Variation of the contents of biphenyl structures in lignins among wood species

Biphenyl structures (5–5 linkages) are one of the significant structural units in plant cell wall lignin. They can potentially play a role in providing branch points in lignins and influence the polymer structure of lignins. Variation of the biphenyl content was investigated for 21 wood species by alkaline nitrobenzene oxidation. The total yields of biphenyl-type products were similar among the six softwoods, and at least 6% of the phenylpropanoid units in lignin were involved in the biphenyl structures (>0.06 per C -C ). The biphenyl product yield of the 15 hardwoods ranged widely (0.002–0.05 per C -C ) and was lower in species having a higher syringyl/guaiacyl ratio (i.e. a species with less guaiacyl units). In addition, the proportion of biphenyl products in all guaiacyl-type products was not constant but decreased as the proportion of syringyl units increased. This indicates that the content of guaiacyl units is not likely the only factor influencing the biphenyl content.