Intermolecular aromatic acid association of an anthocyanin (petanin) evidenced by two‐dimensional nuclear overhauser enhancement nuclear magnetic resonance experiments and distance geometry calculations

A new mechanism is described for intermolecular association of the aromatic acid of the anthocyanin petanin (petunidin 3‐O‐[6‐O‐(4‐O‐E‐p‐coumaroyl‐α‐L‐rhamnopyranosyl)‐β‐D‐glucopyranoside]‐5‐0‐β‐D‐glucopyrano‐side) in cold solution. Petanin was studied in acidified methanolic solution using the nuclear Overhauser enhancement (NOESY) nuclear magnetic resonance technique at −5° and –20°C. Inter‐ and intra‐molecular coumaroyl group NOESY cross‐peaks were observed, and the corresponding proton‐proton distance bounds were used in distance geometry calculations to determine the association of the coumaroyl groups. The orientation of two associated petanin coumaroyl groups was found to be head‐to‐tail along the long coumaroyl group axis. The corresponding orientation along the short coumaroyl group axis could not be determined conclusively; however, the most probable orientation is head‐to‐head. Lack of observed NOESY cross‐peaks between protons of the coumaroyl group and the aglycone indicated absence of the intramolecular coumaroyl group‐aglycone association which has been suggested for other acylated anthocyanins. A structure for a petanin dimer is suggested.