Adsorption, Assembly, and Oligomerization of Aspartic Acid on Pd(111

Article pubs.acs.org/JPCC

Adsorption, Assembly, and Oligomerization of Aspartic Acid on Pd(111) Mausumi Mahapatra, Jerry Praeger, and Wilfred T. Tysoe* Department of Chemistry and Biochemistry and Laboratory for Surface Studies, University of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53211, United States ABSTRACT: The surface chemistry of L-aspartic acid is studied on Pd(111) in ultrahigh vacuum using reflection absorption infrared spectroscopy (RAIRS) and scanning tunneling microscopy (STM) as a function of sample temperature and coverage. Density functional theory (DFT) calculations are carried out to supplement the experimental results. At low temperatures, the dominant surface species was found to be monoaspartate species at all the experimental coverages. The monoaspartate species undergo extensive intermolecular hydrogen-bonding interactions and form extended two-dimensional ordered structures, forming either a honeycomb pattern or linear-row structures. At room temperature, aspartic acid undergoes a self-polymerization reaction to form oligosuccinimide networks, although some individual monoaspartate species are also observed.

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INTRODUCTION Heterogeneous chiral catalysts can be synthesized by modifying nonchiral surfaces using a chiral modifier. However, there are a limited number of examples for which this approach has been exploited successfully, the most notable being the enantioselective hydrogenation of α-keto esters on cinchona-modified platinum catalysts1,2 and the hydrogenation of β-keto esters on tartaric-acid-modified nickel.3,4 Amino acids have also been used as both chiral catalysts5−7 and chiral modifiers8−12 for enantioselective hydrogenation reactions. Alanine13,14 and tartaric acid15,16 have been explored as chiral modifiers on Pd(111), and the resulting enantioselectivity of the modified surface was measured by using propylene oxide17−21 and glycidol22,23 as chiral probe molecules. Scanning tunneling microscopy (STM) showed that both alanine13 and tartaric acid form various structures on Pd(111) and copper24−39 depending on sample temperature and coverage. Alanine self-assembles to form local tetramer structures and dimer rows on Pd(111) due to strong hydrogen-bonding interactions between the carboxylate groups and amine group.13 By correlating the surface structures observed in STM with the measured enantioselectivities, it was concluded that alanine bestows chirality due to the pockets in the tetramers thereby forming chiral templates.14,40 In contrast, under reaction conditions, tartaric acid does not form ordered structures, but individual tartrate species interact with the chiral probe in a 1:1 fashion.16 STM images of tartaric acid on Pd(111) show individual molecules (with a divalent anion) at low coverages, while at higher coverages, oligomeric structures are observed. They are proposed to form due to the hydrogen-bonding interactions between two −OH groups and between the COOH····OH groups of the bitartrate species (with a monovalent anion). Thus, alanine and tartaric acid © XXXX American Chemical Society

behave differently on Pd(111) due to the presence of different functional groups on both compounds. Aspartic acid incorporates bonding moieties of both tartaric acid and alanine, having two COOH groups as in tartaric acid and an −NH2 group as in alanine. The three-dimensional structure (Figure 1a) contains three distinct functional groups; the α-amino (α-NH2), α-carboxyl (α-COOH), and β-carboxyl (β-COOH) groups, thereby providing additional loci for hydrogen-bonding interactions that could potentially form extended chiral networks. This would address one of the central problems in designing heterogeneous chiral catalysts, namely, the presence of unmodified surface sites that yield racemic products to reduce the overall enantioselectivity of the catalyst. This is not an issue in the commonly used homogeneous chiral catalyst as all of the active sites are modified by a chiral ligand.41−44 This problem is mitigated in the case of cinchonamodified platinum catalysts where the hydrogenation activity is larger at modified than unmodified sites.45−48 Accordingly, in this work, the surface chemistry of aspartic acid is investigated on Pd(111) with the motivation of gaining insights into whether it can be used as an extended chiral templating structure. Note that aspartic acid has been used as a chiral modifier for Ni catalysts for the enantioselective hydrogenation of β-keto esters.49,50 There are many examples of UHV-based studies of aspartic acid on Ni(111) using reflection−absorption infrared spectroscopy (RAIRS), STM and temperature-programmed desorption (TPD).51−55 On nickel, aspartic acid polymerizes to form Received: April 13, 2017 Revised: May 22, 2017

