Measurement of diffusion in Langmuir monolayers by single-particle tracking

There is a great amount of literature available indicating that membranes are inhomogeneous, complex fluids. For instance, observation of diffusion in cell membranes demonstrated confined motion of membrane constituents and even subdiffusion. In order to circumvent the small dimensions of cells leading to weak statistics when investigating the diffusion properties of single membrane components, a technique based on optical microscopy employing Langmuir monolayers as membrane model systems has been developed in our lab. In earlier work, the motion of labeled single lipids was visualized. These measurements with long observation times, thus far only possible with this method, were combined with respective Monte-Carlo simulations. We could conclude that noise can lead in general to the assumption of subdiffusion while interpreting the results of single-particle-tracking (SPT) experiments within membranes in general. Since the packing density of lipids within monolayers at the air/water interface can be changed easily, inhomogeneity with regard to the phase state can be achieved by isothermal compression to coexistence regions. Surface charged polystyrene latexes were used as model proteins diffusing in inhomogeneous monolayers as biomembrane mimics. Epifluorescence microscopy coupled to SPT revealed that domain associated, dimensionally reduced diffusion can occur in these kinds of model systems. This was caused by an attractive potential generated by condensed domains within monolayers. Monte-Carlo simulations supported this view point. Moreover, long-time simulations show that diffusion coefficients of respective particles were dependent on the strength of the attractive potential present: a behavior reflecting altered dimensionality of diffusion. The widths of those potentials were also found to be affected by the domain size of the more ordered lipid phase. In biological membrane systems, cells could utilize these physical mechanisms to adjust diffusion properties of membrane components.