Highly Sensitive Method to Isolate Photocurrent Signals from Large Background Redox Currents on Protein‐Modified Electrodes

Purification and preparation of Rhodobacter sphaeroides reaction centers for photocurrent measurements and atomic force microscopy characterization

The formation of defined surfaces consisting of photosynthetic reaction centers (RCs) in biohybrid solar cells is challenging. Here, we start with the production of engineered RCs for oriented binding. RCs are deposited onto gold electrodes, and 6-mercapto-1-hexanol (MCH) is used to displace multilayers and non-specifically adsorbed RCs. The resulting electrode surfaces are analyzed for photocurrent generation using an intensity-modulated light and lock-in amplifier. Atomic force microscopy (AFM) is used to characterize the surface and the formation of RC structural assemblies.

For complete details on the use and execution of this profile, please refer to Jun et al. (2021).

•

Express and purify RC membrane proteins from Rhodobacter sphaeroides

•

Deposit RCs and MCH on gold electrodes for formation of structural assemblies

•

Measure RC photocurrents with an intensity-modulated LED and lock-in detection

•

Prepare single crystal gold electrodes for AFM of RCs and image processing

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

The use of biomacromolecules is a nascent development in clean alternative energies. In applications of biosensors and biophotovoltaic devices, the bacterial photosynthetic reaction center (RC) is a protein-pigment complex that has been commonly interfaced with electrodes, in large part to take advantage of the long-lived and high efficiency of charge separation. We investigated assemblies of RCs on an electrode that range from monolayer to multilayers by measuring the photocurrent produced when illuminated by an intensity-modulated excitation light source. In addition, atomic force microscopy and modeling of the photocurrent with the Marcus-Hush-Chidsey theory detailed the reorganization energy for the electron transfer process, which also revealed changes in the RC local environment due to the adsorbed conformations. The local environment in which the RCs are embedded significantly influenced photocurrent generation, which has implications for electron transfer of other biomacromolecules deposited on a surface in sensor and photovoltaic applications employing a redox electrolyte.

Introduction of the Menaquinone Biosynthetic Pathway into Rhodobacter sphaeroides and de Novo Synthesis of Menaquinone for Incorporation into Heterologously Expressed Integral Membrane Proteins

Quinones are redox-active molecules that transport electrons and protons in organelles and cell membranes during respiration and photosynthesis. In addition to the fundamental importance of these processes in supporting life, there has been considerable interest in exploiting their mechanisms for diverse applications ranging from medical advances to innovative biotechnologies. Such applications include novel treatments to target pathogenic bacterial infections and fabricating biohybrid solar cells as an alternative renewable energy source. Ubiquinone (UQ) is the predominant charge-transfer mediator in both respiration and photosynthesis. Other quinones, such as menaquinone (MK), are additional or alternative redox mediators, for example in bacterial photosynthesis of species such as Thermochromatium tepidum and Chloroflexus aurantiacus. Rhodobacter sphaeroides has been used extensively to study electron transfer processes, and recently as a platform to produce integral membrane proteins from other species. To expand the diversity of redox mediators in R. sphaeroides, nine Escherichia coli genes encoding the synthesis of MK from chorismate and polyprenyl diphosphate were assembled into a synthetic operon in a newly designed expression plasmid. We show that the menFDHBCE, menI, menA, and ubiE genes are sufficient for MK synthesis when expressed in R. sphaeroides cells, on the basis of high performance liquid chromatography and mass spectrometry. The T. tepidum and C. aurantiacus photosynthetic reaction centers produced in R. sphaeroides were found to contain MK. We also measured in vitro charge recombination kinetics of the T. tepidum reaction center to demonstrate that the MK is redox-active and incorporated into the Q_A pocket of this heterologously expressed reaction center.