The Electronic Structure and Adsorption Geometry of - American

J. Phys. Chem. B 2008, 112, 13655–13660

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The Electronic Structure and Adsorption Geometry of L-Histidine on Cu(110) Vitaliy Feyer,*,† Oksana Plekan,‡,† Toma´sˇ Ska´la,† Vladimı´r Cha´b,§ Vladimı´r Matolı´n,| and Kevin C. Prince† Sincrotrone Trieste S.C.p.A., Area Science Park, Strada Statale 14, km 163.5, I-34012 BasoVizza, Trieste, Italy, Institute of Physics, Academy of Sciences of the Czech Republic, CukroVarnicka´ 10, 16253 Prague 6, Czech Republic, and Charles UniVersity, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, VholesˇoVicˇka´ch 2, 18000 Prague 8, Czech Republic ReceiVed: June 27, 2008; ReVised Manuscript ReceiVed: September 5, 2008

The adsorption of L-histidine on clean and oxygen-covered Cu(110) surfaces has been studied by soft X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The valence band spectra, carbon, nitrogen and oxygen 1s XPS and N K edge absorption spectra were measured for submonolayer, monolayer, and multilayer films. The spectra provide a detailed picture of the electronic structure and adsorption geometry at each coverage. In the monolayer, the histidine molecules are randomly oriented, in contrast to the submonolayer regime, where the molecules are coordinated to the copper surface with the imidazole functional group nearly parallel to, and strongly interacting with, the surface. The π*/σ* intensity ratio in NEXAFS spectra at the nitrogen edge varies strongly with angle, showing the imidazole ring is oriented. Adsorption models are proposed. 1. Introduction In many fields ranging from medicine to nanotechnology, bioinorganic interfaces are of great interest. In addition, amino acids are important building blocks of proteins, and information about adsorption of these molecules on metal surfaces can provide an understanding necessary for the construction of sensors and similar devices. Histidine is an amino acid that consists of amino, imidazole (IM), and carboxyl groups. The schematic structure of histidine (C6H9N3O2) is shown in Figure 1. The imidazole side chain contains two nitrogen atoms with different properties: the ring amino nitrogen (N3) is bound to hydrogen and donates its lone pair to the aromatic ring and as such is slightly acidic, whereas the imino nitrogen (N2) donates only one electron pair to the ring so it has a free lone pair and is basic. The IM ring can readily switch between various protonated or unprotonated states.1,2 It has been found that interaction of imidazole with copper at zero oxidation state yielded metal imidazolates that covered the metal surfaces as a passive thin layer.3 Therefore, imidazole-containing compounds can have important applications as corrosion inhibitors and as adhesion promoters, particularly on copper. A variety of experimental surface science techniques such as reflection absorption infrared spectroscopy (RAIRS), photoelectron diffraction, XPS, and NEXAFS, together with theoretical approaches such as density functional theory (DFT), have been used to study the adsorption of amino acids on the Cu(110) surface.1-16 It has been found 4-11 that at low coverage, glycine, alanine, norvaline, and lysine interact with the Cu(110) surface via the oxygen atoms of the deprotonated carboxylate * Corresponding author. Tel.: +39 0403758287. Fax: +390403758565. E-mail: [email protected]. † Sincrotrone Trieste S.C.p.A. ‡ Permanent address: Institute of Electron Physics, 88017 Uzhgorod, Ukraine. § Academy of Sciences of the Czech Republic. | Charles University.

