Effect of the Orientation Factor

Often a value of = 2/3 is assumed, which corresponds to the situation when there is rapid, isotropic rotation of the donor and acceptor molecules. Randomly oriented dipoles that remain fixed during the singlet lifetime give = 0.476. When required, the range of values for can be estimated by polarization measurements [139]. A comprehensive discussion on the theory and effects of the orientation factor is given in [140]. 

In distance measurements using RET, there is often concern about the effects of the orientation factor K. At present, there is no way to measure K, short of detomina-tion of the X-ray crystal structure, in which case the distance would be known and thus there would be no reason to use energy transfer. However, it is possible to set limits on K, which in turn sets limits on the range of possible D-A distances. These limits are determined from the anisotropies of the donor and acceptor, which reflect the extent of orientational avenguig toward the dynamic average of... 

The general effect of the orientation factor is to lower the effective aspect ratio. When 5=1, the equation reduces to the Nielsen equation. Fredrickson and Bicerano [15] applied the same logic with nanoparticles that were modeled as disks. An excellent review of these models, including the Cussler-Aris model [16], is provided by Manias et al. [17]. 

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