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 illumi-nation, 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-oxidizedelectron donor or by the oxidation of the electron acceptor. In this work we have explored in moredetail the factors affecting the photocurrent generation in commercially available screen-printed elec-trochemical cells containing an electrolyte solution where RC proteins and suitable mediators are sol-ubilized. In particular, the role of the applied potential and the influence of conc entration and structureof acceptor and donor molecules have been assessed. We show that efficient generation of cathodicphotocurrents in a three electrode configuration occurs at an applied potential of 0.0 V versus quasi-refAg (the open circuit potential of the system measured in the dark) in presence of ferrocenemethanol anddecylubiquinone, which proved to guarantee high performances as electron donor and acceptorrespectively. Moreover, we employed a set of differential equations, describing reaction and diffusionprocesses, for modelling with high accuracy the chronoamperometry profiles recorded at variable RCconcentrations. This model allowed us to estimate the kinetic parameters relevant to the chemical andelectrochemical reactions triggered by light and to get a snapshot of the electrolyte composition in thebulk and electrode surroundings at different times from the light exposure. The characteristic timecourse of the photocurrent, showing a fast rise to a peak value followed by a slower decay, has beentherefore explained as the result of the strict interconnection between the dynamical processes involved