Adsorption of Amino Acids and Dipeptides to the Hydrophobic Polystyrene Interface Studied by SFG and QCM: The Special Case of Phenylalanine

Aqueous solutions of the amino acids l-phenylalanine, l-lysine, and l-glycine and the homo- and heterodipeptides comprising these amino acids were studied by vibrational sum frequency generation (SFG) and quartz crystal microbalance (QCM) at the hydrophobic polystyrene interface. The phenyl ring of the phenylalanine side chain was determined to adsorb preferentially in a nearly flat geometry relative to the hydrophobic surface based on the concentration dependence of the SFG spectra and symmetry arguments. The amount of adsorbed dipeptide follows a hydrophobic series at concentrations well below monolayer formation, as determined by QCM. However, at higher concentrations, adsorbate–adsorbate interactions play a significant role in the adsorption, and adsorption no longer follows a hydrophobic series. These changes in the quantitative adsorption from QCM correlate with changes in the SFG spectra for phenylalanine, lysyl-phenylalanine, and glycyl-phenylalanine, but not for lysyl-lysine, which shows the most striking adsorbate interaction effect.