Complete NMR assignment and analysis of molecular structural changes of β–O–4 lignin oligomer model compounds in organic media with different water content

Abstract

Abstract The conformation of lignin in solvents is major key factors governing the physicochemical properties of aromatic polymers. However, the conformational changes of lignin in good and poor solvents is not clearly understood at the molecular level. In this study, short- (DP 2.77) and long-chain (DP 4.49) lignin oligomer model compounds composed solely of interunit β–O–4 bonds with a narrow polydispersity were synthesized, and their NMR spectra were recorded to evaluate the molecular structural changes induced by addition of water to an organic solvent. The spectral signals were completely assigned in DMSO-d 6 and D2O by applying 2D 1H–13C HSQC, HMBC, and long-range heteronuclear single quantum multiple bond correlation (LR-HSQMBC). The conformation of the long- and short-chain lignin models were analyzed by 2D 1H–1H ROESY. In all the solvent systems, consisting of DMSO-d 6 and containing 0–90% volume of acetic acid-d 4 buffer in D2O (pD 5.0), the lignin models were found to have folded conformations, but more compact structures were observed in D2O compared with DMSO-d 6.