Dear Maher,

The anthocyanin pigments occur widely in nature and the central oxygen containing ring has two characteristic structural effects. Firstly, visual spectrum colour occurs when a large chain of alternating single and double valency links is present in the chemical structure and clearly, the central oxygen containing ring is a the central component part of such a train . The point is, such structures generate colour by means of selective light absorbing resonance bands in the visible spectrum and the chain must be long enough to resonate at a low enough frequency.

Secondly, one of the four valency electrons of the oxygen is currently un-paired. It thus carries an electron accepting positive charge. This in turn enables further chemical reactions and many more possible anthocyanin structures.

Thanks for the valuable feedback.