Variations in tryptophan fluorescence intensities confirm that copper(II) interacts with a-synuclein, a protein implicated in Parkinson's disease. Trp4 fluorescence ;decay kinetics measured for the F4W protein show that Cu(II) binds tightly (K-d similar to 100 nM) near the N-terminus at pH 7. Work on a F4W/H50S mutant indicates that a histidine imidazole is not a ligand in this high-affinity site.