Hindered diffusion in polymer-tethered phospholipid monolayers at the air-water interface: A single molecule fluorescence imaging study

Recently, our group was the first to apply the technique of single molecule fluorescence imaging toward homogeneous phospholipid monolayers at the air -water interface to study the lateral diffusion properties within these sys tems at the single molecule level.(1) Here, we present measurements on Lang muir monolayers of amphiphiles at the air-water interface of a specific het erogeneity to explore problems of hindered lateral diffusion of biomolecule s observed in complex biomembranes. The lateral mobility of fluorescence-la beled phospholipids in mixtures of phospholipids and lipopolymers was inves tigated via single molecule tracking at different lipopolymer molar concent rations. In agreement with recent fluorescence recovery after photobleachin g experiments,(2) we found that the diffusion behavior of phospholipids wit hin polymer-tethered monolayers is characterized by different regions depen ding on the lateral mobility of lipopolymer molecules. At a low lipopolymer molar concentration, the diffusion coefficient of phospholipids is indepen dent of the concentration of tethered lipids (lipopolymers), thereby showin g no signs of obstructed diffusion. Clear signs of obstructed diffusion are observed, however, if polymer chains of adjacent lipopolymers interact wit h each other. Our experiments showed that single molecule fluorescence imag ing is a powerful experimental tool to study obstructed diffusion in model membranes.