Photo-degradation of trans-caffeic acid in aqueous solution and influence of complexation by metal ions

• The photo-degradation of metal complexes of caffeic acid is compared to the photo-degradation of free caffeic acid.
• The kinetics and mechanism of trans-caffeic acid photo-degradation are investigated.
• The first route is a photo-isomerization which then undergoes a cyclization to form the esculetin.
• Two other routes lead to protocatechuic acid and vinylcatechol formation.
• Metal cations such as Al(III), Pb(II) and Cu(II) can influence the rate and mechanism of caffeic acid photo-degradation.

The photo-degradation of metal complexes of caffeic acid was compared to the photo-degradation of free caffeic acid by using UV–vis spectroscopy and HPLC–ESI-mass spectrometry. This article reports first the determination of the products that are formed from the photo-degradation of trans-caffeic acid in aqueous solution and the investigation of the mechanism by a kinetic approach. The good fit between the model and the experimental concentration profiles confirms the photo-isomerization route of the molecule to cis-caffeic acid which then undergoes a cyclization to form the esculetin photo-product. In addition, it reveals, for the first time, another route of major importance leading to the product vinylcatechol. The presence of oxygen leads to an increase of the photo-isomerization rate. Then we report that metallic cations such as Al(III), Pb(II) and Cu(II) can influence the rate and mechanism of caffeic acid photo-degradation. Al(III) ions slow down the photo-degradation whereas Pb(II) and Cu(II) ions have a promoter effect on the production of esculetin. In all cases, the photo-isomerization is reduced by the presence of metal ions and the formation of vinylcatechol does not occur.