Evidence for self‐association of the anthocyanin petanin in acidified, methanolic solution using two‐dimensional nuclear overhauser enhancement NMR experiments and distance geometry calculations

A new mechanism is decribed for self‐association of anthocyanins in solution. Petanin, petunidin 3‐O‐[6‐O‐(4‐O‐E‐p‐coumaroyl‐α‐L‐rhamnopyranosyl)‐β‐D‐glucopyranoside]‐5‐O‐β‐D‐glucopyranoside, was studied in acidified methanolic solution using the nuclear Overhauser enhancement (NOESY) nuclear magnetic resonance technique. Intra‐ and inter‐molecular NOESY cross‐peaks were observed, and the corresponding proton‐proton distance bounds were used in distance geometry calculations to determine the stacking of the petanin aglycones. The orientation of two self‐associated petanin aglycones was found to be head‐to‐tail along both the long and the short aglycone axis. Lack of observed NOESY cross‐peaks between protons of the coumaroyl group and the aglycone indicated absence of intramolecular stacking of the stable petanin molecules. Non‐coplanarity between the planes of the benzopyrylium and the phenyl rings was also indicated.