Resonant energy transfer process

FIGURE 14. The recombination and resonant energy transfer processes in the PDHS/ PDPS sample. (Reprinted in Ref. 90.)... 

More sophisticated designs involved semiconductor quantum dots with fluorescent protein receptors immobilized on the surface [146], The binding site of the protein receptor is occupied with an efficient fluorophore. On excitation a series of FRET (Forster resonant energy transfer) processes takes place excitation energy is transferred from the core of the quantum dot to the fluorescent protein and subsequently to the fluorophore. On substrate binding only one FRET step takes place and luminescence of the receptor is observed [146], In the simplest sensor architecture the protein contains bound quencher. Upon interaction with analyte the quencher is liberated and luminescence of the quantum dot is observed (Figure 16.25c). 

Actually, 5.7.7. only applies to dd-coupling processes which are resonant. We must also classify resonant energy transfer processes as defined in... 

The theoretical dipole-dipole transition probabilities calculated from Dexter s formula are 0.6 sec-i for 3% Gd and 3% Tb. Comparing this result with the experimentally determined energy transfer probability (817 sec i) given in Table 13 we see that the experimental results do not agree with the theory of the resonant energy transfer process. 

Time resolved fluorescence measurements have become an important tool in applied fluorescence spectroscopy. Recently, it has been pointed out that the controlled manipulation of fluorescence decay rates opens a new dimension in applied fluorescence spectroscopy. The fluorescence decay rate depends on two independent contributions, the pure rachative rate and the nonradiative rate. The latter one can be influenced by the well known Forster-type resonant energy transfer processes, while the radiative rate can be changed if the molecules are embedded or close to media comprising a dielectric constant markedly different from vacuum. Especially metal nanostructures have been used to alter both decay paths of fluorescent molecules. Apart from a change of those two rates, the absorption cross-section might also be altered. 

With this convention, we can now classify energy transfer processes either as resonant, if IA defined in equation (A3.13.81 is small, or non-resonant, if it is large. Quite generally the rate of resonant processes can approach or even exceed the Leimard-Jones collision frequency (the latter is possible if other long-range potentials are actually applicable, such as by pennanent dipole-dipole interaction). 

C. Resonance energy transfer. The excitation energy can be transferred by resonance energy transfer, a radiationless process, to a neighboring molecule if their energy level difference corresponds to the quantum of excitation energy. In this process, the quantum, or so-called exciton, is transferred. 

If the energy is transferred by trivial emission/reabsorption, it will lengthen the measured lifetime of the donor emission, not shorten it as happens in resonance energy transfer. This comes about because intervening absorption and emission processes take place prior to the final fluorescence emission (the reabsorption cannot take place until the photon has been emitted) the two processes do not compete dynamically, but follow in a serial fashion. In FRET, such an emission/reabsorption process does not occur, and the fluorescence lifetime of the donor decreases. This is an experimental check for reabsorption/reemission. 

Rehm, M., Dussmann, H., Janicke, R. U., Tavare, J. M., Kogel, D. and Prehn, J. H. (2002). Single-cell fluorescence resonance energy transfer analysis demonstrates that caspase activation during apoptosis is a rapid process. Role of caspase-3. J. Biol. Chem. 277, 24506-14. 

A detailed theory of energy transfer by the Coulombic mechanism was developed by Forster, so the process is often referred to as Forster resonance energy transfer (FRET). According to the Forster theory, the probability of Coulombic energy transfer falls off inversely with the sixth power of the distance between the donor and the acceptor. For... 

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