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Crystalline Order of a Water/Glycine Film Coadsorbed on the (104) Calcite Surface Uta Magdans,*,† Xavier Torrelles,‡ Klaus Angermund,§ Hermann Gies,† and Jordi Rius‡ Faculty of Geosciences, Department of Geology, Mineralogy and Geophysics, Ruhr-UniVersitaet Bochum, UniVersitaetsstr. 150, 44780 Bochum, Germany, Institut de Ciencia de Materials de Barcelona, Campus de la UniVersitat Autonoma de Barcelona, 08193 Bellaterra, Spain, and Max-Planck-Institut fuer Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Muelheim a. d. Ruhr, Germany ReceiVed December 19, 2006. In Final Form: February 15, 2007 For biomineralization processes, the interaction of the surface of calcite crystals with organic molecules is of particular importance. Especially, biologically controlled biomineralization as in exoskeletons of mollusks and echinoderms, e.g., sea urchin with single-crystal-like spines and shells,1-3 requires molecular control of seed formation and growth process. So far, experiments showing the obvious influence of organic molecules on the morphology and habit of calcite crystals have demonstrated the molecular dimension of the interaction.4-7 Details of the kinetics of growth and dissolution of mineral surfaces influenced by additives are available,8,9 but other experimental data about the structure of the organic/inorganic interface on the atomic scale are rare. On the other hand, complicated organic macromolecules which are involved in biomineralization are numerous, with only a small fraction solved in structure and function so far.10-13 Therefore, model systems have to be designed to provide a basic understanding for the interaction process.14 Using grazing incidence X-ray diffraction combined with molecular modeling techniques, we show that glycine molecules order periodically on the calcite (104) face in competition with the solvent water when exposed to an aqueous solution of the most simple amino acid. In contrast to the general concept of the charge-matching fit of organic molecules on mineral surfaces,4,14 glycine is not attached to the calcite surface directly but substitutes for water molecules in the second hydration layer.
Introduction Biomineralization is a widespread phenomenon on earth. About 50 phyla produce a large variety of minerals with very different forms and functions.15 Although most of the biominerals are characterized in detail, the underlying principles and processes of the growth of biominerals are not yet well-understood.13 This is due to the extreme complexity of the interplay between the organic constituents (cells, organic framework matrices, soluble macromolecules) and the inorganic part (ion concentration, nucleation, crystal growth). However, the application of molecular genetic techniques to study biomolecules, e.g., molluscan shell * Corresponding author. E-mail address:
[email protected]. † Ruhr-Universitaet Bochum. ‡ Institut de Ciencia de Materials de Barcelona. § Max-Planck-Institut fuer Kohlenforschung. (1) Raup, D. M. In Physiology of Echinodermata; Boolootian, R. A., Ed.; Wiley-Interscience Publishers: New York, 1966; pp 379-395. (2) Weiner, S.; Addadi, L.; Wagner, H. D. Mater. Sci. Eng. C 2000, 11, 1-8. (3) Magdans, U.; Gies, H. Eur. J. Mineral. 2004, 16, 261-268. (4) Mann, S.; Didymus, J. M.; Sanderson, N. P.; Heywood, B. R.; Samper, E. J. A. J. Chem. Soc., Faraday Trans. 1990, 86 (10), 1873-1880. (5) Meldrum, F.; Hyde, S. T. J. Cryst. Growth 2001, 231, 544-558. (6) Albeck, S.; Weiner, S.; Addadi, L. Chem.sEur. J. 1996, 2 (3), 278-284. (7) Aizenberg, J.; Albeck, S.; Weiner, S.; Addadi, L. J. Cryst. Growth 1994, 142, 156-164. (8) Teng, H. H.; Dove, P. M.; Orme, C. A.; De Yoreo, J. J. Science 1998, 282, 724-727. (9) Teng, H. H.; Dove, P. M.; De Yoreo, J. J. Geochim. Cosmochim. Acta 1999, 63 (17), 2507-2512. (10) Aizenberg, J.; Hanson, J.; Koetzle, T. F.; Weiner, S.; Addadi, L. J. Am. Chem. Soc. 1997, 119, 881-886. (11) Ameye, L.; Compere, P.; Dille, J.; Dubois, P. Histochem. Cell. Biol. 1998, 110, 285-294. (12) Illies, M. R.; Peeler, M. T.; Dechtiaruk, A. M.; Ettensohn, C. A. DeV. Genes EVol. 2002, 212, 419-431. (13) Marin, F.; Luquet, F. C. R. PaleVol. 2004, 3, 469-492. (14) Addadi, L.; Weiner, S. Proc. Natl. Acad. Science U.S.A. 1985, 82, 41104114. (15) Lowenstam, H. A.; Weiner, S. On biomineralization; Oxford University Press: New York, 1989; Chapter 1.
