A modified alkaline nitrobenzene oxidation (NBO) method was developed to enable the analysis of the biphenyl structures of cedar wood lignin. The most essential point of the process is a modified work-up process in pyridine and a prolonged gas chromatography analysis of the silylated products. By applying this mean to cedar wood meal, a novel biphenyl product, dehydrovanillin-vanillic acid (1-carboxy-1′-formyl-4,4′-dihydroxy-3,3′-dimethoxy-5,5′-biphenyl), was detected together with known products, dehydrodivanillin and dehydrodivanillic acid. The highest total yield of biphenyl products was detected when NBO was carried out for 2-4 h at 170°C. The workup procedure was slightly modified so that the biphenyl products can be quantified. The NBO conversion rates of biphenyl linkages of lignin were also examined with a biphenyl-type lignin model compound, and this gave rise to 75% biphenyl-type NBO degradation products. Under the same condition, the total yield of vanillin and vanillic acid (VA) from a non-condensed-type β-O-4 model compound was 89%. Because the latter did not exhibit any peak of the biphenyl products, it can be concluded that all the biphenyl products obtained by NBO (0.17 mmol g -1 ) were from the lignin of the native wood. It was calculated that at least 6.7 of 100 phenylpropanoid units of cedar lignin were involved in biphenyl structures even if the NBO conversion rate was not taken into consideration. The synthesis and analytical data of 11 lignin model compounds is described.

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 C6-C3). The biphenyl product yield of the 15 hardwoods ranged widely (0.002–0.05 per C6-C3) 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.