Investigating peptide adsorption and orientation using surface analysis techniques

Understanding the interactions between proteins and surfaces is a vital component of biomaterials engineering. The fate of the implant, whether it will be integrated or rejected by the body, and whether it will perform its desired function, is contingent on favorable interactions with the body's proteins. Fouling by adsorption of non-specific proteins can lead to fibrous encapsulation and a thrombogenic material can lead to life-threatening blood clots.;Proteins are complex biomolecules. Thus, unscrambling their many interactions with the surface is challenging. To reduce this complexity, peptides are used as protein mimics so that a smaller number of well-defined interactions can be studied and understood. Here, leucine-lysine (LK) peptides are synthesized in two different sequences to generate two specific secondary structures, an alpha-helix and a beta-sheet. They fold such that they form amphiphilic molecules with a hydrophobic side and a hydrophilic side. This work investigates the interactions of these peptides with two self-assembled monolayer (SAM) surfaces, a hydrophobic, methyl SAM and a hydrophilic, carboxyl SAM.;To study the peptide-SAM interactions label-free, surface-specific techniques were used. X-ray photoelectron spectroscopy (XPS) and surface plasmon resonance (SPR) biosensing are methods that can quantify peptide binding at the SAM surfaces. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) provides information about peptide orientation from the preferential sputtering of ions from the outermost surface. Sum-frequency generation spectroscopy (SFG) vibrational spectroscopy can measure orientation and ordering of the peptide sidechains and backbone. Finally, the orientation of the peptide backbone can be determined using near-edge X-ray absorption fine structure (NEXAFS) spectroscopy.;Adsorption studies using XPS showed that both peptides bound to both surfaces, although they had much stronger affinity for the carboxyl SAM because of the electrostatic interactions between the lysines and the surface. SPR showed that the beta-sheet was irreversibly adsorbed; since the beta-sheet structure is formed by peptide-peptide interactions, this may stabilize the peptides on the surface. Orientation studies with NEXAFS showed the beta-sheet backbone parallel to the surface. ToF-SIMS and SFG results confirmed that both peptides were in their expected orientations with the lysines interacting with the carboxyl surface and the leucines interacting with the methyl surface.