(162x) Investigating Model Peptides on Surfaces Using XPS, SIMS and NEXAFS

The interaction of proteins and peptides with surfaces is an area of great interest for current biomedical research. It has been found through numerous studies that the protein does not remain in its native state when it contacts a surface, and will often partially or fully denature. The presence of these denatured proteins is often the first step in the encapsulation or foreign-body response and can therefore lead to the rejection of synthetic biomaterials by living tissues. It is therefore desirable to examine this phenomenon in detail to learn about the interactions that occur between proteins and surfaces to be able to tailor them to induce integration of biomaterials into the body.

This study uses self-assembled monolayer (SAM) model surfaces with well-defined, extensively studied characteristics. Alkane-thiol SAMs on gold with methyl, carboxylic acid, alcohol and amine terminal functional groups were used. Peptide adsorption onto a plasma-deposited C₃F₆ fluoropolymer were also done for comparisons with previous studies.

The purpose of this study is to study protein-surface interactions at a fundamental level by using model peptides. The model peptides contained leucine (L) and lysine (K) amino acids, which have hydrophobic and hydrophilic R groups respectively. They are arranged in specific sequences to reliably create α-helix or β-sheet secondary structures. The adsorption of these peptides were then studied to isolate the response of a certain protein structure with the surface of interest. These interactions will then be correlated with the interactions of entire proteins with the model SAM surfaces.

Initial adsorption studies with the 14-mer α-helix showed that the adsorption behavior varied greatly depending on the particular SAM surface. For this reason, isotherm experiments were done to determine ideal adsorption conditions. It was found that adsorption onto methyl and carboxylic acid terminated SAMs from phosphate buffered saline caused adsorption in patches with bare spots. There was no adsorption observed on alcohol terminated SAMs, and uniform adsorption on the plasma-deposited fluoropolymer. Adsorption was measured using X-ray photoelectron spectroscopy (XPS). The amount of peptide was tracked by measuring the atomic percent of nitrogen (%N). It was also found that the concentration required to achieve a partial peptide layer with some bare spots on methyl SAMs was 100-fold higher that required for carboxylic acid SAMs.

Angle-dependent XPS studies were done to analyze the patchy phenomenon and also to ensure that the peptides were associating with the terminal end of the SAM monolayer. In addition, near-edge X-ray absorption fluorescence spectroscopy (NEXAFS) was used to probe the organization of these peptides. Adsorption was investigated for three different solution conditions using different buffer salt concentrations to observe the influence of ionic strength on adsorption. On methyl SAMs, it was found that no adsorption was detectable from deionized water, even at concentrations of 1 mg/mL. In contrast adsorption from phosphate buffered saline resulted in XPS concentrations of about 5 %N.