Self-assembled monolayers of hexapeptides on gold: surface characterization and orientation distribution analysis

dMonodisperse hexapeptides that have lipoic acid coupled to the N-terminus were self-assembled on gold substrates. The self-assembled monolayer (SAM) film compositions were investigated by X-ray photoelectron spectroscopy (XPS), angle dependent XPS, and grazing angle Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS). The surface coverage of the self-assembled films was significantly larger than that of control physisorbed films, and the chemisorption between the hexapeptides and gold surface was stable in solvent. The structural results indicated that the hexapeptides helix orientation distribution relative to the surface normal is ambiguous-the data can be explained by either a broad distribution of an on-average perpendicular arrangement or a tight distribution about a single tilt angle. Hexapeptides in which a spin label, 2,2,6,6-tetramethylpiperidine-1-oxyl-4-amino-4-carboxylic acid (TOAC), was incorporated as an amino acid residue exhibit electron spin resonance (ESR) spectra dominated by spin-exchange broadening. This broadening precluded the possibility of using ESR evidence to get insights into hexapeptide orientational order from simulated spectra. ESR suggests that when the spin-label-containing hexapeptide is diluted in the SAM, orientational order is not apparent; the labeled hexapeptide SAM is disordered either structurally or conformationally or both.