The copper chelation of the inhibitors was determined by the UVVis
spectra according to Kubo et al. (2000). The reaction mixture
contained 1800 ml of 0.1 M phosphate buffer (pH 6.8), 1000 ml of DI
water, 100 ml of 0.14 mM CuSO4 solution and 100 ml of 0.050 mM
inhibitor solution. The reaction mixture was incubated at 25°C for 30
min and UV-Vis absorption spectra were measured at 240 to 540
nm.