutral ones (kJ mol-1) as a function of H2O loading (n ) 0-5).
form of glycine as a function of the number of H2O molecules, the present discussion addresses the same topic for the case of the adsorbed glycine. Figure 5 compares the relative stabilities of the most stable structures described in the previous paragraph for the cases of neutral and zwitterionic glycine adsorbed onto silica surface. The comparison starts for the case n ) 0, albeit SG-3 is not a real minimum, in which the neutral form is largely preferred over the zwitterionic one by 64 kJ mol-1. Addition of one H2O molecule (n ) 1) dramatically reduces ∆E to 26 kJ mol-1 (n ) 1), whereas two H2O molecules are already enough to set the neutral and zwitterion populations almost equivalent (n ) 2). For higher H2O loadings (n ) 3-5) the zwitterionic form becomes completely dominant over the neutral one. The present results are at variance with solvation of glycine in the gas phase where at least four H2O molecules are needed to favor the zwitterions over the neutral form. Thus, surface OH groups are indeed mimicking H2O molecules, assisting solvation and stabilizing the zwitterion over the neutral structures. This is also relevant for interpreting experimental spectroscopic studies because the presence of traces of water may seriously alter the equilibrium between neutral vs zwitterionic forms present at the silica surface, stressing the importance of a well-tested and reproducible pretreatment of the silica sample adopted in glycine vapor deposition experiments.37,42
Predicted Vibrational Features and Comparison with Experiments. The last section of this work focuses on the glycine vibrational features of some representative structures described above and comparison with the experimental data available in the literature.42 B3LYP Harmonic Frequencies for Most RepresentatiVe Structures. Let us start by describing the differences between vibrational features of neutral vs zwitterionic glycine in the gas phase. Table 1 displays the most important vibration frequencies for glycine. It is expected that the larger differences between neutral and glycine vibrational features imply the COOH/COOand NH2/NH3+ groups, whereas frequencies involving CH and HCH modes should be less dependent from the glycine state. For instance, neutral glycine shows OH stretching frequency (ν (OHgly)) and COH deformation coupled with the CCH bending (δ(CCH) + δ(COH)) belonging to the acidic carboxyl functionality, while these modes are lost in the zwitterion form, in which the anionic carboxylate fingerprint is due to asymmetric and symmetric stretching within the COO- moiety (ν (CdO)a and δ(CCH) + ν(CdO)s). NH2 modes of the neutral glycine are also deeply altered in the zwitterionic form, which is dominated by the NH3+ features. Inspection of columns Gn and Gz of Table 1 allows us to trace the differences between the two cases. Moving to the vibrational features of SG-1 and SG-3 cases, the first important change compared to the gas-phase glycine frequencies are the ν (OHgly) and ν(NH+) values, both suffering
Glycine Forms at the Water/Silica Interface
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Table 1. Selected Unscaled Harmonic B3LYP Frequencies (cm-1) of Glycine as Neutral (Gn) and Zwitterions (Gz) in the Gas Phase, Neutral (SG-1) and Zwitterion (SG-3) Adsorbed on a Dry Silica Surface, and Neutral (SN5w-1) and Zwitterion (SZ5w-1) Solvated by Five H2O Molecules and Adsorbed on a Silica Surface
a bathochromic shift as a consequence of the H-bonds with the silica surface. The most relevant frequency to trace is the one associated to ν(CdO) since this band usually serves as a fingerprint in the IR spectra of glycine on silica.37,70,71 This mode undergoes a large bathochromic shift in SG-1 (from 1829 to 1746 cm-1), whereas in SG-3 the ν(CdO)a component is almost unperturbed and the ν (CdO)s one shifts hypsochromically. The ν(NH)a/s, δ(HNH), δ(NH3+), and F(NH3+) vibration modes remain moderately affected by adsorption on the silica surface, whereas the ν(CH)a/s and δ(HCH) ones are practically unperturbed. The copresence of H2O molecules deeply alter the spectroscopic picture reported so far for adsorption of glycine on a dry silica surface. The H-bond interactions of H2O molecules with neutral glycine induce large bathochromic shifts of the relevant stretching frequencies of glycine, whereas bending modes suffer moderate hypsochromic shifts as shown by the ν(OHgly) and δ(CCH) + δ(COH) values (see SN5w-1 in Table 1). For the very same reasons, the zwitterion vibrational frequencies associated to the NH3+ moiety (see SZ5w-1 in Table 1) engaged in H-bonds with H2O are deeply affected: the ν(NH+) stretching vibration undergoes a bathochromic shift of about 100 cm-1 with respect to SG-3, whereas the δ(NH3+)a, δ(NH3+)s, and F(NH3+) (70) Basiuk, V. A.; Gromovoy, T. Y.; Golovaty, V. G.; Glukhoy, A. M. Orig. Life EVol. Biosph. 1990-199120, 483. (71) Groenewegen, J. A.; Sachtler, W. M. H. J. Catal. 1974, 33, 176. (72) Stepanian, S. G.; Reva, I. D.; Radchenko, E. D.; Rosado, M. T. S.; Duarte, M. L. T. S.; Fausto, R.; Adamowicz, L. J. Phys. Chem. A 1998, 102, 1041. (73) Derbel, N.; Herna´ndez, B.; Pflu¨ger, F.; Liquier, J.; Geinguenaud, F.; Jaı¨dane, N.; Lakhdar, Z. B.; Ghomi, M. J. Phys. Chem. B 2007, 111, 1470.