proteins,13,16 begins to reveal the structure and functions of the macromolecules involved in the biomineralization process. Despite the diversity of the protein structures solved so far, they share several common properties, e.g., the predominance of two to four amino acids (usually glycine, aspartic acid, or serine) and a modular primary structure, consisting of different functional domains.13 To understand the function of these domains, model systems must be developed and analyzed, in order to obtain information about the interaction mechanisms at the organic/ inorganic interface. Several studies of the influence of organic molecules on crystal surface growth and dissolution mechanisms with atomic force microscopy (AFM) were reported, e.g., on calcium oxalate crystallization17 and calcite growth.8,18 The addition of organic molecules has a large effect on the growth morphologies and on the surface energy, but information about the structure of molecules adsorbed on mineral surfaces is difficult to obtain with AFM.19 Here, surface X-ray diffraction techniques provide a powerful, nondestructive tool to investigate the lateral structure of surfaces and near-surface interfaces of materials on atomic scale. Recently, grazing incidence X-ray diffraction (GIXRD) studies reported atomistic details of the lateral surface structure of the (104) calcite surface and the (100) surface of apatite in dry, humid, and fully hydrated environments under atmospheric conditions.20-22 These studies showed the feasibility (16) Marin, F.; Luquet, F. Mater. Sci. Eng. C 2005, 25, 105-111. (17) Wang, L. J.; Qiu, S. R.; Zachowicz, W.; Guan, X. Y.; DeYoreo, J. J.; Nanchollas, G. H.; Hoyer, J. R. Langmuir 2006, 22 (17), 7279-7285. (18) Teng, H. H.; Dove, P. M. Am. Mineral. 1997, 82, 878-887. (19) Wierzbicki, A.; Sikes, C. S.; Madura, J. D.; Drake, B. Calcif. Tissue Int. 1994, 54, 133-141. (20) Geissbu¨hler, P.; Fenter, P.; DiMasi, E.; Srajer, G.; Sorensen, L. B.; Sturchio, N. C. Surf. Sci. 2004, 573, 191-203. (21) Magdans, U.; Gies, H.; Torrelles, X.; Rius, J. Eur. J. Mineral. 2006, 18, 83-92. (22) Pareek, A.; Torrelles, X.; Rius, J.; Magdans, U.; Gies, H. Phys. ReV. B 2007, 75, 035418 (6 pages).
