Deciphering the Adsorption Mechanisms of RGD Subunits: L-Aspartic Acid on Cu(110)

In this work we present a detailed surface science characterization of L-aspartic acid adsorption on a Cu(110) surface. Aspartic acid is one of the main components of the tripeptide RGD (arginine-glycine-aspartic acid). We replaced the traditional sublimation method to obtain molecular films by dosing aspartic acid directly from an aqueous solution through an electrospray ionization (ESI) device. X-ray photoelectron spectroscopy (XPS) and polarization modulation reflection absorption infrared spectroscopy (PM-RAIRS) evidenced different adsorption states ranging from a submonolayer regime up to multilayers. Molecule-substrate interactions guide the creation of the pattern observed in the submonolayer, but molecule-molecule interactions are prevailing from a certain coverage stage, promoting the overlayer growth while leaving exposed areas of bare copper. This is evidenced by scanning tunneling microscopy (STM) results, showing that single aspartic acid molecules self-organize in a two-dimensional (2D) chiral network at low coverage and start originating new molecular layers even before a saturated monolayer has been reached.