Solvent relaxation in phospholipid bilayers: physical understanding and biophysical applications.

Solvent relaxation (SR) refers to the dynamic process of solvent reorganisation in response to an abrupt change in charge distribution of a dye via electronic excitation. The temporal response can be monitored through the observation of the dynamic StokesΔ shift ν(t) of the dyeΔs emission maximum frequency. The complete time-dependent StokesΔ shift " ν (" ν = ν(0)-ν(∞)) increases with increasing solvent polarity. Linear proportionality between " ν and a dielectric measure of the solvent polarity has been experimentally verified [1]. At ambient temperatures, a typical relaxation process C(t) (C(t) = (ν(t)-ν(∞)) / " ν) in an isotropic polar solvent starts with a fast inertial motion on the 0.05 to 0.5 picoseconds (ps) time-range, followed by rotational and translational diffusion occurring on the pico-to sub-nanosecond (ns) time scale [1]. In pure water an average (integral) solvent relaxation time of about 0.3 ps has been determined [2]. It has been known already for almost thirty years that a significant part of solvent relaxation monitored by dyes associated with phospholipid bilayers occurs on the ns time scale [3]. A quantitative and comprehensive picture of SR in the liquid-crystalline phase of phospholipid bilayers, however, has been presented recently [4]. Within the last ten years SR studies in bilayers became of interest for two different motivations: Firstly, a series of publications appeared demonstrating the benefit of this technique in detecting physiological relevant changes in the phospholipid bilayer organization [5-9]. Subsequently, contributions addressing the question on the origin of slow relaxation components probed in lipid membranes were published [10]. A major requirement for valid application or physical interpretation of solvent relaxation studies in bilayers is the knowledge about the location of the used chromophore. It has been demonstrated [4,6-8] that " ν as well as τ r are strongly dependent on the location of the chromophore within the bilayer. The solvent relaxation time τ r of dyes like 6,8-difluoro-4-heptadecyl-7-hydroxy-coumarin is about 0.4 ns in phosphatidylcholine (PC) small unilamellar vesicles at ambient temperature [4]. Those dyes are probing the external interface of the bilayer. SR in the headgroup region probed by 6-propionyl-2-dimethylaminonaphthalene (Prodan) of PC-bilayers is characterized by τ r value of 1.0 ns [4]. The chromophore of Patman