deformation frequencies suffer hypsochromic shifts of 29, 38, and 86 cm-1, respectively. A different pattern is computed for the ν(CdO) stretching frequency. For the neutral solvated system (see SN5w-1 in Table 1) it undergoes a hypsochromic shift of 23 cm-1 with respect to SG-1, indicating a less strong H bond when H2O is mediating the interaction than those established directly with the OH surface groups. For the SZ5w-1 zwitterion system, however, ν(CdO)a is bathochromically shifted by 42 cm-1 with respect to SG-3 because of the direct H2O · · · COO- H-bonds. The ν(CH)a/s stretching and δ(HCH) bending frequencies remain almost unperturbed (around 20 and 10 cm-1 at most, respectively) because no direct interaction with H2O occurred. Comparing B3LYP Harmonic Frequencies with Experimental IR Measurements. Most experimental studies addressed understanding the interaction of glycine with silica surfaces in both dry and wet conditions are based on accurate recording and interpretation of IR spectra.37,42 Table 2 collects some selected B3LYP harmonic frequencies, which have been scaled to allow for direct comparison with experiments. Frequencies for selected modes have been scaled by a specific scaling factor derived from comparison of the experimental frequencies with the B3LYP harmonic ones for the glycine molecule both in the gas phase and in solution. The comparison for the SG-1 case (see Table 2) was already reported by us in previous work,38 showing excellent agreement with experiment for the ν(CdO) fingerprint band. With respect to the microsolvated systems, experimental data invoke the zwitterionic glycine as the form actually adsorbed at
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Table 2. Experimental and Harmonic B3LYP Frequencies (cm-1) for Gas-Phase Glycine Molecule and When Adsorbed on Dry and Wet Silica Surfacea vibrational mode gas phase
δ(HCH)
δ(HNH)
ν(CdO)
1429 1429 [1459] 0.979 1423 1421 [1451]
1630 1630 [1683] 0.969 1630 1622 [1674]
1784 1784 [1829] 0.975 1699 1702 [1746]
system b
glycine, exp glycine, B3LYP scaling factor glycine/silica, expc SG-1, B3LYP solvated
δ(CCH) + ν(CdO)s
δ(HCH)
δ(NH3+)a
1430 1430 [1385] 1.032 1412 1405 [1362]
1444 1444 [1491] 0.968 1443 1422 [1469]
1637 1637 [1708] 0.958 1617 1619 [1690]
glycine/W, expd glycine/5H2O, B3LYP scaling factor glycine/silica/W, expe SZ5w-1, B3LYP
a The label W refers to an unspecified number of H2O molecules not directly measurable from experiment. Scaled B3LYP frequencies are given as bare numbers; unscaled B3LYP frequencies are listed in brackets. Scaling factors given in italics were computed comparing experimental and harmonic B3LYP frequencies for free-surface glycine (both in the gas phase and solvated). b From ref 72. c From ref 37. d From ref 73. e From ref 42.
