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 Sapp 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, Sapp depends on both order and dynamics. Thus, calibration procedures intended to obtain bilayer microviscosity values from Sapp in the slow motion regime are not reliable. However, Sapp is a useful tool to describe trends in membrane fluidity. [Display omitted] •EPR spectra of stearic acid spin labels in slow motion regime are simulated.•Differences between apparent and true order parameters are discussed.•Erroneous procedures to evaluate membrane microviscosity are pointed out.•Apparent order parameter is not reliable to evaluate membrane microviscosity.•Apparent order parameter is useful to detect trends in membrane fluidity.