报告人：Hou-min Chang 教授（北卡罗莱纳州立大学）
Biphenyl linkage is the second most abundant linkage in softwood lignin; the β-O-4' linkage being the most abundant linkage. That about 20-28 % of monolignols are linked by biphenyl linkage has been demonstrated by UV spectroscopy, permanganate oxidation, 13C NMR and thioacidlolysis followed by 31P NMR. Most of the biphenyl structures in softwood lignin are etherified, indicating that biphenyl structures are formed while monolignols are being transported to the lignifying zone during lignin biosynthesis. Along with 4-5' (~ 3/100 C9 units) and β-6' (~3/100 C9) structures, biphenyl structures are the branching points of lignin structure. Thus, at least one out of four and probably one out of every three C9 units in softwood lignin are branching units. This fact indicates that softwood lignin is most likely a network polymer. Many alkyl-aryl ether bonds are cleaved during ball milling for the isolation of Milled Wood Lignin (MWL), but the number of branching points remain the same. Therefore, softwood milled wood lignin, with a number average DP of 10-20, exists most likely as at least a highly branched polymer and unlikely as a linear polymer. Harwood lignin has less biphenyl structures (~9%), but has more 4-O-5' (~7%) and similar β-6' (~2%) linkages as compared with softwood lignin. Thus, only one out of every six C9 units is a branching point, indicating that hardwood lignin is most likely a highly branched polymer. Hardwood MWL is most likely a branched polymer but the chance of find a few linear oligomers in MWL cannot be ruled out. Two potential branching points are not considered in the above discussion. Both α-O-4' linkage (~1-3% in softwood and ~1-5% in hardwood) and lignin-carbohydrate complexes are also branching points in lignin. While the amounts of α-O-4' linkage are small, the number of LCC linkages in native lignin cannot be determined.
Hou-min Chang (1938), currently emeritus professor in North Carolina State University, obtained his Ph.D. in University of Washington in the field of Wood Chemistry (1968). He was awarded the Sigma Xi Research Award in 1974, NCSU Alumni Association Outstanding Research Award in 1985, TAPPI Research and Development Division Technical Award in 1992, NCSU Alumni Association Alumni Distinguished Professor for Graduate Teaching in 1993 and 1994, Notable Acheivement Award in 2007 and Outstanding Alumni Award of University of Washington Pulp and Paper Foundation in 2010.