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

✍ Scribed by Stéphane Besombes; Jean-Pierre Utille; Karim Mazeau; Danielle Robert; François R. Taravel

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 258 KB
• Volume: 42
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.1317

✦ Synopsis

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/d__T__) and coupling constants (^3^J~HCOH~) were measured in aprotic and protic solutions: DMSO‐d~6~, acetone‐d~6~ and acetone‐d~6~–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 ^1^H, ^1^H and ^1^H, ^13^C 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.