Fluorescence quenching radiationless energy transfer

To perform structural research on a food stuff into which a colorant is incorporated, special properties of fluorescing molecules are exploited fluorescence efficiency, fluorescence lifetime, fluorescence quenching, radiationless energy (Foerster) transfer, stationary or time-dependent fluorescence polarization and depolarization." Generally, if food colorants fluoresce, they allow very sensitive investigations which in most cases cannot be surpassed by other methods. 

Another related phenomenon that results in a lower quantum yield than expected is called concentration quenching. This can occur when a macromolecule, such as an antibody, is heavily labeled with a fluorophore, such as fluorescein isothiocyanate. When this compound is excited, the fluorescence labels are in such close proximity that radiationless energy transfer occurs. Thus, the resulting fluorescence is much lower than expected for the concentration of the label. This is a common problem in flow cytometry and laser-induced fluorescence when attempting to enhance detection sensitivity by increasing the density of the fluorescing label. 

Radiationless energy transfer [see (b) above] from tryptophan to the dihydronicotinamide ring of NADH is calculated to occur with 50% efficiency at 25 A. Thus, the fluorescence of some tryptophan residues in the molecule will be quenched when only one NADH is bound to the tetramer. Binding of each successive NADH molecule to the tetramer will reduce the protein fluorescence by the same factor. The total observed fluorescence for thermodynamically independent binding sites obeys the relationship... 

The fact that the quantum yield in fluorescence is less than one is due to the return of some molecules to the ground state via radiationless energy transfer (see p. 289) and to thermal (collisional) quenching. In photolysis a number of additional factors depending on the reaction mechanism can... 

We consider three decay channels for D in addition to injection Fluorescence (rate constant k ), intramolecular radiationless decay (rate constant k ), and energy transfer quenching within the adsorbed layer (rate constant kg) ... 

Fig. 20. Schematic representation of the reaction coordinate for tryptophan fluorescence quenching induced by hydrogen transfer and aborted decarboxylation. The electronic nature of the Si surface changes character along the Si path due to two avoided crossings between jSi and S2 The first one occurs between the covalent state and the ionic La state along the reaction coordinate that interconverts the i9i-Min and. Si-Exc minima. The second one occurs between the ionic La state and the biradical Bi, state along the tautomerization coordinate that leads to the excited-state tautomerized form S -Taut. This point does not corresponds to a minimum on the potential-energy surface and it is found that evolution along a decarboxylation coordinate leads to a -Si /-So conical intersection, where efficient radiationless decay to the ground state takes place. The values of the relevant structural parameters are given in A. Data from Ref. 102.

A molecule may re-emit absorbed energy as fluorescence radiation within 10 to 10" second, or as phosphorescence which occurs after 10 second or more. These processes can take place in molecules in gaseous, liquid, and solid phases, although not necessarily in all three phases of the same substance. Deactivation or quenching of the excited molecule can occur through radiationless processes such as collision with the walls of the vessel or with other atoms or molecules increase in the vibrational and rotational energies of the molecule or, in solids, transfer... 

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