Quenching Sphere of Action

The Pbisson probability distribution states diat the probability of finding a volume V with n quenchers is 

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N,jV-diethylaniline MTHF, methyl-tetrahydrofuran a The R values in Angstrom units refer to the radius of the Perrin quenching sphere of action ... 

On the other hand, quenching of the Sj state of ZnTTPS proceeds via electron transfer between the fluorophore and 1 within the quenching sphere of action, consistent with the AG° of the charge-separation process. An important outcome of this work is the demonstration that either the Sj state only or both the S, and S2 states can be quenched by judiciously selecting the quenchers used. This has pertinent implications for the design of efQcient molecular logic devices. 

The molecules M and Q can come into contact (within the sphere of action) through their random diffusional motion. The rate constant kD of diffusion controlled encounters is the upper limit for any bimolecular reaction. This must be multiplied by the probability of an encounter leading to reaction (quenching in the present case), and the luminescence quantum yield then follows the Stern-Volmer equation... 

Figure 1 Quenching of ethidium bromide fluorescence by amsacrine in the presence of poly(dA-dT)-poly(dA-dT). ( ) Steady-state intensity and ( ) integrated fluorescence decay (Scxt). (Data compiled from Refs. 2 and 3, respectively.) [EB+] = 2 pM [bp] = 20 pM buffer is 0.01 M SHE (9.4 mM NaCl, 2 mM N-(2-hydroxylethyl)piperazine-N-2-ethan-sulfonate (pH 7), 20 pM EDTA). The solid line is quenching calculated using statistical probabilities to model the binding distribution and allowing complete quenching of only nearest neighbors (sphere of action of radius 3.4 A) [28].

The Combined Stern-Volmer and Perrin Model A model has been proposed by Morishima et al. [97] which takes account of Manning s theory [98] of polyelectrolytes and introduces a modification into the Stem-Volmer equation to describe sphere-of-action (Perrin) quenching this has been termed combined Stern-Volmer and Perrin Analysis and has been adopted [95,96] in an effort to describe quenching of fluorescence from labeled PMAA by T1+ ions, for example. 

This phenomenon is referred as sphere of action within which the probability of quenching is unity. The sphere of action is characterized by a volume v. Determination of this volume is possible with Eq. 9.5 (Frank and Vavilov, 1931)... 

Figure 9.20. Determination of the sphere of action of fluorescence quenching with oxygen of PPIX in myoglobin. Slope 1 yields a value for r = 8.8 A. (R of porphyrin + R of oxygen = 6 + 2 = 8 A.) Slope 2 yields a value for r resulting from oxygen diffusion in the whole protein. At O2 0.056 M, r = 18.2 A. If myoglobin is considered as a sphere, its radius is equal to 19.4 A.

Since any quenching action is a bimolecular process, it is essential that the molecules M and Q should be in relatively close contact, but not necessarily in hard sphere (van der Waals) contact. Theoretical models lead to the distance dependence of the quenching rate constants as exponentials or sixth powers of r, the centre-to-centre distance of M and Q. Since these distance dependences are very steep, it is convenient to define a critical interaction distance r at which the quenching efficiency is, this being the distance at which 50% of the molecules M decay with emission of light (or undergo a chemical reaction) and 50% are quenched by some nearby molecule Q. 

Figure 3.39 The Perrin action volume model of static quenching. Each excited molecule is surrounded by a sphere which can contain one (or several) quenchers (a), or no quencher (b). In (a) quenching is instantaneous, in (b) there is no quenching at all...

This type of static quenching requires relatively high quencher concentrations and it follows the Perrin action sphere model [64]. According to this model, each emitter molecule is surrounded by an active volume (in the general case it needs not be a sphere), such that if there is one quencher molecule at least within this volume, then quenching takes place instantly but molecules which have no quencher within the active volume emit just like those in a sample devoid of quencher. The Perrin model leads to two observable results ... 

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