Insight into Electron Transfer from a Redox Polymer to a Photoactive Protein

Abstract

Biohybrid photoelectrochemical systems in photovoltaic or biosensor applications have gained considerable attention in recent years. While the photoactive proteins engaged in such systems usually maintain an internal charge separation quantum yield of nearly 100%, the subsequent steps of electron and hole transfer beyond the protein often limit the overall system efficiency and their kinetics remain largely uncharacterized. To reveal the dynamics of one of such charge-transfer reactions, we report on the reduction of Rhodobacter sphaeroides reaction centers (RCs) by Os-complexmodified redox polymers (P-Os) characterized using transient absorption spectroscopy. RCs and P-Os were mixed in buffered solution in different molar ratios in the presence of a water-soluble quinone as an electron acceptor. Electron transfer from P-Os to the photoexcited RCs could be described by a three-exponential function, the fastest lifetime of which was on the order of a few microseconds, which is a few orders of magnitude faster than the internal charge recombination of RCs with fully separated charge. This was similar to the lifetime for the reduction of RCs by their natural electron donor, cytochrome c 2 . The rate of electron donation increased with increasing ratio of polymer to protein concentrations. It is proposed that P-Os and RCs engage in electrostatic interactions to form complexes, the sizes of which depend on the polymer-toprotein ratio. Our findings throw light on the processes within hydrogel-based biophotovoltaic devices and will inform the future design of materials optimally suited for this application.

In situ Time-Resolved Spectroelectrochemistry Reveals Limitations of Biohybrid Photoelectrode Performance

“…In robust biological charge-separating systems, the high efficiency at low excitation frequencies is not particularly surprising, even following isolation from the membrane that prevents short-circuits. Rates of excitation energy transfer and charge separation 20 , as well as P + re-reduction under very low photon fluxes 7,19 in the reported excitation rate ranges of 1 s -1 are therefore not affected by the electrode adsorbed configuration, nor does short-circuiting play a substantial role in decreasing efficiency.…”

“…S9) that are often short-lived or even undetected under low light. Previous spectroscopic investigations of biohybrid systems do not take into account the simultaneous interactions of the redox mediators with the electrodes under high light in their measurements, which influence the population of certain RC states and also compromise overall biohybrid efficiency 7,19,20 . To understand how steady-state conditions affect the performance of mITO|cyt c|RC-LH1, we measured the spectroscopic signature of P870 + under continuous illumination during biohybrid operation.…”

“…A challenge in the optimization of photocurrent generation by RC proteins in biohybrid devices is the identification of loss processes of the desired electron flow, as well as the establishment of how bottlenecks in electron flow decrease the proportion of RCs in the productive open state. Absorbance spectroscopy offers a tool to probe electron transfer in RCs, and has been used to assess donor side bottlenecks and electron transfer kinetics in aqueous suspensions (see SI for more details) 7,[18][19][20] . However, what these spectroscopic investigations lack is the investigation of RC operation on an electrode surface, an environment in which mediator-electrode interactions and substrate depletion that can drastically shift limiting processes during biohybrid operation.…”

Correlating structural assemblies of photosynthetic reaction centers on a gold electrode and the photocurrent - potential response

“…The adsorption of RC proteins onto an electrode with the goal to maximize photocurrent generation has been accomplished by deposition in thick layers ( Plumeré and Nowaczyk, 2016 ; Ravi and Tan, 2015 ; Yehezkeli et al., 2014 ; Kamran et al., 2014 ; Mirvakili et al., 2014 ), or with RCs suspended in a redox hydrogel polymer deposited onto the electrode ( Białek et al., 2020a , 2020b ). Although other approaches to orient RCs have been successful in achieving large photocurrents, the possibility of significant random orientations or multilayers of RCs still remains.…”

Charge Transfer and Biocompatibility Aspects in Conducting Polymer-Based Enzymatic Biosensors and Biofuel Cells

“…Some types of redox polymers and redox polymer-based composites can also be applied for this purpose and/or in order to improve the microenvironment required for efficient action of immobilized enzymes or other redox-able proteins [ 18 , 19 , 20 , 21 ]. However, from this point of view, conducting polymers seem to be the most promising because they can be used in order to advance charge transfer efficiency in bioelectronics-based devices [ 22 ] and some analytical characteristics of biosensors [ 23 , 24 ]. In addition, conducting polymers have great environmental stability [ 25 ] and are characterized by rather good biocompatibility [ 26 ].…”