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 Incoming links: FRET efficiency

Förster resonance energy transfer is a nonradiative transfer of energy from an excited state of a FRET donor to a nearby FRET acceptor, through coupling of their electric dipoles. The transfer step does not emit a photon that the FRET acceptor subsequently absorbs. It competes with the FRET donor's own fluorescence and other routes out of its excited state. A fluorescent FRET acceptor can then emit fluorescence; an absorbing but nonfluorescent FRET acceptor can still receive the transferred energy.^[ref]

The striking use of FRET is to turn a change in molecular proximity into an optical readout. Proteins carrying a FRET donor and a FRET acceptor may change the readout when they associate, as in the chemotaxis FRET assay; two labels on one protein may report a change in its shape. Interpretation depends on the FRET spectral overlap, FRET dipole orientation factor and labeling geometry, as well as separation. A change in FRET is therefore evidence about proximity under specified conditions, rather than a direct measurement of an arbitrary distance.^[ref]

For a single pair with fixed Förster distance

R_{0}

and label separation

r

E = \frac{1}{1 + ( r / R _{0} ) ^{6}} .

r = R_{0}

, half the FRET donor excitations transfer energy. At

r = R_{0} /2

, the model gives

E = 64/65 \approx 98.46%

; at

r = 2 R_{0}

, it gives

E = 1/65 \approx 1.54%

. The sixth power makes FRET especially useful near the Förster distance, with little sensitivity to further separation once transfer is already negligible. These are model calculations, not measurements of a particular pair.

FRET efficiency is the probability that an excitation of the FRET donor ends in transfer to the FRET acceptor. Write

k_{D} = k_{r} + k_{nr}

for the total decay rate without a FRET acceptor, including radiative fluorescence and other decay, and

k_{T}

for the transfer rate. For constant competing rates,

k_{T} = k_{D} (\frac{R _{0}}{r})^{6}, E = \frac{k _{T}}{k _{D} + k _{T}} = \int_{0}^{\infty} k_{T} e^{- (k_{D} + k_{T}) t} d t .

The integral counts transfer events among all excitations in a population with exponential decay. Substitution gives the separation law above. This short rate argument assumes one FRET donor, one FRET acceptor, weak coupling, an available FRET acceptor, and fixed geometry during the decay; it is not a model for arbitrary interacting labels.^[ref]

FRET sensitized emission is FRET acceptor fluorescence caused by transfer from an excited FRET donor. Excite predominantly the FRET donor and collect both emission bands. Reduced FRET donor emission and increased FRET acceptor emission are useful together, but an acceptor-to-donor emission ratio is not automatically the FRET efficiency: brightness, label abundance and channel collection also enter.^[ref]