Fluorescent Probes Indicate Levels of Stress in Biological Membranes

Polymersomes are a class of artificial vesicles with radii ranging from 50 nm to 5 μm that are made from amphiphilic synthetic block copolymers to form the vesicle membrane. Most reported polymersomes contain an aqueous solution in their core and are useful for encapsulating and protecting sensitive molecules, such as drugs, enzymes, other proteins and peptides, and DNA and RNA fragments. Polymersomes are similar to liposomes, which are vesicles formed from naturally occurring lipids. While having many of the properties of natural liposomes, polymersomes exhibit increased stability and reduced permeability.

For the signaling system, the investigators selected supermolecular porphyrin-based fluorophores to act as molecular rotors. They characterized changes in the optical emission of these near-infrared- (NIR) emissive probes that were embedded within the hydrophobic core of the polymersome membranes. The configuration of entrapped fluorophore depends on the available space within the membrane; in response to increased volume, emission is blue shifted. This feature was used to study how shifts in fluorescence correlated to membrane integrity, imparted by membrane stress.

A paper published in the August 23, 2011, issue of the journal Proceedings of the [US] National Academy of Sciences described how the system was calibrated. The investigators monitored changes in emission of the porphyrin-based fluorophores resulting from 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.

“When you package these porphyrins in a confined environment, such as a polymersome membrane, you can modulate the light emission from the molecules,” said senior author Dr. Daniel Hammer, professor of bioengineering at the University of Pennsylvania. “If you put a stress on the confined environment, you change the porphyrin's configuration, and, because their optical release is tied to their configuration, you can use the optical release as a direct measure of the stress in the environment.”

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