Figure 1. Schematic structure of histidine (C6H9N3O2).

group (COO-) and the nitrogen atom of the amino group (NH2). However, at high coverage, close to a monolayer, the molecules reorient to be anchored to the surface via a single oxygen atom of a sideways-tilted carboxylate moiety. After annealing to ∼425-450 K, these molecules formed adsorption structures where both carboxylic oxygen atoms and the amino groups bond to the surface. These structures were similar to those obtained at low coverage. Sayago et al.12 used photoelectron diffraction to determine that the N atoms of alanine at low coverage were adsorbed on Cu(110) close to atop sites, and the O atoms were also in sites consistent with bonding to single-surface Cu atoms and substantially atop. The amino acid serine has an additional OH group that forms intermolecular hydrogen bonds on copper surfaces.6,13 Proline, in which the amino nitrogen atom is located in a pyrrolidine ring, adsorbs on Cu(110) in an anionic form, bonding largely via O atoms of the carboxylate group, in which the two oxygen atoms are equidistant from the surface, and the N atom of the ring also bonds to the surface. The pyrrolidine ring was tilted at a small angle with respect to the surface plane.14 Cysteine and methionine differ from all other amino acids by the presence of a potentially more reactive sulfur atom in

10.1021/jp805671h CCC: $40.75  2008 American Chemical Society Published on Web 10/04/2008

13656 J. Phys. Chem. B, Vol. 112, No. 43, 2008 the side chain. At low coverage, they show similar chemisorption behavior and interact with the Cu(110) surface via the carboxylate group, with the two oxygen atoms equidistant from the surface. The amino (NH2) group and S atoms also interact with the surface.6,15 Using RAIRS, Mateo Marti et al. reported that histidine preferentially binds to the Cu(110) surface in its HHismolecular form, that is, with the carboxylic acid group deprotonated.1,16 A model has been proposed in which adsorption occurs at three sites: via the two oxygen atoms that are nearly equidistant from the surface; via the N2 nitrogen atom of the IM ring, which adopts an upright position with respect to the surface; and via the primary amino group (NH2), which remains close to the surface.1 In a second model, the NH2 and COO- groups are involved in the interaction with the Cu(110) surface, with the ring close to the surface, but not bonding.16 Preadsorption of oxygen (θ ≈ 0.3) on the copper surface leads to a p(2 × 1) reconstruction. This surface forces a readjustment of the molecular orientation, and interaction of the amino group (N3) of the heterocycle with the Cu-O row of the Cu(110)-O-p(2 × 1) reconstruction was reported. The ring tended to be less upright than in the absence of oxygen, and the molecules did not reorient with increasing oxygen coverage (θ ≈ 0.6).1 In monolayer films, histidine bonds to polycrystalline gold surfaces as anions, that is histidinate, forming strong ionic-covalent bonds with the gold substrate atoms.2 The imino N2 nitrogen atoms in the IM ring and carboxylate groups were involved in the bonding to the substrate. In multilayer films, the molecules were zwitterionic. They formed 3D aggregates bound by strong intermolecular forces and interacted only weakly with the gold substrate. Two types of zwitterions in about equal proportions have been observed with protonated amino groups and protonated IM rings. However, Zubavichus et al. left open the question of the orientation of histidine on the surface. Previous studies of histidine on Au(111) by RAIRS showed that histidine interacted with the surface via the carboxylate group only, and the IM ring was not bound to the surface.16 It appears that additional information about the adsorption geometry of histidine on Cu(110) is required if we are to understand it fully. High-resolution photoelectron spectroscopy has not been used to study the adsorption of histidine on Cu(110) single crystals. Here, we present a combined XPS and NEXAFS studies of histidine adsorbed on Cu(110), which provide additional information about the adsorption geometry and electronic structure. 2. Experimental Section The experiments were performed at the Materials Science Beamline at the Elettra synchrotron light source in Trieste.17 The mechanical movement of the grating and plane mirror has recently been upgraded to provide better reproducibility. The original optical concept of the beamline has been substantially retained. During the experiments, the base pressure in the main chamber was 2 × 10-10 mbar. Photoemission spectra were recorded with an angle of incidence of the synchrotron light of 30° with respect to the surface normal and an emission angle of 30°, using the multichannel Phoibos analyzer of the beamline. The total energy resolution of the analyzer and incident photons was estimated to be 0.1 and 0.45 eV at 43 and 490 eV, respectively. The binding energy (BE) was calibrated by measuring the Fermi edge with the same monochromator and analyzer parameters (photon energy and pass energy) after each

