Fluorescence-tagged monolignols: synthesis, and application to studying in vitro lignification

Fluorescence-tagged coniferyl alcohols, coniferyl alcohol γ-coupled by ethylenediamine spacers to dimethylaminocoumarin or nitrobenzofuran fluorophores, were tested as photoprobes to study the oxidase-mediated polymerization of monolignols. The fluorescent coniferyl alcohol derivatives readily underwent peroxidase-catalyzed in vitro copolymerization with coniferyl alcohol to yield fluorescent dehydrogenation polymers, the backbone polymers of which were structurally indistinguishable from polymers formed solely from coniferyl alcohol. To illustrate the use of the photoprobes, we successfully monitored in real time the complexation of coniferyl alcohol with horseradish apoperoxidase by Förster resonance energy transfer (FRET) using the protein-tryptophan near the active site and a dimethylaminocoumarin moiety as donor and acceptor fluorophores. Furthermore, mixtures of fluorescence-tagged and normal coniferyl alcohols readily diffused into isolated maize cell walls and reacted with wall-bound peroxidases to form in muro artificial lignins that could be visualized by fluorescence microscopy. Thus we anticipate that fluorescence-tagged monolignols will be useful for in vitro and in vivo studies of cell wall lignification.