Fluorescence quenching collisional mechanisms

Kasha M (1952) Collisional perturbation of spin-orbital coupling and the mechanism of fluorescence quenching—a visual demonstration of the perturbation. J Chem Phys 20 (l) 71-74. doi 10.1063/1.1700199... 

Mechanisms of fluorescence quenching are usually divided into two groups collisional and complex formation these are also referred to as dynamic and static mechanisms, respectively. The distinction between dynamic and static mechanisms can get blurred, as can the distinction between collision complexes and subsequent more specific interactions (see discussion of diffusion controlled reactions in section 7.4). In addition to these mechanisms there can be internal quenching if the optical density of the solution is high. This last phenomenon, which is the so-called inner filter effect, is only a nuisance and provides no useful information. 

Fluorescence quenching occurs by a variety of mechanisms. Static quenching describes the situation when the interaction between the fluorescent molecule (F) and the quencher (Q) takes place in the ground state. Dynamic quenching occurs when they undergo a collisional process during the lifetime of the excited state of the fluorescent molecule, as shown below ... 

The acetone-sensitized photodehydrochlorination of 1,4-dichlorobutane is not suppressed by triplet quenchers (20), but the fluorescence of the sensitizer is quenched by the alkyl chloride (13). These observations imply the operation of a mechanism involving collisional deactivation, by the substrate, of the acetone excited singlet state (13,21). This type of mechanism has received strong support from another study in which the fluorescence of acetone and 2-butanone was found to be quenched by several alkyl and benzyl chlorides (24). The detailed mechanism for alkanone sensitization proposed on the basis of the latter work invokes a charge-transfer (singlet ketone)-substrate exciplex (24) and is similar to one of the mechanisms that has been suggested (15) for sensitization by ketone triplets (cf. Equations 4 and 5). 

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. 

In systems where only dynamic quenching occurs, then steady-state fluorescence intensities can be measured instead of lifetimes/101 103-,07) In experiments where comparisons are being made (i.e., for a comparison of different experimental conditions or types of membrane), it is important that the lifetime of the fluorophore (r0) is not affected by the experimental conditions. Fluorescence intensities can be obtained much more rapidly and without specialized instrumentation. Blatt and Sawyer(101) have employed a relationship essentially the same as Eq. (5.20) in this way. They have pointed out that since the quenching mechanism is collisional, the partition coefficient that is derived is a partition coefficient of the quencher into the immediate environment of the fluorophore and is therefore a local Kp. It is therefore possible to investigate the partition coefficient gradient across the lipid bilayer by using a series of probes, such as the anthroylstearates,(108) located at different depths. In their method, Eq. (5.20) has the form... 

Silver ions cause strong quenching of protein fluorescence by at least two distinct mechanisms collisional quenching and energy transfer to Ag+-mercaptide absorption bands.415 The effect was studied in detail for both sulfhydryl and non-sulfhydryl proteins and had a number of practical applications including the determination of SH groups and as a probe of binding sites. 

Another class of fluorescence sensors are those molecules which display changes in lifetime in response to the analyte of interest. Such sensors are distinct from collisionally quenched sensors, such as the ruthenium complexes for oxygen, in that the mechanism is not necessarily collisional quenching. The lifetimes of such sensors can increase or decrease in response to analyte. 

Any phenomenon which results in a change of fluorescence intensity wavelength, anisotropy, or lifetime can be used for sensing. The simplest mechanism to understand is collisional quenching (Figure 19.6, middle). In this case, one identifies a fluorophoie which is quenched by the analyte. Collisional quenching results in a decrease in the intensity or lifetime of the fluorophore, either of which can be used to determine the analyte concentration. 

Use of collisional quenching as the sensing mechanism requires the fluorescent probe to be sensitive to quenching by the desired analyte. Collisional quenching results in a decrease in intensity and lifetime, which can be described by the Stem-M>Imerequatii ,... 

Chiral amines have been shown to be able to quench the fluorescence of several fluorophores. The first cases were reported by Irie and coworkers, who described that the collisional quenching of (/ )-binaphthyl by A, A -dimethyl-l-phenylethyl-amine was dependent on the stereochemistry of the latter [50]. The mechanism of this process corresponds to a series of subsequent events formation of an encounter complex between the excited fluorophore and the quencher, which can then give rise to a tight exciplex ([A-F ] in Fig. 2a) or to an electron transfer ion pair [ A -F ] (Fig. 2a). This can explain the strOTig dependence of both quenching and enantios-electivity uprm the polarity of the solvent observed in these studies [126]. 

The fact that the quantum yield in fluorescence is less than one is due to the return of some molecules to the ground state via radiationless energy transfer (see p. 289) and to thermal (collisional) quenching. In photolysis a number of additional factors depending on the reaction mechanism can... 

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