Quenching measurement

Time-resolved luminescence quenching measurements using the probe Tb(pyridine-2,6-dicarboxylic acid)i and the quencher bromophenol blue show the existence of micellar clusters in AOT-based w/o microemulsions. The fast exchange appearing over several microseconds was attributed to intracluster quenching, whereas the slow exchange on the millisecond time scale was attributed to intercluster exchange [243]. 

In general, reduction potentials of nucleobases have been studied much less than their oxidation potentials, and in particular water-based data are rather lacking [2, 35]. We therefore listed the available polarographic potentials measured in dimethylformamide and data obtained from pulse radiolysis studies or fluorescence quenching measurements. From the data in Table 1, it is evident that the pyrimidine bases are most easily reduced. The reduction potential of the T=T CPD lesion is close to the estimated value of the undamaged thymine base [34, 36]. 

We turn now to intensity quenching measurements (/o// versus [Q]). As we will show, these measurements are even less sensitive for detecting complex models than are lifetime measurements. On the plus side, however, they show a remarkable ability... 

Our results demonstrated clearly that the lifetime data are more sensitive to subtleties of the micromechanistic photophysics. In this case we were able to establish inadequacies of the two-component model that were not detected by intensity quenching measurements alone. It is also clear that resolution of the detailed mechanism in these complex polymer systems will require even better lifetime data than we are able to obtain with a conventional flash lamp-based time-correlated photon counting system. 

While no spectroscopic evidence of a ground-state complex between anthracene and carbon tetrachloride, naphthalene or 1,2-benzanthracene and carbon tetrabromide has been found, Nemzek and Ware [7] were unable to explain their steady-state fluorescence quenching measurements with the parameters deduced from the determination of the time-dependent rate coefficients unless a ground-state complex was present. This cannot be regarded as a satisfactory and consistent analysis because the time-dependent rate coefficient would be modified by the presence of the initial distribution of quencher and fluorophor in the ground state. 

Cabaniss, S.E. and Shuman, M.S. (1988) Fluorescence quenching measurements of copper-fulvic acid binding. Anal. Chem., 60, 2418-2421. 

In order to extract rate constants for electron transfer from observed fluorescence quenching measurements the following mechanistic scheme is commonly used. In this scheme, presented here in the case of an excited anionic donor, kd and k d are the rate constants for the diffusive formation and dissociation of an encounter complex within which the electron transfer takes place (kel). 

It was of course of interest to review in this context the use of the Rehm-Weller empirical equation, Eq. (6), versus the Marcus model of Eq. (5). The method used to correlate the results for the jl-naphtholate quenchings was thus applied to the important Weller series of fluorescence quenching measurements made in acetonitrile [88] and this shows that 60 values (on a total number of 65) fit quite well with a mean intrinsic activation barrier of 5.5 Kcal. M 1 [96] (see Fig. 3). Moreover, this value is not far from the expected value for the outer contribution (AGJ), as calculated by Eq. (7) and it has often been underlined that the solvent reorganization term is effectively the dominant contribution in... 

When dealing with organic anions, a problem often encountered is to find correct values for the oxidation potential. Electrochemical methods frequently lead to irreversible values generally related to the high reactivity of the radical species formed after oxidation. One way to circumvent this problem is to use a series of electron transfer quenching measurements involving the anion and to adjust the oxidation potential in order to fit the results to a Marcus correlation. This has been done using the Weller equation to calculate the oxidation potentials of a series of borate anions [93] and the Marcus model in the case of naphtholate anions [96], A reasonable estimation of E0I, with discrepancies of 0.2 V as a maximum can be obtained by this method. 

The results of fluorescence quenching measurements indicated that a rather large portion of the protein can be penetrated by the physiological ligands of the... 

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. 

Experimentally, the most relevant issue for obtaining systematic results is the availability of well-characterized apertures, a point that cannot be overestimated, Obviously, the results depend sensitively on the tip and more precisely on the structure of the aperture. This reveals the importance of the probe in single-molecule near-field fluorescence microscopy in general not only in lifetime measurements. In view of the fluorescence quenching measurements mentioned at the beginning of this... 

These data demonstrate that QEext(EL) / QEext(PL) = 0.5. This is a lower limit for the ratio. Note that the EL data were obtained at electric fields of 3-4 X 105 V/cm at these high electric fields, field-induced quenching measurements show a reduction in PL efficiency by 10% [39, 40]. Thus, QEext(EL) / QEext(PL) > 0.5. 

Fluorescence Quenching Measurements Addition of an external species, known as a quencher (Q), which is capable of deactivating the excited state through collision can provide information concerning the extent/degree of exposure of a fluorescent species. The process is outlined below... 

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