Side chain, chain length, and sequence effects on amphiphilic peptide adsorption at hydrophobic and hydrophilic surfaces studied by sum-frequency generation vibrational spectroscopy and quartz crystal microbalance

The surface molecular structure and adsorbed mass of a series of model amphiphilic peptides have been studied in situ with surface-specific sum frequency generation (SFG) vibrational spectroscopy and quartz crystal microbalance (QCM) at the hydrophobic polystyrene and hydrophilic silica solid-water interface. The peptides are designed to form alpha-helical (XY14) and beta-strand (XY7) secondary structures at an apolar interface and contain hydrophobic (X) and charged (Y) amino acids with sequence Ac-XYYXXYXX-YYXXYX-NH2 or Ac-XYXYXYX-NH2, respectively. The X and Y combinations are leucine (L) and lysine (K), alanine (A) and lysine (K), alanine (A) and arginine (R), and phenylalanine (F) and arginine (R). One additional peptide with sequence Ac-LKKLLKL-NH2 (LK7 R) was synthesized. These peptides allow for the study of how chain length (LK14 vs LK7 alpha and LK7 beta), amino acid side chain character (LK vs AK vs AR vs FR), and sequence (LK7 alpha-helix vs LK7 beta-strand sequences) affect adsorption. The SFG spectra of adsorbed peptides on polystyrene typically show CH resonances characteristic of the hydrophobic side chains oriented at the interface. On polystyrene, the molecular orientation the side chain of alanine is more sensitive to changes in peptide chain length and sequence than that of leucine or phenylalanine. The SFG spectra of adsorbed peptides on silica show no distinct peptide modes, with the exception of the LK14 peptide, where an amide A NH mode is observed. The results demonstrate that SFG vibrational spectra can fingerprint specific amino acid ordering occurring at the polystyrene interface and secondary structure ordering at the silica interface. QCM data indicates that all peptides except LK7 beta adsorb onto both hydrophobic polystyrene and hydrophilic silica surfaces, even when SFG active modes are not observed.