Abstract
Catechins, a class of flavonoids found in foods and beverages such as wine and tea, exhibit potent antioxidant properties that contribute to various health benefits.[1] Self-association of catechins, occurring through both transient and covalent mechanisms, plays a critical role in determining their stability, bioavailability, and biological activity. [2],[3] This ability to form supramolecular complexes may modulate their absorption and effectiveness, with important implications for optimizing catechin-rich foods and developing novel functional foods and nutraceuticals.
In this study, we present a quantitative analysis of the hydrogen-deuterium (H/D) exchange kinetics of the C6 and C8 hydrogens in (+)-catechin, monitored using ¹H NMR spectroscopy. While the presence of heavy water is not significant in natural systems, it serves as an instrumental tool to investigate the exchange mechanisms in this model system. At low concentrations, the exchange follows a two-step mechanism with pseudo-first-order rate constants (𝑘1, 𝑘2), showing a slight preference for deuterium incorporation at C6 over C8 at 298 K and pD 6.[4] Despite theoretical prediction, increasing (+)-catechin concentration accelerates the observed H/D exchange, suggesting a concentration-dependent mechanism. This effect is not attributed to pD changes but rather to (+)-catechin self-association. NMR chemical shift perturbations coupled with relaxation-based and diffusion NMR experiments reveal a monomer-dimer equilibrium, where dimerization alters the local environment of exchangeable protons, facilitating faster H/D exchange.
Based on these findings, we propose a comprehensive kinetic model that integrates both deuteration and self-association of (+)-catechin, aligning with the growing interest in H/D exchange at carbon centers in polyphenolic´compounds.[5] Furthermore, complementary LC-MS data provide insights into how reversible self-association influences irreversible oligomerization, leading to changes in dimeric procyanidins profiles, in terms of conversion rate and stereochemical preference.