10.1021/la0636659 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/30/2007
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of surface X-ray diffraction for analyzing the atomic structure of mineral surfaces and interfaces in environmental conditions. Half of the amount of biominerals are calcium compounds, especially calcium carbonates, which also occur in the environment as rock-forming minerals or sediments. The stable and most abundant form of CaCO3, geologically and as a biomineral, is calcite. Therefore, calcite provides a suitable substrate to study organic/mineral interaction. Before understanding complicated adsorbate/substrate interactions, the basic structure and behavior of the surface substrate atoms must be understood. The structure of the (104) surface in atomic resolution has been studied previously by a number of researchers with techniques requiring ultrahigh vacuum such as LEED and XPS,23 but also with AFM24-27 and X-ray reflectivity28-30 allowing for environmental conditions. Whereas all studies working in vacuum conditions neglect the interaction with atmospheric gases and liquid films, and thus are of limited relevance for surface-related properties involving the environment, AFM and reflectivity experiments are performed in environmental conditions, but are restricted in spacial resolution. No atomic resolution AFM study is reported from calcite with hydrate layers, whereas reflectivity measurements only reveal a projection of the 2D structure into 1D without information on the lateral order. There are also a number of atomistic simulations studying anhydrous and completely hydrated calcite surfaces,31-34 as well as the energetics of sorption and desorption processes of water molecules on mineral surfaces.35 Information about the electron density profile perpendicular to the surface is available from reflectivity measurements; additional information about the lateral surface structure is obtained from non-specular diffraction patterns, called crystal truncation rods (CTR). Intensity in CTRs is highly sensitive to surface or near-surface scatterers and, except for the position of the Bragg peaks, without interference from the bulk diffraction signal. Third-generation synchrotron sources provide an adequate beam flux and intensity for the poor signal-to-noise ratio along these rods. The analysis of the experiments reveals the full details of the structure in atomic resolution of the surface and nearsurface ranges including the solvent molecules.
Methods Surface Diffraction. Surface diffraction experiments were accomplished at the surface diffraction beam line ID03 at the ESRF, Grenoble. The beamline setup is described by Ferrer and Comin.36 The experimental setup and performance of the measurements were carried out according to previous experi(23) Stipp, S. L. S.; Hochella, M. F., Jr. Geochim. Cosmochim. Acta 1991, 55, 1723-1736. (24) Ohnesorge, F.; Binnig, G. Science 1993, 260, 1451-1456. (25) Stipp, S. L. S.; Eggleston, C. M.; Nielsen, B. S. Geochim. Cosmochim. Acta 1994, 58 (14), 3023-3033. (26) Liang, Y.; Lea, A. S.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1996, 351, 172-182. (27) Jordan, G.; Rammensee, W. Geochim. Cosmochim. Acta 1998, 62 (6), 941-947. (28) Chiarello, R. P.; Wogelius, R. A.; Sturchio, N. C. Geochim. Cosmochim. Acta 1993, 57, 4103-4110. (29) Chiarello, R. P.; Sturchio, N. C. Geochim. Cosmochim. Acta 1995, 59 (21), 4557-4561. (30) Fenter, P.; Geissbu¨hler, P.; DiMasi, E.; Srajer, G.; Sorensen, L. B.; Sturchio, N. C. Geochim. Cosmochim. Acta 2000, 64 (7), 1221-1228. (31) De Leeuw, N. H.; Parker, S. C. J. Chem. Soc., Faraday Trans. 1997, 93 (3), 467-475. (32) De Leeuw, N. H.; Parker, S. C. J. Phys. Chem. B 1998, 102, 2914-2922. (33) Wright, K.; Cygan, R. T.; Slater, B. Phys. Chem. Chem. Phys. 2001, 3, 839-844. (34) Kerisit, S.; Parker, S. C.; Harding, J. H. J. Phys. Chem. B 2003, 107, 7676-7682. (35) Kerisit, S.; Parker, S. C. Chem. Commun. 2004, 1, 52-53. (36) Ferrer, S.; Comin, F. ReV. Sci. Instrum. 1995, 66 (2), 1674-1676.
