Structural Investigation of 3,4-Dihydroxyphenylalanine-Terminated Propanethiol Assembled on Gold

Abstract

3,4-Dihydroxyphenylalanine-terminated propanethiol (DOPA-PT), an amino acid DOPA linked to 3-mercaptopropionic acid through an amide bond, is adsorbed and self-assembled to polycrystalline gold surfaces. The structure of the adsorbates was characterized by means of X-ray photoelectron spectroscopy (XPS), infrared reflection−absorption spectroscopy (IRAS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Strong molecular binding of a DOPA derivative on gold surfaces through the sulfur atom was attained. Angle-dependent XPS results showed that the aromatic ring is oriented away from the gold surface. Both IRAS and NEXAFS results showed parallel orientation of the CO bond of the amide moiety relative to the gold surface. Hydrogen bonding between amide moieties is achieved, and it seemed to provide additional orientation stabilization. Deduced orientation of the amide moiety on the short alkyl chain (or the peptide plane) is assumed to give the average orientation of the main molecular axis. The main molecular axis is estimated to have an average tilt angle of approximately 37° relative to the gold surface normal based on NEXAFS results. The aromatic ring exhibits a preferred orientation with an average tilt angle of about 64°. The experimental results showed that DOPA-thiol with amide linkage is able to self-assemble and form a layered structured film consisting of a layer of alkane chains with a gauche conformation beneath an oriented layer of DOPA.