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DOI: 10.1021/acs.jpcc.7b03519 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

The Pd(111) substrate was cleaned using a standard procedure consisting of cycles of argon ion sputtering and annealing in 3 × 10−8 Torr of oxygen at 1000 K, and the cleanliness was judged either using Auger spectroscopy or TPD after dosing with oxygen, where the absence of CO desorption indicated that the sample was carbon free. L-Aspartic acid (Aldrich, 99% purity) was dosed onto the sample using a lab-built Knudsen source attached to the respective vacuum chambers. The aspartic acid source was generally outgassed overnight at 400 K to remove contaminants, particularly water, and the temperature of the source was adjusted to yield reasonable dosing rates. The temperature of the Knudsen source was maintained between ∼400 and ∼410 K for the STM experiments and between 390 and 395 K for the RAIRS experiments. Density functional theory (DFT) calculations were performed with the projector augmented wave (PAW) method64,65 as implemented in the Vienna ab initio simulation package, VASP.66,67 The exchange correlation potential was described using the generalized gradient approximation (GGA) of Perdue, Burke and Ernzehof.68 Hydrogen-bonding interactions are reproduced well (within ∼4 kJ/mol) using this functional, although the accuracy deteriorates as the hydrogen bonds deviate from linear.69 A cut off of 400 eV was used for the plane-wave basis set, and the wave functions and electron density were converged to within 1 × 10−5 eV. The first Brillouin zone was sampled with a 4 × 4 × 1 Γ-centered k-point mesh. Geometric relaxations were considered to be converged when the force was less than 0.02 eV/Å on all unrestricted atoms.

Figure 1. (a) Three-dimensional ball-and-stick model of aspartic acid. (b) Reaction scheme depicting the polycondensation of aspartic acid to polysuccinimide.

oligosuccinimide species (Figure 1b) at surface defects, and at higher coverages, the surface is covered with monoaspartate species, which bind to the surface through the amine group and carboxylate oxygens. It has been speculated that the origin of enantioselectivity in the Ni/aspartate system is the formation of oligosuccinimide clusters at low coverage which create docking sites for methylacetoacetate.55 The orientation and bonding of aspartic adsorption from solution phase have been studied in detail on rutile TiO2 films and γ-Al2O3.52,56,57 On both surfaces, the dominant adsorption geometry is one in which both carboxylate groups of aspartic acid are involved in bonding to the surface. Recently, density function theory (DFT) calculations have been performed for aspartic acid adsorption on a rutile (110) surface,56 and these suggest that the strongest adsorption occurs when both the amino and carboxyl groups of aspartic acid form a bidentate coordination to two surface Ti atoms. The chiral separation and resolution of aspartic acid has been extensively studied on copper,58,59 and chiral autoamplification has been found on Cu(111).60,61 Kinetic analyses of the autoamplification process suggest the participation of aspartic acid clusters.61 In the following, RAIRS is used to characterize the bonding and orientation of aspartic acid on Pd(111), while STM is used to explore the surface structures and compare them with those of alanine and tartaric acid. Finally, DFT calculations are used as an aid in assigning surface structures.

