Photo-induced electron transfer in supramolecular materials of titania nanostructures and cytochromec

Abstract

In the present paper, we report on the molecular interaction and photochemistry of TiO₂ nanoparticles (NPs) and cytochrome c systems for understanding the effects of supramolecular organization and electron transfer by using two TiO₂ structures: P25 TiO₂ NPs and titanate nanotubes. The adsorption and reduction of cytochrome c heme iron promoted by photo-excited TiO₂, arranged as P25 TiO₂ NPs and as nanotubes, were characterized using electronic absorption spectroscopy, thermogravimetric analysis, and atomic force microscopy. In an aqueous buffered suspension (pH 8.0), the mass of cytochrome c adsorbed on the P25 TiO₂ NP surface was 2.3 fold lower (0.75 μg m⁻²) than that adsorbed on the titanate nanotubes (1.75 μg m⁻²). Probably due to the high coverage of titanate nanotubes by adsorbed cytochrome c, the low amount of soluble remaining protein was not as efficiently photo-reduced by this nanostructure as it was by the P25 TiO₂ NPs. Cytochrome c, which desorbed from both titanium materials, did not exhibit changes in its redox properties. In the presence of the TiO₂ NPs, the photo-induced electron transfer from water to soluble cytochrome c heme iron was corroborated by the following findings: (i) identification by EPR of the hydroxyl radical production during the irradiation of an aqueous suspension of TiO₂ NPs, (ii) impairment of a cytochrome c reduction by photo-excited TiO₂ in the presence of dioxane, which affects the dielectric constant of the water, and (iii) change in the rate of TiO₂-promoted cytochrome c reduction when water was replaced with D₂O. The TiO₂-promoted photo-reduction of cytochrome c was reverted by peroxides. Cytochrome c incorporated in the titanate nanotubes was also reversibly reduced under irradiation, as confirmed by EPR and UV-visible spectroscopy.