The interpeptidic exchange of Cu(II) between biologically relevant peptides like Gly-His-Lys (GHK) was measured through proximity static fluorescence quenching of a noncoordinating tryptophan residue by Cu(II). The inability to spectrally distinguish between starting and final Cu(H(-1)GHK)(+) complexes by the current methods was solved by the replacement of noncoordinating lysine for tryptophan in the starting complex, Cu(H(-1)GHW). Because the apoGHW is the only fluorescent species, the recovered fluorescence is directly proportional to the [Cu(II)](exchanged) between GHW and GHK. The apparent second-order rate constants of the exchanges from Cu(H(-1)GHW) to GHK and DARK are 1.6 (+/- 0.2) x 10(2) and 5.0 (+/- 0.7) x 10(1) M-1 s(-1), respectively. The easy-to-implement kinetic fluorescent method described here for Cu(II) interpeptidic exchange can be expanded to other biological systems.