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RESULTS Infrared spectra were collected following adsorption of Laspartic acid on a Pd(111) surface at ∼100 K as a function of increasing dosing times and the results are shown in Figure 2. The spectrum, measured after dosing for 1200 s to produce the largest coverage, shows a number of peaks that are summarized along with their assignments in Table 1 and compared with the

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EXPERIMENTAL AND THEORETICAL SECTION Infrared spectra were collected in a chamber as described elsewhere.62 The spectra were collected using a Bruker Vertex infrared spectrometer operating at a resolution of 4 cm−1, using a liquid-nitrogen-cooled, mercury cadmium telluride detector, and spectra were typically collected for 1000 scans. STM experiments were performed as described elsewhere,63 and images were acquired using an electrochemically etched tip made from recrystallized tungsten wire. All presented images were processed by background subtraction except where specifically indicated. Temperature-programmed desorption (TPD) experiments were carried out as described previously.15

Figure 2. Infrared spectra of L-aspartic acid adsorbed on Pd(111) at 97 K as a function of exposure, where the dosing times are displayed adjacent to the corresponding spectra. B


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The Journal of Physical Chemistry C frequencies found previously for aspartic acid by Wolpert and López-Navarette et al.70,71 Table 1. Vibrational Frequencies and Assignments of LAspartic Acid on Pd(111) at ∼100 K vibrational frequency at 97 K/cm−1

vibrational frequency from ref 70 (cm−1)

vibrational frequency from ref 71 (cm−1)

1800 1788 1722

1729

1691

1710 1630 (broad) 1413 1350 1310 1246 1169 1132 1051 991 937

1652 1395 1357 1306 1221 1163 1120 1079 980

1407 1359 1336, 1307 1259

1081 991 935

assignments ν(CO) α-carboxylic acid (isolated) ν(CO) γ-carboxylic acid (isolated) ν(CO) α-carboxylic acid (H-bonded) ν(CO) γ-carboxylic acid (H-bonded) δas(NH2) δip(COH) ω(CH2) ω(CH2), δ(CH) ν(CO) τ(CH2), δ(CH) δ(CH) ν(CN), ν(C−C) ν(C−C) δop(COH)

Figure 3. Infrared spectra of L-aspartic acid adsorbed on Pd(111) at 97 K as a function of annealing temperature, where the temperatures are displayed adjacent to the corresponding spectra. The positions of the polysuccinimide infrared features are indicated at the bottom of the figure.

significantly, except for variations in intensity suggesting some changes in adsorbate orientation. Further heating results in a diminution in intensity of almost all the aspartic acid infrared modes, which essentially disappear on heating to just above room temperature. The exception is the persistence of features at ∼1730 and 1406 cm−1 that only disappear when the surface is heated above 500 K. The assignment of these features will be discussed in greater detail below. However, it has been reported previously that aspartic acid dehydrates to polysuccinimide on Ni(111),55 and the positions of the features measured for polysuccinimide have been marked on Figure 3.79 The dehydration of aspartic acid adsorbed on Pd(111) is followed by TPD, and the results are displayed in Figure 4. Water (at 18 amu) desorbs in a sharp

Initial spectral assignments were made based on the group frequencies for amino acids,70,72,73 and the remaining features were assigned based on the spectra of aspartic acid. Accordingly, peaks between ∼1700 and 1800 cm−1 are assigned to CO stretches of the carboxylic acid group. They occur as two intense features centered at ∼1800 and 1715 cm−1 assigned to non-hydrogen-bonded and hydrogen-bonded COOH groups, respectively.74,75 The presence of hydrogen-bonded COOH groups is confirmed by a δop(COH) mode at 937 cm−1, which is only detected for hydrogen-bonded carboxylic acid.72 These peaks undergo a small additional splitting of ∼10 cm−1 due to the slightly different environments of the α and distal (γ) COOH groups,73 giving rise to two doublets at 1788 and 1800 cm−1 and 1710 and 1722 cm−1. The broad feature at ∼1650 cm−1 is assigned to an NH2 group. Small features at ∼1850 cm−1 are due to a low coverage of carbon monoxide adsorbed from the background.76 The intensities of all the infrared features grow with increasing exposure, suggesting that aspartic acid adsorbs on the surface at low temperatures as the neutral molecule. The infrared spectra undergo several distinct changes as the sample is heated (Figure 3). Initial changes occur as the sample is heated to ∼150 K and above. The first is the disappearance of the low-frequency component of the doublet at 1788 and 1800 cm−1 so that after heating to ∼150 and 180 K the peak is centered at ∼1800 cm−1 which implies that the γ-COOH group preferentially dehydrogenates. This is accompanied by the appearance of additional intensity as a shoulder at ∼1400 cm−1, indicated by a blue line, indicative of a carboxylate species, and is typical behavior for carboxylic acids on transition-metal surfaces, which deprotonate to form carboxylate species.77,78 A similar frequency is found for alanine on Pd(111).40 The remaining skeletal modes of aspartic acid do not change

