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Resonance transfer

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Resonance transfer

Postby kumar985 » Fri Jun 07, 2013 12:03 pm

What is resonance transfer ?
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Re: Resonance transfer

Postby jonmoulton » Fri Jun 07, 2013 3:20 pm

Once case of resonance transfer is when an electron in an excited state drops to a more stable state and passes the energy to another electron, exciting that electron to a higher-energy state. An example of this with relevance to biological assays is Förster resonance energy transfer, or FRET. https://en.wikipedia.org/wiki/F%C3%B6rster_resonance_energy_transfer

In FRET, a fluorescent molecule (fluorochrome) is excited with light; for now, let's say it's blue light. Instead of simply emitting the excitation energy at a longer wavelength, let's say green light, it instead passes the energy (the "exciton") to another fluorochrome that has an excitation energy matching the emission energy of the first fluorochrome; in this case, let's say the second fluorochrome excites with green light and emits with red. Finally, the remaining excitation energy is emitted by the second fluorochrome as red light.

This energy transfer can only occur if the two fluorochromes are very close to each other in space, so it can be used to probe for proximity. For instance, if you have two biomolecules that you think dock together, you can put the first fluorochrome on one and the second fluorochrome on the other, positioned so that if they dock as expected the fluorochromes will be in close proximity. When you test the molecules separately, the first labelled biomolecules absorbs blue and emits green, while the second labelled biomolecule absorbs green and emits red. If you mix them in solution and find the mixture can absorb blue and emit red, that shows that the two different fluorochromes are being held in close proximity and supports the hypothesis that the two biomolecules do indeed dock together.

You might wonder where the extra energy goes -- blue was absorbed, red emitted, some extra energy went somewhere. The Jablonski diagram on the Wikipedia page (cited above) shows the change graphically. After the initial excitation, the electron hands off some of the excitation energy to some lower-energy transitions (I don't specifically know what these are; it might be transitions like changing the rotational frequency around single bonds). The ability to pass a fraction of the electron's excitation energy to another energy-accepting structure is a characteristic of fluorescent compounds and without that characteristic, they would simply emit photons of the same energy as the photons that that excited them. The interesting thing about fluorescent compounds is this ability to peel off some of the excitation energy and then emit the new photon at a lower energy. In the case of FRET, this energy-splitting process happens (at least) twice, at least once in each of the participating flurochromes.
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