Fluorescence quenching stabilization

Although no such correlation has been established for fluorescence quenching by conjugated dienes,50 the wide range of quenching constants obtained experimentally51 may be explained in terms of exciplex stability if this depends on the -orbital overlap of the interacting species. The steric requirements in this case appear to be very critical. 

Further indication of the minor contribution of charge-transfer to the stability of t-l-diene or alkene exciplexes is provided by the relative insensitivity of fluorescence quenching rates to donor ionization potential (Table 8). A plot of log kq versus Pip for quenching of t by dienes and vinyl ethers is shown in Fig. 6. Data for the two families of quenchers fall on different lines, both of which have slopes which are less steep... 

The participation of the singlet states of dyes such as rose bengal (RB, tetraiodotetrachlorofluorescein, sodium salt) and MB during the sensitized oxygenation of amines was demonstrated by Davidson (40). Fluorescence quenching Stem-Volmer constants were reported for several amines, halide anions, and for 3-carotene and were consistent with charge transfer stabilization of an exciplex quenching intermediate. 

In choosing a fluorescent tag, the most important factors to consider are good absorption, stable excitation, and efficient, high-quantum yield of fluorescence. Some fluorophores, such as fluorescein, exhibit fluorescent quenching, which lowers the quantum yield over time. Up to 50% of the fluorescent intensity observed on a fluorescein-stained slide can be lost within 1 month in storage. AMCA, by contrast, has much better stability, but all fluorophores lose some intensity upon exposure to light or upon storage. 

Ryan, D.K. and Weber, 3.H., 1982. Fluorescence quenching titration for determination of complexing capacities and stability constants of fulvic acids. Anal. Chem., 54 986-990. 

Grimm, D.M., Azarraga, L.V, Carreira, L.A. and Susetyo, W. (1991) Continuous multiligand distribution model used to predict the stability constant of Cu(II) metal com-plexation with humic material from fluorescence quenching data. Environ. Sci. Technol, 25,1427-1431. 

To be successfully used as labels in biological assays, rare earth complexes should possess specific properties including stability, high light yield, and ability to be linked to biomolecules. Moreover, insensitivity to fluorescence quenching is of crucial importance when working directly in biological fluids. When complexed with cryptates, however, many of these limitations are eliminated. 

A second nonlinear fluorescence quenching data treatment method developed by Ventry and Ryan may also be used to extract conditional stability constants, and ligand concentrations from titrations of FA with Cu (23). The model designed is a modification of the original Stem-Volmer theory defined by equation 7 which accounts for either static or dynamic quenching of fluorescent species. 

Even if the rationalization of all the data is a difficult problem, the K values in Table 10 for the amines which serve as proton acceptors show that the availability of the lone pair of the proton acceptor is an important factor in affecting the complex stability. Probably, the mechanism of fluorescence quenching becomes different from the charge transfer mechanism when the hydrogen bond is the main interaction147. 

In Scheme 16, D and A are proton donor molecules and proton acceptor molecules, respectively. The quantity K represents the stability of the complex DH A, formed by hydrogen bonding interactions in the ground state. For the 2-naphthylamine/pyridine system, the K value in hexane (obtained by fluorescence quenching measures) is 0.6 dm3 mol-1. This value is reduced by using benzene as a solvent (K = 0.2 dm3 mol-1) while, in cyclohexane, K = 12. The observed variations confirm the importance of solvents in influencing solute/solute interactions149. 

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