EVIDENCE FOR SELF-ASSOCIATION OF THE ANTHOCYANIN PETANIN IN ACIDIFIED, METHANOLIC SOLUTION USING 2-DIMENSIONAL NUCLEAR OVERHAUSER ENHANCEMENT NMR EXPERIMENTS AND DISTANCE GEOMETRY CALCULATIONS

A new mechanism is described for self-association of anthocyanins in solution. Petanin, petunidin 3-O-[6-O-(4-O-E-p-coumaroyl-alpha-L-rhamnopyranosyl)-beta-D-glucopyranoside]-5-O-beta-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.