Adsorption and photooxidation of SARS-CoV 2 spike amino acids on TiO2 polymorphs

Abstract

This cumulative doctoral thesis investigates the molecular interactions between biologically relevant amino acids, L-cysteine and L-asparagine, and well-defined titanium dioxide (TiO2) surfaces, with emphasis on adsorption mechanisms and photocatalytically driven oxidation processes. This work was initiated within the CORAERO project (Grant KA1-Co-06), which investigated airborne transmission routes of the SARS-CoV-2 and explored photocatalytic strategies for viral inactivation. TiO2 was selected as a model photocatalyst due to its established antimicrobial activity against a broad range of pathogens. Despite this proven performance, the molecular mechanisms governing interactions between biomolecules and TiO2 surfaces remain unclear. This work examines how representative biomolecular building blocks, namely amino acids, adsorb and interact on TiO2 surfaces and how these processes contribute to chemical pathways leading to the inactivation of larger biological systems such as viruses.

A surface science approach allows adsorption geometries, surface coordination, and light-driven transformations to be resolved with atomic scale detail. Single crystalline rutile TiO2(110) and anatase TiO2(101) surfaces serve as model systems, enabling a controlled comparison between two technologically relevant TiO2 polymorphs. The amino acids L-cysteine and L-asparagine were selected as representative protein building blocks with chemically distinct functional groups in their side chain. The experimental methodology combines ultrahigh vacuum (UHV) sample preparation, scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared reflection-absorption spectroscopy (FT-IRRAS) methods, supported by density functional theory (DFT) calculations. Cysteine adsorption on rutile TiO2(110) is considerably more complex than previously assumed. Three adsorption configurations coexist at room temperature: a deprotonated or zwitterionic bridging bidentate DP(O,O)/ZW(O,O) carboxylate geometry, a deprotonated DP(O,N) configuration involving the amino group, and a mixed DP(O,S) configuration in which sulphur directly coordinates to surface Ti sites without C-S bond cleavage. STM reveals adsorption confined to Ti5c rows and dimer formation even at low coverage. XPS identifies both thiol (S-H) and thiolate (S-Ti) species confirming bond formation between the thiol group and the surface and together with FT-IRRAS demonstrate multiple protonation states for the amino group.

Cysteine adsorption on anatase TiO2(101) shows a different behaviour compared with rutile TiO2(110). STM reveals adsorption both on terraces and at step edges. XPS and FT-IRRAS indicate a high abundance of neutral NH2 species together with sulphur present in both S-H and S-Ti species, consistent with adsorption geometries involving direct coordination of nitrogen and sulphur to surface Ti atoms. Density functional theory calculations identify two nearly degenerate adsorption configurations, molecular M(N,S) and DP(O,N), with additional contributions from DP(O,S), whereas bridging carboxylate configurations such as DP(O,O)/ZW(O,O) are largely suppressed. Under UV illumination in air, cysteine undergoes selective photooxidation at the thiol group of the side chain, progressing from thiol species through intermediate oxidation states to fully oxidized sulfonic acid. This work provides the first experimental evidence of cysteine adsorption and photooxidation on anatase TiO2 single crystals.

The adsorption of L-asparagine on rutile TiO2(110) and anatase TiO2(101) further highlights the influence of surface structure on amino acid binding. On both polymorphs, XPS and FT-IRRAS show adsorption through a deprotonated carboxylate group while the amide side chain remains intact. However, the relative stability of adsorption geometries differs between the two surfaces. Rutile TiO2(110) predominantly stabilizes a bridging bidentate (O,O) configuration, as observed by STM as adsorbates aligned along Ti5c rows. In contrast, anatase TiO2(101) stabilizes a larger proportion of (O,N) geometries, consistent with the larger Ti5c–Ti5c separation of the anatase surface, which favors adsorption involving simultaneous coordination of oxygen and nitrogen atoms. As a consequence, XPS shows a higher proportion of protonated NH3+ species on rutile, whereas on anatase the amino group more frequently binds directly to surface Ti atoms and therefore remains in the neutral NH2 form.

Together, these results establish molecular-level adsorption models for cysteine and asparagine on rutile TiO2(110) and anatase TiO2(101) and reveal a selective stepwise sulphur photooxidation pathway for cysteine on anatase leading to sulfonic acid formation. They demonstrate that differences in Ti5c–Ti5c spacing between rutile and anatase determine the stability of (O,O) and mixed (O,N)/(N,S) adsorption geometries and thereby control biomolecule binding and photocatalytic reactivity on TiO2 surfaces.