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 t ype of spectroscopy has a natural time scale set by the magnitude of t he interactions that determine the spectral width. For fluid membranes , the lateral diffusion of the lipid molecules then implies an associa ted distance scale over which the measurements are averaged. These fac tors have an influence on the interpretation of spectroscopic measurem ents and on whether or not the spectroscopic technique is capable of d istinguishing neighbouring domains from each other. NMR occupies a spe cial place among spectroscopies because its time scale is so long. Som e examples are given of the conceptual role of spectroscopic time and distance scales with regard to the inference of domains in membranes f rom NMR or other spectroscopic studies, or the apparent failure to det ect 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.