title: Single-molecule microscopy on model membranes reveals anomalous diffusion.
abstract: The lateral mobility of lipids in phospholipid membranes has attracted numerous experimental and theoretical studies, inspired by the model of Singer and Nicholson (1972. Science, 175:720-731) and the theoretical description by Saffman and Delbrück (1975. Proc. Natl. Acad. Sci. USA. 72:3111-3113). Fluorescence recovery after photobleaching (FRAP) is used as the standard experimental technique for the study of lateral mobility, yielding an ensemble-averaged diffusion constant. Single-particle tracking (SPT) and the recently developed single-molecule imaging techniques now give access to data on individual displacements of molecules, which can be used for characterization of the mobility in a membrane. Here we present a new type of analysis for tracking data by making use of the probability distribution of square displacements. The potential of this new type of analysis is shown for single-molecule imaging, which was employed to follow the motion of individual fluorescence-labeled lipids in two systems: a fluid-supported phospholipid membrane and a solid polymerstabilized phospholipid monolayer. In the fluid membrane, a high-mobility component characterized by a diffusion constant of 4.4 microns2/s and a low-mobility component characterized by a diffusion constant of 0.07 micron2/s were identified. It is proposed that the latter characterizes the so-called immobile fraction often found in FRAP experiments. In the polymer-stabilized system, diffusion restricted to corrals of 140 nm was directly visualized. Both examples show the potentials of such detailed analysis in combination with single-molecule techniques: with minimal interference with the native structure, inhomogeneities of membrane mobility can be resolved with a spatial resolution of 100 nm, well below the diffraction limit.
authors.affiliation: Institute for Biophysics, University of Linz, Austria.
authors.forename: G J
authors.initials: GJ
authors.lastname: Schütz
authors.forename: H
authors.initials: H
authors.lastname: Schindler
authors.forename: T
authors.initials: T
authors.lastname: Schmidt
mesh_terms: D004058:Diffusion; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry*; D005456:Fluorescent Dyes; D007700:Kinetics; D008051:Lipid Bilayers / Q000737:chemistry*; D008081:Liposomes / Q000737:chemistry*; D008856:Microscopy, Fluorescence / Q000379:methods; D008956:Models, Chemical; D008968:Molecular Conformation; D010713:Phosphatidylcholines / Q000737:chemistry*; D010714:Phosphatidylethanolamines / Q000737:chemistry*; D012235:Rhodamines / Q000737:chemistry*; D012680:Sensitivity and Specificity
chemical_list: D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D008081:Liposomes; D010713:Phosphatidylcholines; D010714:Phosphatidylethanolamines; D012235:Rhodamines; C108056:triethylammonium N-(6-tetramethylrhodaminethiocarbamoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine; C009434:tetramethylrhodamine isothiocyanate; C028694:1-palmitoyl-2-oleoylphosphatidylcholine; D004134:Dimyristoylphosphatidylcholine