Time and Distance Scales of Membrane Domain Organization

Time and distance scales in membranes are discussed in relation to the inference of domain structure from spectroscopic measurements. Each type of spectroscopy has a natural time scale set by the magnitude of the interactions that determine the spectral width. For fluid membranes, the lateral diffusion of the lipid molecules then implies an associated distance scale over which the measurements are averaged. These factors have an influence on the interpretation of spectroscopic measurements and on whether or not the spectroscopic technique is capable of distinguishing neighbouring domains from each other. NMR occupies a special place among spectroscopies because its time scale is so long. Some examples are given of the conceptual role of spectroscopic time and distance scales with regard to the inference of domains in membranes from NMR or other spectroscopic studies, or the apparent failure to detect domains believed to be present. These examples include mixtures of phospholipids with cholesterol and/or protein molecules. In addition to time scales being set by line width and line shape considerations, the study of relaxation times within a given spectroscopy carries its own characteristic insights into motional correlation times.