Thermally Regulated Reversible Formation of Vesicle-Like Assemblies by Hexaproline Amphiphiles

Peptides composed of hexaproline and glutamic acid (P₆E) or lysine (P₆K) as C-terminal units show thermally promoted aggregation, affording vesicle-like assemblies upon heating to 80 °C. The aggregation is analyzed by dynamic light scattering (DLS), with number-averaged diameters of ca. 600 and 300 nm, respectively, for P₆E and P₆K. NMR studies reveal that upon heating the amount of NMR-visible species is reduced to ca. 50% and that an important conformational change is experienced by the molecules in solution. Circular dichroism (CD) shows that at 20 °C the peptides present a polyproline II (PP-II) conformation which is disorganized upon heating. Scanning electron microscopy for samples which were fast frozen at 80 °C reveals vesicle-like assemblies. Using pyrene as a fluorescence probe, a critical aggregation concentration of ca. 30 μM was estimated for P₆E, while that of P₆K was above 0.6 mM. The aggregation process is found to be fully reversible and could serve as a basis for development of stimuli responsive carriers.