The DFRC Method for Lignin Analysis. 6. A Simple Modification for Identifying Natural Acetates on Lignins

By modifying the derivatization followed by reductive cleavage (DFRC) procedure to be completely acetate-free, the presence and regiochemistry of natural acetates on lignins can be determined. Derivatization is conducted with propionyl bromide in propionic acid instead of acetyl bromide in acetic acid; reductive cleavage uses zinc in propionic acid instead of in acetic acid, and the final derivatization step uses propionic anhydride instead of acetic anhydride. Applying the modified procedure to lignins or cell walls from kenaf bast fibers proved that their lignins are highly γ-acetylated, mainly on syringyl units, supporting results of previous NMR work. Application to isolated hardwood lignins or cell walls confirmed reported low-level acetylation and elucidated its γ-regiochemistry. The DFRC method therefore provides a convenient screen for many types of lignin acylation.