Adsorption Structure of Glycyl-Glycine on Cu(110) - The Journal of

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Adsorption Structure of Glycyl-Glycine on Cu(110) Vitaliy Feyer,*,† Oksana Plekan,†,⊥ Nataliya Tsud,‡ Victor Lyamayev,†,# Vladimı´r Cha´b,§ Vladimı´r Matolıń,‡ Kevin C. Prince,† and Vincenzo Carravetta| Sincrotrone Trieste S.C.p.A., Area Science Park, Strada Statale 14, km 163.5, I-34012 BasoVizza, Trieste, Italy, Department of Surface and Plasma Science, Faculty of Mathematics and Physics, Charles UniVersity, V HolesˇoVicˇkaćh 2, 18000 Prague 8, Czech Republic, Institute of Physics, Academy of Sciences of the Czech Republic, CukroVarnicka´ 10, 16253 Prague 6, Czech Republic, and CNR-Institute of Chemical Physical Processes, Via Moruzzi 1, 56124 Pisa, Italy ReceiVed: April 1, 2010; ReVised Manuscript ReceiVed: May 17, 2010

Multilayer, monolayer, and submonolayer films of the dipeptide glycyl-glycine on the Cu(110) surface have been studied by a combination of soft X-ray photoelectron spectroscopy, near-edge X-ray absorption fine structure spectroscopy, and density functional theory calculations. Detailed models of the electronic structure and adsorption geometry at each coverage have been proposed. After adsorption, the molecules bind to the copper surface in two forms, zwitterionic and anionic, and the latter is dominant after annealing. In the monolayer, the anionic form of the dipeptide coordinates to the surface via the oxygen atoms of the carboxylate or peptide group and amino nitrogen atom, whereas in the submonolayer regime, the amino group interacts with the surface through a hydrogen atom. 1. Introduction Amino acids link together by peptide bonds to form polypeptides and proteins, which play crucial roles in virtually all biological processes. Because of their biological importance, there is a growing interest in studies of the interaction between organic compounds and surfaces, and this includes compounds with peptide bonds. Within the field of biosensors, and integration of biology and electronics, in particular, there is interest in the adsorption of amino acids and peptides on metal surfaces,1-16 and this interest has partially motivated the present study. The dipeptide, glycyl-glycine (Gly-Gly), consists of two glycine (Gly) molecules joined by a single peptide bond. Gly is the simplest amino acid and is the most basic model compound for understanding the adsorption of other amino acids and more complex compounds. A variety of experimental surface science techniques such as reflection absorption infrared spectroscopy (RAIRS), X-ray photoelectron diffraction, X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, together with theoretical approaches such as density functional theory (DFT) have been used to study the adsorption of Gly on the Cu(110) surface.1-4 However, studies dedicated to chemisorption of small peptides on well-defined surfaces have mostly begun to appear only recently.4,5,8-14 Using RAIRS, it has been found2,5 that at low coverage Gly interacts with the Cu(110) surface only via the oxygen atoms of the deprotonated carboxylate group (COO-) in an upright orientation. However, as the coverage increased, the carboxylate * Corresponding author. Tel: +39 0403758287. Fax: +390403758565. E-mail: [email protected]. † Sincrotrone Trieste S.C.p.A. ‡ Charles University. § Academy of Sciences of the Czech Republic. | CNR-Institute of Chemical Physical Processes. ⊥ Present address: Aarhus University, Department of Physics and Astronomy, Ny Munkegade 120, 8000 Aarhus C, Denmark. # Permanent address: Institute of Electron Physics, Universitetska St. 21, 88017 Uzhgorod, Ukraine.

group was bonded in a more asymmetric geometry, and at saturation, the molecules reorient to be anchored to the surface via a single oxygen atom of a sideways-tilted carboxylate moiety and the nitrogen atom of the amino group (NH2). After annealing a saturation coverage to ∼420 K, these molecules formed adsorption structures where both carboxylate oxygen atoms and the amino groups bond to the surface. It was not possible to distinguish whether the adsorbed Gly molecules at low coverage were in anionic (NH2CH2COO-) or zwitterionic (NH3+CH2COO-) form on the basis of RAIRS because of the similar vibrational frequencies of -NH2 and -NH3+ groups. The XPS, NEXAFS, and DFT showed that the most stable adsorption structure of a Gly monolayer (ML) on Cu(110) is deprotonated where both oxygen atoms were essentially chemically equivalent, with the O-O bond azimuthally oriented along the close-packed Cu [11j0] rows. Also, the nitrogen of the amino group was found to be involved in a chemisorption bond with a Cu atom in a neighboring [11j0] row.1,3 In contrast with the ML, Gly and its di- and tripeptides in thick films were found to be mostly in zwitterionic form,17-21 as measured by XPS, NEXAFS and electron energy loss spectroscopies (EELS). However because these studies were of thick layers, there was no information about the role of the peptide bond in adsorption of the chemisorbed species. Nevertheless, the CdO functionalities of the amide group (CONH) of peptides can play an important role in the adsorption process. Barlow et al.4 showed that trialanine and trileucine bond to the Cu(110) surface through terminal carboxylate ions and amino and amide groups. At low coverage, trialanine molecules were randomly adsorbed and isolated from each other, whereas close to monolayer coverage, the adsorbed molecules formed an adsorbate phase with intermolecular hydrogen bonding occurring across the surface. In addition, at higher coverages, a saturated bilayer phase was created with the perpendicular oriented CdO moiety of the peptide groups being involved in the strong interlayer hydrogen bonding.

