Analysis of vertical fluorescence resonance energy transfer from the surface of a small-diameter sphere

Fluorescence resonance energy transfer (FRET) measurements have been used to analyze fluorophore separations in a number of varying geometries, including small particles and extended surfaces. This study focuses on the geometry created by a donor extended above the surface of a small sphere (radius < R0), where the acceptors are integrated into the sphere surface. The model of this geometry was based on an amphipathic molecule with its lipophilic region integrated into a detergent micelle and its hydrophilic region extending outward from the micelle surface, where the donor fluorophore is attached to the hydrophilic region of the molecule. Based on random acceptor incorporation into the micelle, a Poisson distribution was used to calculate the distribution of acceptor probes across the micelle population. The model converges to RET on a flat surface when the radius of the micelle exceeds 0.8 R0. The model was also used to simulate FRET data showing that the positions of donors above the micelle surface could be uniquely resolved. Experimental verification of the model was achieved in a sulfobetaine palmitate micelle with fluorescein isothiocyanate donors attached to detergent-solubilized lipopolysaccharide (LPS) and lipophilic Fast-DiI acceptors. The use of steady-state analysis allowed resolution of cases in which donors were located at different distances from the surface. Combining steady-state with excited-state lifetime analysis allowed resolution of cases where there was a combination of distances. Given the large number of biomolecules that interact with lipids, this approach may prove generally useful for defining molecular conformation.