Structural characteristics of (-)-epigallocatechin-3-gallate inhibiting amyloid Aβ42 aggregation and remodeling amyloid fibers

To elucidate the structural requirements by which EGCG analogs inhibit A beta 42 protein aggregation and remodel amyloid fibers, the molecular interactions between A beta 42 and four EGCG analogs, epigallocatechin-3-gallate (EGCG), (-)-gallocatechin gallate (GCG), (-)-epicatechin-3-gallate (ECG) and (-)-epigallocatechin (EGC), were investigated by thioflavin T fluorescence (ThT), circular dichroism (CD), atomic force microscopy (AFM), differential scanning calorimetry (DSC) and BCA protein assay. Results revealed that the four EGCG analogs had the ability to prevent the increase of beta-sheet content and inhibit A beta 42 fibrillation when added in the lag and growth phases of A beta 42 fibrillation process. When added in the equilibrium phase, the four EGCG analogs can disaggregate the preformed protofibrils/fibrils to oligomers and unfold or partially unfold oligomers. It was also observed that EGCG showed the highest inhibitory effect on A beta 42 fibrillation, followed by GCG, ECG and EGC. From the values of IC50, kinetic parameters, secondary structures, thermo-stability and solubility measurement, a reasonable conclusion can be preliminarily drawn that the structural contribution efficiency of EGCG to inhibit A beta 42 aggregation and remodel A beta 42 amyloid fibrils decreases by the order of 30-hydroxyl group of trihydroxyphenyl ring > gallol ester moiety > stereoisomer. The findings in this work provide the structure based molecular interaction mechanism between EGCG analogs and A beta 42 amyloid protein.