= Förster resonance energy transfer
{c}
= FRET
{c}
{synonym}
<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>.<Advanced Fluorescence Microscopy Techniques—FRAP, FLIP, FLAP, FRET and FLIM by Hellen C. Ishikawa-Ankerhold and others>[§3.1: nonradiative dipole coupling and a potentially nonfluorescent acceptor]{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.<Advanced Fluorescence Microscopy Techniques—FRAP, FLIP, FLAP, FRET and FLIM by Hellen C. Ishikawa-Ankerhold and others>[§3.1 and §3.3: dependence on distance and orientation; inter- and intramolecular applications]{ref}
For a single pair with fixed <Förster distance> $R_0$ and label separation $r$, the <FRET efficiency> is
$$
E=\frac{1}{1+(r/R_0)^6}.
$$
At $r=R_0$, half the <FRET donor> excitations transfer <energy>. At $r=R_0/2$, the model gives $E=64/65\approx98.46\%$; at $r=2R_0$, it gives $E=1/65\approx1.54\%$. 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.
\Image[/forster-distance-transfer-efficiency.png]
{title=<FRET efficiency> versus separation relative to the <Förster distance>}
{description=Original single-pair <mathematical model> with fixed <Förster distance>. The shaded interval is $0.2\leq E\leq0.8$. The curve does not establish a universal working range or justify the <electric dipole> approximation for overlapping labels.}
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