A Molecular Rotor that Measures Dynamic Changes of Lipid Bilayer Viscosity Caused by Oxidative Stress

Oxidation of cellular structures is typically an undesirable process that can be a hallmark of certain diseases. On the other hand, photooxidation is a necessary step of photodynamic therapy (PDT), a cancer treatment causing cell death upon light irradiation. Here, the effect of photooxidation on the microscopic viscosity of model lipid bilayers constructed of 1,2-dioleoyl-sn-glycero-3-phosphocholine has been studied. A molecular rotor has been employed that displays a viscosity-dependent fluorescence lifetime as a quantitative probe of the bilayer's viscosity. Thus, spatially-resolved viscosity maps of lipid photooxidation in giant unilamellar vesicles (GUVs) were obtained, testing the effect of the positioning of the oxidant relative to the rotor in the bilayer. It was found that PDT has a strong impact on viscoelastic properties of lipid bilayers, which 'travels' through the bilayer to areas that have not been irradiated directly. A dramatic difference in viscoelastic properties of oxidized GUVs by Type I (electron transfer) and Type II (singlet oxygen-based) photosensitisers was also detected.