Catechin Photolysis Suppression by Aluminum Chloride under Alkaline Conditions and Assessment with Liquid Chromatography–Mass Spectrometry

Tea is rich in catechins and aluminum. In this study, the process of catechin photolysis was applied as a model for examining the effects of aluminum chloride (AlCl₃) on the structural changes of catechin and the alteration of aluminum complexes under blue light irradiation (BLI) at pH 8 using liquid chromatography and mass spectrometry techniques. Additionally, the effects of anions on catechin upon the addition of AlCl₃ and treatment with BLI were also studied. In this study, when 1 mM catechin was treated with BLI, a superoxide anion radical (O2•−) was generated in an air-saturated aqueous solution, in addition to forming a dimeric catechin (proanthocyanidin) via a photon-induced redox reaction. The relative percentage of catechin was found to be 59.0 and 95.7 for catechin treated with BLI and catechin upon the addition of 1 mM AlCl₃ treated with BLI, respectively. It suggested that catechin treated with BLI could be suppressed by AlCl₃, while AlCl₃ did not form a complex with catechin in the photolytic system. However, under the same conditions, it was also found that the addition of AlCl₃ inhibited the photolytic formation of O2•−, and reduced the generation of proanthocyanidin, suggesting that the disconnection of proanthocyanidin was achieved by AlCl₃ acting as a catalyst under treatment with BLI. The influence of 1 mM fluoride (F−) and 1 mM oxalate (C2O42−) ions on the photolysis of 1 mM catechin upon the addition of 1 mM AlCl₃ and treatment with BLI was found to be insignificant, implying that, during the photolysis of catechin, the Al species were either neutral or negatively charged and the aluminum species did not form a complex with anions in the photolytic system. Therefore, aluminum, which is an amphoteric species, has an inherent potential to stabilize the photolysis of catechin in an alkaline conditions, while suppressing the O2•− and proanthocyanidin generation via aluminum ion catalysis in the catechin/Al system under treatment with BLI.