Solvent relaxation of fluorescent labels as a new tool for the detection of polarity and rigidity changes in membranes

Since solvent relaxation (SR) exclusively depends on the physical properties of the dye environment, SR spectroscopy of defined located labels in amphiphilic assemblies accomplishes the characterisation of specific domains. The most accurate way to characterise SR is the determination of the time-dependent Stokes shift. The time course of the Stokes shift, expressed as a solvent relaxation time, gives information about both the rigidity and polarity of the dye environment. The absolute value of the Stokes shift following the excitation is correlated with the polarity of the probed region. The validity of this approach for the investigation of phospholipid bilayers is illustrated by listing the parameters influencing the SR kinetics of appropriate membrane labels: membrane curvature, percentage of phosphatidylserine (PS) in small unilamell vesicles (SUV), addition of Ca²⁺ ions, binding of vitamin-K dependent proteins, percentage of diether-lipids in phosphatidylcholine (PC)-vesicles, and temperature. Presented at the Czech-Israeli-German Symposium “Dynamical Processes in Condensed Molecular Systems”, Prague, Czech Republic, 26–30 May 1997.