Feyer et al. change in the beamline setting. The O 1s XPS spectra were measured with the same Phoibos analyzer with Al KR radiation as the excitation source and the total energy resolution was 0.95 eV. The NEXAFS spectra were taken at the N K edge using the nitrogen KVV Auger yield at normal (90°), grazing (10°) incidence, and at the magic angle (55°) of the photon beam with respect to the surface. The energy resolution in NEXAFS was estimated to be 0.3 eV. The polarization of light from the beamline has not been measured, but is believed to be between 80 and 90% linear, as the source is a bending magnet. The sample was mounted with its [001] crystallographic direction in the vertical direction, that is, parallel to the manipulator axis. During the course of the experiment, the crystal rotated slightly in its holder (10°). No significant changes were noted in spectra taken before and after this rotation. Thus the light was incident along, or close to the [1j10] azimuth. The raw NEXAFS data were normalized to the intensity of the photon beam, measured by means of a high transmission gold mesh in the beam. The Cu(110) single crystal was cleaned using standard procedures: cycles of Ar ion sputtering (kinetic energy 0.5 keV), followed by flashing to 870 K. The surface order and cleanliness were monitored by LEED (low-energy electron diffraction) and XPS. Contaminants (such as C, N, and O) were below the detection limits. L-Histidine (99.5%) was obtained from Sigma Aldrich and used without further purification. Histidine was deposited on the surface in a separate preparation chamber, with base pressure 5 × 10-9 mbar (mainly water), using a homemade Knudsen cell type evaporator. Before the deposition, the histidine powder was degassed in vacuum at 385 K, and then heated to 443 K and dosed onto the surface. The deposition rate was approximately 1.0 monolayer (ML) in 200 s. The ML was prepared by adsorbing about ∼1.5 ML and then flashing to 375 K to desorb the weakly adsorbed histidine species. Subsequent heating to 450 K did not change the areas of the C 1s, N 1s and O 1s XPS signals, whereas annealing to 475 K led to partial decomposition and desorption of histidine from the surface, which was indicated by a decrease in intensity and substantial changes in the XPS spectra. Amino acids and many other organic molecules have a tendency to dissociate when exposed to ionizing radiation, so to check for radiation damage, valence band spectra of the multilayer at 43 eV were measured. No spectral changes were observed after 1 h, so we believe that histidine is reasonably stable under our experimental conditions. Although we used only one enantiomer, our spectroscopic methods are not sensitive to chiral effects, so we drop the prefix L from the description of histidine. The oxygen modified Cu(110) surface was prepared in the main chamber by exposing the surface to oxygen for 5 min at PO2 )1 × 10-6 mbar, leading to a p(2 × 1) LEED structure.1 3. Results and Discussion 3.1. Valence Band Spectra of Histidine. The valence band spectra of the multilayer, monolayer and submonolayer of histidine on Cu(110) measured at photon energy 43 eV are shown in Figure 2. The copper d band lies in the interval 2 to 6 eV approximately and partially overlaps part of the spectrum of histidine. Difference spectra have been calculated but at low coverage the spectra are less reliable in this energy interval as they may contain residual structure due to the d band. This is because at this energy, the observed valence band spectrum is due to direct k conserving transitions. The disordered layer

Structure and Geometry of L-Histidine on Cu(110)

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Figure 2. Valence spectra of histidine adsorbed on Cu(110) and annealed to the temperature indicated. Photon energy 43 eV. (a) θ ) 1.5 and 1.0 ML; (b) θ ) 0.4 ML; (i) before adsorption; (ii) after adsorption; (iii) spectra formed by subtraction of (i) from (ii).