the silica surface,42 a fact which is supported by the B3LYP higher stability of SZ5w-1 with respect to SN5w-1 by about 46 kJ mol-1. The presence of H2O at the surface complicates the vibrational spectrum because the stretching and bending modes of water molecules deeply involved in H-bond interactions become shifted and broad, covering the 3900-3500 and 1800-1700 cm-1 ranges, respectively. Accordingly, the ν(CdO)a as well as other important bands useful for comparison, such as ν(NH+), are hidden by the bands associated to water molecules, so that other frequencies must be used. To avoid interference with the water bands, only the δ(CCH) + δ(CdO)s, δ(CCH), and δ(NH3+)a bands in the 1600-1300 cm-1 region can be safely adopted as fingerprints. Comparison with the B3LYP results only refers to the SZ5w-1 structure, the latter being the most stable one at the highest H2O loading, assumed here as the most representative structure adsorbed at the silica surface. Comparison between experiment and B3LYP data of Table 2 shows a maximum deviation of about 30 cm-1. The B3LYP-δ(NH3+)a bending frequency is in excellent agreement with the measured one, supporting the view by which H2O molecules solvate the NH3+ moiety. Similar agreement is reported for the δ(CCH) + ν(CdO)s vibrational frequency, again supporting the view of the COO- group completely solvated by H2O molecules (similarly to the NH3+ moiety), thus ruling out any direct glycine/silica interaction. For the δ(HCH) bending frequencies, the agreement is less stringent.
Conclusions The all-silica hydroxylated (001) edingtonite surface, mimicking a real amorphous silica surface outgassed at 800 °C, has been adopted to model by periodic B3LYP calculations adsorption of glycine, either from the gas phase or when in water solution. The latter state is simulated using a progressive microsolvation approach in which 1-5 H2O molecules are coadsorbed with glycine at the silica surface. Both neutral and zwitterionic glycine states have been studied, and their relative stability was analyzed as a function of the coadsorbed number of H2O molecules. The rationale to solvate glycine in its two states was to establish efficient H-bonds and exploit the electrostatic complementarities between glycine/water, water/silica, and silica/glycine phases. The most interesting points emerging from this work are as follows. (i) When glycine adsorbs on silica from the gas phase it prefers to stick as a neutral form, the zwitterion one being much higher in energy and an actual transition state.38 The most stable adduct exhibits two H-bonds between the CdO group and
the surface SiOH groups and an extra one between the carboxyl OH and the SiOH groups. (ii) When glycine is microsolvated, H2O molecules bridge the interaction between glycine and the silica surface via H-bond interactions. This happens as a function of H2O loading, and already with four H2O molecules no direct contact between glycine and silica is established. (iii) Comparison between energies of the microsolvated neutral and zwitterion glycine/silica cases reveals that for n ) 0 and 1 glycine sticks as a neutral form, for n ) 2 neutral and zwitterions are almost degenerate, and for n ) 3-5 the zwitterionic case is largerly the preferred one. (iv) Comparison between scaled harmonic B3LYP vibrational frequencies and FTIR experiments of gas-phase glycine adsorbed on silica supports the SG-1 model as the one which maximizes the agreement with the measured spectroscopic features. In the presence of H2O the SZ5w-1 model, in which solvation of NH3+ and COO- moieties prevents any direct contact between glycine and silica, exhibits computed vibrational features in remarkably good agreement with the FTIR experiment recorded for adsorption of glycine from water solution. (v) The agreement of our results with both the experimental IR measurements42 and the accurate ab initio MD of alanine on amorphous silica41 indicates that the progressive microsolvation strategy employed in this work may be a reliable and computationally feasible approach to simulate by means of static calculations adsorbant/ adsorbate interfaces in the presence of a relatively low amount of water, resembling moderate drying conditions. When simulation deals with the contact between bulk water/glycine solution and silica one can no longer rely on the present approach and MD techniques become essential to properly study the features of these systems. Acknowledgment. Financial support from the Italian Ministry MIUR (Project COFIN2006, Prot. 2006032335_005), the Regione Piemonte (Bando ricerca scientifica Piemonte 2004, Settore: Nanotecnologie e nanoscienze, “Materiali nanostrutturati biocompatibili per applicazioni biomediche”), and INSTM (progetto PRISMA 2002, “Nanostructured oxidic materials for the adsorption and the catalysis”) is gratefully acknowledged. A.R. is indebted to the Ramo´n Areces Foundation for a postdoctoral fellowship at the University of Torino. Supporting Information Available: Coordinates of the optimized structures described in the text as well as those relative to higher energy configurations. This material is available free of charge via the Internet at http://pubs.acs.org. LA8029352
