ELECTROPEP · Electronic devises from peptides

Final Activity Report Summary - ELECTROPEP (Electronic Devises from Peptides)

This ToK project was aimed at establishing new research competence in the development and understanding of bio-molecular electronic devices. This project brought together the specialities of biochemistry and synthetic chemistry in designing and synthesising redox active peptide molecular wires and specialities in molecular nanoelectronics in integrating these molecules into electrical junctions down to the single molecule level. State-of-the-art electrical measurements were made on these electrical junctions based on peptides using scanning probe microscopy and electrochemistry. Electrochemistry was important in charactering the electron transport characteristics of well-defined peptide films on gold electrodes. An important sequence and model system in our studies was a peptide chain whose conformation and extension could be changed with solution pH. This modified peptide was specially designed as a biomolecular electrical "component" whose electrical properties could be controlled through solution pH. This sequence with an appended redox group was synthesised and the interfacial properties of monomolecular films of this peptide were investigated, including its electron transfer characteristics using electrochemical methods. Since redox active peptides have formed a major strategic part of the ELECTROPEP project, as a means of incorporating electrical functionality into peptide molecular wires, we have developed an alternative strategy for studying self-assembled monolayers (SAMs) of peptides molecules having a redox active chain. It consisted of the in-situ modification, by the redox active group, of the pre-formed monolayer of peptide. Using this strategy, investigations on well organised monolayers were then possible, and this avoids some time- and reactants- consuming steps inherent to the organic synthesis. Electron transport across such peptide monolayers was studied. A key objective of the project was the measurement of the single molecule electrical properties of the peptides, and the correlation of these data with electrochemical results. We used a technique employing a scanning tunnelling microscope (STM) which enables the formation of single molecule junctions involving peptides which are formed between the gold STM tip and gold surface. This provides a state-of-the-art means of investigating the electrical properties of single bio-molecules. This represents a key method in the nascent field of single biomolecular electronics which enables a reliable characterisation of the electrical properties of single bio-molecules. While it was initially believed that only sulphur terminated molecules were suitable for these types of experiments, a number of recent publications, including one from our team, have shown that other terminal groups are able to bind to gold. Of particular interest in the present case, it has been shown that both the -NH2 and the -COOH groups, present at the extremities of peptides molecules, can attach to gold in molecular junctions. To further explore this issue we conducted fundamental study on the single molecule conductance of alkanedicarboxylic acids. It was demonstrated that Au surface HOOC-(CH2)n-COOH Au tip junctions can be formed, and their single molecule conductance be measured ( Journal of Physical Chemistry C 2008, 112, 3941-3948). We have also measured the single molecule conductance of a number of differing peptide sequences to establish factors influencing reliable junction formation and the electrical properties of single peptide molecular wires. To capitalise on this success we are continuing with further single molecule measurements of peptide sequence using the scanning tunnelling microscope.

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