10.1021/jp102922g  2010 American Chemical Society Published on Web 05/28/2010

Adsorption Structure of Glycyl-Glycine on Cu(110) In common with single amino acids, peptides can interact with the surface in a variety of chemical states dependent on the charges associated with the functional groups. Valleé et al.13,14,16 showed that from acidic solution, glycine-containing tripeptides adsorb on the gold surface in their cationic form. At moderate pH, the molecules were found to be mostly zwitterionic, whereas from basic solution, the dominant species observed were anionic. It has been found that amino acids and their small peptides self-organize to form a number of well-ordered 2D structures at the metal surface, which are dependent on coverage and temperature.5,14 Using scanning tunneling microscopy, Wang et al.10 also reported that ordered chiral structures of isolated diphenylalanine chains can be formed by utilizing terephthalic acid molecules as linkers. It appears that additional information is required on the chemisorption of glycine peptides on well-defined surfaces. Our interest here is to investigate the adsorbed molecules at saturated coverage where molecules may interact strongly with one another and at submonolayer coverage where there is space between them, and the molecule-surface interaction may dominate adsorption. We present a combined XPS and NEXAFS study of multilayer, monolayer, and submonolayer coverages of the dipeptide Gly-Gly, adsorbed on Cu(110), and provide information about the impact of the peptide bond on the adsorption geometry and electronic structure at each coverage. 2. Experimental Details The experiments were performed at the Materials Science Beamline at the Elettra synchrotron light source in Trieste,22 and the experimental details are very similar to those in ref 15. Photoemission spectra were recorded with an angle of incidence of the synchrotron light of 60° with respect to the surface normal and at normal emission (NE) geometry. The total energy resolution of the analyzer, incident photons, and natural broadening in the photoemission measurement was estimated to be 0.1 and 0.45 eV at 100 and 490 eV photon energy, respectively. The O 1s XPS spectra were measured with Al KR radiation as the excitation source, and the total energy resolution was 1.0 eV. The sample was mounted with its [001] crystallographic direction in the horizontal plane, that is, perpendicular to the manipulator rotation axis. The NEXAFS spectra were taken at the N and O K-edge using the nitrogen and oxygen KVV Auger yield at normal (NI, 90°), grazing (GI, 10°) incidence, and at the magic angle (MA, 55°) of the photon beam with respect to the surface. 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. Thereore, in the NEXAFS experiment, the E-vector was perpendicular to the close-packed copper rows and along the [001]-direction at normal incidence, and nearly normal to the surface ([110]direction) at grazing incidence. The energy resolution in NEXAFS was estimated to be 0.8 eV. The raw NEXAFS data were normalized to the intensity of the photon beam. The Gly-Gly sample was supplied by Sigma Aldrich in the form of crystalline powder with minimum purity of 99% and was used without further purification. Gly-Gly was deposited on the surface in a separate chamber, with base pressure ∼10-9 mbar (mainly water), using a homemade Knudsen cell type evaporator. Before the deposition, the Gly-Gly was degassed in vacuum at ∼380 K and then heated to 410 K and dosed onto the surface. The deposition rate was ∼1 ML in 360 s. The ML was prepared by adsorbing about ∼1.4 to 1.5 ML of Gly-Gly and then flashing to 420 K to desorb the weakly adsorbed,

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Figure 1. Theoretical model of Gly-Gly on the Cu(110) surface.

second layer Gly-Gly. Subsequent annealing at this temperature did not change the areas of the C 1s, N 1s, and O 1s XPS signals, and only annealing up to 500 K led to partial decomposition and desorption of Gly-Gly from the surface, which was indicated by a decrease in intensity and substantial changes in all core level spectra. Weakly adsorbed Gly-Gly molecules in multilayers were found to desorb slowly from samples at room temperature (300 K); however, the chemisorbed monolayer and submonolayer (0.4 ML) were stable. We checked for radiation damage by monitoring the valence band and C 1s XPS spectra measured at 100 and 490 eV photon energy, respectively. No spectral changes were observed after 1 h, so we believe that Gly-Gly is reasonably stable under our experimental conditions. The Cu(110) single crystal was cleaned using standard procedures.15 The oxygen-modified Cu(110) surface was prepared in the main chamber by exposing the surface to oxygen for 5 min at 1 × 10-6 Torr, leading to a sharp p(2 × 1) LEED structure.6 3. Computational Details The adsorption geometry of Gly-Gly on Cu(110) has been studied theoretically by DFT calculations using the StoBe code23 on a model system formed by Gly-Gly interacting with a cluster of 82 Cu atoms. (See Figure 1.) An “all electron” description was adopted for Gly-Gly and for the eight central atoms of the surface, whereas an effective core potential (ECP) was used to describe the remaining 74 Cu atoms of the cluster. The basis sets employed were TZVP (triple-ζ valence plus polarization) basis24 for the organic molecule, Wachters basis (8s,6p,4d)25 for the “all electron” Cu atoms, and ECP basis26 for the Cu atoms with pseudopotential. The Perdew-Burke-Ernzerhof density functional27 was used for both the exchange and the correlation terms. The adsorbate geometry was optimized by keeping the Cu cluster rigid at the experimental geometry of bulk Cu metal, whereas all atoms of Gly-Gly could freely move. The O 1s, N 1s, and C 1s ionization potentials of adsorbed Gly-Gly have been computed by ∆SCF calculations for each ionization site using the DALTON code28 with Ahlrichs VDZ basis set29 and the different optimized geometries of Gly-Gly on a reduced cluster containing 20 surface Cu atoms, as shown in Figure 2. 4. Results and Discussion 4.1. Calculation of Optimized Adsorption Geometries. In the present computational study, several geometries for the

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Figure 2. Proposed adsorption geometries of Gly-Gly on Cu(110) in anionic (AN) and zwitterionic (ZW) forms. Sticks between substrate atoms indicate the [11j0] direction.