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ments.21 The sample was prepared by cleaving a calcite single crystal of high optical quality from Durango, Mexico, in air. Chemical analysis of the single crystal was directly inferred from the analysis of the bulk lattice parameters during the refinement of the orientation matrix of the sample. Their refinement agrees within their respective statistical errors with those of the pure natural calcite.37,38 The topology of the cleavage surface was analyzed with optical microscopy and with AFM. It consists of large terraces, several micrometers wide, separated by macrosteps with an average height of about 100 nm (image available in the Supporting Information). A piece of about 10 × 6 × 2 mm without visible steps and defects was immediately mounted in a polyethylene chamber with 2 kapton windows coupled to the six-circle diffractometer. The chamber was then flushed with dry N2 gas in constant flow. The adsorbate solutions, first, pure destilled water, and, in the second step, the aqueous glycine solution, were dropped and spread out on the surface to a film of about 1 mm thickness with a pipet. To prevent evaporation of the solution, the humidity in the sample chamber was enhanced to 95% relative humidity (RH) by bubbling the N2 gas through a heated water vessel. The temperature and humidity in the cell were monitored and maintained constant during the experiment at 23 °C and 95% RH. Two sets of CTRs were measured, one for the calcite surface covered only with a film of pure water and one with a film of aqueous solution of glycine (gly) molecules on the surface. The integrated intensities of the CTRs were determined from rocking scans. Integration and correction of the measured data is described by Vlieg.39 Averaging of the data with plane group symmetry pg according to the surface symmetry yielded 284 and 289 nonequivalent data points corresponding to 5 CTRs and 6 CTRs for the calcite-water interface and for the calcite-gly-water interface, respectively. Specular CTRs were measured in stationary scanning mode to directly obtain peak intensities, which are proportional to the structure factor at each (0, 0, L) value of the measurement. They were used to monitore changes in the interface structure when introducing the gly molecules to the surface and were not included in the quantitative analysis. The aqueous glycine solution had a pH of about 6, so that calcite would normally dissolve, but the experiment was carried out with a stationary thin film of solution on the calcite surface, so that an equilibrium state at the interface appeared. The surface, monitored by reference scans in constant time steps, remained stable during both experiments. Structure Refinement. The conventional hexagonal unit cell of calcite (a1 ) a2 ) 4.990 Å, c ) 17.0615 Å, space group R-3c37) was transformed into an orthorhombic unit cell, so that the hexagonal (104) face corresponds to the (001) face of the orthorhombic unit cell with a ) 8.095 Å, b ) 4.990 Å, and c ) 24.286 Å. The orthorhombic unit cell parameters calculated from the hexagonal unit cell correspond within statistical error to the unit cell lengths refined from the experiment. All of the following crystallographic data refer to the orthorhombic unit cell setting. The calcite-solution interface structure was determined through a least-squares (LS) fit of the experimental structure factors with structure factors calculated from a structure model consisting of the calcite orthorhombic unit cell. Bulk atom positions were used as initial positions. The structure of the (37) Graf, D. L. Am. Mineral. 1961, 46, 1283-1316. (38) Markgraf, S. A.; Reeder, R. J. Am. Mineral. 1985, 70, 590-600. (39) Vlieg, E. J. Appl. Cryst. 1997, 30, 532-543.
