Simple Estimation of Forster Resonance Energy Transfer (FRET) Orientation Factor Distribution in Membranes

Because of its acute sensitivity to distance in the nanometer scale, Forster resonance energy transfer (FRET) has found a large variety of applications in many fields of chemistry, physics, and biology. One important issue regarding the correct usage of FRET is its dependence on the donor-acceptor relative orientation, expressed as the orientation factor kappa(2). Different donor/acceptor conformations can lead to kappa(2) values in the 0 <= kappa(2) <= 4 range. Because the characteristic distance for FRET, R-0, is proportional to (kappa(2))(1/6), uncertainties in the orientation factor are reflected in the quality of information that can be retrieved from a FRET experiment. In most cases, the average value of kappa(2) corresponding to the dynamic isotropic limit (<kappa(2)> = 2/3) is used for computation of R-0 and hence donor-acceptor distances and acceptor concentrations. However, this can lead to significant error in unfavorable cases. This issue is more critical in membrane systems, because of their intrinsically anisotropic nature and their reduced fluidity in comparison to most common solvents. Here, a simple numerical simulation method for estimation of the probability density function of kappa(2) for membrane-embedded donor and acceptor fluorophores in the dynamic regime is presented. In the simplest form, the proposed procedure uses as input the most probable orientations of the donor and acceptor transition dipoles, obtained by experimental (including linear dichroism) or theoretical (such as molecular dynamics simulation) techniques. Optionally, information about the widths of the donor and/or acceptor angular distributions may be incorporated. The methodology is illustrated for special limiting cases and common membrane FRET pairs.