Advanced structural analysis of lignin - a strategy for improving functionalisation of lignin

Lignin is the second most abundant of all the natural polymers that comprise plant biomass. Its challenging structural and chemical properties still present the main obstacle to its widespread use in chemical, pharmaceutical, and medicinal applications. An rather exact knowledge about i) its chemical features in terms quantitative data for the intrinsic functional groups, ii) its structural features in terms of quantitative data regarding interunit bonding motifs and non-phenolic end-groups, as well as iii) its key molecular mass data, namely the number average molecular weight Mn, the weight average molecular weight Mw, and the polydispersity PD, are the prerequisites for achieving reasonable tailored chemical functionalisation for targeted high value-added applications. As part of our on-going efforts to develop reliable and easy-to-adopt protocols and general data analysis schemes for the characterisation of the plant-derived polyphenols lignins and tannins, as preparation of their chemical modification for targeted applications, we systematically evaluated methods widely used for lignin characterisation, focussing on elements that normally give rise to misunderstanding, misinterpreting and false results. We compared different advanced quantitative NMR techniques (QQ-HSQC, HSQC0, 31P NMR) for the analysis of lignins, and quantified often under-estimated error sources in the chromatographic analyses, such as i) the nature of calibration of the analytical set-up, the influence of the nature of the detector used for recording, and the effect on the molecular masses caused by solubility-enhancing functionalisation methods used in lignin chromatography. By combining and correlating the different analyses, we obtain unprecedented levels of accuracy in the structural and chemical characterisation of lignins and tannins.