HOMO-NUCLEAR AND HETERO-NUCLEAR 2D NMR TECHNIQUES - UMAMBIGUOUS STRUCTURAL PROBES FOR NONCYCLIC BENZYL ARYL ETHERS IN SOLUBLE LIGNIN SAMPLES

A number of two-dimensional NMR experiments are evaluated with respect to their ability to provide unambiguous evidence for the presence or absence of non-cyclic benzyl aryl ether (alpha-O-4) structures in solu ble lignin samples. The most suitable techniques, in terms of both sen sitivity and structural information content were the homonuclear Hartm ann-Hahn (HOHAHA) and heteronuclear multiple quantum coherence (HMQC) techniques. By spiking an acetylated Pinus radiata MWL sample with an oligomeric alpha-O-4 ether model compound, it was possible to determin e a detection limit of <0.3 structures per 100 lignin C9 units for bot h the HOHAHA and HMQC techniques. From this, and other work, it can be shown that if alpha-O-4 structures are present in MWL samples, they a re present at a level below the detection limit of these experiments. The implications of these results are discussed in terms of lignin bio synthesis and reactivity.