Fluorescence Quenching As A Parameter For Measuring Complex Formation Between Metal Ions And Aromatic Amino Acids And Peptides

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Abstract
The ultraviolet fluorescence of tryptophan and tyrosine is quenched when they are complexed with Cu++ and Ni++. Titrations of fixed amounts of the amino acids or their peptides with increments of metal ion performed in dilute buffered solutions, yield curves from which the binding constants of the 1:1 complexes can be determined by curve fitting. The association constants found by this method agree with those obtained by potentiometric titration. The binding of these metal ions to glycyl-L-tryptophan was also measured. Zn++, while not quenching tryptophan fluorescence upon binding, competes with Cu++, thus permitting the Zn++-tryptophan association constant to be found. Unlike potentiometric titration, fluorescence quenching would detect binding even when the metal ion does not displace H+. Inherent limitations of the method involve collisional quenching and self absorption artifacts at high reactant concentrations.