Abstract
Photosynthetic reaction centres show promise for biomolecular electronics as nanoscale solar-powered batteries and molecular diodes that are amenable to atomic-level re-engineering. In this work the mechanism of electron conduction across the highly tractable Rhodobacter sphaeroides reaction centre is characterized by conductive atomic force microscopy. We find, using engineered proteins of known structure, that only one of the two cofactor wires connecting the positive and negative termini of this reaction centre is capable of conducting unidirectional current under a suitably oriented bias, irrespective of the magnitude of the bias or the applied force at the tunnelling junction. This behaviour, strong functional asymmetry in a largely symmetrical protein–cofactor matrix, recapitulates the strong functional asymmetry characteristic of natural photochemical charge separation, but it is surprising given that the stimulus for electron flow is simply an externally applied bias. Reasons for the electrical resistance displayed by the so-called B-wire of cofactors are explored.
Highlights
Photosynthetic reaction centres show promise for biomolecular electronics as nanoscale solar-powered batteries and molecular diodes that are amenable to atomic-level re-engineering
The need to understand how electrons tunnel through individual protein molecules under an applied bias has been a driver in the development of techniques such as conducting atomic force microscopy (C-AFM) and scanning tunnelling microscopy that can probe the electrical properties of single molecules fixed between a substrate and an electrically conductive probe tip[6]
Application of C-AFM to oriented Rba. sphaeroides reaction centres (RCs) in the dark and under an applied external bias of appropriate polarity has revealed that current will flow through the protein from the electron donor side to the electron acceptor side, with the current amplitude growing with increasing bias, but unlike a great many peptides or redox proteins little or no current is seen if a reverse bias is applied, irrespective of its magnitude[18,19,20]
Summary
Photosynthetic reaction centres show promise for biomolecular electronics as nanoscale solar-powered batteries and molecular diodes that are amenable to atomic-level re-engineering. We find, using engineered proteins of known structure, that only one of the two cofactor wires connecting the positive and negative termini of this reaction centre is capable of conducting unidirectional current under a suitably oriented bias, irrespective of the magnitude of the bias or the applied force at the tunnelling junction This behaviour, strong functional asymmetry in a largely symmetrical protein–cofactor matrix, recapitulates the strong functional asymmetry characteristic of natural photochemical charge separation, but it is surprising given that the stimulus for electron flow is an externally applied bias. RCs act as solar batteries, using harvested light energy to separate charge between an electron donor at one end of the protein and an acceptor at the other[8,9,10,11] This multi-step process occurs with a very high quantum efficiency (charges separated across the RC protein per photon absorbed) and, in biology, creates a potential difference that powers an external linear or cyclic electron transfer system. These polypeptides have a similar fold but are only B33% identical[39], facilitating microscopic asymmetry in a structure with macroscopic symmetry
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