Lignin content versus syringyl to guaiacyl ratio amongst poplars

Two oxidation techniques that afford high yields of monomers and dimers were used to more accurately estimate the syringyl to guaiacyl (S:G) ratio of hardwood lignins. Permanganate oxidation of the woodmeal after a CuO pre-hydrolysis step gave poor results and this was attributed to preferential oxidation and degradation of syringyl nuclei by CuO. However, this procedure did provide a good estimate of the percentages of both S and G phenylpropane (C9) units that were uncondensed. When the total S and G products from nitrobenzene oxidation (NBO) of the uncondensed fractions were corrected, credible S:G ratios were obtained. These ratios were in good agreement with results from KMnO4 oxidation of dissolved kraft lignin without CuO pre-hydrolysis. The corrected NBO method was used to determine the S:G ratio of 13 poplars, and the values ranged from 1.01 to 1.68. Unlike results from other investigations, an excellent linear correlation (R2 = 0.846) was obtained for a decreasing lignin content (28% to 16.5%) with an increase in the S:G ratio.