The chemical structure of milled-wood lignins fromEucalyptus globulus,E. nitens,E. maidenii,E. grandis, andE. dunniiwas investigated. The lignins were characterized by analytical pyrolysis, thioacidolysis, and 2D-NMR that confirmed the predominance of syringyl over guaiacyl units and only showed traces ofp-hydroxyphenyl units.E. globuluslignin had the highest syringyl content. The heteronuclear single quantum correlation (HSQC) NMR spectra yielded information about relative abundances of inter-unit linkages in the whole polymer. All the lignins showed a predominance of β-O-4′ ether linkages (66–72% of total side-chains), followed by β-β′ resinol-type linkages (16–19%) and lower amounts of β-5′ phenylcoumaran-type (3–7%) and β-1′ spirodienone-type linkages (1–4%). The analysis of desulfurated thioacidolysis dimers provided additional information on the relative abundances of the various carbon-carbon and diaryl ether bonds, and the type of units (syringyl or guaiacyl) involved in each of the above linkage types. Interestingly, 93–94% of the total β-β′ dimers included two syringyl units indicating that most of the β-β′ substructures identified in the HSQC spectra were of the syringaresinol type. Moreover, three isomers of a major trimeric compound were found which were tentatively identified as arising from a β-β′ syringaresinol substructure attached to a guaiacyl unit through a 4-O-5′ linkage.

In situ analysis of lignin by 2D NMR of whole plant material was carried out by swelling finely ball-milled samples in deuterated dimethylsulfoxide (DMSO-d₆ ) and sonicated so that a gel was formed in the NMR analysis tube. Solution HSQC NMR spectra of different plant materials representative for hardwood (Eucalyptus globulus), softwood (Picea abies), and non-woody plants (Agave sisalana) are presented here. The spectra show signals corresponding to those of the main plant constituents, such as lignin and polysaccharides. The lignin signals were assigned by comparing the HSQC spectra of the whole plant materials with the HSQC spectra of their respective milled-wood lignins (MWLs). In general terms, the major lignin structural features, such as the relative abundances of the main lignin substructures, the syringyl/guaiacyl ratios and the extent of γ-acetylation of the lignin side-chain observed in the HSQC spectra of the whole plant materials, matched those obtained from the HSQC spectra of the isolated MWLs. Therefore, this technique, which needs only minor amounts of lignocellulosic material and minimal sample preparation, can be useful for the rapid screening of plant lignins without the need for tedious and time-consuming lignin isolation procedures.

This perspective article is focused on the presentation of the latest advances in NMR methods and applications that are behind the exciting achievements in the understanding of glycan receptors in molecular recognition events. Different NMR-based methodologies are discussed along with their applications to scrutinize the conformation and dynamics of glycans as well as their interactions with protein receptors.