Submitted to: International Symposium on Wood and Pulping Chemistry
Publication Type: Proceedings
Publication Acceptance Date: 12/9/1998
Publication Date: N/A
Citation: N/A Interpretive Summary:
Technical Abstract: It has long been assumed that lignins are not optically active. Recently, their accepted derivation from non-controlled radical coupling reactions has been challenged, and it is relevant to ascertain unequivocally if lignins are optically active. Dimeric lignin fragments, in which the bonds originally formed by the radical coupling steps remain unaltered, have been nisolated from pine wood by derivatization followed by reductive cleavage (the DFRC method). Non-stereospecific chemical reactions that have occurred at other sites in the molecules do not alter chirality at the sites of original coupling, as is proven for phenylcoumarans. Determination of optical activity in these compounds therefore reflects optical activity in the original lignin. Dimers derived from beta-5-, beta-beta-, beta-1-, and beta-O-4-units were examined for optical activity by circular dichroism (CD) and chiral HPLC. In no case was any optical activity detected. CD of beta-5-derived dimers following enantiomeric separation by chiral HPLC demonstrated the sensitivity of the method. As circular dichroism is associated only with UV absorption bands, it was possible to seek optical activity in isolated; restricting the analysis of underivatized lignin isolates to above 250 nm allowed the sensitive 280 nm peak to be scanned while keeping well clear of carbohydrate absorptions. No optical activity could be detected in isolated lignins from pine, kenaf, maize, or a CAD- deficient pine mutant. The negative results from the lignins, and the strong supporting observations from isolated dimers, confirm that representative lignins are not optically active.