Determination of molecular motion in membranes using periodic pattern photobleaching.

The lateral diffusion of a fluorescent phospholipid probe in oriented multibilayers of dimyristoylphosphatidylcholine has been measured by observing the redistribution of fluorescence after photobleaching of the membranes in a periodic pattern of parallel stripes. The diffusion constant D of the fluorescent lipid was found to vary between 1.5 X 10(-11) cm2/sec at 9.6 degrees and 2.0 X 10(-10) at 22.5 degrees in the monoclinic phase. Preliminary studies of dipalmitoylphosphatidylcholine liposomes in the Lbeta' and Pbeta' phases yielded diffusion constants of the order of 10(-11) cm2/sec. These data are relevant to earlier discussions of the rate of complement activation by hapten-sensitized liposomal membranes [Brûlet, P. and McConnell, H. M. (1976) Proc. Natl. Acad. Sci. USA 73,2977--2981; Parce, J. W., Henry, N. and McConnell, H.M. (1978) Proc. Natl. Acad. Sci. USA 75, 1515--1518)]. We have also used this method to study the motion of fluorescent antibodies bound to murine EL-4 tumor cells. Pattern photobleaching techniques have the advantages that cellular or liposomal translation has no major adverse effect on the measurements, that certain nondiffusive motions can be detected and characterized, and that diffusive or other motions can be recorded photographically.