Fluorescence polarization is a technique used to measure molecular orientation and mobility using polarized light and a fluorescent tracer. Absorption of polarized light will be most probable for molecules whose transition dipoles are parallel to the plane of polarization. Other molecules will absorb light to a lesser extent and those perpendicular to the incident plane of polarization, will not absorb. Often the depolarization of the emitted (I) light is given in terms of fluorescence anisotropy(r = (I_║ - I_┴)/ (I_║+2I_┴)) which can be converted to polarization (P) values by r=2P/(3-P). Just the random orientation of the molecules leads to a depolarization (r₀ = 2/5) and when the emitting transition dipole is not parallel to the absorbing transition dipole, r₀ is reduced even further. Additionally, the polarization of the emitted light is affected by the extent of molecular rotation which takes place and generally leads to more depolarization.

     The fundamental power of FP lies in the fact that P is not dependent on the intensity of the emitted light or on the concentration of the fluorophore and assays can be performed in turbid solutions. Sensitivity is generally in the nanomolar range but under certain conditions it is possible to detect picomolar amounts (as with normal fluorescence). FP can also be done with mulitphoton excitation to increase the accuracy of measurements.

Applications:

1. Antibody-antigen interactions
2. Monitoring drug levels in human plasma (Jolley, 1981)
3. Fluorescein-labeled oligonucleotide to detect specific PCR products (emit less polarized fluorescence when unbound to targets)

Pros:

1. The interfilter effect is not a big problem so assays can be performed on colored and turbid solutions
2. FP is relatively inexpensive

Cons:

1. Purity of the fluorescent tracer is critical