Abstract
While syringyl units are the most abundant monolignols in hardwood lignin, their phenolic (i.e. hydroxyl) end group concentration has not been measured. In two uniformly
C-enriched young hardwoods, from beech and oak, the syringyl units were quantitatively investigated by advanced solid-state
C NMR. Small signals of OH-terminated syringyl units were resolved in spectrally edited two-dimensional
C-
C NMR spectra of the two hardwoods. Their distinct peak positions predicted based on literature data were validated via the abundant OH-terminated syringyl units in hydrolyzed
C-beechwood. In a two-dimensional
C-
C exchange spectrum with diagonal-ridge suppression, a well-resolved peak of phenolic syringyl units was observed at the characteristic C-H peak position of syringyl rings, without significant overlap from guaiacyl peaks. Accurate
C chemical shifts of regular and end-group syringyl units were obtained. Through spectrally edited 2D NMR after
H inversion recovery, phenols of condensed tannin complexed with arginine were carefully analyzed and shown to overlap minimally with signals from phenolic syringyl units. The local structure and resulting spin dynamics of ether (chain) and hydroxyl (end-group) syringyl units are nearly the same, enabling quantification by peak integration or deconvolution, which shows that phenolic syringyl end groups account for 2 ± 1 % of syringyl units in beechwood and 5 ± 2 % in oakwood. The observed low end-group concentration needs to be taken into account in realistic molecular models of hardwood lignin structure.