**4.1 Fluorescence spectra of brain cystatin with drug**

Brain cystatin (BC) (1 M) was incubated for 30 min with subsequent higher concentration of donepezil in 0.05 M sodium phosphate buffer pH 7.5 in a final reaction volume of 1 mL at room temperature. The same buffer is used for preparation of drug solutions. Fluorescence measurements were carried out on a Shimadzu Spectroflourimeter model RF-5301PC (Shimadzu, Japan) equipped with a 150 W Xenon lamp and a slit width of 10 nm at 298 K. The fluorescence was recorded in wavelength region 300–400 nm after exciting the protein at 280 nm. The slits were set at 10 nm for excitation and emission. The path length of the sample was 1 cm.
