Molecular rotors as probes for cell viscosity
Viscosity of biological cells is paramount for chemical and biochemical processes as well as the diffusion of molecules. In instances where a photosensitised drug is used such as photodynamic therapy (PDT) of cancer, changes in viscosity have been observed. Scientists of the EU-funded FLUMVI (Fluorescent macrocycles as functional viscosity probes in live cells) project utilised molecular rotors, fluorophores in which fluorescence depends on intramolecular rotation. These molecules are ideal for determining cell viscosity as highly viscous media inhibit intramolecular rotation, resulting in a significant increase in emission intensity and fluorescence lifetime. Using novel classes of functional probes and fluorescence lifetime imaging (FLIM), researchers set out to measure viscosity in cells during normal cell function and PDT. Porphyrazines and porphycenes were the molecules of choice with pz1 demonstrating good cellular uptake, indicating its usefulness for differentiating between different viscosities. Upon irradiation, cells underwent dramatic morphological changes, including cell contraction and detachment concomitant with an increase in pz1 fluorescence lifetime. This clearly indicated that irradiation-induced cell death caused an increase in cell viscosity. Further photophysical experiments showed that the solubility of pz1 increased in the presence of polymers and this might be of great interest for biological applications. With respect to porphycenes, the compound TMPc exhibited molecular rotor properties but its fluorescence intensity and fluorescence lifetime was too small. Application in cell experiments did not provide good FLIM signals from dying cells, indicating their unsuitability for monitoring cell viscosity during PDT. Taken together, the FLUMVI fluorescent probes are expected to find various applications that require quantification of cell viscosity. This will provide fundamental knowledge on the complex dynamics of important biological phenomena.
