Mechanism of Orientation-Dependent Asymmetric Charge Transport in Tunneling Junctions Comprising Photosystem I

Abstract

Recently, photoactive proteins have gained a lot of attention due to their incorporation into bioinspired (photo)electrochemical and solar cells. This paper describes the measurement of the asymmetry of current transport of self-assembled monolayers (SAMs) of the entire photosystem I (PSI) protein complex (not the isolated reaction center, RCI), on two different “director SAMs” supported by ultraflat Au substrates. The director SAMs induce the preferential orientation of PSI, which manifest as asymmetry in tunneling charge-transport. We measured the oriented SAMs of PSI using eutectic Ga–In (EGaIn), a large-area technique, and conducting probe atomic force microscopy (CP-AFM), a single-complex technique, and determined that the transport properties are comparable. By varying the temperatures at which the measurements were performed, we found that there is no measurable dependence of the current on temperature from ±0.1 to ±1.0 V bias, and thus, we suggest tunneling as the mechanism for transport; there are no thermally activated (e.g., hopping) processes. Therefore, it is likely that relaxation in the electron transport chain is not responsible for the asymmetry in the conductance of SAMs of PSI complexes in these junctions, which we ascribe instead to the presence of a large, net dipole moment present in PSI.

Orientation and Incorporation of Photosystem I in Bioelectronics Devices Enabled by Phage Display

“…Differences in log|χ| are an indication of the effect of the PSI/peptide versus possible asymmetry induced by the peptides, which are themselves highly oriented (otherwise they would not be able to orient PSI). Thus, we ascribe the observation of similar values of log|χ| to the low coverage of PSI complexes on RDQ and LAT because, due to the noncovalent nature of the peptide–PSI interaction and the strong affinity of the peptides for ITO, the PSI/peptide complex dissociated and the surface is passivated mainly by free peptide 26. That is, we expect ∆log|χ| to increase monotonically with orientation, but a lower surface coverage of PSI on LAT could lower ∆log|χ|.…”

“…We determined the orientation of single complexes by conducting AFM, large‐area junctions with EGaIn, and under illumination in photovoltaic devices. In all three cases, the collective action of the dipoles intrinsic to the complexes (dark) or the electron‐transport chain (illumination) affected charge‐transport systematically as a function of the direction/degree of orientation 26. Future work will be directed toward exploiting these peptides for integrating PSI in other biosolar and biofuel cells and to functionalize PSI in a mild, noncovalent fashion, which requires only the addition of functionalized peptides and their binding to PSI without any chemical coupling reagent 32.…”

“…In a recent study, SAMs of partially oriented PSI using two different short linkers showed orientation‐dependent electrical properties using EGaIn as a top electrode 26. Additionally, by measuring the charge transport through these two partially oriented PSI layers at temperatures between 198 and 298 K, it was shown that the mechanism of charge transport through PSI is most likely tunneling due to the lack of any apparent thermally activated processes.…”

“…The orientation of PSI on a conducting surface can be determined by c‐AFM, taking advantage of the tendency of the complexes to induce asymmetric I–V curves depending on their orientation. The origin of this rectification on Au substrates was recently elucidated and is most likely due to the permanent dipole of the PSI protein scaffold and not the cofactors of the ETC 26. When PSI is directed with the F B side or PsaC toward the substrate (i.e., the bottom electrode), a higher electrical current is measured at negative bias compared to the electrical current at positive bias giving rise to I–V data that are asymmetric about zero.…”

“…In comparison to macroscopic studies, the ITO surface was functionalized with an SAM of DHAP as described elsewhere 22. The EGaIn measurement protocol is described in detail separately 26. We contacted multiple PSI complexes on monolayers of different peptides—RDQ, ALF, LAT, and IQA—by lowering a conical tip of EGaIn27 (≈25 µm in diameter) and measuring conductance under dark conditions when PSI is not photoactivated.…”