Theoretical analysis of adsorption thermodynamics for charged peptide residues on SAM surfaces of varying functionality.

Cellular response to an implant is largely controlled by protein adsorption because cells directly interact with the adsorbed protein rather than the implant surface. Protein adsorption will occur when the change in Gibbs free energy (Delta G) of the system decreases during the adsorption process. Electrostatic interactions between charged peptide residues presented by a protein's surface and surface functional groups greatly contribute to the Delta G of protein adsorption. In this study, semiempirical molecular orbital calculations were used to theoretically determine the adsorption enthalpy between charged peptide residues [aspartic acid (-1), glutamic acid (-1), and arginine (+1)] and functionalized SAM surfaces [methyl, hydroxyl, amine (+1), and carboxylic acid (-1)]. Additional enthalpic and entropic contributions attributed to water restructuring effects were then approximated based on literature values for functional group solvation and considered along with the calculated enthalpy values to estimate the change in Delta G for each residue/surface system as a function of surface separation distance. The results predict long-range attraction and repulsion to the opposite and same-charge residue/surface systems, respectively, followed by strong short-range repulsion caused by functional group dehydration. Short-range repulsion alone was predicted for the charged residues on the methyl and hydroxyl surfaces. These results provide a theoretical quantitative description of fundamental mechanisms governing protein adsorption behavior and provide a basis for the development of a knowledge-based surface design approach to control biological response.