Figure 4. Data for 18 amu (water) temperature-programmed desorption (TPD) for various exposure of L-aspartic acid adsorbed on Pd(111) at ∼120 K, where the dosing times are displayed adjacent to the corresponding desorption profiles. C


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The Journal of Physical Chemistry C state centered at ∼190 K at lower aspartic acid coverages and in a broader state at higher exposures, with features at ∼170 and 184 K, with a broad tail extending to ∼400 K. The intensity variation of the ∼1788 and 1722 cm−1 infrared features from the spectra plotted in Figure 3 are displayed in Figure 5 as a function of annealing temperature. The peak at

Figure 5. Plot of the peak absorbances of the 1722 (■) and 1788 (●) cm−1 infrared features plotted as a function of annealing temperature taken from the infrared data shown in Figure 4.

∼1722 cm−1 is assigned to both hydrogen-bonded COOH groups and polysuccinimide. The intensity of this feature decrease rapidly on heating to ∼180 K, when water is evolved in the sharp peak centered at ∼184 K (Figure 4) but increases again at higher annealing temperatures, proposed to be due to polysuccinimide formation. This is accompanied by a broad tail in the water desorption profile (Figure 4). However, the ∼1788 cm−1 peak, assigned to isolated COOH groups, shows a monotonic decrease in intensity so that isolated aspartic acid disappears completely by ∼350 K. STM images were collected for various coverages of aspartic acid dosed at ∼140 K on Pd(111). Figure 6a−c shows the surface as a function of increasing coverage, where a honeycomb motif is the most common surface structure observed at all coverages. At low coverages (Figure 6a), some individual molecules are observed which are imaged as bright, oval-shaped features. In addition, one-dimensional ladder structures and small honeycomb islands are also found. With a slight increase in coverage (Figure 6b), the islands grow and are aligned with the centers of the hexagons oriented at ∼15° to the three close-packed crystallographic directions, oriented at 120° to each other, where the islands propagate from step edges and run along the terraces. Figure 6c shows a larger-area STM image for a high coverage of aspartic acid dosed and imaged at 120 K, which again shows extended island growth. Finally, Figure 6d shows a high-resolution image for a smaller scan area which clearly shows the periodicity within the honeycomb islands. A line profile (indicated by a white line) showsperiodicity in the honeycomb island of ∼1.1 nm. Occasionally, the commonly observed honeycomb motifs are imaged with a different contrast, where they appear as linear rows running parallel to each other. Figure 7 shows an example, where a change in image contrast is observed in four consecutive images. Figure 7a displays images of the honeycomb motif seen in Figure 6, and Figure 7b,c shows linear

Figure 6. Large-area scan STM images of L-aspartic acid on Pd(111) dosed and imaged at 120 K. (a−c) Surface coverage is gradually increased. (d) Zoomed-in image and the line profile measurement is shown below (Vb = 0.8 V, It = 200 pA).

