Functional Enzymes in Nonaqueous Environment: The Case of Photosynthetic Reaction Centers in Deep Eutectic Solvents

Deep eutectic solvents (DESs) are emerging as a new class of green solvents with the potential to replace organic solvents in several fundamental and applied processes. In this work, we offer an unprecedented characterization of the behavior of the bacterial photosynthetic reaction center (RC) from Rhodobacter sphaeroides in a series of choline chloride based DESs. RC is a membrane-spanning three-subunit pigment protein complex that, upon illumination, is capable of producing a stable charge-separated state. Thus, it represents the ideal model for carrying out basic studies of protein solvent interactions. Herein, we first report that, in many DES mixtures investigated, RC (a) is stable, (b) is capable of generating the charge-separated state, and (c) is even able to perform its natural photocycle. It proved, indeed, to be effective in reducing quinone molecules to quinol by withdrawing electrons from cytochrome c. As an example of biotechnological application, a photoelectrochemical cell based on DES-dissolved RC has also been designed and successfully employed to generate photocurrents arising from the reduction of the electron-donor ferrocenemethanol.