The UV absorption spectra were recorded by using Ultrospec 4000 spectrophotometer (Pharmacia Biotech) equipped with a 10- mm quartz cuvette. The temperature of the cell compartment was thermostated at 27.5 ± 0.1 ◦C by circulating water from a ther- mostated bath. The pH measurements were made with a Metrohm 780 pH metre using a combined glass electrode.