Antioxidant and Cytoprotective Effects of the Di-O-Caffeoylquinic Acid Family: The Mechanism, Structure–Activity Relationship, and Conformational Effect

In this study, a series of di-O-caffeoylquinic acids (di-COQs) were systematically investigated for their antioxidant and cytoprotective effects towards •OH-damaged bone marrow-derived mesenchymal stem cells (bmMSCs). Five di-COQs were measured using a set of antioxidant assays. The results show that adjacent 4,5-Di-O-caffeoylquinic acid (4,5-COQ) and 3,4-di-O-caffeoylquinic acid (3,4-COQ) always gave lower IC50 values than did non-adjacent di-COQs. In the Fe2+-chelating assay, 4,5-COQ and 3,4-COQ presented greater UV-Vis spectra and darker colors than did non-adjacent di-COQs. In the UPLC-ESI-MS/MS analysis, no corresponding radical adduct formation (RAF) peak was found in the reaction products of di-COQs with PTIO•. In the MTT assay, all di-COQs (especially 1,5-COQ, 1,3-COQ, and 4,5-COQ) dose-dependently increased the cellular viabilities of •OH-damaged bmMSCs. Based on this evidence, we conclude that the five antioxidant di-COQs can protect bmMSCs from •OH-induced damage. Their antioxidant mechanisms may include electron-transfer (ET), H+-transfer, and Fe2+-chelating, except for RAF. Two adjacent di-COQs (4,5-COQ and 3,4-COQ) always possessed a higher antioxidant ability than the non-adjacent di-COQs (1,3-COQ, 1,5-COQ, and 3,5-COQ) in chemical models. However, non-adjacent 1,3-COQ and 1,5-COQ exhibited a higher cytoprotective effect than did adjacent di-COQs. These differences can be attributed to the relative positions of two caffeoyl moieties and, ultimately, to the conformational effect from the cyclohexane skeleton.