FRETing about forster vs. fluorescence

March 2, 2008 at 7:19 pm | | everyday science, open thread, tutorial

fret-plot.jpgUntil 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.


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  1. THANK YOU for clearing this up. I have a biochemist to pwn after arguing with him about it.

    Comment by excimer — March 2, 2008 #

  2. I think RET is an accurate description of the physical process, but I see the term FRET as more of a description of a technique – a technique that uses fluorescent molecules. When FRET is employed, when is it ever not done with fluorescent molecules?

    Comment by matt — March 2, 2008 #

  3. Yeah it’s called an nOe.

    Dipole-dipole nonradiative energy transfer (hence inverse sixth).

    FRET *should* stand for fluorescence, but NIH guidelines say Forster. Because there’s also BRET/LRET where you do a different type of excitation (spin-forbidden) and transfer that energy. Since it’s still a dipole-dipole interaction (spin transfers are magnetic dipoles, not electric dipoles), presumably you can get inverse sixth rules going on, but I’m not 100% sure on that.

    Comment by Darksyde — March 3, 2008 #

  4. that’s a good counter-argument, matt.

    Comment by sam — March 3, 2008 #

  5. Yes, this is my point. FRET is RET with fluorescence, and L/BRET is RET with luminescence (and sometimes fluorescence too). Both techniques depend on the Forster distance, but you can’t call them both “Forster resonance energy transfer”.

    Also, I am not familiar with NIH guidelines on what to call stuff. Do you have a link for that?

    Comment by matt — March 3, 2008 #

  6. I’m gonna go with Forster. Ignore the lazy biologists.

    Comment by psi*psi — March 3, 2008 #

  7. unfortunately, people use “FRET” to describe the physical process all the time!

    Comment by sam — March 3, 2008 #

  8. It’s irrelevant, really, since FRET is often used in fluorescence measurements and there should be some understanding by all that it’s efficacy is always dictated by the Forster distance. Since changing the name does not change the mechanism (though it may mislead one into thinking a photon has transfered) calling it “Fluorescence” energy transfer isn’t all that misleading since the energy from one fluorescent molecule is clearly being transfered to another…

    So, I suppose my point would be, like all arguments involving nomenclature, this is a silly thing to worry about until someone tries to tell you a photon is involved (or doesn’t realize there are other methods to transfer energy from one glowing molecule to another). Then you’ve got my permission to smash their face with a book.

    Comment by Kyle Finchsigmate — March 4, 2008 #

  9. I would like to suggest “Basic Methods in Microscopy,” by Spector and Goldman for smashing faces. It’s considered one of the best by cell biologists. It is soft-cover, though, so there is little chance of permanent damage.

    Comment by J — March 4, 2008 #

  10. I think EET is the most correct, so why the resistance? There are multiple types of resonant energy transfers, and different excitation transfers happening via the Forester mechanism.

    To be facetious, I’ll point out that while the process doesn’t involve a photon (and thus the resistance to the tag “fluorescence”), the process does require a virtual photon to mediate the exchange. So how about vFRET?

    Comment by kendall — March 4, 2008 #

  11. or FEET?

    Comment by sam — March 4, 2008 #


    Comment by anon — April 12, 2008 #

  13. […] so for excited-state proton transfers, ESIPT could be ESInterPT or ESIntraPT. Another example is Förster resonant energy transfer (FRET): because the ambiguity with the “F,” the first word is […]

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  14. […] we should also be able to watch the FRET trace to observe the conformational changes of his arms. In this fashion, we should be able to […]

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  15. here’s another perspective from Valeur:

    “Fluorescence Resonance Energy Transfer”: a misnomer

    The term FRET first appeared in papers relevant to life sciences, as the acronym of “Fluorescence Resonance Energy Transfer.” Another interpretation of FRET was given by van der Meer as “Fluorescence with Resonance Energy Transfer.” However, fluorescence is not involved in resonance ET, which is non-radiative. Moreover, under suitable conditions, Forster’s theory may be applied even to donor–acceptor pairs undergoing triplet–singlet ET.The acronym FRET is now so widely used that the solution to overcome this situation—and a way to acknowledge the author for his important contribution—is to consider that F in FRET means “Forster” or “Forster-type,” the interpretation recommended in the IUPAC Glossary of terms used in photochemistry, rather than “fluorescence.”

    We agree with R. Clegg, who published a comprehensive article on the history of FRET: “there are other modes of energy transfer, and circumstances where Forster transfer is not valid; these require different theoretical foundations. However, reserving “Forster” for the “F” in FRET, whenever we mean Forster transfer, gives credit to the person who made it possible for us to gain valuable, quantitative insight into so many processes at the molecular scale, through relatively easy experiments.”

    Comment by sam — December 16, 2008 #

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