Dimeric anthocyanins from strawberry ( Fragaria ananassa) consisting of pelargonidin 3-glucoside covalently linked to four flavan-3-ols

Abstract

Flavanol–anthocyanin complexes were isolated by successive use of Amberlite XAD-7 chromatography, Sephadex LH-20 gel filtration and preparative HPLC from acidified, methanolic extract of strawberries ( Fragaria ananassa Dutch.). These purple minor pigments were characterized by UV–Vis spectroscopy, 1D and 2D NMR techniques, and electrospray mass spectrometry to be catechin(4α→8)pelargonidin 3- O-β-glucopyranoside ( 1), epicatechin(4α→8)pelargonidin 3- O-β-glucopyranoside ( 2), afzelechin(4α→8)pelargonidin 3- O-β-glucopyranoside ( 3) and epiafzelechin(4α→8)pelargonidin 3- O-β-glucopyranoside ( 4). The stereochemistry at the 3- and 4-positions of the flavan-3-ol units was based on assumption of R-configuration at C-2. Each of the four pigments occurred in the NMR solvent as a pair of rotamers. Proved by cross-peaks in the 1H– 1H NOESY NMR spectra of 1, 2 and 4, the two conformations within each rotameric pair were in equilibrium with each other. Even though 1 and 2 are based on a different aglycone, their structures may be similar to tentatively identified pigments, which have been assumed to contribute to the colour of red wines.