The DFRC method for lignin analysis. 4. Lignin dimers isolated from DFRC-degraded loblolly pine wood

Sixteen lignin dimers were directly isolated by gel permeation and reversed-phase TLC and HPLC followed by reductive cleavage (DFRC) procedure. Their structures were elucidated by GC/MS and NMR. These dimers included representatives from all of the common interunit linkages in softwood lignins (beta-1, beta-beta, 5-5, beta-5, and 5-O-4). The DFRC procedure efficiently cleaves beta-aryl ethers, the major structural unit, so beta-ethers were not found among the dimeric products. All but two of the isolated dimers were guaiacyl-guaiacyl (from coniferyl alcohol-derived units in lignin); small amounts of p-hydroxyphenyl/guaiacyl beta-5 and beta-1 dimers were characterized. Several dimers have benzaldehydes as one unit-since aldehydes are not created by the procedure, they are either present in the native lignin or are formed during the minimal preparation of the wood. The dimers isolated and identified here will eventually allow their quantitation to aid in the structural analyses of wide-ranging plant materials.