Photoinitiated Destruction of Composite Porphyrin-Protein Polymersomes

Bilayer vesicles assembled from amphiphilic diblock copolymers (polymersomes) adopt asymmetric structures when loaded with moderate concentrations (>= 1.5 mg/mL) of horse spleen ferritin (HSF) or its iron-free variant (HSAF). Incorporation of both ferritin and a zinc porphyrin dimer (PZn2) generates photoresponsive vesicles: irradiation with focused tight of near-UV to near-IR wavelengths induces polymersome deformation and destruction on the minute time scale. To investigate this phenomenon, polymersomes were loaded with dye-labeled ferritin and PZn2. Confocal microscopy identified BODIPY-FL-labeled ferritin at the membrane, whereas Cy3-labeled ferritin was found both at the membrane and throughout the aqueous core. Fluorescence recovery after photobleaching (FRAP) experiments confirmed that Cy3- and BODIPY-FL-labeled ferritin and PZn2 exhibited slow diffusion at the membrane, consistent with membrane association. Furthermore, micropipette aspiration experiments revealed increased elastic moduli and altered bending rigidity in vesicles incorporating HSAF. Finally, a small molecule (biocytin) was encapsulated within the ferritin-PZn2 vesicles and released upon exposure to light. These data indicate synergy between ferritin, whose membrane association towers the barrier to deformation, and PZn2, which embeds in the membrane, harvests light energy and produces local heating that may lead to membrane budding. This appears to be a general protein-polymer membrane phenomenon, as replacement of ferritin with bovine serum albumin or equine skeletal myoglobin resulted in vesicles with similar asymmetric morphology and photosensitivity.