Water/Glycine Film on (104) Calcite Surface
constituents of the solution near the calcite surface was analyzed starting from different interface models, varying the number of ordered layers, molecule position and occupation, and symmetry elements. The best LS fit of these models yielded the refined interface structure. The LS refinement was carried out with the program ROD,40 modified to allow the refinement of the position and rotation of rigid molecules (about 40 parameters). The quality of the LS fit is given as the normalized χ2 parameter. The roughness of the surface was also refined during the fit, using the β-model of Robinson.41 The initial position and conformation of the gly molecules on the calcite surface were determined from force field simulation studies. Here, a gly zwitterion was docked on a calcite surface slab exhibiting the structure of the hydrated calcite surface determined in the previous experiment.21 The simulation resulted in two symmetry-equivalent positions of gly molecules with respect to the surface pattern, related by glide plane symmetry normal to [010]. The arrangement from the simulation was used as the starting model in the structure refinement of the CTR data set. Molecular Dynamics Simulation of the H-Positions. The molecular mechanics and molecular dynamics simulations were carried out with Discover3 in the module InsightII from Accelrys, Inc.,42 using the CVFF-Forcefield. Partial charges were calculated with MOPAC. The refined structure model of the experiment was used to build a calcite slab with six unit cells in the x- and y-directions, respectively, and one unit cell in the z-direction, generating a surface area of about 48 Å × 30 Å and about 24 Å depth. This size provides for the atoms of the 2 × 2 inner unit cells a cutoff radius of at least 10 Å related to the (001) plane, so that border effects can be neglected for the inner part of the slab. Only this inner surface area was used after the simulation for analysis of the atomic structure and the energy calculation. All atom positions except H-positions were fixed. The initial structure was energyminimized to get a convenient starting structure for the molecular dynamics (MD) simulation. The MD simulation was split in two parts: (1) equilibration phase, a simulation of 5 ps with 1 fs time step at 298 K with a Boltzmann energy distribution; (2) simulation phase, a simulation of 20 ps with 1 fs time step at 298 K. Every 2 ps, the actual structure was saved and separately energyminimized. The resulting frames were sorted by energy, and hydrogen bond lengths and angles were analyzed. Simulation of the Long-Range Order for the Adsorbate Layer. Doubling the orthorhombic unit cell in the a and b directions leads to a calcite supercell consisting of four unit cells with 16 Å × 10 Å surface area. Each unit cell can have occupation with either the gly position 1 or the symmetry-equivalent gly position 2, respectively, which results in sixteen different occupations of the adsorbate layer above the supercell. The number of the supercell occupation possibilities is reduced to seven independent supercells by building an actual crystal slab and deleting the supercells generating the same patterns. The remaining calcite supercells were used to build calcite crystal slabs with the respective occupations, and to each, the MD simulation procedure described before was applied.
Results The sorption experiment was carried out in two steps. First, the calcite surface was covered with pure water. This provided a reference for the second measurement, where the glycine (40) Vlieg, E. J. Appl. Cryst. 2000, 33, 401-405. (41) Robinson, I. K. Phys. ReV. B 1986, 33 (6), 3830-3836. (42) See www.accelrys.com for more detail.
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Figure 1. Structure model of the hydrated calcite surface with the orthorhombic unit cell setting. The refined interface structure is displayed in grayscale, whereas bulk atom positions are shown in pale gray.
molecules were introduced into the water film exposed to the mineral surface. Hydrated Calcite Surface Structure. The completely hydrated calcite surface was studied previously by Geissbu¨hler et al.20 The structure model resulting from our measurement is in general agreement with the structure model of Geissbu¨hler et al., consisting of two laterally ordered monolayers of water on the slightly relaxed mineral surface, with two water molecules per surface unit cell area (Figure 1; for refinement parameters, see Supporting Information). Our structure refinement resulted in considerably shorter distances between the surface and the two water layers of 1.9(1) Å and 3.1(1) Å, respectively, compared to 2.3(1) Å and 3.5(1) Å of Geissbu¨hler et al.,20 leading to Ca-O distances between the surface Ca2+ cations and the water O of 2.4(1) Å, which agrees very well with the Ca-O coordination bond length in the bulk structure and also with simulations.34 The differences in layer distances and coordination lengths can be explained by the different relaxation models of the calcite surface atoms, especially the rotation and tilt of the surface carbonate groups. Furthermore, the use of rigid water molecules allowed us to include the H-atoms in the refinement which were obtained from molecular dynamics studies of the structure model. The water molecules are tilted toward the surface, so that at least one H-atom per water molecule forms a hydrogen bond with an O-atom of a neighboring surface carbonate group. The experimental average O-H distances (1.6(1) Å and 2.0(1) Å) are in good agreement with O-H distances from simulations (1.68 Å and 2.01 Å).34
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Figure 2. Crystal truncation rods (CTRs) of the hydrated calcite surface (open circles) and in aqueous glycine solution (filled circles). The patterns are shifted for clarity. The fits (solid lines) were obtained from the respective structure models. CTRs are labeled with the integer indices (HK), where L refers to the component of the momentum transfer Q perpendicular to the surface. Bragg peaks occur at integer values of L, with systematic absences for (H0L), H ) 2n + 1.
