Paper BI-TuP20
Determination of Standard-State Adsorption Free Energy for Peptide-Surface Interactions by SPR Spectroscopy without Peptide-Peptide Interaction Effects

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

As an approach to gaining fundamental insights into the thermodynamics of protein-surface interactions, we are quantitatively determining the standard state free energy of peptide-surface interactions using a host-guest peptide model in the form of TGTG-X-GTGT, where the T (threonine) and G (glycine) flanking sequences are the host residues and X represents a variable guest residue. Alkanethiol self-assembled monolayers (SAMs) with a broad range of polymer-like functionalities are being used as the adsorbent surfaces. With this experimental model, we are able to determine contributions of individual mid-chain amino acid residues on peptide adsorption behavior. The most common way of determining the standard state free energy of adsorption of a peptide to a surface is by use of the Langmuir equation fitted to an adsorption isotherm. This method, however, has the inherent problem of the influence of peptide-peptide interactions at the interface. These effects can substantially affect the shape of the isotherm, leading to errors in the calculated values of the standard state free energy of adsorption. We have developed a new approach to solve this problem and provide accurate, quantitative measurements of the standard state adsorption free energy using surface plasmon resonance (SPR) spectroscopy based on chemical potential relationships vs. the classical Langmuir equation. In addition, we have also developed a very simple, automated method of accounting for bulk shift effects, which can be problematic when doing SPR adsorption studies. These novel methods will be presented along with results for a series of peptide adsorption systems using these methods.