Paper BI+AS+NS-ThA7
XPS, ToF-SIMS and NEXAFS Investigation of Peptide Adsorption onto SAMs

Thursday, October 18, 2007, 4:00 pm, Room 609

The interactions between proteins and surfaces are critical to the success or failure of implants in the body. When adsorbed onto a synthetic surface, proteins often denature which can trigger the foreign-body response. It is therefore essential to develop methods to examine these interfacial phenomena. This work uses X-ray photoelectron spectroscopy (XPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS) and near-edge X-ray absorption spectroscopy (NEXAFS) to characterize the structure of α-helix and β-sheet peptides adsorbed onto self-assembled monolayers (SAMs). The α-helix peptide is a 14-mer made up of lysine (K) and leucine (L) residues with a hydrophobic periodicity of 3.5. The β-sheet peptide is a 15-mer also made up of L and K residues with a hydrophobic periodicity of 2. A β-sheet peptide with the same structure but with valine (V) substituted for L was also studied since V has a higher tendency than L to form β-sheet structures. All peptides have the hydrophobic side-chains on one side of the peptide and the hydrophilic on the other. The SAMs studied were thiols on gold containing the ω-groups -CH₃, -OH, -COOH, -NH₃⁺ and -CF₃. XPS nitrogen atomic percent was used to measure adsorption isotherms for the peptides. The α-helix peptide forms a monolayer (8.2% N) on the COOH-terminated SAM at an adsorption concentration 50 times lower than on the CH₃-terminated SAM (0.01 mg/mL compared to 0.5). The surface coverage on the CH₃ SAMs appeared to be patchier compared to the COOH SAMs, since they had standard deviations of 2-3% N. Atomic force microscopy images of the adsorbed peptides were generated to examine this patchiness. Also, the adsorption process depended on buffer salt concentration. Little peptide adsorption was detectable on the methyl SAMs when adsorbed from a 0.1x buffer. ToF-SIMS was used to investigate the ratio of K to L characteristic mass fragments at 84 and 86 m/z, respectively. The 84/86 (K/L) ratio on CH₃ SAMs (1.2) was the same, within experimental error, as the ratio on COOH SAMs (1.1). Polarization dependent NEXAFS experiments at the nitrogen K-edge indicated the β-sheet was lying down on the SAM surfaces. The α-helical peptide exhibited significantly less polarization dependence than the β-sheet peptide, probably due to the different structure of the backbone amide groups in the α-helical peptide.