Sensing Membrane Stress with Near IR-Emissive Porphyrins

Abstract

Internal-strain sensors, embedded within a membrane, can enhance our ability to monitor membrane dynamics and predict its failure. Development of these probes with model membrane systems may provide a tool to elucidate areas of membrane stress or stretching in cellular membranes. Molecular rotors are one such class of mechano-optical molecules that have potential as intrinsic stress sensors. Supermolecular porphyrin-based fluorophores demonstrate this capability since the twisting of various subunits of the dye can lead to significant shifts in the fluorophore emission wavelength. We encapsulated these hydrophobic porphyrin-based fluorophores into bilayer vesicle membranes made of diblock-copolymers (polymersomes) and characterized changes in the optical emission of these near-infrared (NIR) emissive probes in response to membrane stress. The conformation of entrapped fluorophore depends on the available space within the membrane. We systematically changed the available space for the fluorophore in the polymersome membrane by changing the amount of fluorophore loading as well as increasing the membrane volume by using a larger molecular weight diblock copolymer. In response to increased volume for the fluorophore, emission is blue shifted. Using this property, we studied how shifts in fluorescence correlate to membrane integrity, imparted by membrane stress produced through a range of physical and chemical perturbations, including surfactant-induced lysis, hydrolytic lysis, thermal degradation, and applied stress by micropipette aspiration. With the latter aspiration studies, we find that changes in the NIR probe's emission accompany measured changes in polymersome membrane tension. Studies conducted with this NIR probe and model membrane suggest that molecular rotors, such as these, can potentially be used in future studies to monitor intrinsic stress in a variety of membranes, ranging from synthetic membranes, to monitor the delivery of drugs or in vivo rheological changes, to cellular membranes, to monitor membrane stress during motility or transport processes.