Self-assembling monolayers of helical oligopeptides on gold with applications in molecular electronics

A generic strategy is outlined for preparing a chiral functionalised surface using self-assembled monolayers (SAMs) of helical oligopeptides for intended application in molecular electronics, sensors and catalysis. Helical peptides have been designed and synthesised with 3 Met or 2 Cys residues per peptide, placed in the sequence so that in a helical conformation their side-chains are on the same side of the helix and organised to bind to gold by self-assembly of the thioether or thiol functions. Circular dichroism (CD) of the Met-containing peptides showed that they are mainly helical in organic solvents and FT-RAIRS (reflection-absorption infrared spectroscopy) of their SAMs on gold confirmed that they are aligned with the helix axes parallel to the surface. However complete coverage of the surface is dependent critically on the preparation of the gold. CD of an oligopeptide containing 2 Cys residues suggested that the conformation was a mixture of helix and random coil. Films of this peptide adsorbed from different solvents were characterised by ellipsometry and cyclic voltammetry. Ethanol and methanol led to multilayer formation indicating protic solvents destabilise the helical structure, allowing formation of intermolecular disulfide bonds. By contrast the aprotic solvent acetonitrile led to formation of a close-packed self-assembled monolayer suitable for elaboration into a functionalised supramolecular architecture.