anionic (AN) and zwitterionic (ZW) forms of Gly-Gly adsorbed on the Cu(110) surface have been optimized by the previously described DFT calculations on the model cluster. The computed adsorption geometries of Gly-Gly on the Cu(110) surface most relevant for the following discussion are reported in Figure 2, whereas the full set of calculated geometries is summarized in Figure 1 of the Supporting Information. The geometrical parameters for the carboxylic and the peptide plane of more stable structures are summarized in Table 1, and all coordinates of adsorbates optimized by DFT calculations are given in Table 1 of the Supporting Information. We have considered both bidentate (AN1-AN5) and monodentate (AN6, AN7, and AN8) coordination of the carboxylic group with the copper surface. However, following previous experimental and theoretical studies1-3 of Gly adsorbed on Cu(110) and the present experimental evidence that the O 1s XPS spectrum of the adsorbate shows a single broad peak instead of the three peaks observed in the same spectrum of Gly-Gly in the gas phase,30 it may be argued convincingly that the carboxylic group is deprotonated in the adsorbate. For this reason the most favored geometries are those with bidentate coordination of the carboxylate group with two neighboring Cu

TABLE 1: Geometrical Parameters for the Carboxylic and the Peptide Plane of More Stable Structuresa COO-

HN-CdO

geometries

tilt

azimuthal

tilt

azimuthal

AN1 AN2 AN2a AN4

87.0 59.8 51.1 48.9

3.4 12.9 5.5 2.5

89.3 42.7 40.6 23.5

4.6 11.1 41.7 5.9

Note: “tilt” ) angle (degrees) between the normal to the carboxylic plane or the peptide plane and the normal to the surface; “azimuthal” ) angle (degrees) between the projection of the normal to the carboxylic plane or the peptide plane and the direction [001] perpendicular to the close-packed copper rows on the surface. a

atoms along the close-packed [11j0] direction; such an interaction is particularly strong because the Cu-Cu distance (2.556 Å) is very close to that of the O atoms in -COO-. The structures with monodentate bonding of the carboxylic group (AN6, AN7, and AN8) have a weaker interaction with the surface and show a splitting in the O 1s spectrum that is in less good agreement with the present experiment.

Adsorption Structure of Glycyl-Glycine on Cu(110)

Figure 3. Valence band spectra of Gly-Gly adsorbed on Cu(110) and annealed to the temperature indicated. Photon energy 100 eV. (a) Clean surface, 0.4 ML; (b) 1.4 and 1 ML; (c) difference and gas phase spectra.30 The difference spectrum was formed by subtraction of the clean surface spectrum from the Gly-Gly spectrum at 1.4 ML.

In the AN2a, AN4a, and AN5a adsorption geometries, we consider the interaction of the NH2 group with the surface through a hydrogen atom (N-H · · · Cu), whereas in the adsorbed AN2b form, hydrogen loss from the bonded amino group was tested. The adsorbate geometries in Figure 2 have in common the bidentate coordination of the -COO- group with two Cu atoms, which is the only bond in AN1, whereas in AN4, the terminal amino group is also bonded to the surface, and in AN2, there is the further bonding of the -CO group of the peptide bond. The geometry of the zwitterionic form is, in general, rather close to that of the corresponding anionic form, apart from the missing interaction of the amino terminal group (protonated in the zwitterions) with the surface. It can also be observed that in optimized ZW2, the bond of the -CO group is also broken, and the final geometries of ZW2 and ZW4 are, in fact, very similar. The presence of multiple coordination points in AN2 and AN4, in comparison with AN1, is in favor of a larger interaction energy, but the approach of Gly-Gly to the surface to increase the number of bonds involves a distortion of the -COO- group from the direction normal to the surface plane, whereas the normal geometry gives the lowest energy for the bidentate interaction of the carboxylate. The energetic balance is such that the AN2 geometry turns out to be the most stable anionic structure, according to the SCF calculations with the “all electron” Cu(20) cluster but with a small energy difference with respect to AN1 of just a few kilocalories per mole. 4.2. Valence Level Spectra. The valence band spectra of Gly-Gly films and of the clean Cu(110) surface measured at photon energy 100 eV as well as a gas-phase spectrum taken at the same photon energy30 are shown in Figure 3. The gas-phase spectrum, Figure 3c, is referred to the vacuum level, and it has been shifted by 5.0 eV to align the majority of peaks; the surface spectrum is referred to the Fermi edge. The shift of 5.0 eV is due to the work function of the sample plus relaxation shifts in the condensed state. Most of the peaks observed in the gasphase spectrum are present in the spectrum of the multilayer Gly-Gly, although the peaks show broadening due to solid-state effects. The copper d band lies in the interval 2 to 6 eV approximately and partially overlaps some of the electronic states of the

