Probing the antioxidant activity of polyphenols by CIDNP: from model compounds to green tea and red wine

Polyphenols occur naturally in a vast variety of plants. One of their predominant properties is their antioxidant activity. To provide a deeper understanding of the antioxidant mechanism, (1)H CIDNP spectroscopy (CIDNP=chemically induced dynamic nuclear polarization) is used to study model hydrogen abstraction reactions with four catechin-based polyphenols: catechin (CA), gallocatechin (GC), epigallocatechin (EGC), and epigallocatechin gallate (EGCG). The experiments involve photoinduced hydrogen-atom transfer to a hydrogen abstractor (e.g., excited isopropylthioxanthone) followed under steady-state conditions and in a time-resolved fashion (resolution 500 ns-1 ms). It is found that hydrogen abstraction is an essentially stochastic process with a slight preference for the B rings in the catechin-based polyphenols. Remarkably, analogous reactivity patterns could be followed in the "real systems", green tea and red wine. We also show that CIDNP can be used as a semiquantitative tool to assess chemical reactivity.