Figure 3. N K-edge NEXAFS spectra of histidine adsorbed on clean and oxygen-modified Cu(110) surfaces and annealed to the temperature indicated, measured at grazing (GI), normal incidence (NI), and the magic angle (MA): (a) θ ) 1.0 ML, (b) θ ) 0.4 ML.

breaks the symmetry of the surface so that momentum conservation parallel to the surface is no longer conserved. The only valence band spectra of histidine previously reported are those of films on polycrystalline Au surfaces.2 The spectra of a multilayer sample and a monolayer of histidine, Figure 2a, are similar to those on the gold surface, and we follow the assignments of Zubavichus et al.2 Feature A was assigned to photoemission from nonbonding molecular orbitals localized on the amino group (N1) and imino nitrogen (N2) as well as oxygen lone pairs. The peaks B and C are related to the molecular orbitals of π and σ symmetry, which are strongly delocalized in the IM plane (the notation refers to the local symmetry of the ring, not the whole molecule). In the valence band spectra of multilayer and monolayer histidine, no significant change can be observed (Figure 2a). However, at submonolayer coverage after annealing, the B and C bands split

(Figure 2b). This is attributed to chemisorption induced shifts of the component orbitals. 3.2. NEXAFS Spectra of Monolayer and Submonolayer Histidine. The NEXAFS spectra of multilayers of histidine were not measured as they are not expected to differ from those of thin films reported by Zubavichus et al.18 Figure 3a shows typical N K edge NEXAFS spectra of histidine adsorbed on Cu(110) at a coverage of 1.0 ML. Four well separated features marked A, B, C, and D are observed. The peak A at 399.5 eV is attributed to the N2 1s f π* transition for the imino nitrogen in the IM ring. The second peak B at 401.2 eV is due to a transition of a ring amino (N3) nitrogen 1s electron to the π* resonance. The energy positions are in agreement within 0.2 eV with published data of polycrystalline powder films of histidine.18 This assignment is also supported by studies of solidstate IM, where the peaks were observed at 400.15 and

13658 J. Phys. Chem. B, Vol. 112, No. 43, 2008 401.5 eV.19 The two broad features C and D centered at 406.3 and 412.3 eV are attributed to transitions of 1s electrons of all three nitrogen atoms to σ* (N-C) shape resonances.9,19 The weak shoulder at 402.6 eV is also assigned to a σ* (N-C) resonance of N1 and it has been observed in NEXAFS spectra of alanine adsorbed on Cu(110),9 and in the gas phase.20,21 After flashing the histidine monolayer to a higher temperature, 425 K (Figure 3a) the spectra change only a little: peak A moved to higher energy and was centered at 400.0 eV (A′ in the spectra). As was mentioned above the A resonance is related to the imino nitrogen (N2) in the IM ring, and therefore the shift in energy ∼0.5 eV (A′) is due to the stronger interaction of the IM ring with the copper surface via the N2 atom. In the monolayer spectra (Figure 3a), there is no strong angular dependence of the π*/σ* intensity ratio, indicating that the IM ring in the monolayer regime is randomly oriented. The NEXAFS spectra at lower coverage (∼0.4 ML) in Figure 3b are different from the monolayer spectra. Also shown are spectra of the oxygen covered surface, which will be discussed below. The spectra do not show the A resonance, but peak A′ at 400.0 eV is the dominant peak, which suggests strong interaction via the N2 imino nitrogen atom of the IM ring with the copper surface. The spectra change substantially with angle: the π* (A′ and B) resonances are stronger than the σ* resonances at grazing incidence (GI), while at normal incidence (NI) the π*/σ* intensity ratios reverse, and at the magic angle (MA), almost equal intensity is observed. This evidence and the absence of the A resonance suggest that the IM ring of histidine strongly interacts with the copper surface via the N2 nitrogen atom, with the ring roughly parallel to the surface. It is not exactly parallel because some intensity is observed at normal incidence. Flashing of the submonolayer of histidine to 425 K (Figure 3b) almost extinguishes the intensity of the π* resonances at NI, whereas at GI, the B resonance is invisible and one strong broad peak A′ is observed. This suggests that after annealing the IM ring is lying more nearly parallel to the surface, and it interacts with the surface via the two (N2 and N3) nitrogen atoms in the IM ring. Annealing the histidine submonolayer to 375 K produces an intermediate spectrum, with the ring tilted closer to the surface than before annealing. A coverage of about 0.4 ML was also deposited on an oxygen modified O-p(2 × 1) surface and spectra are shown in Figure 3b, curves “O-p(2 × 1)”. At GI the spectrum, shows one strong peak at 400.2 eV due to the transition to π* resonance, which suggest the same chemical state of two nitrogen atoms in IM ring. Oxygen adsorbed on Cu(110) is known to be a good hydrogen acceptor (Brønsted-Lowry base) and readily reacts with ammonia22 and amines23 to form water and other products. Therefore we propose that the N3 nitrogen in the IM ring loses a H atom and to become an imino nitrogen. At NI the π* resonances are stronger than for the same coverage on a clean surface (Figure 3b), which suggests a more upright orientation of the ring than on the clean surface. 3.3. Core Level XPS Results. XPS is site specific and so offers the possibility to determine which atoms of the molecule interact with the surface. The C 1s, N 1s and O 1s core level spectra of multilayer, monolayer and submonolayer coverages of histidine adsorbed on the Cu(110) surface are shown in Figures 46. The multilayer spectrum resembles the corresponding spectrum of histidine adsorbed on polycrystalline Au.2 The highest BE peak A of the C 1s spectrum of the multilayer of histidine (Figure 4a) is centered at 288.7 eV and assigned to the carboxylate carbon. The five carbon atoms of the IM ring and side chain contribute to the strong peak B located at lower

