Conformational study of a guaiacyl β‐O‐4 lignin model compound by NMR. Examination of intramolecular hydrogen bonding interactions and conformational flexibility in solution

Abstract

Intramolecular H-bonding interactions were investigated in solution for the threo and erythro diastereomeric forms of a guaiacyl β-O-4 lignin model compound by using the NMR data obtained from hydroxyl protons. Temperature coefficients of the chemical shifts (dδ/dT) and coupling constants (³J_HCOH) were measured in aprotic and protic solutions: DMSO-d₆, acetone-d₆ and acetone-d₆–water. The NMR parameters do not support the existence of strong and persistent intramolecular H-bonds that could participate in the stabilization of the guaiacyl β-O-4 structure in solution, but instead indicate that intermolecular H-bonds to solvent predominate. 1D NOE experiments nevertheless revealed the presence of a direct chemical exchange between the hydroxyl protons, suggesting the possible existence of weak and transient intramolecular H-bonding interactions. The conformational flexibility of the threo structure was also investigated in acetone solution from the measurement of long-range ¹H, ¹H and ¹H, ¹³C coupling constants and from NOESY experiments. The NMR data are not consistent with any single conformation, indicating that different conformers co-exist in solution. The experimental results support the conformational flexibility predicted by molecular dynamics simulations performed in a previous study. Finally, both experimental and theoretical approaches indicate that weak intramolecular H-bonds can exist transiently in solution, breaking and reforming as the β-O-4 molecule undergoes conformational interconversion, but cannot be invoked as possible means of conferring rigidity to the β-O-4 structure. Copyright © 2004 John Wiley & Sons, Ltd.