Photosynthesis powers life on earth, harvesting sunlight photons converting and storing them into chemical bonds and it does it very efficiently. Indeed Photosynthetic complex quantum-yield i.e. photon to electron conversion efficiency, is higher than 95% outperforming any human-made device. Unraveling this process is key to develop new solar renewable energy strategies. Scientific community have devoted a huge effort to grasp the features of this process: the molecules involved, their structure, their biological function. Biophysical tools have revealed how photons are absorbed, how the radiation energy is transferred with-in the complex, how excited protein co-factors leads to charge separation and how this charged is transferred with-in the photosynthetic complexes and out of them.
We present our work on the electron transfer process towards and into the membrane Photosystem-I (PSI) complex protein. Making use of an electrochemical set up, we deposit the proteins into a polarizable gold electrode and mimic the protein to protein electron transfer process. Under light exposure, we record the photo-current output of the protein complex deposited monolayer. Going a little bit further, we exploit the capabilities of the Scanning Tunneling Microscope (STM) probing the PSI complexes (8 nm wide) at the single molecule level. STM is operated in electrochemical set-up which allow us: i) keep proteins under physiological buffer to retain their biological function as much as possible; and ii) modify the sample electrode and microscope probe electrochemical potential to mimic the redox potential of PSI and PSI electron transfer partners. This allow us to probe PSI electron transfer properties at the single molecule level. This way, we have access to current versus distance decay constant of the electron transfer process. Our experiments have revealed that, at the right potentials, electrons can be transferred over several nanometers distance in aqueous medium.