The different types of dynamic quenching

When a protein possesses two or several Trp residues and when quenchers such as iodide, cesium or acrylamide are used and if all the Trp residues are not accessible to the quencher, the Stern-Volmer equation yields a downward curvature. In this case, we are in presence of a selective quenching (Fig. 4.4B). The linear part of the plot allows calculating the value of the Stern-Volmer constant corresponding to the interaction between the quencher and the accessible Trp residues. The fluorescence of these residues has been quenched. 

Upon complete denaturation and loss of the tertiary slructure of a protein, all Tip residues will be accessible to the quencher. In this case, the Slcrn-Volmer plot wjll display an upward curvature. 

The Myb oncoprotein Innds spediically DNA by a dc nain that consisia of three imperfecl repeats, Ri R2 and R3, each containing 3 tryptophans. The tryptophan fluorescatce of the Rj Rj dnn is used to monitor structural flexibility dianges occurring upon n IA binding to R2 R  

Upon complex foimaflon with a drable stranded DNA, the intensity of Tip residiies of R.2 R) decreases by about 50%. This intenaQr decrease is accompained by a shift of the flucwescence emis on ma mum from 336 to 332 tun (Fig. 4.6) and by a decrease in the mean fluorescence lifetime from 1.46 to 0.71 ns. These results indicate that Tip residues environment is not the same in presence c DNA. I lowevcr, tins does not mean necessaiify that the envirenment of all the six tiypb ihans has been modified. 

Right panel Experimental Trp Huorescence intensity dec of free R2R3 (-) and of ihe RiRi 

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