Self-assembling peptides: A combined XPS and NEXAFS investigation on the structure of two dipeptides Ala–Glu, Ala–Lys

The two dipeptides AE (Lalanine–Lglutamic acid) and AK (Lalanine–Llysine), that constitute the “building blocks” of the 16-unit self-complementary amphiphilic oligopeptide EAK16, have been investigated by XPS (X-ray photoelectron spectroscopy) and NEXAFS (near-edge X-ray absorption fine structure) spectroscopy. Thin films of both dipeptides on TiO2, a distinguished biocompatible surface, were prepared by incubation from aqueous solutions. Thick films of dipeptides on inert Au substrates were also studied for comparison. The chemical structure and composition were investigated by XPS spectroscopy; furthermore, molecular orientation of dipeptides on TiO2 was checked by angular dependent NEXAFS measurements at both C–K and N–K edges. In order to yield some insight on adsorption geometry and molecular orientation MD (molecular dynamic) simulations were also carried out.The performed molecular and electronic characterization of AE and AK provides an excellent model for the interpretation of more complex peptide spectra.