Function of the photosynthetic reaction center from Rhodobacter sphaeroides in deep eutectic solvents

Deep eutectic solvents (DESs) are emerging as a new class of green solvents, with the potentiality of replacing organic solvents in many applications both at industrial and laboratory scale. The intriguing possibility of carrying out bio-catalytic reactions has been recently explored using hydrolases with promising results [1]. In this work we aim to offer a complete characterization of the behavior of the bacterial photosynthetic reaction center (RC) from Rhodobacter sphaeroides in a series of choline chloride-based DESs. The stable charge-separated state of RC, attained under illumination, represents the first step of light energy conversion into chemical energy in phototrophic organisms and a number of reports have recently enlightened the possibility of effectively exploiting RC photo-activity in bio-devices. The employment of non-aqueous solvents would then open the way to a wider range of technological applications. Moreover, the fully characterized RC is the ideal model for carrying out basic studies of protein-solvent interactions, due to its information-rich optical spectrum and its light-triggered enzymatic activity. Herein we report that RC (a) is stable in all the DESs tested, (b) is able to generate the charge-separated state under illumination, and (c) even to perform its natural photocycle. We have indeed demonstrated in DES environment that RC can effectively promote under light the reduction of quinone molecules by withdrawing electrons from cytochrome c. Finally, as an example of biotechnological application, a photo-electrochemical cell based on DES- dissolved RC has been designed and successfully employed to generate photocurrents arising from the reduction of the electron-donor ferrocene-methanol.