Everyday Scientist

FRETing about forster vs. fluorescence

Until recently, I thought FRET stood for “Förster resonance energy transfer”; I figured that “fluorescence resonance energy transfer” was a bastardization used by biologists. But a friend challenged me on that point, claiming that fluorescence was more specific and meaningful than Förster. I was all confused.

My reasoning was this: Förster’s equation for long-range dipole-dipole nonradiative energy transfer is a specific case of RET; other cases (e.g. Dexter electron exchange) have different mechanisms and follow different scaling laws. Moreover, because fluorescence is not necessary in the FRET mechanism, I thought it was misleading.

But how true is all that? Does Dexter ET count as RET? Is FRET the only way to transfer the potential to fluoresce from one molecule to another? My friend claimed that Dexter should not be called RET, because it is electron exchange instead of Coulombic.

So I refer to the experts.

Bernard Valeur, in Molecular Fluorescence, says:

The term resonance energy transfer (RET) is often used. In some papers, the acronym FRET is used, denoting fluorescence resonance energy transfer, but this express is incorrect because it is not the fluorescence that is transferred but the electronic energy of the donor. Therefore, it is recommended that either EET (excitation energy transfer or electronic energy transfer) or RET (resonance energy transfer) are used.

That doesn’t help solve the Förster vs. fluorescence dilemma, but instead adds another term (EET, gross) to throw into the mix. But I think this sorta supports my using “Förster” because “fluorescence” is misleading and too broad. Anyway, good ol’ Bernard goes on to carefully describe the different RET mechanisms and formulas.

So what does Joseph R. Lakowicz say? First, he calls it “fluorescence resonance energy transfer,” but then echoes Valeur that RET is a preferable term because “the process does not involve the appearance of a photon.” But Lakowicz also differentiates RET and Dexter electron exchange (because the latter is purely quantum-mechanical).

In Turro’s Modern Molecular Photochemistry, the energy-transfer chapter starts right in with a “Golden Rule” for the transitions between states, and demonstrates that the probability includes an exchange term and a Coulombic term. (Valeur’s book also includes a nice mathematical explanation of the two terms; it might even be in Lakowicz somewhere!)

So now I’m mostly reconvinced that FRET should be Förster resonance energy transfer, not fluorescence. That is, RET is the general term for nonradiative excitation-energy transfers, and FRET is a specific mechanism—the specific mechanism applied in practically all biophysical measurements using RET to study distances.

What do you think? Am I way off?

UPDATE: IUPAC says I’m right.