J. Phys. Chem. C, Vol. 114, No. 24, 2010 10925 molecules. Two shoulders at 1.85 and 2.23 eV appear at lower BE, close to the 3d orbitals of copper, and there are no corresponding features in the gas-phase spectrum. These features could arise from the adsorbate or from direct transitions in the Cu bulk and so have been further investigated by measuring the VB spectra at different photon energies at NE and also at 100 eV of photon energy and various emission angles (Figure 2 of the Supporting Information). For the low BE shoulder at 1.85 eV, no peaks corresponding to 3d orbitals of copper were observed in this BE range for the clean surface, and the shoulder is present at all energies and angles. Therefore, it is assigned to states due to the interface formation between Gly-Gly and the Cu(110) surface. The feature at 2.23 eV may be tentatively attributed to another interface feature, but it may contain additional intensity due to 3d Cu photoelectrons scattered by the molecular layers. It is not always present and disappears at some angles or energies, as expected for a bulk state. We have obtained the difference spectrum in Figure 3c by subtracting the spectrum of the clean surface from the measured multilayer spectrum. The assignment of features in the spectrum follows our previously reported results of Gly-Gly in the gas phase30 and published results31,32 and is given in the Supporting Information. 4.3. Core Level Spectra. 4.3.1. Computed XPS Spectra. The computed O 1s, N 1s, and C 1s ionization potentials for geometries shown in Figure 2 are listed in Table 2, whereas calculations of all the optimized geometries in the anionic and zwitterionic forms of Gly-Gly adsorbed on the Cu(110) surface are summarized in Table 2 of the Supporting Information. The theoretical values of ionization potentials cannot be directly compared with the experimental ones because of the missing “work function” that makes the theoretical results more directly comparable to gas-phase results. Because the AN and ZW models are slightly different, a direct comparison of AN and ZW binding energies could also be affected by the same kind of problem of having different reference energies. It is then more appropriate to discuss the results in terms of differential chemical shifts among nonequivalent atoms in different adsorbates of analogous (anionic or zwitterionic) character and compare them with the free Gly-Gly in gas phase. 4.3.2. Multilayer and Monolayer Films. The C 1s, N 1s and O 1s core level spectra of 1.4 ML (multilayer) and 1 ML of Gly-Gly adsorbed on the Cu(110) surface are shown in Figure 4. For comparison the computed spectra for the most stable geometries AN2, AN4, and their zwitterionic forms (ZW2, ZW4) are also presented in the same Figure. The measured multilayer spectra resemble the previous spectra of Gly-Gly films adsorbed in situ by thermal evaporation onto a clean Si(111)7 × 7 surface.21 Our calculation shows that in the zwitterionic forms the C2 carbon atom contributes to the low energy band B, whereas the other three carbon atoms are responsible for the high energy feature A in the C 1s spectra. (See Table 2 and Figure 4a.) In the AN adsorbate forms, two bands are also calculated but with different contributions: the deprotonated carboxylate (-COO-) and amide (-CONH-) carbon atoms of Gly-Gly contribute to the highest BE peak A, whereas we assign the second peak B to the ionization of 1s electrons of the two methylene carbon atoms (-CH2-). (See Table 2 and Figure 4a.) This is in agreement with the experimentally observed, relatively large decrease in intensity on the high energy side (peak A) of the spectrum when the monolayer is formed. Comparison with the theoretical spectra then supports the presence of some zwitterions on the surface in the multilayer film.

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TABLE 2: Theoretical C 1s, N 1s, and O 1s Binding Energies in electronvolts of Gly-Gly in Different Adsorbate Statesa Core-hole

C1

C2

C3

C4

N1

N2

O1

O2

O3

AN1 ZW1 AN2 ZW2 AN4 ZW4 AN2a AN4a gas phase30

294.93 294.75 294.80 294.51 294.29 294.66 294.12 294.37 297.08

291.81 292.73 292.42 292.40 291.97 292.52 292.24 291.93 293.18

294.54 294.49 294.52 294.50 294.71 294.35 294.43 294.50 295.00

291.98 295.40 293.01 294.23 292.62 294.61 292.67 291.95 292.76

404.95 410.33 405.67 409.01 405.28 408.90 404.56 404.36 405.72

405.22 407.01 406.31 406.65 405.81 406.86 406.08 405.60 405.86

535.91 535.87 536.04 535.64 535.78 535.73 535.78 535.84 538.31

536.01 536.03 536.14 535.77 535.84 535.92 535.74 535.92 540.31

535.97 537.66 537.07 537.68 536.75 537.70 537.03 536.39 536.27

a

Atoms are numbered as shown in Figure 2, AN1 structure.

Figure 4. (a) C 1s and (b) N 1s core levels measured at 490 eV and (c) O 1s measured with Al KR radiation, of 1.4 and 1 ML Gly-Gly adsorbed on Cu(110) and annealed to the temperature indicated. Points: data; solid lines: theoretical curves. Theoretical spectra for the most stable geometries AN2, AN4, ZW2, and ZW4 are shown below the experimental curves. The computed C 1s, N 1s, and O 1s spectra are shifted by -5.9, -6.0, and -4.45 eV, respectively.

In the multilayer spectrum, the peaks A and B are at 288.75 and 286.7 eV, respectively, whereas in the monolayer spectrum, after heating to 420 K, an energy shift toward lower BE of about ∼0.3 eV is observed. (See Figure 4a.) The energy position of these peaks is in good agreement with a previously reported study of Gly-Gly films, where peaks corresponding to A and B were observed at 288.9 and 286.6 eV, respectively.21 The energy separation of the two peaks in the spectra of both the monolayer and multilayer is ∼2.0 eV, and it is significantly smaller than that observed in the gas-phase spectrum, ∼3.0 eV.30 This

difference is due to the fact that in contrast with the gas phase, the Gly-Gly molecules are zwitterionic in multilayer films. We therefore conclude that also in the monolayer sample the adsorbed species on Cu(110) are ionized. Similar energy differences were previously reported in C 1s spectra of simpler amino acids such as Gly and alanine adsorbed on Cu (110),1,33 and again, smaller separations between maxima compared with the corresponding spectra in the gas phase were observed.34-36 The energy separation of the two bands in the calculated C 1s spectra is ∼2 eV for AN2 and AN4 and ∼3 eV for AN1,



Figure 5. (a) N K-edge and (b) O K-edge NEXAFS spectra of 1 ML of Gly-Gly adsorbed on Cu(110), measured at GI, NI, and MA.