Calcite Surface with Aqueous Glycine Solution. The refined structure model of the calcite-water interface provided a basis for the analysis of the calcite-glycine interface in aqueous solution. The specular CTRs of both measurements look similar, as expected, due to the small difference in scattering contribution of the glycine molecules (Figure 2). Both scans show a broad distribution of the data points coming from background subtraction and are, therefore, not suitable for quantitative analysis. The lower intensity of the glycine CTR indicates an enhanced roughness of the surface, caused by the competitive adsorption of gly and water molecules on the surface. Introducing glycine molecules to the sorbate layer of the calcite surface in aqueous environment changes the nonspecular CTRs significantly in the surface-sensitive range between Bragg peaks. The measured CTRs and the best fit of the data are shown in Figure 2. When exposed to aqueous glycine solution, the interface is composed of a laterally ordered monolayer of water a distance of 1.9(1) Å from the surface with two water molecules per surface unit cell area, followed by a second layer with one gly and one water molecule per surface unit cell in a distance of 3.1(1) Å, also with perfect lateral order (Figure 3). The surface of the calcite substrate is only slightly relaxed within the topmost 3-4 layers, similar to what has already been observed for the water film. The translational and rotational parameters of the atom species of the topmost four layers can be found in the Supporting Information. In agreement with the calcite-water interface structure, the hydration layer consists of two water molecules per unit cell, which complete the O-coordination sphere of the surface Ca cations. The interaction between surface atoms and the glycine molecules is not strong enough to replace water molecules in the first hydration layer. Glycine molecules are accommodated only in the second layer. The fit of the structure model improved significantly when the glide plane symmetry of the calcite surface was also included for the adsorbate layers. The glide plane, therefore, constrains the sorbate molecules in both layers. This was already observed independently as result of the atomistic simulation of docking the glycine zwitterion on the calcite surface. However, the structure refinement yielded site occupation factors of 0.5 for the two equivalent glycine and water positions in the
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Figure 3. Structure model of the orthorhombic calcite unit cell in aqueous glycine solution. The two different motifs of occupation in the adsorbate layer are displayed separately in panels a and b. Hydrogen positions are restrained and not refined. The layers perpendicular to the c-axis are labeled with A1 and A2 for the adsorbate and with L1 to L4 for the calcite crystal, respectively.
second adsorbate layer. As net occupation in the mixed adsorbate layer, one glycine and one water molecule per unit cell were obtained, which is in agreement with the available space for sorbate molecules in the second layer. Hence, two motives of adsorption exist, which are shown in Figure 3. Incoherent addition of two domains with different calcite relaxations in the surface model resulted in a fit with a χ2 of 2.71, whereas the coherent model yielded a fit with χ2 ) 1.58. Thus, the two occupation possibilities for one unit cell add coherently. Long-Range Order of the Adsorbate Layer. The whole calcite crystal surface can be covered with theoretically innumerable versions of combinations of the two occupations with one gly and one water molecule per unit cell, determined from the structure refinement. However, the strong periodical order of the surface is likely to induce long-range order in the adsorbate layers. From force-field simulations, we calculated the energy of a calcite slab, varying the occupation of the adsorbate layer with different arrangements of the two gly occupations of the calcite unit cell. The energy differences of the occupation patterns provide information about the probability of the appearance of long-range order in the adsorbate layer. However, the energy of this system strongly depends on the position of the H-atoms and the formation of hydrogen bonds. Therefore, the