New bioactive flavonoid glycosides with antioxidant activity from the stem bark of Olax subscorpioidea Oliv

Abstract

A previously unreported gallocatechin glycoside, (2 R,3S) 4′-O-methyl-gallocatechin-3-O-α-ʟ-rhamnopyranoside (1) and an unseparable mixture of two previously undescribed dihydromyricetin glycosides, (2 R,3R) 4′-O-methyl-dihydromyricetin-3-O-α-ʟ-rhamnopyranoside (2a) and (2 R,3S) 4′-O-methyl-dihydromyricetin-3-O-α-ʟ-rhamnopyranoside (2 b) along with three known compounds were isolated from the n-butanol soluble fraction of the stem bark of Olax subscorpioidea Oliv. Their structures were elucidated by detailed spectroscopic analyses, including ¹H NMR, ¹³C NMR, ¹H-¹H COSY, HSQC, HMBC, NOESY, HR-ESI-MS and chemical methods. The crude ethanol extract, the fractions, and some of the isolated compounds were screened for their antioxidant and antibacterial activities. They showed significant antioxidant activities with EC₅₀ ranging from 6.29 to 18.19 µg/mL in 2,2-diphenyl-1-picrylhydrazyl (DPPH) method and EC₅₀ ranging from 85.77 to 86.39 mmol FeSO₄/g in ferric reducing antioxidant power (FRAP) methods compared with 2.29 µg/mL and 3.52 mmol FeSO₄/g for the positive control (ʟ-ascorbic acid). Nevertheless, no inhibition was observed against the tested bacterial strains at a MIC less than 256 μg/mL.

Graphical Abstract

Keywords:

• Olax subscorpioidea
• structural elucidation
• gallocatechin
• dihydromyricetin
• flavonoid glycosides
• antioxidant activity

Conflict of interest

The authors declare no conflict of interest.

Additional information

Funding

We are grateful to the Alexander Von Humbolt Foundation (AvH), Bonn, Germany for the financial support of the present work.