Polycyclic Aromatics Observed in Enzymatic Lignin by Spectral Characterization and Ruthenium Ion-Catalyzed Oxidation

It is generally considered that lignin is a three-dimensional amorphous polymer consisting of methoxylated phenylpropane structures. However, high yields of monomer structural units of lignin cannot be obtained through various ways, which inspired us to gain insights into the structures of lignin. Herein, enzymatic lignin (EL) was directly characterized by a solid-state ¹³C nuclear magnetic resonance spectrometer and Fourier transform infrared spectrometer and then subjected to ruthenium ion-catalyzed oxidation. According to the spectral characterization, it can be inferred that multi-ring aromatic clusters exist in EL because of the aromatic bridgehead carbon ratio of 0.136. Based on the results of ruthenium ion-catalyzed oxidation of the EL, it can be deduced that (1) double- and triple-aromatic ring clusters exist in the EL besides the traditional phenylpropane single-aromatic ring clusters, and (2) some aromatic rings with long-alkyl chain substituents exist in the EL, which is quite different from the traditional cognition of lignin. This investigation provides a new insight into the structure of EL.