Table 4 Semiquantitative 2D-HSQC NMR spectra analysis of structural characteristics of lignin

Lignin interunit linkages^a (%)

 β-Aryl ether

80.2

87.2

96.0

 Phenylcoumaran

1.2

1.3

1.7

 Resinol

7.3

3.5

0.7

 Cinnamyl alcohol

11.3

8.0

1.6

 Total

100

100

100

 Condensation degree (β–β/β-O-4)

0.091

0.040

0.007

Lignin aromatic units^b

 H (%)

20.1 (124.6)

11.8 (10.1)

11.9 (6.2)

 G (%)

55.1 (340.7)

65.4 (56.2)

53.2 (27.8)

 S (%)

24.8 (153.3)

22.7 (19.5)

35.0 (18.3)

 Total

100 (618.6)

100 (85.8)

100 (52.3)

 S/G ratio

0.45

0.35

0.66

p-Hydroxycinnamates^c

 p-Coumarate (%)

31.1

23.4

18.6

 Ferulates (%)

24.8

15.2

12.4