INTERMOLECULAR AROMATIC ACID ASSOCIATION OF AN ANTHOCYANIN (PETANIN) EVIDENCED BY 2-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 3-0-[6-O-(4-0-E-p-coumaroyl-alpha-L-rhamnopyranosyl)-beta-D-glucopyranoside]-5-O-beta-D-glucopyranoside) in cold solution. Petanin was studied in acidified methanolic solution using the nuclear Overhauser enhancement (NOESY) nuclear magnetic resonance technique at -5-degrees and -20-degrees-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.