Acacia mearnsii (Fabaceae) contains acacia polyphenols, which are a complex mixture of proanthocyanidins that are mainly composed of 5-deoxycatechin units. In this study, an aqueous extract of A. mearnsii bark was fractionated and the α-amylase inhibitory activity of each fraction was evaluated. The 13C NMR and MS data and the pyrolysis products obtained from the active and inactive fractions were compared. The spectroscopic results clearly indicated that fractions with strong inhibitory activity contained proanthocyanidin oligomers with catechol-type B-rings rather than pyrogallol-type B-rings. HPLC analysis of the pyrolysis products showed peaks for pyrocatechol were only observed in the mixtures obtained from the fractions with high inhibitory activities. In addition, (+)-pinitol was isolated as a major polyol of the extract at a level comparable with that of sucrose.
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