Figure 7. (a−d) Four consecutive STM images collected for the same scan area, which shows switching between two different image contrasts. (a) Original honeycomb motif (Vb = 1 V, It = 200 pA), (b) and (c) parallel line structures (Vb = 0.8 V, It = 200 pA), and (d) original honeycomb motif restored (Vb = 0.47 V, It = 180 pA).

structures, which are oriented at ∼15° to the close-packed crystallographic directions. In Figure 7d, the original honeyD


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The Journal of Physical Chemistry C comb motif is restored. The spacing between the rows is ∼0.47 nm. While the images are collected with a slightly different tip bias and the changes in image contrast could be a result of applying different biases, similar changes in image contrast are also observed during scanning with the same bias (data not shown). Consequently, the changes are attributed to the adsorption dynamics where the molecules slightly rearrange over the period of scanning, possibly by influencing the hydrogen-bonding interactions. In addition to the commonly observed honeycomb motifs, another striking feature is seen which consists of patches of parallel rows as shown in Figure 8. The domains run at angles

Figure 9. (a−d) Four time-lapse images collected for the same surface after dosing L-aspartic acid at 120 K. The growth of the ordered domains increases as a function of time.

more tightly on the surface compared to the honeycomb pattern. The formation of those domains increases under positive tip bias conditions and scan time so that the possibility of tip-induced changes cannot be eliminated. Finally, STM images are collected after dosing Pd(111) at room temperature and then cooling to 120 K for imaging. Figure 10a−c shows the structures formed at 300 K as a function of coverage. At low coverages, the molecules decorate Figure 8. STM image of L-aspartic acid on Pd(111) showing patches of parallel rows. The images are collected when the surface is both dosed and imaged at ∼120 K. (Vb = 1 V, It = 300 pA).

of ∼30° to or are aligned with the close-packed crystallographic directions, where an example of a domain at ∼30° is displayed at the top of the image and another along the direction is seen at the bottom left corner of the image. The spacing between the rows for domains at ∼30° to the direction are well-defined, and a line profile measurement across the linear features is shown in Figure 8 yields a periodicity of ∼0.72 nm. Note that this periodicity is different from that seen in Figure 7b,c (of ∼0.47 nm). In addition, the rows have different orientations with respect to the direction, and individual protuberances are seen along the chains in Figure 7b,c that are not evident in Figure 8 and are therefore due to a different structure. The domains parallel to the direction are much less well-defined, thus precluding a periodicity from being measured. Some individual isolated aspartate species are observed between the domains. The domains grow with repeated scanning over the same surface as shown in time-lapse images (Figure 9). Figure 9a shows the presence of individual aspartate species along with ordered domains, which consist of linear rows. Streaked lines appear near the domain boundary as well as near the individual species, which suggest molecular motion. With consecutive scans, the number of domains increases and fewer individual species are observed. This structure allows molecules to pack

Figure 10. STM images of L-aspartic acid on a Pd(111) surface dosed at ∼300 K and then cooled to ∼120 K for imaging. (a−d) Surface structure as a function of an increase in coverage. Imaging conditions for (a) and (b) are Vb = 0.4 V, It = 100 pA and for (c) and (d) are Vb = 1 V, It = 120 pA. E


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Figure 11. Depiction of the two most-stable structures of aspartate species on Pd(111) calculated by DFT, showing the top and side views for (a) γcarboxylate (b) α-carboxylate.

step edges (Figure 10a). In addition, individual molecules and small clusters are also observed on the terraces. At a slightly higher coverage (Figure 10b), the surface consists mostly of clusters, but individual molecules are also found. This is consistent with the remaining 1788 cm−1 intensity present at ∼300 K (Figure 5). At higher coverages, as shown in Figure 10c,d, the formation of complex, networklike structures and pores of different sizes are observed. This network structure originates from step edges and grows toward terraces. At the bottom of Figure 10d, individual molecules are present, while the top portion consists of a complex network structure. The middle portion shows the presence of two-dimensional row structures. It is also found that the same network structure is obtained when a surface is dosed at 80 K and annealed to ∼300 K (data not shown).