Feyer et al.

Figure 4. XPS of C 1s core levels for histidine adsorbed on clean and oxygen modified Cu(110) surfaces and annealed to the temperature indicated. Photon energy 490 eV. (a) θ ) 1.5 and 1.0 ML; (b) θ ) 0.4 ML.

Figure 5. XPS of N 1s core levels for histidine adsorbed on clean and oxygen modified Cu(110) surfaces and annealed to the temperature indicated. Photon energy 490 eV. (a) θ ) 1.5 and 1.0 ML; (b) θ ) 0.4 ML. Points, data; solid lines, fitted curves.

BE, and the ratio of the areas A:B ) 1:5. After annealing to 375 K and forming one ML of histidine, the peak due to carboxylate (A) shifted by 0.6 eV toward lower BE, whereas peak B moved by 0.3 eV, with the same area ratio A:B ) 1:5. After annealing to 425 K, the general spectral shape is retained (Figure 4a). The C 1s spectrum of low-coverage histidine on clean Cu(110) is shown in Figure 4b. As for the monolayer regime, after annealing the peak assigned to the carboxylic carbon C1 is at BE 288.1 eV, Figure 4a, and the other carbon atoms in the

Structure and Geometry of L-Histidine on Cu(110)

Figure 6. XPS of O 1s core level spectra measured with Al KR radiation. Histidine adsorbed on Cu(110) and annealed to the temperature indicated: (a) θ ) 1.5 and 1.0 ML; (b) θ ) 0.4 ML.

