Measurement of diffusion in Langmuir monolayers by single-particle tracking

Author :Carsten Selle, Florian Ruckerl, Douglas S. Martin, Martin B. Forstner and Josef A. Kas
Publication :Physical Chemistry Chemical Physics
Publisher :The Owner Societies
Volume :6
Pages :5535-5542
Year :2004

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.