(232c) Modeling the Dynamic Surface Activity of Folding/unfolding α-Helical Peptide

We have designed a set of tunably surface active peptides, where the folded form of the peptide is amphiphilic and the unfolded form is not. The peptides that have been designed are α-helical containing 23 amino acids with variation in the number and distribution of hydrophobic and charged amino acids. The designs incorporate hydrophobic residues on one side (leucines and alanines) and hydrophilic residues on the opposite side so that helices are surface active. The secondary structure has been characterized by using circular dichroism spectropolarimetry, and we show that the peptide has a transient secondary structure as a function of salt concentration. We hypothesize that we can control the equilibrium structure, α-helix and random coil, and, thus, control the surface activity.

The behavior of the peptide at air-water interface is characterized by pendant drop/bubble method and modeled accordingly. The two partial differential equation, one for α-helix and one for random coil, are solved simultaneously, resulting in an equation for surface concentration. This part of the work focuses on the measurement of the folding/unfolding rates of peptide and rate constants for the kinetic steps of adsorption of peptide between air/water interface and the aqueous bulk sublayer adjacent to the surface. Kinetics constants are determined in dynamic experiments in which a clean surface is contacted with a peptide in salt solution, and α-helical peptide diffuses towards and adsorbs onto the interface. Here, we hypothesize that only folded peptide adsorbs at the air/water interface. The surface tension changes as α-helical peptide adsorption is measured. These measurements are compared to predictions of kinetic-diffusive transport models in order to infer the kinetic coefficients, folding rates as well as diffusion coefficients. The resulting kinetic equation used is can be reduced to the Ward and Tordai equation as the reaction term goes to zero.