Single molecule fluorescence imaging of phospholipid monolayers at the air-water interface

Langmuir films of amphiphilic biomolecules represent promising systems to s tudy diffusional properties in model membranes under controlled area condit ions. Here, we present for the first time single molecule fluorescence imag ing experiments on phospholipid monolayers at the air-water interface. The technique is used to track the lateral diffusion of-single molecules of N-( 6-tetramethylrhodaminethiocarbamoyl)-1,2-dihexadecanoyl-sn-glycero-3-phosph oethanolamine, triethylammonium salt (TRITC-DHPE), in phospholipid monolaye rs of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-aimyristoy l-sn-glycero3-[phospho-rac-(1-glycerol)] (sodium salt) (DMPG) at different areas per phospholipid molecule. Our tracking data of the averaged mean-squ are displacement indicate for both phospholipids that surface flow could be suppressed significantly. The diffusion behavior of TRITC-DHPE in DMPC and DMPG monolayers is characterized by unobstructed diffusion and can be desc ribed well by a free area model. Our experiments show that single molecule fluorescence imaging can be successfully applied to monolayers of amphiphil es at the air-water interface. This opens the door to future studies of hin dered diffusion in monolayers of specific heterogeneity.