Charge Transfer to Photosystem I Through Hydroxyl-Terminated Alkanethiol SAM Length Modification

Photosystem I (PS I) is a supra-molecular photosynthetic protein complex that functions as a biological
photodiode. Upon exposure to light, PS I charge separates between the reaction center (P700 on
the lumenal side) and the Fe-S clusters (FA,FX,FB on the stromal side). The highly efficient photo-
electrochemical activities of PS I (with 100% quantum efficiency over 54% of the solar spectrum) make it
an ideal candidate for next-generation bio-hybrid electronic devices. Specifically, our interest in using PS
I for future hybrid photovoltaic (PV) device fabrications requires optimal encapsulation of these proteins
onto chemically grafted self-assembled monolayer (SAM) substrates. Our previous results indicate
that various experimental parameters alter the surface attachment dynamics of PS I, deposited from
colloidal aqueous buffer suspensions onto OH-terminated alkanethiolate/Au SAM substrates, thereby
allowing us to tailor the complex structural arrangements of PS I which affect their electron transfer and
capture pathways. We present surface topographical, combined with electrochemical characterizations
of PS I/Au SAM substrates, to elucidate the controlled monolayer surface attachment-as well as
the directional attachment dynamics of PS I to ensure proper photo-activated electronic activities.
Specifically, light induced directional electron transfer by surface immobilized PSI is demonstrated via
direct electrochemistry measurements from PS I complexes assembled on varying carbon chain lengths
(C6, C9, C8, C11 alkanethiolate SAM /Au surfaces). The results reveal that the supporting thiol brush
density and chain length, due to their non-optimal packing or large tunneling distance through the SAM,
can act as the critical bottleneck for the electron transfer process from the Au surface to the oxidized
P700+ reaction center of PS I. Similarly, on the stromal side, the electron donation of the reduced
FB- to the surrounding electrolyte solution is enhanced by the presence of a suitable oxidizing agent
or the addition of a soluble electron scavenger. This work provides a detailed and critical in sight into
the optimization of the rate limiting electron transfer processes at the interfaces between the donor
surfaces and P700+ as well as the FB- and the electrolyte. In turn , such studies can reveal significant
information regarding the orientation of PS I complexes on various donor substrates.