Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/3/1999
Publication Date: N/A
Citation: Interpretive Summary: A major component in all terrestrial plants, lignin is a polymer that limits digestion of plants by animals, and must be removed from wood to make paper. It is different than many other biological polymers in that its structure is not only complex but non-uniform - it is possible that no two lignin molecules in a plant are the same. Recently, how lignin is made eby the plant has become of elevated interest, particularly with bioengineering approaches to reducing or changing the lignin in plants for improved utilization. We sought to test whether lignin derives from carefully controlled enzymatic reactions or more random chemical reactions by examining its optical activity. Enzyme-controlled reactions usually produce products that are optically active; that is where a molecule is only one of two possible non-superimposable mirror images, rather like our left and right hands. Chemical reactions, by contrast, produce both possibilities and the products are not optically active. Using two methods, we find no evidence for any optical activity in a lignin and a variety of lignin isolates. We therefore conclude that lignins are produced in the plant by non-controlled reactions. This produces a variety of structures which are beneficial for defense purposes. It also provides us with significant opportunities to engineer lignin to have more desirable properties and improve our utilization of valuable plant resources.
Technical Abstract: Lignins have long been assumed to be optically inactive, but there has been little rigorous proof. Recently, their accepted derivation from non- (enzymatically)-controlled radical coupling reactions has been challenged, and it is relevant to ascertain unequivocally whether lignins are, or are not, optically active. Dimeric lignin fragments, in which the bonds originally formed by the radical coupling steps remain unaltered, were isolated from plant material by 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. No optical activity could be detected in dimers derived from beta-5-, beta-beta-, beta-l- or beta-0-4-units by circular dichroism (CD) and chiral HPLC. CD of beta-5-derived dimers following enantiomeric separation by chiral HPLC readily demonstrated the sensitivity of the method. optical activity could not be detected in isolated lignins from various plants either. For lignins, however, optical activity could only be detected above about the 5% level under the conditions used. Nevertheless, the negative results from the lignins, and the strong supporting observations from isolated dimers, confirm that representative lignins are not optically active.