Estimating the lignin composition of sugarcane hybrids with varied lignin content by pyrolysis-GC/MS

Lignin plays an important role in lignocellulosic biomass recalcitrance. In grasses, lignin is usually the HGS type: p-hydroxyphenyl (H), guaiacyl (G) and syringyl (S). In order to evaluate the lignin composition and S/G ratio in sugarcane, we examined the chemical composition of sugar cane internodes with varied recalcitrance and lignin contents and their precursors composition. The components proportion was determined by sulfuric acid hydrolysis and the lignin precursors were determined by pyrolysis GC/MS. Sugarcane internodes from six hybrids were divided into three different stem fractions. Treatment with sulfuric acid 72% (w/w) revealed a lower content of lignin in the center of stems (14-22% in pith fractions) that increased in the rind fraction (20-22%). On the other hand, glucan content decreased from pith (40-55%) to rind (40-44%). Pyrolysis-GC/MS showed that lignin composition varied along the internodes and depended on the hybrid. A high level of H lignin was detected; however it might be overestimated owing to the presence of hydroxycinnamic acids esterified to xylans and lignin - the pith and rind fractions contained the highest (30-34%) and the lowest (23-33%) H lignin contents, respectively. The S/G ratio increased from pith (0.87 – 2.11) to rind (1.88 – 2.90), except in hybrid 58, which rind had the lowest S/G ratio among its fractions (2.04). This S/G ratio gradient matched the differential recalcitrance along the internode, where pith and rind were the least and most recalcitrant regions, respectively.