IM ring and side chain contribute to the broad peak. The C 1s spectrum of chemisorbed alanine (annealed to 450 K) on the Cu(110) surface showed peaks at 288.2, 286.2, and 285.7 eV and were assigned to the carboxylic, central and methyl carbon respectively.9 After annealing to 425 K, the C 1s spectrum is dramatically different from the spectrum after deposition (see Figure 4b); the peak due to the carboxylic carbon is at the same BE as after adsorption, whereas the core levels of the other carbon atoms are located in the sharp peak centered at 285.1 eV. The similar BE of the carbon atoms in the IM ring and side chain suggest stronger interaction with the copper surface and/or loss of H atoms. The intensity ratio of peak B to the peak due to carboxylic carbon (A) in C 1s spectrum after annealing to 425 K is 5:1, indicating that the skeletal structure of the molecule is still intact. The N 1s spectra of a monolayer and multilayer of histidine are shown in Figure 5a. The spectrum of the multilayer consists of two strong features, a broad peak at 401.15 eV (A) and a narrower peak centered at 399.6 eV (B). By fitting with two Gaussian profiles, we obtain an intensity ratio of 2:1. We assign peak B to the imino nitrogen (N2) and the lower BE part of A to the ring amino nitrogen atom (N3) and amino nitrogen (N1). The higher BE part of feature A has been assigned to protonated nitrogen atoms of amino groups in the IM rings.2 These assignments are supported by previous studies of alanine adsorbed on the Cu(110) surface8 and imidazole in the solid state.18 After annealing of multilayer histidine to 375 K, the relative intensity of peak A, especially the higher BE part, is substantially reduced, which implies reduction of the degree of nitrogen protonation of histidine with decreasing film thickness. By fitting with two Gaussian profiles, we obtain an intensity ratio of A:B ) 1:1.7, which suggests that about 60% of ring amino has lost hydrogen to become imino. After annealing to the higher temperature of 425 K, the new feature C at 398.6 eV appeared, which indicates strong interaction of nitrogen with the Cu surface. Fitting with three Gaussian profiles gives an intensity ratio of A:B:C ) 3:5.1. We propose that after annealing to 425 K, about 30% of histidine in the 1 ML

J. Phys. Chem. B, Vol. 112, No. 43, 2008 13659 overlayer is chemisorbed on the Cu surface via imino nitrogen atoms (N2) of the IM ring. This evidence is supported by the corresponding NEXAFS spectrum (Figure 3a), which was discussed above. The N 1s spectrum of histidine after adsorption of 0.4 ML is shown in Figure 5b. The spectrum was fitted using three Gaussian peaks with energy 400.35, 399.6, and 398.6 eV. We assign the higher BE peaks to amino N1 nitrogen, the peak at 399.6 eV to N3 nitrogen, and both are not bonded to the copper surface. The third peak at 398.6 eV is due to chemisorbed imino (N2) nitrogen atoms. This assignment is supported by evidence of a new feature C in the monolayer spectrum after annealing to 425 K (Figure 5a) and the absence of the A peak, whereas a new feature A′ appears in the corresponding NEXAFS spectrum (Figure 3b). After annealing to 425 K (Figure 5b), the spectrum has changed and the higher BE peak due to ionization of N1 1s appears at 399.9 eV, which indicate a stronger interaction (chemisorption) than after adsorption of amino nitrogen on the copper surface at 300 K. The peak due to the amino nitrogen (N1) of chemisorbed alanine on the Cu(110) surface was at 399.75 eV,9 which supports our assignment. The feature due to the ring amino (N3) nitrogen is moved to lower BE by ∼1.0 eV and is located at the same BE as chemisorbed imino nitrogen (N2). This evidence suggests that after annealing the amino nitrogen (N3) has lost H and strongly interacts with the surface. However, also deprotonation can take place and leave charge in the IM ring of histidine, and the two nitrogen atoms in the imidazole anion are indistinguishable.24 Strong interaction of the two nitrogen atoms in the IM ring was confirmed by the observation in the NEXAFS spectra of adsorbed histidine annealed to 425 K (see Figure 3b), which showed one π resonance peak, and has been discussed above. The C 1s and N 1s spectra of a low coverage of histidine on the oxygen modified O-p(2 × 1)-Cu(110) surface are shown in Figures 4b and 5b, and a different chemical state of histidine is observed after adsorption, compared with the clean surface. The similarity of the C 1s spectra of a submonolayer of histidine on the oxygen-modified surface (Figure 4b) to the core level spectrum of the oxygen-free surface after annealing to 425 K suggests a similar chemical state of carbon in both cases. Furthermore the N 1s spectrum of histidine adsorbed on the p(2 × 1) surface is closer to the corresponding spectrum after annealing of histidine on a clean surface to 425 K, however a big change in the chemical state of amino nitrogen (N1) at the higher BE is observed. The peak due to both the chemisorbed nitrogen atoms in the IM ring (N2 and N3) on the Cu(110) surface remains the same, while the peak due to N1 moved by ∼0.5 eV toward lower BE. We suggest that preadsorbed oxygen leads to removal of H from amino nitrogen atom (N3), whereas the energy shift is due to N1-HsO hydrogen bonding. The spectral shape of the O 1s core hole of multilayers and the monolayer after annealing to 375 and 425 K is retained and only a small shift of ∼ 0.3 eV toward lower BE is observed after annealing to the two temperatures, Figure 6a. The fwhm of the multilayer peak is about 1.8 eV and narrows to 1.65 eV after annealing to 425 K. Histidine is known to exist as a zwitterion in the solid state, and this is reflected in the single O 1s peak of (COO-) rather than two separate peaks for oxo and hydroxy forms of oxygen. The small spectral change for the monolayer suggests that the carboxylic acid group is also deprotonated in the monolayer, and the shift is probably a final state shift reflecting more efficient screening by the metallic substrate. In Figure 6b, the O 1s spectra of a 0.4 ML of histidine on a clean surface are shown, and only one peak is present at 532 eV