whereas in the monolayer spectrum, the separation is ∼2 eV. This comparison suggests that the Gly-Gly molecules in the ML are preferably bonded to the Cu(110) surface in the AN2 and AN4 geometries. There is a discrepancy between the theory and experiment in the relative intensity of peaks A and B, which are predicted to be equal, but experimentally, peak B is stronger. We do not know the reason for this but speculate that it may be due to intermolecular interactions such as hydrogen bonding. The N K-edge NEXAFS spectrum of glycine has been shown to be influenced by hydrogen bonding,37 and we have observed shifts in N 1s binding energy in the gas phase due to the same effect.30 We conjecture that on a surface, hydrogen bonds may also affect the C 1s binding energy, although we would have expected more obvious effects on the N 1s core level. Unfortunately a theoretical examination of this issue is beyond the scope of the present work because a calculation for multiple Gly-Gly molecules on a cluster is very demanding. The N 1s spectra of the multilayer and monolayer of GlyGly are shown in Figure 4b. The spectrum of the multilayer consists of a peak centered at 400.45 eV and a prominent shoulder at 402.2 eV. From our calculation, we assign the higher BE feature to the protonated amino nitrogen atom (N(1)Η3+). The chemical shift between N1 and N2 is calculated to be from 3 eV in ZW1 to 2 eV in ZW4 (Table 2, Figure 4b). The previous theoretical and experimental studies of thick Gly-Gly films and other amino acids support this assignment.7,15,21,33 The nitrogen atom (N2) of the peptide group and the unprotonated amino nitrogen (N1) contribute to the stronger peak. The N 1s spectrum of gaseous Gly-Gly shows a single broad peak; however, in comparison with the N 1s spectrum of Gly and related compounds, a difference of ∼0.3 to 0.5 eV between the ionization potentials of N1 and N2 is expected with the core hole in the N2 atom at higher BE.17,30 This chemical shift is also predicted by the present calculations for the AN forms. (See Table 2.) The N 1s spectrum of a multilayer film was fitted with two Gaussian profiles, and an intensity ratio of 0.35:1 was obtained (Figure 4b). This suggests that in 1.4 ML of Gly-Gly adsorbed on Cu(110), ∼50% of the molecules are in zwitterionic form. After annealing to 420 K, the intensity decreased, and the peak corresponding to the zwitterionic form vanished, which implies reduction of the degree of amino nitrogen protonation of Gly-Gly with annealing, decreasing film thickness, or both. (See Figure 4b.) A new, very weak shoulder in the monolayer N 1s spectrum appears at lower BE, which we associate with strong interaction of the amino nitrogen atom (N1) with the copper surface, and this structure will be further discussed in the section regarding the 0.4 ML Gly-Gly sample.

The core level BE of oxo and hydroxyl oxygen atoms of the carboxylic groups (COOH) of Gly-Gly and Gly in the gas phase differ by 1.8 eV.35,36 Therefore, the single O 1s peak in the spectra of multilayer and monolayer Gly-Gly is a further confirmation of deprotonation of the carboxylic group and formation of the carboxylate group (-COO-) in the adsorbed species (Figure 4c), and it is also in agreement with the calculated chemical shifts. (See Table 2.) The broad O 1s peak of the multilayer is at 532.05 eV. A small shift of only ∼0.2 eV toward lower BE is observed after the formation of the monolayer (annealing to 420 K), whereas the spectral shape is retained (Figure 4c). The full width at half-maximum (fwhm) of the multilayer peak is ∼1.85 eV and narrows to 1.75 eV after annealing to 420 K. (See Figure 4c.) On the basis of the experimental and theoretical evidence of the O 1s spectra, we conclude that in the multilayer film at least two different species are present, which may be a combination of a weakly adsorbed zwitterionic (NH3+CH2CONHCH2COO-) and a strongly adsorbed anionic (NH2CH2CONHCH2COO-) form. In the monolayer film, the adsorbed species could be mainly anionic, although the evidence of the O 1s spectrum is not clear. Stronger evidence of this conclusion is provided by the C 1s and N 1s spectra (Figure 4b) presented above. We have also studied the geometry and electronic structure of the adsorbed species using NEXAFS. The N and O K-edge absorption spectra of Gly-Gly monolayer on Cu(110) were measured as a function of the incidence angle of the photon beam with respect to the sample surface, and they are shown in Figure 5. Four well-separated features are observed in the N K-edge NEXAFS spectra of monolayer Gly-Gly (Figure 5a). The lowenergy feature at 400.05 eV is associated with N-H bonds of the amino group (-NH2) and has a large contribution of Rydberg character.1,17 This assignment is supported by the previous NEXAFS study of Gly adsorbed on Cu(110)1 as well as by the EXAFS spectra of free and adsorbed ammonia on Cu(110), where a prominent feature at ∼401 eV was observed and associated with the 4a1 orbital.1 This low-energy feature in the N K-edge spectrum of adsorbed Gly-Gly shows a polarization dependence (Figure 5a), where it is stronger at NI and is very weak or vanishes at GI. The local symmetry of the -NH2 moiety, including the nitrogen lone pair, is Cs, and the mirror plane passes through the C and N atoms and between the two hydrogen atoms. The symmetric 4a1 orbital of ammonia corresponds to a symmetric a′ orbital in the Cs group. The 1s f a′ transition is forbidden when the (antisymmetric) electric vector is perpendicular to the mirror plane. Because the transition vanishes at GI, this suggests that the mirror plane of the amino

