Single molecule conduction of engineered cytochrome b562 bonded to metallic electrodes

Abstract

Measuring single molecule conductance is a fundamental step in order to realise the basic elements of future electronic circuits. This work describes the use of an engineered electron transfer protein, cytochrome 6562 (cyt 6562), as a single molecule junction point between a gold surface and a metallic tip through defined thiol-metal interactions. Two separate cysteine residues were introduced in the cyt b562 amino acid sequence at strategic positions the single-molecule conductivity of the two double cysteine mutants SH-SA and SH-LA was investigated using atomic force microscopy (AFM), scanning tunnelling microscopy (STM), current-volt age (TV) and current-distance (I-z) experiments. The haem binding properties of the cysteine variants were similar to that of the wild-type cyt 6562 confirming that the mutations had not altered the protein's core properties. AFM and STM studies revealed that the SH-SA and SH-LA molecules bound to gold electrode in defined orientations, dictated by the thiol-pair utilised. A strong and stable interaction between the proteins bearing the thiol groups and a Au(lll) surface was achieved, and a single-molecule conductance of 1 nS w is measured in air. In contrast, the unengineered wild-type cyt 6562 bound much less robustly to the gold surface and the measured conductance was at least one order of magnitude less. Crucially, using electrochemical STM (EC-STM) approaches a change in conductance of the cytochrome over different overpotentials was observed, demonstrating that the molecule can act as an electrochemical gate. The protein became most conducting when the substrate potential was set close to the redox potential of the protein. The electrochemical, I-V and I-z STM measurements sug gested a two-step model for electron transfer. This study illustrates the possibility of exploiting a haem binding nrotein directly adsorbed onto a conducting surface as a nanoelectronic element and offers nw perspectives for future biomolecular electronic circuits