Spectroscopic investigation on the interaction characteristics and inhibitory activities between baicalin and acetylcholinesterase

Restoring neurotransmitter acetylcholine (ACh) levels by inhibiting acetylcholinesterase (AChE) has become the primary treatment for the cognitive deficits of Alzheimer's disease. The inhibitory effects of flavonoids on AChE have attracted great interest among researchers, but few reports have focused on the interaction characteristics of inhibitor with AChE. In this work, the interaction of inhibitor baicalin with AChE was studied for the first time by fluorescence spectroscopy coupled with UV spectroscopy and circular dichroism (CD) techniques under near physiological conditions. The fluorescence quenching rate constants and binding constants for baicalinaEuro'AChE system were determined at different temperatures. The fluorescence quenching of AChE by baicalin is due to static quenching and energy transfer. The distance and the binding constant (K (a)) between baicalin and AChE were estimated to be 2.85 nm and 9.772 x 10(4) L mol(-1) at 298 K. The results of thermodynamic parameters, Delta H (-174.6 kJ mol(-1)), Delta S (-489.9 J mol(-1) K-1) and Delta G (-28.61 +/- -23.71 kJ mol(-1)), indicated that van der Waals interaction and hydrogen bonding played a major role for baicalinaEuro'AChE association. Synchronous fluorescence spectral change of AChE showed the binding sites mainly are focused on tryptophan moiety. Circular dichroism spectra showed that baicalin caused a secondary structure change of AChE. Synchronous fluorescence and three-dimensional fluorescence studies showed that the presence of baicalin could change the conformation of AChE during the binding process. Baicalin showed an obvious inhibitory activity, with 1.24 x 10(-4) mol L-1 for 50% inhibition of AChE activity (IC50). The in intro studies suggest that the baicalin could decrease catalytic activity of AChE, it is helpful for restoring and rebalancing neurotransmitter ACh levels.