Apparent static quenching

The values of K0 and Ksv obtained by using eq 9 are given in Table 4. The ground-state complex considered in eq 7 includes not only the CT complex but all kinds of complexes that may lead to apparent static quenching. Therefore, usually K0 is larger than KCT as can be seen from Table 4 (although there are a few exceptions). 

This type of apparent static quenching is usually interpreted in terms of a hete of action within which the prd)ability of quenching is unity. The modified form of die Stero- blmer equafiem which describes tlus situation is... 

NATA and DHE, respectively. These values imply that quench concentrations near 03M are required to quench one-half of the fluorophores by a static p x)ces8. Such a weak association suggests that the fluorophores and quenchers do not actually form a ground-state complex. Instead, it seems that the apparent static con nent is due to the quencher being adjacent to the fluorophore at the moment of excitation. These closely spaced fluorophore-quencher pairs are immedi ely quenched and thus appear to be dark com dcxes. 

The modified Stem-Volmer expression (Eq. 5.58) also gives a straight line in the case of a homogeneous system with either purely dynamic or purely static quenching (Fig. 5.7C, solid line). A mixture of dynamic and dynamic quenching gives a nonlinear plot, but the curvature may be apparent only at high quencher concentrations (Fig. 5.7C, dotted line). The slope and abscissa intercept of such a plot must, therefore, be interpreted cautiously and with comparisons to the curvature of an ordinary Stem-Volmer plot. 

In the presence of static quenching, deviations from the linearity of the Stern-Volmer plots for emission and photoreaction quenching are expected, and the lifetime may apparently result unquenched [5]. The experimental quenching constant is related to the rate constants of the quenching process in the encounter (figure 11) by the following equation... 

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. 

We have previously used the Cu ion as a convenient cationic quencher for polyanion - bound chromophores. In these previous papers it was found that the apparent second order rate constant was extremely high, presumably because of the electrostatic attraction between Cu and the polyelectrolyte. The quenching was found previously to be largely static in that the total fluorescence intensity was quenched very efficiently but there was little or no lifetime quenching. 

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