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group in the monolayer is lying almost parallel to the surface. Therefore, the carbon-amino bond is also parallel to the surface. The peak at 401.7 eV is assigned to the N2 1s f π*CONH resonance, where the partial π character of the N-C bond is due to conjugation with the adjacent CdO bond, as discussed for formamide by Ishii and Hitchcock.38 The assignment is in agreement with previous EELS and NEXAFS measurements of Gly-Gly in the solid state17-19 and in solution.39 The dipole moment of the π*CONH orbital is oriented perpendicular to the peptide plane, and the stronger intensity of the corresponding peak at GI indicates that the peptide plane is oriented essentially parallel to the Cu(110) surface. This evidence supports the AN4 geometry rather than the AN2 form. The broad features at ∼405.8 and 413.0 eV are assigned to the σ*C-NH2 and σ*CONH resonances related to the C-N bonds.17 The intensity of these bands changes with the incidence angle. They are stronger when the polarization vector is parallel to the surface (at NI), suggesting that the C-N bonds are lying down and close to parallel to the surface. The O K-edge spectra of a monolayer Gly-Gly film on Cu(110), measured at various photon incidence angles, are shown in Figure 5b. Three features are observed at 532.7, 535.7, and 545.0 eV. On the basis of the EELS17 and NEXAFS studies of Gly-Gly in the gas phase30 and in the solid state,19 two overlapping resonances due to the carboxylate O 1s f π*COOand to the peptide group O 1s f π*CONH transitions are expected in the π* region, that is, the peak at 532.7 eV. The N K-edge NEXAFS have established above that the plane of the peptide group is parallel to the surface, so we expect the O spectrum to have a similar angular dependence. The lack of variation in intensity of the peak at 532.7 eV with angle is consistent with the overlap of two resonances with opposite angular dependence. Therefore, we conclude that the plane of the carboxylate group is tilted at a steep angle with respect to the surface. It agrees well with the present calculated geometries. (See Table 1.) Hasselstro¨m et al.1 in their O K-edge NEXAFS spectra of Gly adsorbed on the Cu(110) surface observed two resonances, a stronger one at ∼533 eV and a smaller one at 536 eV. O K-edge NEXAFS spectra of alanine adsorbed on the same surface also showed two resonances at similar energies.33 These two low-energy resonances showed the same polarization dependence for both Gly and alanine.1,33 The authors1 noted that the bonding of the carboxyl group of Gly on the copper surface gives rise to a splitting of the π-system into bonding, antibonding, and nonbonding Cu-adsorbate orbitals, and the feature observed at ∼536 eV corresponds in this model to the antibonding combination. On the basis of this, we associate the peak at 535.7 eV in the O K-edge NEXAFS spectra (Figure 5b) with a carboxylate-Cu interface state. However, this feature in the Gly-Gly spectrum shows a different symmetry compared with Gly. In the present studies, the VB spectra showed a feature close to the Fermi level, which is considered to be an interface state, because of the formation of a bond between Gly-Gly and the Cu(110) surface. (See Figure 2.) The broad, higher energy feature at 545.0 eV is assigned to the σ*C-O resonance associated with both the carboxylate and peptide groups.17 On the basis of the XPS and NEXAFS studies, we propose that Gly-Gly in the monolayer interacts with the Cu(110) surface in anionic form (NH2CH2CONHCH2COO-) via oxygen atoms. The C-N bond of the amino group and the plane of the peptide group are lying at a small angle to the surface, whereas the plane of the carboxylate group is tilted with a steep angle. All in all, there is better agreement between theory and experiment for AN4 where Gly-Gly lies flat on the surface interacting with

Feyer et al. carboxylate O atoms and amino N atom, but AN2 also could provide a reasonable interpretation of the spectra. 4.3.3. Submonolayer Film. The C 1s, N 1s, and O 1s spectra of 0.4 ML Gly-Gly on the clean and oxygen-modified (O-p(2 × 1)) Cu(110) surfaces are shown in Figure 6. The C 1s spectrum resembles that of the multilayer and monolayer spectra and consists of two prominent peaks at 288.6 and 286.75 eV. However, after annealing to 420 K, the C 1s spectrum is greatly changed (Figure 6a): the high BE peak due to the carboxylate and amide carbon atoms shifts toward lower BE, whereas the peaks corresponding to the methylene carbon atoms show a smaller energy shift. This probably indicates that after annealing, the carboxylate and peptide groups more strongly interact with the Cu(110) surface. However, the theoretical calculation does not predict a strong chemical shift of the amide carbon atom for the considered adsorbate geometries (Table 2, Figures 4a (AN) and 6a (AN2a)). The C 1s spectrum of a freshly deposited 0.4 ML Gly-Gly layer on the oxygen-modified O-p(2 × 1)-Cu(110) surface is similar to the spectrum after annealing (Figure 6a). These data suggest that either annealing to higher temperature or oxygen preadsorption induces the Gly-Gly molecules to interact in similar geometries with the copper surface. A natural choice for the activation barrier that needs to be overcome is removal of the hydrogen from the carboxylic group, the amino group, or both, which could be either thermally removed or chemically removed by preadsorbed oxygen (AN2b). In the second geometry, the interaction of the amino group takes place via a hydrogen atom (for example, AN2a). The latter is found to be more probable based on the calculated and measured N 1s spectra. (See discussion below.) Therefore the experimental C 1s, N 1s and O 1s spectra have been compared with the computed ones for an adsorbed Gly-Gly molecule in the AN2a geometry. The N 1s spectrum of Gly-Gly after adsorption of 0.4 ML is shown in Figure 6b. The spectrum was fitted using two Gaussian peaks with energy 400.48 and 398.65 eV. As for the monolayer, the N1 nitrogen atom of the amine group and the N2 atom of the peptide group contribute to the peak at higher BE, whereas we attribute the lowest BE peak (∼398.65 eV) to the interaction of N1 with a Cu atom through a hydrogen atom (N1-H · · · Cu). (See Figure 6b and Table 2.) Previous studies of adsorption of amino acids and related compounds also reported energy shifts in N 1s spectra due to direct nitrogen-copper interaction,1,3,15,33,40 but they did not consider NH-Cu interaction. Our calculations show that the chemical shift is due to the amino nitrogen interaction via a hydrogen atom, as in the AN2a or AN4a geometries. The computed chemical shift is similar, ∼1.5 and 1.25 eV for AN2a and AN4a, respectively, and in reasonable agreement with experimental observation (∼1.8 eV). (See Figure 6b and Table 2.) In the geometry assuming complete removal of a hydrogen atom from the amino group (AN2b), the computed chemical shift between N1 and N2 is significantly larger ∼4 eV. (See Table 1 of the Supporting Information.) We conclude that AN2a and AN4a are important forms present at low coverage. The fit analysis showed that after deposition, GlyGly adsorbs on the Cu(110) surface in anionic form (AN2 and/ or AN4) (65%) and via hydrogen atom interaction (AN2a and AN4a) (35%). After annealing to 420 K, all Gly-Gly species interact with the Cu(110) surface via the hydrogen of the amino group; as a result, the intensity of the lowest BE peak is equal to that of the peak due to the N2 nitrogen atom. (See Figure 6b.) The N 1s spectrum of Gly-Gly adsorbed on the oxygenmodified O-p(2 × 1) surface is close to that obtained for the



