Convertible electron transfer pathways of cytochrome c at TiO2 quantum electrode

In this work, biofunctional ultrathin TiO2 quantum wires (about 3 nm), self-assembled into flat-cable-like stacks'', are synthesized by a simple template-free wet chemical method with high uniformity and on a large scale. On the self-assembled TiO2 nanowire (SA-Nw-TiO2) coated electrode, cytochrome c (cyt c) undergoes a fast, reversible (or quasi-reversible) and sensitive electron transfer process that is closely related to the relative ionic strength and the hydrated state of SA-Nw-TiO2 surface. At the suitably hydrated SA-Nw-TiO2 electrode in an optimum bulk ionic environment, electrons are reversibly transferred at the heme edge on the basis of an effective orientation of cyt c. It is interesting to note that in a suitable ionic microenvironment, a convertible electron transfer occurs in the following three steps: electron transfer driven partial unfolding of cyt c, electron transfer, and thermodynamics driven refolding. In addition, the partially unfolded cyt c was proven to exist only in the quasi-reversible electron transfer process, and the multi-step pathway of electron transfer can be converted into the single-step pathway at the fully hydrated SA-Nw-TiO2 electrode.