Analysis of the structure and morphology of oligopeptide SAM using XPS and AFM

Oligopeptides such as PSP1 (Tyr-Ala-Gly-Tyr-Cys) and PSP2 (His-Ala-Ser-TyrSer-Cys), which interact strongly with a typical cationic porphyrin derivative, are synthesized. The structure and morphology of PSP1 and PSP2 monolayers self-assembled on Au surface have been investigated by using X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). XPS of the monolayers provides evidence that the primary adsorbate species are bonded to Au through the sulfur atom comprised in the species. It is revealed from XPS spectroscopy and AFM images that PSP2 is self-assembled better on Au surface than PSP1. This experimental result is well consistent with the prediction based on steric energies of Au-Peptide bond.