Accessing the position-specific O/O ratios of lignin monomeric units from higher plants with highly selective hydrogenolysis followed by GC/Py/IRMS analysis.

The O/O of lignin at bulk, molecular and positional levels can be used to extract valuable information about climate, plant growth environment, plant physiology, and plant metabolism. Access to the individual oxygen isotope compositions (δO) in the lignin monomeric units is, however, challenging as depolymerization of lignin to release the monomeric units may cause isotope fractionation. We have developed a novel method to measure the δO of the three oxygens (O-3, O-4 and O-5) attached to the aromatic ring of the monomeric units (bearing no oxygen in their side chains) releasable by highly selective WC/AC (tungsten carbide supported by activated carbon)-catalyzed hydrogenolysis of lignin. O-4 is obtained by measuring the δO of H-type monomeric unit, while O-3 and O-5 can be calculated following isotope mass balance between H, G and S-type monomeric units measurable simultaneously with GC/Py/IRMS (gas chromatography-pyrolysis-isotope ratio mass spectrometry). The measurement precisions are better than 1.15 mUr and 4.15 mUr at molecular and positional levels, respectively. It was shown that there were a δO > δO > δO isotopic order in the herbaceous plant lignin and an (inclusive) opposite order in woody plant lignin. Such differences in isotopic order is likely to be caused by the fact that both L-tyrosine, which carries an O-enriched leaf water signal, and L-phenylalanine, which carries mainly a molecular O isotopic signal, serve as the precursors for lignin biosynthesis in herbaceous plants while only the latter serves as precursor for lignin biosynthesis in woody plants. We have highlighted the potential application of such molecular and positional levels isotopic signals in plant physiological, metabolic, lignin biosynthetic and climate studies.