p-coumaroylated syringyl units in maize lignin: Implications for beta-ether cleavage by thioacidolysis

Recent NMR studies of lignin isolated from maize demonstrated that p-coumarate esters are attached exclusively to the gamma-position of phenylpropane side chains. Thioacidolysis/desulphuration experiments have revealed that p-coumarate units are attached primarily (ca 90%) to syringyl moieties in maize lignin. In model studies with guaiacylglycerol and syringylglycerol-beta-guaiacyl ethers, cleavage of beta-ether linkages by thioacidolysis was reduced 40% by gamma-acylation of phenylpropane side chains with p-coumarate. Our results indicate that gamma-p-coumarate esters significantly reduce the yields of syringyl products recovered after thioacidolysis of grass lignins.