THE DFRC METHOD FOR LIGNIN ANALYSIS - 4 - LIGNIN DIMERS ISOLATED FROMDFRC-DEGRADED LOBLOLLY-PINE WOOD

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