## 3 Results
The strongest peak appeared at 284.9 eV, assigned to the aromatic ring of Tyr, labeled C5 (-C\({}_{6}\)H\({}_{5}\)-). Four other peaks were assigned corresponding to CH\({}_{2}\)- (283.7 eV: C6), C-NH\({}_{2}\) (285.3 eV: C7), COO- (288.4 eV: C8), and C-OH (287.1 eV: C9), respectively. These observations suggested the presence of an electronically neutral (C3 and N1) amine group for Leu but a positively charged (C7 and N2) form of the amine for Tyr [19]. Concurrently, the changes in the N 1s and O 1s core-level spectra at the \(\sim\)0.20 ML level of coverage were determined using the same methods for the two amino acids adsorbed on the Cu(110) surface. The N 1s core-level spectrum of \(\sim\)0.2 ML Leu on the Cu(110) surface in Figure 2b showed a single peak at 398.5 eV (marked as N1). Considering the binding energy, N1 was assigned to the neutral N atom in  
Figure 2: (**a**–f) C 1s, N 1s, and O 1s core-level spectra for Leu (**a**,**b**, and **c**) and for Tyr (**d**,**e**, and **f**), in each case on the Cu(110) surface with an \(\sim\)0.20 ML deposition: the dots indicate experimental values, and the solid lines represent the results of peak fittings. the Leu molecule [20; 21; 22]. The N 1s core-level spectrum obtained for \(\sim\)0.20 ML Tyr adsorbed on the Cu(110) surface, as in Figure 2e, also showed a single peak, but at 400.5 eV. This result was indicative of the N-ditive bonding structure for the adsorbed Tyr. Finally, the O 1s peaks of the adsorbed Leu and Tyr molecules were determined and compared. Figure 2c shows the O 1s core-level spectrum obtained after deposition of \(\sim\)0.20 ML Leu on the Cu(110) surface. Both oxygen atoms of Leu adsorbed equivalently on the Cu(110) surface, as indicated by the O 1s spectrum showing a single peak (marked as O1 at 530.5 eV). This equivalent adsorption was apparently a result of the Cu(110) surface having an equivalent dimer. The O 1s core-level spectrum of Tyr on the Cu(110) surface, shown in Figure 2f, was found to display two oxygen peaks: one peak (marked as O2 at 530.9 eV) being similar to that for Leu and corresponding to the two deprotonated carboxyl oxygens adsorbed equivalently on the Cu(110) surface and the other peak (marked as O3 at 532.9 eV) being the phenol oxygen. In previous experiments of Leu adsorbed on a Ge(100) surface, the O 1s peaks were divided into two components, O-Ge and -C=O, which could be done because of the chemical inequivalence of the oxygen atoms in such Leu resulting from the zwitterionic character of the Ge(100) surface [13; 23]. The analyses of the C 1s, N 1s, and O 1s core-level high-resolution photoelectron spectra taken together revealed that, at low coverage, the Leu adsorbed on the Cu(110) surface adopted the O-H dissociation bonded structure, whereas the Tyr system showed the O-H dissociated-N dative bonded structure. DFT calculations were also performed to determine the most stable adsorption structural features of Leu and Tyr on the Cu(110) and their corresponding adsorption energies. The same calculation procedure was employed for both amino acid molecules. Figure 3 shows these two DFT-optimized structural features from the front and top, with respect to calculating the adsorption energies. As shown in Figure 3, the O-H dissociation bonded structure, containing a neutral N, was calculated to be the most stable adsorption structure for Leu on the Cu(110) surface, with an Eads of \(-\)45.23 kcal/mol, rather than the O-H dissociated-N dative bonded structure with Eads of \(-\)19.86 kcal/mol, as seen Figure 3a,c. Taking into account the stability of the structure of Leu adsorbed on the Cu(110) surface, as predicted using the DFT calculations, together with the experimental HRPES results, the O-H dissociation bonded structure was found to be most stable for Leu in the low-coverage regime. Using the same method, the adsorption structure of Tyr on the Cu(110) surface was also calculated.