Different Types of Dynamic Quenching

3 Linear plot, i.e., fluorescence is heterogeneous and the accessibility of residues to the inhibitor slightly differs (Eftink and Ghiron 1976). 

In a selective quencher, the fluorophores fraction accessible to the quencher can be calculated. Let us consider two populations of fluorophore, one accessible to the quencher and the second inaccessible. At high quencher concentrations, fluorescence of the accessible fluorophores is completely quenched. Thus, residual fluorescence originates from inaccessible fluorophores, i.e., fluorophores buried in the hydrophobic core of the protein. 

The fluorescence intensity Fq recorded in the absence of quencher is equal to the sum of the fluorescence intensities of populations accessible (Fa) and inaccessible (Ft,) to the quencher  

In the presence of quencher, only fluorescence of the accessible population decreases according to the Stern-Volmer equation. The fluorescence recorded at a defined quencher concentration [Q] is 

Accessibility of a fluorophore to the solvent and thus to the quencher depends on its position within the protein. For example, buried Trp residues should have a lower accessibility to the solvent than those present at the surface. 

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