Effective lattice behavior of fluorescence energy transfer at lamellar macromolecular interfaces

Fluorescence energy transfer between donors and acceptors confined to macromolecular interfaces is considered. In particular, we discuss two theoretical models for the ensemble-average fluorescence intensity decay of the donor when both fluorophores are incorporated into a planar (e.g., lamellar) interface. The first model is based on a continuous distribution of donor and acceptor molecules on a two-dimensional surface, whereas the other assumes a discrete distribution of fluorophores along the nodes of a two-dimensional square lattice. Results for the discrete model show that the fluorescence intensity kinetics of a donor depends strongly on the geometry of the molecular distribution (i.e., the lattice constant) and the photophysics of fluorophores (i.e., critical radius of the energy transfer). Furthermore, a "discrete molecular distribution" might manifest itself in the experimental data as an increase in the apparent dimensionality of the energy transfer with increasing acceptor concentration. Altogether, the experimental and theoretical underpinnings indicate the enormous potential of using fluorescence energy-transfer kinetics for revealing structural features of molecular ensembles (i.e., geometry, shape) based on a single experimental measurement. However, further understanding the effects of restricted geometries on the fluorescence energy transfer is required to take full advantage of this information. Basic theoretical considerations to that end are provided.