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

Abstract

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