Biomolecules at the amorphous silica/water interface: Binding and fluorescence anisotropy of peptides

We investigate binding of the tripeptides Lys-Trp-Lys (KWK) and Glu-Trp-Glu (EWE) to the amorphous silica surface using atomistic simulations. These peptides were chosen because they were previously utilized in experiments measuring binding affinity and steady-state fluorescence anisotropy from the indole chromophore of the tryptophan residue. Our simulations were performed using silica with surface change density of -0.8 elementary charges per square nanometer, which is expected at neutral pH. Even though positive charged KWK binds more strongly to the negatively charged silica surface, EWE also forms bound complexes with the surface that are stable for at least 15ns of simulation, in agreement with the experiments which revealed evidence for binding of both KWK and EWE to silica. Binding mechanisms include a wide variety of electrostatic interactions, as well as hydrophobic interactions between the indole group and hydrophobic areas of the heterogeneous silica surface. The long-time limit of the fluorescence anisotropy of tryptophan is calculated from the simulations in order to help interpret the recent experiments. We identify several factors which control the magnitude of the fluorescence anisotropy for each binding configuration.