Homo- and hetero-nuclear 2D NMR techniques: Umambiguous structural probes for non-cyclic 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 (α-O-4) structures in soluble lignin samples. The most suitable techniques, in terms of both sensitivity and structural information content were the homonuclear Hartmann-Hahn (HOHAHA) and heteronuclear multiple quantum coherence (HMQC) techniques. By spiking an acetylated Pinus radiata MWL sample with an oligomeric α-O-4 ether model compound, it was possible to determine a detection limit of <0.3 structures per 100 lignin C9 units for both the HOHAHA and HMQC techniques. From this, and other work, it can be shown that if α-O-4 structures are present in MWL samples, they are present at a level below the detection limit of these experiments. The implications of these results are discussed in terms of lignin biosynthesis and reactivity.