acid group and the hydrogen atom of the amine. This structure yields an adsorption energy, calculated from the difference in energy of the structure shown in Figure 11a and the sum of the energies of the relaxed Pd(111) slab and gas-phase aspartic acid, −74 kJ/mol. In Figure 11b, the α-carboxylic acid group deprotonates and binds to the surface via the oxygen atoms of the α-carboxylate and nitrogen atom of the amine group. In this case, the C−C molecular backbone is close to perpendicular to the surface, and the γ-carboxylic acid group slightly tilts toward the surface so that internal hydrogen bonding can occur in a similar manner as in structure Figure 11a. The calculated adsorption energy for the structure in Figure 11b is −64 kJ/mol; the DFT calculations are consistent with the observation of γ-carboxylate species in the infrared spectra. It appears that the adsorption geometry in structure shown in Figure 11a is less strained than that in Figure 11b, thus leading to ∼10 kJ/mol difference in binding energy. If the deprotonation reaction follows an Evans−Polanyi relationship,80 then this will result in a lower activation barrier and a preferential deprotonation of the γCOOH group as found experimentally. Similar η3 adsorption geometries are observed for alanine on Pd and Cu,13,24,28,29,81−83 where bonding to the surface occurs through the oxygen atoms of the carboxylate and the amine group. STM studies carried out following low-temperature adsorption show the presence of honeycomb islands. The origin of the honeycomb motif is proposed to be due to selfassembly through hydrogen-bonding interactions between the carboxylate groups with the amine group. A structural model for the formation of the honeycomb islands is proposed based on the most stable aspartate monomer structure (Figure 11a). DFT calculations were carried out for an aspartate hexamer structure, the basic repeat unit being a honeycomb island, and

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DISCUSSION The infrared data indicate that L-aspartic acid adsorbed at ∼100 K and heated above ∼150 K preferentially deprotonates the γCOOH group to form a monoaspartate species. In order to explore this in greater detail, DFT calculations were performed for isolated monoaspartate species on a 6 × 6 Pd(111) slab with a coadsorbed hydrogen atom located on a 3-fold hollow site. Two different initial monoaspartate structures were considered and the resulting relaxed geometries are shown in Figure 11. In Figure 11a, the γ-carboxylic acid group is deprotonated and binds to the surface through the oxygen atoms of both the γ-carboxylate and the nitrogen atom of the amine group. The oxygen and nitrogen atoms bind to palladium atop sites, and the C−C molecular backbone is close to parallel to the surface. The α-carboxylic acid group tilts slightly toward the surface, which allows internal hydrogen bonding to occur between the oxygen atom of the carboxylic F


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The Journal of Physical Chemistry C the converged geometry is shown in Figure 12, yielding a hexamer stabilization energy of −49 kJ/mol. Hydrogen

Figure 13. Proposed model for the experimentally observed linear-row aspartic acid domain structures for domains oriented at ∼30° to the crystallographic directions.

bonding interactions between the carboxylate group and the amine group (indicated by green dotted lines). The individual rows hydrogen-bond to each other through the carboxylic acid group of one row with the carboxylate oxygen of the adjacent row (as shown in Figure 13 by green dotted lines). The structure of the domains that propagate along the directions is less clearly defined but likely formed from similar assemblies as shown in Figure 13. Substantial spectral changes occur as the sample is heated to higher temperatures (Figure 3). All features due to L-aspartic acid decrease in intensity to yield peaks at ∼1730 and 1406 cm−1 (Figures 3 and 5) due to a dehydration reaction (Figure 4) to produce oligomeric species that appear to nucleate at step edges (Figure 10). The final product formed after heating to ∼342 K (Figure 3) is assigned to oligosuccinimides.79,85,86 Thermal dehydrogenation of aspartic acid occurs in two steps. The first occurs at ∼450 K via the intermediate formation of aspartic anhydride, which then reacts with aspartic acid to form a peptide bond, subsequently dehydrating to polysuccinimide.85 However, the oligomerization temperature is significantly lower than this on Pd(111) (Figure 4) and on Ni(111).55 The results in Figures 4 and 5 suggest that the hydrogen-bonded L-aspartic acid undergoes an initial dehydration step at ∼180 K, followed by a slower dehydration reaction at higher temperatures. The initial loss and subsequent increase in the intensity of the 1722 cm−1 peak (Figure 5) suggests the presence of an intermediate species, in accord with the model proposed above. Succinic anhydride is characterized by an intense CO stretching mode at ∼1780 cm−1, evident in the spectrum in Figure 3, but its intensity decreases monotonically with annealing temperature (Figure 5). It also has a peak at ∼920 cm−1, not evident in Figure 3.87 However, Figure 3 shows two features at ∼1640 and 1580 cm−1 (indicated by red lines 3) that can be assigned to the Amide I and II peptide bands,88 which disappear at the oligosuccinimide features appear.