calculation of the energy was coupled with a molecule dynamics simulation, where the H-positions were refined. The energy calculation of the simulated occupations of the adsorbate layer yielded surface coverage where every unit cell has the same occupation with the same motif (Figure 4). Mixed occupations of the two motifs were found to be energetically more unfavorable. The interface structure is furthermore stabilized by an extended network of hydrogen bonds, mostly bridges from the hydration layer to the surface and the adsorbate glycine layer. Bond lengths are summarized in Table 1. The interface structure of glycine on calcite exhibits various similarities to the hydrated calcite surface, as could be expected
Water/Glycine Film on (104) Calcite Surface
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Table 1. Interlayer Distances, Ca Coordination, and Hydrogen Bond Lengths of the Refined Structure Models hydrated surface
aqueous glycine solution
A1 - surface plane A2 - surface plane Ca2+ - OH2O, A1 Ca2+ - OH2O, A2 Ca2+ - OCOO-
1.9(1) Å; [2.3(1) Å] 3.1(1) Å; [3.5(2) Å]20 2.4(1) Å; [3.0(1) Å]20 3.6(1) Å
A1 OH2O-H‚‚‚OCO3 A1 OH2O-H‚‚‚A2 OH2O A1 OH2O-H‚‚‚A2 OCOOA1 OH2O-H‚‚‚A2 N A2 N-H‚‚‚A1 OH2O
2.5-3.2 Å 2.4 Å
20
1.9(1) Å 3.1(1)/4.3(1)a Å; [3.4(1) Å]b 2.45(9) Å 4.6(1) Å 3.5(1), 4.1(1), 4.7(1), 4.9(1)c Å; [3.2(1), 4.5(1) Å]d
Hydrogen Bonds 2.8-3.0 Å 3.0 Å 3.0-3.2 Å 3.0-3.1 Å 3.0-3.1 Å
a z-coordinate of CR atom of glycine/z-coordinate of O of water molecule. b z-coordinate of CR atom of glycine from MM simulation. c Coordination distances of both O atoms of the carboxylate group to the two Ca cation neighbors. d Coordination distances of both O atoms of the carboxylate group to the next Ca cation neighbors from MM simulation.
Figure 4. Result of the MD simulation of the experimental structure model of the calcite-glycine interface. Only the hydrogen positions were optimized.
from the similarity of the specular CTR (Figure 2). The hydration layer remains on the surface, the Ca coordination is filled by O-atoms of water molecules and not replaced by the O-atoms of the carboxylate groups of glycine. The gly molecules are placed in a second adsorbate layer a distance of 3.1(1) Å above the surface, which agrees with the distance of the second water monolayer of calcite with a pure water film of 3.1(1) Å (Table 1). Also, the Ca-O distances between the surface Ca-cations and the O-atoms of the water molecules in the hydration layer coincide with 2.4(1) Å and 2.45(9) Å, respectively. The gly zwitterion is oriented with the carboxyl group slightly tilted toward the surface, forming the shorter Ca-O distances of 3.5(1) and 4.1(1) Å from the nearest Ca-cations, whereas the other O pointing up is 4.7(1) and 4.9(1) Å away from the neighboring Ca-cations. The shortest Ca-OCOO- distance of 3.5(1) Å coincides with the Ca-OH2O distance of the second ordered water layer of the hydrated calcite surface of the same value (see Table 1). Apart from the weak interaction between the surface and the gly molecules, the amino acid “binds” not like a chelate-like
formation, but rather monodentate, with one O of the COO group positioned comparatively equally between two cations. The calcite surface in contact with aqueous glycine solution is relaxed but not reconstructed. The lateral as well as the normal surface relaxations decrease with increasing depth, passing into the bulk structure at about 12 Å depth. In the topmost four layers, the carbonate groups are rotated about z by angles between -0.5° and 3.4° and tilted toward the surface by (2.8 ( 0.9)° in the first layer and away in the following layer by (9.5 ( 1.1)°, (4.8 ( 1.2)°, and (2.6 ( 1.2)°, respectively. The surface of calcite in aqueous glycine solution shows smaller relaxations compared to the surface structure of the hydrated calcite crystal. The roughness parameter β41 refined for the water-calcite interface and the glycine-water-calcite interface is 0.01(1) for both structure models, yielding a rms roughness of 0.3(1) Å. This value is very low compared to the surface roughness of calcite determined from X-ray reflectivity of 2.6 Å.29 The scattered signal from different terraces adds coherently, so the roughness refined from the experimental data describes the roughness of the terraces, not the overall roughness of the