Design and modelling of a photo-electrochemical transduction system based on solubilized photosynthetic reaction centres

The bacterial photosynthetic reaction centre (RC) is a membrane spanning protein that, upon illumination, promotes the reduction of a ubiquinone molecule withdrawing electrons from cytochrome c2. This photo-activated reaction has been often exploited, in suitably designed photoelectrochemical cells, to generate photocurrents sustained by the reduction at the working electrode of the photo-oxidized electron donor or by the oxidation of the electron acceptor. In this work we have explored in more detail the factors affecting the photocurrent generation in commercially available screen-printed electrochemical cells containing an electrolyte solution where RC proteins and suitable mediators are solubilized. In particular, the role of the applied potential and the influence of concentration and structure of acceptor and donor molecules have been assessed. We show that efficient generation of cathodic photocurrents in a three electrode configuration occurs at an applied potential of 0.0 V versus quasi-ref Ag (the open circuit potential of the system measured in the dark) in presence of ferrocenemethanol and decylubiquinone, which proved to guarantee high performances as electron donor and acceptor respectively. Moreover, we employed a set of differential equations, describing reaction and diffusion processes, for modelling with high accuracy the chronoamperometry profiles recorded at variable RC concentrations. This model allowed us to estimate the kinetic parameters relevant to the chemical and electrochemical reactions triggered by light and to get a snapshot of the electrolyte composition in the bulk and electrode surroundings at different times from the light exposure. The characteristic time course of the photocurrent, showing a fast rise to a peak value followed by a slower decay, has been therefore explained as the result of the strict interconnection between the dynamical processes involved.