Preparation of synthetic lignins with superior NMR characteristics via isotopically labeled monolignols

Abstract

Synthetic lignins are particularly valuable for studying aspects of lignification, plant cell wall cross-linking, and lignin structure. If they are not too highly polymeric, they are soluble in normal lignin solvents and amenable to solution-state NMR studies. However, in the application of inverse-detected correlation experiments, particularly the popular HMQC and HMBC experiments, the spectra have annoying T₁-noise ridges. These artifacts make it difficult to locate correlation peaks that are near the methoxy signal in the proton dimension. One solution is to use gradient-enhanced NMR but that requires additional hardware that is not yet ubiquitous. An alternative is to produce monolignols in which the atoms of the methoxy group are NMR-invisible. We have accomplished this by preparing coniferyl and sinapyl alcohols using ¹³C-depleted deuterated methyl iodide (¹²C²H₃I). The methods, which incorporate steps simpler than have been used previously for labeled monolignols, are sufficiently low cost and straightforward that these monomers can be utilized for any synthetic lignins destined for NMR studies. The NMR spectra of lignins derived from these ‘methoxy-less’ monomers are markedly superior to their normal-monomer counterparts. Several popular NMR experiments are illustrated for synthetic lignins derived from normal vs. isotopically labeled coniferyl alcohol, along with some useful experiments that have not been seen in lignin-related publications to date.