The adsorption of a 14-amino acid amphiphilic peptide, LK14, which is composed of leucine (L, nonpolar) and lysine (K, charged), on hydrophobic polystyrene (PS) and hydrophilic silica (SiO2) was investigated in situ by quartz crystal microbalance (QCM), atomic force microscopy (AFM), and sum frequency generation (SFG) vibrational spectroscopy. The LK14 peptide, adsorbed from a pH 7.4 phosphate-buffered saline (PBS) solution, displayed very different coverage, surface roughness and friction, topography, and surface-induced orientation when adsorbed onto PS versus SiO2 surfaces. Real-time QCM adsorption data revealed that the peptide adsorbed onto hydrophobic PS through a fast (t < 2 min) process, while a much slower (t > 30 min) multistep adsorption and rearrangement occurred on the hydrophilic SiO2. AFM measurements showed different surface morphologies and friction coefficients for LK14 adsorbed on the two surfaces. Surface-specific SFG spectra indicate very different ordering of the adsorbed peptide on hydrophobic PS as compared to hydrophilic SiO2. At the LK14 solution/PS interface, CH resonances corresponding to the hydrophobic leucine side chains are evident. Conversely, only NH modes are observed at the peptide solution/SiO2 interface, indicating a different average molecular orientation on this hydrophilic surface. The surface-dependent difference in the molecular-scale peptide interaction at the solution/hydrophobic solid versus solution/hydrophilic solid interfaces (measured by SFG) is manifested as significantly different macromolecular-level adsorption properties on the two surfaces (determined via AFM and QCM experiments).

No abstract

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 R-helical (XY 14 ) and β-strand (XY 7 ) secondary structures at an apolar interface and contain hydrophobic (X) and charged (Y) amino acids with sequence Ac-XYYXXYXX-YYXXYX-NH 2 or Ac-XYXYXYX-NH 2 , 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-NH 2 (LK 7 R) was synthesized. These peptides allow for the study of how chain length (LK 14 vs LK 7 R and LK 7 β), amino acid side chain character (LK vs AK vs AR vs FR), and sequence (LK 7 R-helix vs LK 7 β-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 LK 14 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 LK 7 β adsorb onto both hydrophobic polystyrene and hydrophilic silica surfaces, even when SFG active modes are not observed.