Figure 12. Depiction of the proposed structure of aspartate hexamer structure of the experimentally observed honeycomb motif following aspartic acid adsorption at low temperatures. The repeat units of the honeycomb structure are aspartate hexamers on Pd(111) surface where the structure was optimized using DFT calculations. The hexamer structure comprises β-aspartate species. The hydrogenbonding interactions between the β-carboxylate oxygen and −NH2 group are indicated by green lines, and the interaction between the αcarboxylic acid group and the γ-carboxylate oxygen is shown by yellow lines.

bonding occurs within the hexamer between the γ-carboxylate oxygen of one aspartate (η2) and the −NH2 group of the other (η3) (highlighted by green dotted lines in Figure 12). Additional hydrogen-bonding interaction occur between the α-carboxylic acid group with the carboxylate oxygen, which completes the hexamer (highlighted by yellow dotted lines in Figure 12). Note, however, that tilting of the carboxylate to form hydrogen bonds would produce vibrational frequencies ∼1600 cm−1, for example at ∼1606 cm−1 for alaninate species on Pd(111)40 and at 1613 cm−1 on Cu.84 This spectral region is, however, obscured by the broad and intense NH2 mode, although there is evidence in Figure 3 of a weak and relatively sharp mode at this frequency. These hexamer units propagate on the surface to form the two-dimensional honeycomb islands as shown in Figure 12. The spacing and orientation of the proposed structure match well with the experimentally observed STM images. The other type of observed ordered overlayer consists of domains of linear rows (Figures 8 and 9), which is also proposed to be driven by similar OCO···H2N hydrogenbonding interactions. Taking into account the row spacing and orientation of the domains with respect to the substrate crystallographic direction, a model is proposed for the formation of the linear-row structures (Figure 13), based on the monomer geometry shown in Figure 11a. The aspartate species self-organize into linear rows to optimize the hydrogen-

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CONCLUSIONS At low temperatures, the aspartic acid monolayer consists of aspartate species which self-assemble to form hexamers, which G


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occur either in a honeycomb pattern or linear rows. The pores of the honeycomb may provide chiral reaction sites where the extended hydrogen-bonded, two-dimensional network will ensure that all surface sites are modified. In this context, it is interesting to note that the autoamplification of the chirality of aspartic acid on copper has been proposed to occur through the participation of small aspartic acid clusters that are suggested to contain between 8 and 12 aspartic acid molecules.61 At room temperature, aspartic acid polymerizes to form oligosuccinimide networks via a two-step dehydration reaction likely through the formation of an intermediate with a peptide bond.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (414) 229-5222. Fax: (414) 229-5036. E-mail: wtt@ uwm.edu. ORCID

Wilfred T. Tysoe: 0000-0002-9295-448X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support of this work by the U.S. Department of Energy, Division of Chemical Sciences, Office of Basic Energy Sciences, under grants DE-FG02-03ER15474 and DE-SC008703/0003. We thank Professor Michael Weinert for advice on carrying out density functional theory calculations.

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REFERENCES

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Adsorption, Assembly, and Oligomerization of Aspartic Acid on Pd(111

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