AN EVALUATION OF CHARGE EFFECTS ON THE QUENCHING OF TRYPTOPHAN FLUORESCENCE IN SMALL PEPTIDES BY IODIDE ION

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Abstract

Abstract— Charge effects on the quenching of tryptophan fluorescence in small peptides by iodide ion have been analyzed by the conventional “static” quenching model and by a recently proposed competitive quenching model. The former involves a fit of the quenching data using two quenching parameters—one for dynamic and one for static quenching contributions. The latter model involves a single parameter fit in which the fitting parameter is the characteristic rate constant for quenching of the fluorescent state. Both models indicate a clear charge effect on the efficiency of quenching by iodide ion. However, the static model results are obscured by the interdependence of the two fitting parameters and the fact that the true physical meaning of the static parameter is uncertain. Rate constants derived from the competitive model can be converted into relative quenching efficiencies. These efficiencies, which vary by more than a factor of two for the molecules studied, are greatest when the positive charge is on the tryptophan and least when this residue contains a negative charge.

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