pH and solvent effects on the copigmentation reaction of malvin with polyphenols, purine and pyrimidine derivatives

The influence of pH on the copigmentation reaction of malvin has been investigated from an experimental and theoretical viewpoint. The general equation for the copigment effect, when monitored by visible absorption spectrometry, is derived and it is shown to be in good agreement with results obtained in the case of three different copigments, namely chlorogenic acid, caffeine and adenosine. In particular, we demonstrate that association of malvin with the copigment occurs for all coloured malvin species and the corresponding stability constants are given. A few tannins and a few purine or pyrimidine derivatives have also been tested for their ability to act as copigments; some were shown to associate quite strongly with malvin. On the effect of the solvent on the extent of the copigmentation phenomenon, we reached an interesting conclusion: no solvent is better than water. Such a result seems to indicate that the strength of the copigment effect parallels the cohesion of the hydrogen-bonded tetrahedral network of water molecules.