2, Centre for Nanoscale BioPhotonics, Adelaide, South Australia, Australia
Bio-inspired molecular electronics is a particularly intriguing paradigm, as charge transfer in proteins/peptides, for example, plays a crucial role in energy storage and conversion processes in all living organisms. However, the structure and conformation of even the simplest protein is complex, and as such, model synthetic peptides containing well-defined geometry and pre-determined functionality, present as ideal platforms to mimic nature for the elucidation of fundamental biological processes, while also advancing the design and development of single-peptide electronic components and other devices.
We present studies on intramolecular electron transfer in synthetic peptides of well-defined helical conformation and also ill-defined geometry, using electrochemical techniques and constrained density functional theory simulations. Two definitive electron transfer pathways are apparent, the nature of which is dependent on secondary structure. Electrochemical results indicate that peptides constrained by either Huisgen cycloaddition, ring-closing metathesis or lactam-bridge exhibit remarkable positive formal potential shifts (> 460 mV) and significant electron transfer rate constant drops (up to 15-fold), which represent two distinct electronic ‘on/off’ states. The additional backbone rigidity imparted by the side-bridge constraints leads to an increased reorganization energy barrier to restrict the torsional motions necessary for facile intramolecular electron transfer along the backbone. A clear mechanistic transition from hopping to superexchange, stemming from side-bridge gating, is apparent. The electronic properties of peptides can be fine-tuned through both structural and chemical manipulation, to reveal an interplay between backbone rigidity and electron rich side-chains on electron transfer. The side-bridge constraints provide an additional electron transport pathway, to provide two distinct forms of quantum interferometers. The effects of destructive quantum interference occur essentially through the backbone and the additional tunnelling pathway provided by the side-bridge in the constrained β-strand peptide, as evidenced by a correlation between electrochemical measurements and molecular junction conductance simulations for both linear and constrained β-strand peptides. In contrast, an interplay between quantum interference effects and vibrational fluctuations is revealed in the linear and constrained helical peptides.
Collectively, these findings not only augment our fundamental knowledge of charge transfer dynamics and kinetics in peptides, but also open up new avenues to design and develop functional bio-inspired electronic devices, such as on/off switches and quantum interferometers, for practical applications in molecular electronics. These studies also provide an opportunity to develop peptide-based sensors for detecting biological Zn2+ and also protein-protein interactions, aspects of which will also be discussed.