In situ, fluorescence lifetime-based measurements of cell membrane micromechanics

Microscopic in situ measurements of the mechanical properties of lipid bilayers were derived from the mean and variance of the fluorescence lifetime distributions of 19-dioctadecyl-3,3,3939-tetramethylindocarbocyanine perchlorate (DiI). In this method, DiI, incorporated into membranes, acts as a membrane-targeted molecular rotor whose fluorescence lifetime is sensitive to local lipid viscosity. A new model was developed in which changes in area per lipid were derived from the first and second moments of a stretched exponential distribution of fluorescence lifetimes of DiI, which were subsequently used to compute mean area per lipid and its variance, quantities directly related to bilayer compressibility and bending moduli. This method enabled molecular scale assays of surface micromechanics of membrane-bound entities, such as nanoliposomes and human red blood cells.