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( 4a ! 8)pelargonidin 3-O-b-glucopyranoside (1), epicatechin(4a ! 8)pelargonidin 3-O-b-glucopyranoside (2), afzelechin(4a ! 8)pelargonidin 3-O-b-glucopyranoside (3) and epiafzelechin(4a ! 8)pelargonidin 3-O-b-glucopyranoside (4). The stereochemistry at the 3- and 4-positions of the flavan-3-ol units was based on assumption ofR-configuration at C-2. Each of the four pigments occurred in theNMR solvent as a pair of rotamers. Proved by cross-peaks in the 1H–1HNOESYNMRspectra 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.