Investigating Membrane Heterogeneities with Fluorescence Energy Transfer
Kevin B. Towles and Nily Dan. Drexel University, 310 S. 10th St., Philadelphia, PA 19107
Inhomogeneous partitioning of fluorescent probes between membrane heterogeneities, or domains, can lead to significant changes in the efficiency of energy transfer between probes. In this paper we develop a quantitative model to relate time-resolved Forster resonance energy transfer (FRET) data to the size of membrane domains. We expand upon a classic approach to the "infinite phase separation limit" and formulate a model to account for the presence of monodisperse disks of finite dimensions within a two dimensional infinite planar bilayer. The model was applied to a simulated model membrane composed of dimyristolphosphatidylcholine (DMPC)/cholesterol within the liquid disordered (l_d) and liquid ordered (l_o) coexistence regime. Two fluorophores, one preferentially partitioning into each phase, were added to simulate time-resolved FRET data. Domain sizes ranging from 5-50 nm were simulated, showing clear differences in the efficiency of energy transfer as a function of domain size within this range. Time resolved data were were fit with both a single parameter (domain diameter only) and a full five parameter fit. Each fitting procedure yielded similar results for the domain diameter; however, analysis of the multi-parameter fit provided a more complete picture as to the experimental applicability of such a technique. We show, using off-lattice simulations and a quantitative analysis, that the method can identify the presence of domains with a diameter of 5-50 nm, with approximately 20% error over a wide range of liquid-ordered fractional coverages.