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Feyer et al. atoms with a small tilt angle of the IM ring with respect to the surface, as indicated in Figure 7a. This is in agreement with a previous study by Mateo Marti et al. using RAIRS.1 However, after annealing to 425 K the N3 nitrogen lost a H atom, and both nitrogen atoms of the IM ring (N2 and N3) and the nitrogen atom of the amino group (N1) bond to the surface covalently, Figure 7b. The carboxylate group (COO-) forms a strong ionic bond to the Cu(110) surface. This model suggests that histidine bonds to the surface in its HHis- ionic form. The previous RAIRS study suggested a small difference in adsorption of histidine on a clean and oxygen modified surface, with less upright orientation in the last case. However the present studies show that on a O-p(2 × 1)-Cu(110) surface not only the N2 nitrogen and carboxylate group are involved in the interaction but preadsorbed oxygen on the surface reacts with H from histidine which then also bonds to the surface via the dehydrogenated N3 nitrogen atom, with the IM ring tilted to a more upright orientation.

Figure 7. Proposed model for histidine adsorbed on Cu(110) at coverage θ ) 0.4 ML: (a) after adsorption; (b) after annealing to 425 K.

BE. This indicates that for both a monolayer and low coverage, histidine interacts via the carboxylate group. The two oxygen atoms are in very similar chemical states as no chemical splitting is observed. The shift toward lower BE in the C, N, and O 1s core hole spectra of the high and low coverage adsorbed histidine can be explained by a net negative charge of the adsorbed histidine species or by final state screening effects.2 In the multilayer, the molecules are randomly oriented and strongly interact with each other via Coulomb forces and H-bonding, and are only weakly bound to the surface via van der Waals forces. However after annealing and at low coverage they adsorb in anionic form and are more strongly coordinated to the copper surface. A large chemical shift of the C 1s and N 1s core level spectra with respect to corresponding multilayer spectra is observed indicating a stronger interaction with the metal surface at low coverage (see Figures 4 and 5). 4. Conclusions The electronic structure and adsorption geometry of histidine on Cu(110) have been investigated using high-resolution synchrotron radiation based photoelectron spectroscopy. On the basis of our NEXAFS and XPS data, we conclude that at high coverage, the IM side chains of the histidine molecules are randomly oriented, because no strong angular dependence of the π*/σ* intensity ratio was observed. The molecules bind to the copper surface via the carboxylate group, and the IM ring lies close to the surface. This last conclusion is supported by the observation of a single O 1s peak in XPS and features in the N 1s spectra due to two amino and imino nitrogen atoms. After annealing to 375 K, one ML remains on the surface, the IM ring was tilted close to the surface, and loss of the H atom from the N3 nitrogen in the IM ring was observed. A new feature (C) in the N 1s spectrum of the monolayer heated to 425 K was observed, indicating the strong interaction of the IM ring with the copper surface via the imino nitrogen atom (N2). At low coverage histidine interacts with the Cu(110) surface only via the carboxylate group and ring imino (N2) nitrogen

