Quenching constants

This order agrees with that of quenching constant (Aqr) vaiues of the fluorescence of amines by AN. That is, the easier the reaction between an excited aromatic tertiary amine and the ground state AN, the faster the initiation. 

Table 2 lists the apparent Stern-Volmer quenching constants (Ksv) for APh-x, estimated from the initial slopes of the Stern-Volmer plots, along with the... 

These findings can be attributed to the increase in the local concentration of MV2+ on the APh-x molecular surface caused by eletrostatic interactions. In contrast, the quenching constants for MV2 + and SPV show no such large difference in the SDS micellar and AM systems. The addition of NaCl reduces the value of kq to about one-third that for the quenching of APh-9 (APh-x with 9 mol% Phen units) by MV2 + in a salt-free solution. This effect is mainly accounted for by the screening of electrostatic attraction between APh-9 and MV2+. 

In order to clear up the mechanism of inactivation of excited states, we examined the processes of quenching of fluorescence and phosphorescence in PCSs by the additives of the donor and acceptor type253,2S5,2S6 Within the concentration range of 1 x 1CT4 — 1 x 10"3 mol/1, a linear relationship between the efficiency of fluorescence quenching [(/0//) — 1] and the quencher concentration was found. For the determination of quenching constants, the Stem-Volmer equation was used, viz. 

This makes us conclude that the process of quenching is associated with an electron transfer. The efficiency of phosphorescence quenching by acceptors follows, as well, the growth of electron affinity of the latter. Phosphorescence quenching constants are two orders of magnitude lower than fluorescence quenching constants. This indi-... 

In some cases it is possible to obtain a measure of the association constant for intercalation directly from fluorescence quenching data. This method is applicable when the dynamic quenching of the hydrocarbon fluorescence by DNA is small and when the intercalated hydrocarbon has a negligible fluorescence quantum yield compared to that of the free hydrocarbon. If these conditions are met, the association constant for intercalation, Kq, is equal to the Stern-Volmer quenching constant Kgy (76) and is given by Equation 1. 

Figure 4. Stern-Volmer plots and quenching constants derived from the fluorescence quenching of DMA (T), 1,2,3,4-tetra-hydro-BA ( ), 5,6-dihydro-BA (A), 8,9,10,11-tetrahydro-BA ( ) and anthracene ( ) by DNA in 15% methanol at 23° C. Emission and excitation wavelengths and details concerning the experimental conditions are given in refs. 12 and 14. The open symbols, o and V, show I /I for 1,2,3,4-tetrahydro-BA and DMA respectively in denatured DNA([P04"] 4.4 x 10 4 M).

Table II. Intercalation Association Constants and Stern-Volmer Quenching Constants for Benzo[a]pyrene Metabolites and Metabolite Model Compounds3 with Calf Thymus DNA...

Stern-Volmer Quenching Constants. d Measured in 15% methanol. Taken from refs. 15 and 18. e Measured in 2.5% DMSO. Taken from ref. 9. 

In the presence of earlier generations, the luminescence quenching of aqueous solutions of Ru(L)32+ by methyl viologen (Figure 13.11) was found to follow Stern-Volmer bimolecular kinetics with a quenching constant /cq 5 x 109 M-1s-1, typical for the bimolecular quenching in homogeneous solutions. This... 

It should be emphasized that time-resolved experiments are required for unambiguous assignment of the dynamic and static quenching constants. 

The definition used depends on the phenomenon under study. For instance, the intensity-averaged lifetime must be used for the calculation of an average colli-sional quenching constant, whereas in resonance energy transfer experiments, the amplitude-averaged decay time or lifetime must be used for the calculation of energy transfer efficiency (see Section 9.2.1). 

Figure 4.7 shows the best fits to the experimental data using Eq. (4.11). Although the data are fit within experimental error, the two-state model is certainly just an approximation. More complex distributions of sites with different quenching constants could fit the data. The success of the two-state model is not surprising given the well-known ability of two exponentials to accurately mimic complex decay curves (see above). Further, r data indicate that a more complex model is needed for a full description. 

Table 10.4. Photophysical Properties (Quantum Yield (<2>), Lifetime (to), Wavelength at Absorption (Aexc), and Emission Maximum (Aem)) of Chloride Probes and Their Quenching Constants (Ks h...

RNA. Both proteins contain no tryptophan S8 contains three tyrosines, and S15 contains two tyrosines. The tyrosine emission of these two proteins represents a case in which the quantum yield is higher in the native than the denatured protein. The average fluorescence lifetime, however, was little affected by denaturation no change was observed for S8, and the average lifetime decreased about 12% for S15. The collisional quenchers 1 and Cs + had essentially equivalent access to the tyrosines of both proteins, either in the native or denatured state, and comparison of the bimolecular quenching constants with that of free tyrosine suggested that the tyrosines were all well exposed. The mechanism for the reduction in quantum yield upon denaturation, apparently a static interaction, has not been elucidated. 

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