Biomimetic Enzymatic Oxidative Coupling of Barley Phenolamides: Hydroxycinnamoylagmatines

Oxidative coupling of hydroxycinnamoylagmatines in barley (Hordeum vulgare) and related Hordeum species is part of the plant defense mechanism. Three linkage types have been reported for hydroxycinnamoylagmatine dimers, but knowledge on oxidative coupling reactions underlying their formation is limited. In this study, the monomers coumaroylagmatine, feruloylagmatine, and sinapoylagmatine were each incubated with horseradish peroxidase. Their coupling reactivity was in line with the order of peak potentials measured: sinapoylagmatine (245 mV) > feruloylagmatine (341 mV) > coumaroylagmatine (506 mV). Structure elucidation of fourteen in vitro coupling products by NMR and MS revealed that the three main linkage types were identical to those naturally present in Hordeum species, namely, 4-O-7′/3-8′, 2-7′/8-8′, and 8-8′/9-N-7′. Furthermore, we identified two linkage types that were not previously reported for hydroxycinnamoylagmatine dimers, namely, 8-8′ and 4-O-8′. We conclude that oxidative coupling by horseradish peroxidase can be used for biomimetic formation of natural antifungal hydroxycinnamoylagmatine dimers from barley.