surface. Comparing the simulated and experimentally found position and orientation of the gly molecule adsorbed on the calcite surface, we observed a large shift of the molecule’s position of ∼1.1 Å in x- and ∼2.5 Å in y-direction, respectively. Additionally, in the refinement, the gly molecule was rotated counterclockwise about z by about 30° and tilted toward the surface by 7°. Despite the change in position, the gly surface distances and the Ca-O coordination lengths are reasonable and only slightly different (see Table 1). The lateral shift of the gly position during the structure refinement can be attributed to the difference between the treatments of the aqueous environment in the simulation and in the structure refinement. In the simulation, the aqueous environment was modeled implicitly by choosing the dielectric constant of water for the environment. This leads to an average isotropic shielding of surface charges. In the experiment, the water molecules are explicitly included as rigid molecules, exhibiting a strongly anisotropic dipolar moment, which strongly influences the interactions in the mineral-adsorbate interface. The successful derivation of a reasonable structure model of the calcite-adsorbate interface from the experimental data set proves the applicability of the GIXRD method for the investigation of mineral-organic interfaces. The method is highly sensitive to the atomic-scale interface structure. By applying simulation techniques to experimental results, additional structural information can be obtained, e.g., H-positions, the formation of hydrogen bond networks, and predictions about the probability of different structures in the adsorbate layer can be made.
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In aqueous solution, the adsorption of the dissolved molecular species is in competition with the formation of a hydration layer on the calcite surface. The hydration layer shields the surface, so that the ordering of the amino acid molecules is determined mostly by van der Waals interaction and hydrogen bonding. Our experiment showed that in aqueous solution the dissolved gly molecules have no direct interaction with the surface. This is probably the case for other amino acids, too, despite the presence of functional groups, e.g., carboxylate groups, or their good geometric and stereochemical fit with the mineral surface charge distribution. However, our experimental results are valid for the calcite surface terraces. The atomic structure of the (104) face is very stable, as we observed no indication for a surface reconstruction and only small relaxations of the topmost atom layers. The stability and rigidity of this surface is an important feature allowing us to study this surface experimentally with surface X-ray techniques, but it also could be responsible for the fast and stable adsorption of a water layer. Other surfaces, e.g., the (001) surface, would (43) Orme, C. A.; Noy, A.; Wierzbicki, A.; McBride, M. T.; Grantham, M.; Teng, H. H.; Dove, P. M.; DeYoreo, J. J. Nature (London) 2001, 411 (6839), 775-779.
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be a very interesting substrate also, but the surface roughness and size of single crystals available are not sufficient for surface diffraction experiments. Other experimental techniques, e.g., AFM, show the formation of etch pits ad changes in the surface morphology, where additives, e.g., amino acids like aspartate,18,43 interact with dislocations and step edges on the surface. Here, the experimental approach is even more difficult, so that the mechanisms of interactions of organic molecules on rough, stepped surfaces on the atomic scale are not yet accessible with experimental techniques. Acknowledgment. We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities, and we would like to thank H. Kim and E. Paiser for assistance in using beamline ID03. Supporting Information Available: Two tables with initial atom coordinates and refined structure parameters of the hydrated calcite surface and the glycine/water/calcite interface structure model, respectively; one table showing initial Cartesian coordinates of water and glycine molecules; one AFM image of the calcite (104) surface. This material is available free of charge via the Internet at http://pubs.acs.org. LA0636659
