XPS and ToF-SIMS Investigation of α-Helical and β-Strand Peptide Adsorption onto SAMs

Abstract

14-mer α-helix and a 15-mer β-strand oligopeptides composed of leucine (L) and lysine (K) were used to investigate peptide adsorption and orientation onto well-defined methyl and carboxylic acid terminated self-assembled monolayer (SAM) surfaces with X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS showed both peptides reached monolayer thickness on both SAMs, but significantly higher solution concentrations were required to reach this coverage on the methyl SAMs. This shows that the peptides adsorb more strongly onto the carboxyl-terminated SAMs. The excess oxygen detected by XPS and the H3O+ signal detected by ToF-SIMS for the SAMs with adsorbed peptides indicated that water molecules are associated with the adsorbed peptides, even under ultra-high vacuum conditions. Changes in the amount of L and K fragments detected by ToF-SIMS indicate the β-strand oriented differently on the two SAMs. The L side-chains were preferentially associated with the methyl-terminated SAM and the K side-chains were preferentially associated with the carboxyl SAM. In contrast, little change in the ToF-SIMS K/L ratio was observed for the α-helix peptide absorbed on the two SAMs, indicating ToF-SIMS was not as sensitive to orientation of the α-helix peptide.

Comparison Between Empirical Protein Force Fields for the Simulation of the Adsorption Behavior of Structured LK Peptides on Functionalized Surfaces

“…These results match the solution structure results for that FF (Fig. 4a, d), and most closely match experimental findings [9][10][11]50]. As in the case of the LKb7 peptides, it appears that the methyl-group parameters in CHARMM22 FF permit strong binding of the peptide with minimal disruption of the peptide's internal structure.…”

“…7c), thus suggesting a broader range of orientations with AMBER94. Experimentally, LKb7 peptides have been shown to form stable b-sheet structures with the L amino acids adsorbed closer to the surface than the K amino acids on hydrophobic surfaces by DeGrado and Lear [7] (air-water interface and an apolar surface), by Phillips and coworkers [8] (hydrophobic polystyrene), and by Castner and coworkers [9,11] (CH 3 -SAMs). In addition, using infrared spectroscopy, DeGrado and Lear indicated that the b-sheet formed by the LKb7 on an apolar surface occurred in an antiparallel structure, while using sum frequency generation (SFG), Castner and coworkers [11] indicated very distinct separation between the L and K amino acid residues on a CH 3 -SAM surface using a 15 amino acid alternating LK peptide (LKb15) as opposed to LKb7.…”

“…Experimentally, DeGrado and Lear [7] reported that LKb7 peptides formed stable antiparallel b-sheet structure on a hydrophilic quartz surface using CD and infrared spectroscopy, while both Castner and coworkers [9,11] (LKb15 peptides on COOH-SAM surface) and Phillips and coworkers [8] (LKb7 peptides on hydrophilic silica) found that these LK peptides oriented with K residues adsorbed closer to the surface than the L residues using SFG, timeof-flight secondary ion mass spectrometry (ToF-SIMS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and using SFG, respectively. Comparisons between our simulation results with the available experimental findings thus show that while all three FF were able to correctly predict the orientation of the LKb7 peptides on the COOH-SAM surface, none of the FFs were able to predict the formation of stable b-sheet structure, thus indicating problems with force field parameterization for this type of system.…”

“…Recently, a larger set of experimental studies has been published by Phillips and coworkers [8] on the adsorption behavior of this same set of LK peptides to both hydrophobic polystyrene and hydrophilic silica surfaces. Additionally and most recently, Castner and coworkers [9][10][11][12] have completed a comprehensive set of experiments studying the conformations and orientations of similar LK peptides adsorbed to a variety of material surfaces, including a hydrophobic methyl-terminated self-assembled monolayer (SAM) surface and a negatively charged carboxylic acid-terminated SAM on a gold substrate. The peptide-SAM surface studies completed by Castner and coworkers, coupled with results from similar previous studies, provide an excellent opportunity for the evaluation of different FFs in order to determine which FF is most capable of accurately representing peptide adsorption behavior for these types of systems.…”

Sum frequency generation and solid-state NMR study of the structure, orientation, and dynamics of polystyrene-adsorbed peptides

“…We have also studied the structure and orientation of LKα14 on PS, silica, self-assembled monolayers (SAMs), and fluorocarbon (FC) surfaces using SFG combined with other spectroscopic methods (9,16,18,(41)(42)(43). These studies show that adsorbed LKα14 has an α-helical secondary structure, and on the hydrophobic surfaces (PS and FC) the leucine side chains were oriented towards, and the lysine side chains oriented away from, the surface.…”