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

Abstract

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.

Imaging tumor microscopic viscosity in vivo using molecular rotors

“…We have previously demonstrated using two independent molecular rotors that PDT of cells causes a large viscosity increase in vitro 30 31 . Furthermore, we have investigated the mechanism of this process using a third independent molecular rotor, BODIPY1, incorporated into model lipid bilayers 34 . In all cases we found that photooxidation of lipids or cellular components caused by PDT results in a large increase in viscosity.…”

“…Importantly, fluorescence ratiometric and lifetime detection from molecular rotors allowed to overcome difficulties associated with an unknown fluorophore concentration and thus enabled quantitative viscosity mapping to be performed. These measurements provided the wealth of biologically relevant information on model lipid membranes 17 18 19 , bacterial 20 21 22 and eukaryotic cells and cellular organelles 23 24 25 26 27 28 29 30 31 32 33 , and allowed viscosity monitoring during lipid (photo)oxidation 34 , cell death 31 , bacterial sporulation and deactivation 20 21 , and bacterial membrane viscosity changes in response to variations in temperature 22 . While the fluorescence-based approaches, including molecular rotors, allowed the measurements of viscosity and diffusion on a single cell level in vitro , the in vivo viscosity monitoring has not yet been realized.…”

Enhancing the Viscosity‐Sensitive Range of a BODIPY Molecular Rotor by Two Orders of Magnitude

“…Fortunately, relatively fast and convenient measurements of viscosity can be achieved with an emerging class of fluorescent compounds—molecular rotors . Previously, they have been used for viscosity sensing in polymers, polymersomes, aerosols, model lipid bilayers, protein aggregates, and live cells . Within cells, viscosity measurements have been performed in mitochondria, lysosomes, an endoplasmic reticulum, and a plasma membrane .…”

“…BODIPY‐C 12 (Figure ) and its variations containing other ether groups as substituents are arguably the most widely used molecular rotors . These rotors have similar photophysical properties, and their main advantage is monoexponential fluorescence decay, which simplifies data analysis and interpretation .…”

Molecular Viscosity Sensors with Two Rotators for Optimizing the Fluorescence Intensity–Contrast Trade‐Off

“…Synthetic molecular rotors have high potential in various photonic, electronic, and biological applications . In molecular rotors, rotational groups (“rotators”), linked by either covalent bonds or noncovalent coordination bonds to a part of the molecular rotors, are rotated along an axle. In π‐conjugated molecular rotors, the rotation of rotators is affected by the π‐electron delocalization characteristics, resulting in a change of photophysical properties related to electronic states, such as fluorescence and charge transfer .…”