Figure 6. Core level spectra of (a) C 1s and (b) N 1s measured at photon energy 490 eV and (c) O 1s measured with Al KR radiation of 0.4 ML Gly-Gly adsorbed on the clean and oxygen modified (O-p(2 × 1)) Cu(110) surfaces and annealed to the temperature indicated. Points: data; thin solid lines: fitted curves; thick solid lines: theoretical spectra for AN2a geometry. (See Figure 2.) The computed C 1s, N 1s, and O 1s spectra are shifted by -5.9, -6.0, and -4.45 eV, respectively.

0.4 ML film after annealing (without oxygen) to 420 K, except for a small energy shift toward low BE. (See Figure 6b.) We suggest that preadsorbed oxygen leads to the formation of mostly anionic adsorbed Gly-Gly with the amino group bonded via hydrogen atoms. In Figure 6c, the O 1s spectra of 0.4 ML Gly-Gly films on the clean and O-p(2 ×)-Cu(110) surfaces are shown. The O 1s spectra of the deposited and annealed 0.4 ML Gly-Gly showed only one peak at ∼532 eV BE, and after annealing, an energy shift of 0.4 eV is observed. The single peak (Figures 4c and 6c) indicates that all oxygen atoms in the 1 and 0.4 ML samples are in very similar chemical states because no chemical splitting is observed. This assignment and the energy shifts observed after annealing are also in agreement with the calculated O 1s spectrum. The O 1s spectrum of Gly-Gly on O-p(2 ×)-Cu(110) exhibits two features, one at low BE due to the preadsorbed oxygen and the other at higher BE corresponding to Gly-Gly (Figure 6c). Oxygen adsorbed on Cu(110) is known to be a good hydrogen acceptor (Brønsted-Lowry base) and readily reacts with ammonia41 and amines42 to form water and other products, which can desorb from the surface. This is visible in the O 1s

spectrum as a reduction of the intensity of the O-p(2 × 1) peak after deposition of 0.4 ML Gly-Gly. (See Figure 6c.) The N K-edge NEXAFS spectra of 0.4 ML Gly-Gly adsorbed on clean and oxygen-modified Cu(110) are shown in Figure 7. As for the monolayer spectra, four well-separated resonances are observed. However, a strong difference in the spectra is observed at lower photon energy, where resonances associated with N-H bonds of the amino group (-NH2) are present. This is in agreement with the appearance of a low BE peak in the N 1s spectra of 0.4 ML Gly-Gly, related to interaction of the N1 nitrogen atom with Cu via a hydrogen atom. (See Figure 6b.) The lowest energy feature at the N K-edge spectra showed a polarization dependence as well as an energy shift (∼0.25 eV) due to the interaction of amino nitrogen (N1) with preadsorbed oxygen. As was mentioned above, this resonance is symmetric with respect to the local symmetry of the amino group; therefore, the stronger peak at GI in the spectrum of the 0.4 ML film suggests that the plane of the amino group is at a steep angle. The absence of this feature in the spectrum of Gly-Gly on O-p(2 × 1)-Cu(110) measured at NI indicates that the N-H bonds are lying almost perpendicular to the surface. The peak at 401.7 eV is assigned to the N2 1s f π*CONH resonance

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Feyer et al.

Figure 7. N K-edge NEXAFS spectra of 0.4 ML Gly-Gly adsorbed on the (a) clean and (b) oxygen-modified (O-p(2 × 1)) Cu(110) surface measured at GI, NI, and MA.

Figure 8. O K-edge NEXAFS spectra of 0.4 ML Gly-Gly adsorbed on the (a) clean and (b) oxygen-modified (O-p(2 × 1)) Cu(110) surface measured at GI, NI, and MA.

and relates to the peptide group and did not show a strong angular dependence. This suggests that the peptide plane of the adsorbed molecules is randomly oriented or the peptide moiety of the two groups of Gly-Gly tilt with an angle of ∼45° with respect to the [11j0] plane, and there are equal numbers of Gly-Gly molecules tilted with opposite directions. The preadsorbed oxygen on the O-p(2 × 1)-Cu(110) surface creates a more ordered structure, and as a result, the N2 1s f π*CONH resonance is stronger at GI and vanishes at NI. We propose that the majority of peptide moieties are lying parallel to the surface. (See Figure 7b.) As for the N K-edge monolayer spectra, the broad bands at ∼405.8 and 413.0 eV are attributed to the σ*C-NH2 and σ*CONH resonances.17 The O K-edge NEXAFS spectra of 0.4 ML Gly-Gly adsorbed on the clean and oxygen-modified Cu(110) surface are presented in Figure 8. The energies and assignments are the same as those proposed for the Gly-Gly monolayer. (See Figure 5b and Section 4.3.2.) The strong peak at 530.0 eV and broadening of the main feature at 533 eV in the O K-edge NEXAFS spectra of 0.4 ML Gly-Gly adsorbed on the O-p(2 × 1)-Cu(110) are associated with preadsorbed oxygen (Figure 8b).43 In both cases, the spectra did not show strong polarization dependence. As argued above, the small polarization dependence is consistent with a geometry in which the C-O bonds of the carboxylate and peptide groups in 0.4 ML Gly-Gly have opposite orientation with respect to the surface, and π*COO- and π*CONH transitions related to carboxyl and peptide groups compensate each other and add up to an angle-independent intensity. Alternatively, C-O bonds

