SELECTIVE C-13 ENRICHMENT OF SIDE-CHAIN CARBONS OF OLEANDER LIGNIN TRACED BY C-13 NUCLEAR-MAGNETIC-RESONANCE

As one of the nondestructive analytical methods, carbon 13 nuclear mag netic resonance (C-13-NMR) spectroscopy has been used widely in lignin chemistry. However, serious difficulties were encountered in the assi gnments of signals of lignin carbons because of their extensive overla ps and low natural abundance of carbon 13. To overcome these difficult ies, lignin precursors, specifically C-13-enriched at side chain carbo ns (syringin-[alpha-C-13], syringin-[beta-C-13], and syringin-[gamma-C -13]) were synthesized, and these lignin precursors were administered to cut shoots of a growing oleander (Nerium indicum Mill.) tree to pre pare the lignin in which side chain carbons of syringylpropane units w ere enriched with carbon 13 in situ. Milled wood lignins (MWLs) were p repared from the newly formed xylems. In the C-13-NMR spectra of the C -13-enriched MWLs, enhanced signals corresponding to alpha-, beta-, an d gamma-carbons of syringylpropane units, were assigned exactly and le d us to an easy elucidation of the structure of oleander lignin. It wa s shown that syringylpropane nuits are connected with guaiacyl units t hrough linkages including at least beta-O-4 and beta-5 bands.