Tyrosine residues enhance cross-linking of synthetic proteins into lignin-like dehydrogenation products

Synthetic proteins composed of lysine (polylysine, PL) and a random co-polymer of lysine and tyrosine (polylysine/tyrosine, PLT) were incorporated into lignin-like dehydrogenation polymers (DHPs) formed by the peroxidase-catalysed polymerisation of coniferyl alcohol. The yield of water-insoluble DHPs was greater in the presence of the tyrosine-containing PLT than PL. Indeed, the increase in total yield with PLT was often greater than could be accounted for if all the protein added had become incorporated into the DHPs. A comparison of the Fourier transform infra-red spectra of the DHPs formed in the absence and presence of synthetic proteins provided evidence that the presence of PLT had enhanced the incorporation of coniferyl alcohol, albeit in a less cross-linked form, into DHPs. The insolubilisation of PL and PLT was suggested by the presence of absorption bands in the infra-red spectra of their respective DHPs that are characteristic of protein/amide moieties. The insolubilisation of PLT was confirmed by the release of material from PLT-DHPs by digestion with trypsin. Therefore, this study provides evidence that the presence of tyrosine residues may enhance the cross-linking of proteins into lignin.