in the two groups tilt with an angle of ∼45°, and there are equal numbers of Gly-Gly molecules tilted with opposite directions with respect to the [11j0] plane, and in this case, the relative phase shifts of ∼90° in angular dependencies of π* resonances from the two different CONH and COO- groups compensate each other. In summary, Gly-Gly at 0.4 ML coverage bonds with the surface via oxygen atoms of the carboxylate and peptide groups as in the ML but with an additional interaction via the hydrogen atom of the amino group, and the most credible adsorbate structures are AN2a and AN4a. The preadsorbed oxygen forces a tilt of the peptide group to almost parallel with respect to the surface, whereas on the clean surface, this moiety lies randomly or divided in two domains tilted at ∼45° with respect to the [11j0] plane. 5. Conclusions In the present studies, two experimental techniques, XPS and NEXAFS, and two theoretical approaches, DFT calculations for the optimization of the adsorbate structure and ∆SCF calculations for the simulation of the spectra, have been used to study the adsorption of the simplest peptide, Gly-Gly, on a clean and an oxygen-covered O-p(2 × 1) Cu(110) surface. In the monolayer film, the molecules bond to the surface via oxygen atoms and the nitrogen atom of the amino group, whereas in the 0.4 ML film, the Gly-Gly is found to interact with a similar geometry but with an additional interaction of the amino group with the surface via hydrogen atoms. Annealing to 420 K, or

Adsorption Structure of Glycyl-Glycine on Cu(110) preadsorbed oxygen, induces the formation of the anionic species; however, after low-temperature adsorption on a clean surface the zwitterionic form of Gly-Gly has also been found in the multilayer film. The N and O K-edge NEXAFS spectra measured at various incident angles allowed us to propose detailed adsorption models of the adsorbed species for monolayer and 0.4 ML Gly-Gly coverage, which are in reasonable agreement with the stable calculated geometries of the adsorbate. Acknowledgment. V.L. thanks the Abdus Salam International Center for Theoretical Physics for financial support under the Elettra-ICTP scheme. We gratefully acknowledge the assistance of our colleagues at Elettra for providing good quality synchrotron light, and we particularly thank Toma´sˇ Ska´la for his help. The Materials Science Beamline is supported by the Ministry of Education of the Czech Republic under grant no. LC06058. Supporting Information Available: Proposed adsorption geometries of Gly-Gly on Cu(110), valence spectra of Gly-Gly adsorbed on Cu(110) and annealed to 420 K to give a coverage of 1 ML, Coordinates (au) of adsorbates optimized by DFT calculations, and theoretical C 1s, N 1s, and O 1s binding energies in eV of Gly-Gly in different adsorbate states. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Hasselstro¨m, J.; Karis, O.; Weinelt, M.; Wassdahl, N.; Nilsson, A.; Nyberg, M.; Pettersson, L. G. M.; Samant, M. G.; Sto¨hr, J. Surf. Sci. 1998, 407, 221. (2) Barlow, S. M.; Kitching, K. J.; Haq, S.; Richardson, N. V. Surf. Sci. 1998, 401, 322. (3) Nyberg, M.; Hasselstro¨m, J.; Karis, O.; Wassdahl, N.; Weinelt, M.; Nilsson, A.; Pettersson, L. G. M. J. Chem. Phys. 2000, 112, 5420. (4) Barlow, S. M.; Haq, S.; Raval, R. Langmuir 2001, 17, 3292. (5) Barlow, S. M.; Raval, R. Surf. Sci. Rep. 2003, 50, 201. (6) Mateo Marti, E.; Methivier, Ch.; Dubot, P.; Pradier, C. M. J. Phys. Chem. B 2003, 107, 10785. (7) 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. (8) Tomba, G.; Lingenfelder, M.; Costantini, G.; Kern, K.; Klappenberger, F.; Berth, J. V.; Colombi Ciacchi, L.; De Vita, A. J. Phys. Chem. A 2007, 111, 12740. (9) Lingenfelder, M.; Tomba, G.; Costantini, G.; Colombi Ciacchi, L.; De Vita, A.; Kern, K. Angew. Chem., Int. Ed. 2007, 46, 4492. (10) Wang, Y.; Lingenfelder, M.; Classen, T.; Costantini, G.; Kern, K. J. Am. Chem. Soc. 2007, 129, 15742. (11) Monti, S.; Carravetta, V.; Battocchio, C.; Iucci, G.; Polzonetti, G. Langmuir 2008, 24, 320. (12) Polzonetti, G.; Battocchio, C.; Dettin, M.; Gambaretto, R.; Di Bello, C.; Carravetta, V.; Monti, S.; Iucci, G. Mater. Sci. Eng. C. 2008, 28, 309. (13) Valleé, A.; Humblot, V.; Me´thivier, C.; Pradier, C. -M. Surf. Sci. 2008, 602, 2256. (14) Valleé, A.; Humblot, V.; Me´thivier, C.; Pradier, C. -M. J. Phys. Chem. C 2009, 113, 9336.

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