Regarding the measurement of microviscosity in lipid bilayers by EPR.

Simulations of electron paramagnetic resonance spectra of stearic acid spin labels in lipid bilayers are used to illustrate the fact that the apparent order parameter S calculated from the spectral shape does not coincide with the true order parameter S in the case of slow motions. While S reflects a static property as the degree of order of the lipid chains, S depends on both order and dynamics. Thus, calibration procedures intended to obtain bilayer microviscosity values from S in the slow motion regime are not reliable. However, S is a useful tool to describe trends in membrane fluidity.