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Synthesis and Characterization of a Leucine-Based Block Co-Polypeptide: The Effect of the Leucine Zipper on Self-Assembly.

The self-assembly behavior of an ABC triblock copolypeptide consisting of poly(ethylene oxide-b-(leucine-s-valine)-b-lysine) (PEO-PLV-PK) was examined via dynamic light scattering in dilute aqueous solution. Leucine is a hydrophobic, α-helix forming polypeptide that exhibits a "zipper effect" in coiled-coil dimers. We hypothesize that the specific interaction afforded by the leucine zipper dominates the thermodynamics of self-assembly through the side-by-side ordering of α-helices, which drives vesicle formation in a polymer with only 6 wt % hydrophobic content. Additionally, a multitude of assembly sizes and morphologies were attainable from a single polymer, depending on the solution processing method. Thermodynamic effects of the leucine zipper can be interpreted, in part, from solubility parameters determined from molecular modeling. The combination of synthesis, solvent processing, and computational studies helps to elucidate the thermodynamic effects of this unique assembly motif on classical self-assembly processes.