Experimental and Theoretical Research on the Effect of Coupling Heat and pH on the Structure and Antioxidant Activity of Cyanidin-3-O-glucoside from Black Soybean Coat.

BACKGROUND Since anthocyanin has good coloration and antioxidant properties, many studies have focused on exploring the stability and antioxidant activity of anthocyanin. The objective of this work was to study effects of pH and temperature on the bioactivity of cyanidin-3-O-glucoside(C3G) and ultra performance liquid chromatography-photodiode array-electrospray ionization-quadrupole-time of flight mass spectrometry (UPLC-PDA-ESI-Q-TOF MS) and density functional theory (DFT) were used to explain the mechanism of structural transformation of C3G affecting their bioactivity at the molecular level. RESULTS During the heating process at pH 2.2 to 7.0，the flavylium cation content of C3G decreased from 92.71% to 51.64% and the chalcone content increased from 7.29% to 30.61%. The quinoidal base and first discovery of the degradation product of the C3G, 1-(3,4-dihydroxy-phenyl)-2-(3, 4,5-trihydroxy-6-hydroxymethyl-tetrahydro-pyran-2-yloxy)-ethanone, were only detected in heated samples at pH 7.0. DFT revealed the antioxidant mechanism was mediated by sequential proton loss electron transfer (SPLET) and the antioxidant activity of C3G in pH 5.0 and 7.0 environments was higher than that in the pH 2.2 environment. CONCLUSIONS The results revealed the thermal degradation products of C3G included catechin, 3,4-dihydroxybenzoic acid, 2,4,6-trihydroxy-benzaldehyde and 1-(3,4-Dihydroxy-phenyl)-2-(3,4,5-trihydroxy-6-hydroxymethyl-tetrahydro-pyran-2-yloxy)-ethanone. C3G had higher antioxidant activity in weakly acidic to near-neutral environments and the reactive sites were most likely at the 4'-OH and 5-OH sites. This article is protected by copyright. All rights reserved.

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