Credit: © 2008 ACS

The rate of diffusion of a species in solution is affected by the viscosity of the medium. A number of important biological processes depend on this rate: for example, the speed at which chemical signals are passed on, or the efficiency of a biomolecular conversion in which a short-lived intermediate is involved. However, measurement of the viscosity at the microscale has remained a challenge. Now, a team from Imperial College London and King's College London has used the fluorescent properties of a molecular rotor to map the viscosity inside cells.

Marina Kuimova and co-workers demonstrated1 that the speed of rotation about a sterically hindered single bond can be used to monitor local viscosity. The change in fluorescence relies on the different conformational states of the rotor: if the two groups at either end of the rotor can become coplanar, there is no fluorescence because a non-radiative decay of the excited state is possible. Fluorescence is maximized when the molecule is in a more energetically favourable twisted conformation. Higher local viscosities therefore result in an increased fluorescence lifetime as the rate of rotation — and how quickly it can adopt the coplanar conformation — is decreased.

Kuimova and colleagues produced maps of viscosity within cells by growing them in the presence of the molecular rotor. A variety of fluorescent molecular rotors have previously been reported, which should enable viscosity measurements to be made in a number of specific cell targets.