Computational studies on conformation, electron density distributions, and antioxidant properties of anthocyanidins

Computational studies carried out at density functional theory levels are able to provide reliable chemical information about medium sized compounds as anthocyanins and their aglycons (anthocyanidins). Thus, they indicate that the most stable tautomers in aqueous solution for the main anthocyanidins (excluding pelargonidin) are deprotonated at C4' in the neutral forms, while deprotonations at C5 and C4' characterize the most stable anions in solution. QTAIM electron density analysis (overviewed in brief in the methods section) shows that Lewis structures usually employed give rise to unreliable atomic charges. Thus: (1) The positive charge spreads throughout the whole cation, and is not localized on any specific atom or set of atoms; (2) Neutral forms can be described as enolates where the negative charge is counterbalanced in a different way to that indicated by the typical resonance forms; and (3) The negative charge of anions is mainly spread among three regions of the molecule: the two deprotonated sites and the C9-O1-C2 area. The analysis of a group of complexes formed by a model of cyanin with four common metalic cations (Mg(II), Al(II), Cu(II), Zn(II)), shows: (1) the preference for tetracoordination in Zn(II) and Cu(II) complexes, (2) higher affinity for Cu(II) than for the other metals here studied, and (3) the distortion of electron density in the cyanin ligand affects the whole molecule. This distortion can be described as a continuous polarization where, even, in some cases, the atomic electron populations of those atoms of the ligand that are more directly involved in bonding to the metal increase.