Adsorption of Proline and Glycine on the TiO₂(110) Surface: A Density Functional Theory Study

Abstract

The optimal adsorption modes for the amino acids glycine and proline on the ideal TiO(2)(110) surface are investigated by using density functional theory (PBE) applying periodic boundary conditions. Binding modes with anionic acid moieties bridging two titanium atoms after transferring a proton to the surface are the most stable configurations for both molecules investigated-similar to previous results for carboxylic acids. In contrast to the latter compounds, amino acids can form hydrogen bonds via the amino group towards the surface-bound proton; this provides an additional stabilisation of 15-20 kJ mol(-1). Zwitterionic binding modes are less stable (by 10-20 kJ mol(-1)) and are less important for proline. Neutral modes are energetically even less favourable. Calculations of vibrational frequencies and core-level shifts complement the adsorption study and provide guidance for future experimental investigations. Control of the computational parameters is crucial for the derivation of accurate results. The layout and thickness of the slab model used are also shown to be decisive factors. Calculations with a different GGA-functional (PW91) provide very similar relative energies, although the absolute energies change by about 20 kJ mol(-1). Results derived with the hybrid functional PBE0 show an even greater stabilisation of the anionic binding modes with respect to the zwitterionic modes. A previously observed discrepancy between experimental and theoretical results for glycine could be solved, although the experimentally proposed free rotation of the C-C bond could not be reproduced.

Adsorption and self-assembly of bio-organic molecules at model surfaces: A route towards increased complexity

“…Tonner [293]confirmed the trend found by Ojamae [289], but a new anionic adsorption mode ( Figure 78b) was identified as being even more stable by 9 kJmol -1 (0.1 eV/molecule) compared to the previous minimum configuration, this time in agreement with the STM study of Qiu and Barteau which suggested adsorption in the anionic form. Among the other considered configurations, the anionic bifunctional adsorption with COO and NH 2 binding to Ti atoms in a bridging mode was found 23 kJ/mol (0.2 eV/molecule) less stable than the most favoured configuration.…”

“…In contrast, proline appears to be strongly adsorbed on the defected (001) surface, as evidenced by a shift of the desorption temperature from 350 K on stoichiometric TiO 2 (001) to 530-550 K on the oxygen-defected surface. Theoretical calculations reveal that proline shows similar adsorption behaviour than glycine but zwitterionic forms and coordination via the nitrogen atom are much less viable [293]. The most stable mode for both molecules is theone schematized in Figure 77, with an anionic amino acid moiety bridging two surface titanium atoms via the carboxyl oxygen atoms and a proton transferred to an oxygen atom at the surface.…”

“…DFT-Optimized adsorption geometries of glycine on TiO 2 (110), most stable configurations found by a) Ojamae et al [289](by 9 kJ/mol) b) Tonner [293].…”

“…The most stable mode for both molecules is theone schematized in Figure 77, with an anionic amino acid moiety bridging two surface titanium atoms via the carboxyl oxygen atoms and a proton transferred to an oxygen atom at the surface. This is similar to carboxylic acids, but the amino acids are able to form hydrogen bonds with the surface, thus stabilizing the adsorbed molecules by 15-20 kJmol -1 (0.15 eV) [293]. Ataman et al [309] used XPS to study the adsorption of L-cysteine on a rutile TiO 2 (110) surface at room temperature and at T=−65 °C.…”

Adsorption of organic molecules at the TiO2(110) surface: The effect of van der Waals interactions

“…DFT calculations by Tonner resulted in a similar tilting stabilisation effect of the surface bound hydrogen atom and the nitrogen atom of the amino group [11]. It is interesting that in the case of calculations including vdW interactions, the "vertical" configurations AN(OO) and ZW(OO) are not minima in the adsorption landscape.…”