The solution-state NMR experiments were all performed on Bruker Advance II 800 MHz spectrometer equipped with a cryogenically cooled probe (TCI) with z-axis gradients (Bruker BioSpin, Billerica, MA). NMR data were processed and analyzed with Topsin 2.1.6 software (Bruker). The lignin was dissolved in DMSO-d6 and transferred into 5-mm NMR microtubes for one-dimensional 1 H NMR, one-dimensional 13C NMR and two-dimensional 13C-1 H heteronuclear
single quantum coherence spectroscopy (HSQC) [20]. The conditions for one-dimensional 1 H NMR spectra were as follows: 32 scans, acquisition time of 0.99 s, and relaxation delay of 8.00 s. The conditions for one-dimensional 13C NMR spectra were as follows: 4096 scans, acquisition time of 0.23 s and relaxation delay of 8 s. The conditions for two dimensional HSQC spectra were as follows: 32 scans, acquisition time of 0.33 s and relaxation delay of 0.90 s. All experiments were carried out at 333 K.