Surface modification of polypyrrole via affinity peptide: quantification and mechanism.

Abstract

The conducting polymer, polypyrrole (PPy), is an exciting substrate for basic research to study cell behavior in the presence of exogenous electric fields and has potential for clinical application as a coating for neural electrodes. However, typical surface modification approaches (e.g., copolymerization of pyrrole and pyrrole derivatives, post-polymerization modification, doping with functional biomolecules) reduce the electrical conductivity and hence the utility of the material. In previous work, a 12 amino acid peptide, T59, was evolved to bind to PPy and provide a novel means of surface modification that does not reduce the bulk conductivity. In the current study, we have used equilibrium binding assays and force spectroscopy to determine that the affinity of T59 to PPy is 92.6 ± 21.4 nM, with a maximum surface density of 5.1 ± 1.7 femtomoles per cm2 by equilibrium binding assay, and that the off-rate of this binding pair is 2.5 s-1, with a calculated on-rate of 2.6 × 107 M-1 s-1. This suggests that T59 possesses a high affinity for PPy with binding that occurs in a rapid equilibrium. We also present evidence in support of a proposed mechanism of binding for T59 to PPy; an association of an aspartic acid (D8) and the delocalized positive charge on the backbone of oxidized PPy is indicated as the key interaction. Understanding the binding parameters and mechanism of this novel bio/synthetic interface is important as we try to understand how best to employ PPy-based materials.