Acknowledgment. O.P. thanks the Central European Initiative for a fellowship. We gratefully acknowledge the assistance of our colleagues at Elettra for providing good-quality synchrotron light. The Materials Science Beamline is supported by the Ministry of Education of the Czech Republic under Grant LC06058. References and Notes (1) Mateo Marti, E.; Methivier, Ch.; Dubot, P.; Pradier, C. M. J. Phys. Chem. B 2003, 107, 10785. (2) Zubavichus, Y.; Zharnikov, M.; Yang, Y.; Fuchs, O.; Heske, C.; Umbach, E.; Tzvetkov, G.; Netzer, F. P.; Grunze, M. J. Phys. Chem. B 2005, 109, 884. (3) Xue, G.; Dong, J.; Sun, Y. Langmuir 1994, 10, 1477. (4) Barlow, S. M.; Kitching, K. J.; Haq, S.; Richardson, N. V. Surf. Sci. 1998, 401, 322. (5) Nyberg, M.; Hasselstro¨m, J.; Karis, O.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Pettersson, L. G. M. J. Chem. Phys. 2000, 112, 5420. (6) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201. (7) Kang, J.-H.; Toomes, R. L.; Polcik, M.; Kittel, M.; Hoeft, J.-H.; Efstathiou, V.; Woodruff, D. P.; Bradshaw, A. M. J. Chem. Phys. 2003, 118, 6059. (8) Barlow, S. M.; Louafi, S.; Le Roux, D.; Williams, J.; Muryn, C.; Haq, S.; Raval, R. Surf. Sci. 2005, 590, 243. (9) Jones, J.; Jones, L. B.; Thibault-Starzyk, F.; Seddon, E. A.; Raval, R.; Jenkins, S. J.; Held, G. Surf. Sci. 2006, 600, 1924. (10) Humblot, V.; Me´thivier; C.; Pradier, C.-M. Langmuir 2006, 22, 3089. (11) Humblot, V.; Me´thivier, C.; Raval, R.; Pradier, C.-M. Surf. Sci. 2007, 601, 4189. (12) Sayago, D. I.; Polcik, M.; Nisbet, G.; Lamont, C. L. A.; Woodruff, D. P. Surf. Sci. 2005, 590, 76. (13) Iwai, H.; Emori, A.; Egawa, C. Surf. Sci. 2006, 600, 1670. (14) Mateo Marti, E.; Barlow, S. M.; Haq, S.; Raval, R. Surf. Sci. 2002, 501, 191. (15) Mateo Marti, E.; Methivier, Ch.; Pradier, C. M. Langmuir 2004, 20, 10223. (16) Mateo Marti, E.; Quash, A.; Methivier, Ch.; Dubot, P.; Pradier, C. M. Colloid Surf., A 2004, 249, 85. (17) Vasˇina, R.; Kolarˇ´ık, V.; Dolezˇel, P.; Myna´rˇ, M.; Vondra´cˇek, M.; Cha´b, V.; Sleza´k, J.; Comicioli, C.; Prince, K. C. Nucl. Instrum. Methods Phys. Res. A 2001, 467-468, 561. (18) Zubavichus, Shaporenko, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem A 2005, 109, 6998. (19) Apen, E.; Hitchcock, A. P.; Gland, J. L. J. Phys. Chem. 1993, 97, 6859. (20) Morita, M.; Mori, M.; Sunami, T.; Yoshida, H.; Hiraya, A. Chem. Phys. Lett. 2006, 417, 246. (21) Feyer, V.; Plekan, O.; Richter, R.; Coreno, M.; Prince, K. C.; Carravetta, V. J. Phys. Chem. A 2008, 112, 7806. (22) Guo, X.-C.; Madix, R. J. Surf. Sci. 1997, 387, 1. (23) Davies, P. R.; Edwards, D.; Parsons, M. Surf. Sci. 2007, 601, 3253. (24) Xu, G.; Dai, Q.; Jiang, S. J. Am. Chem. Soc. 1988, 110, 2393.

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