TABLE 4.
Structural characteristics (lignin inter-unit linkage types, end-groups, γ-acylation, aromatic units, and S/G ratio, cinnamate content, and tricin content) from integration of 1H/13C correlation signals in the HSQC spectra of the whole cell walls (CW) of rice husks and rice straw and their isolated MWLs.
| Rice husks |
Rice straw |
|||
| CW | MWL | CW | MWL | |
| Lignin inter-unit linkages (%) | ||||
| β–O–4′ aryl ethers (A/A′) | – | 65 | – | 78 |
| β–5′ Phenylcoumarans (B) | – | 23 | – | 12 |
| β–β′ Resinols (C) | – | 4 | – | 4 |
| β–β′ Tetrahydrofurans (C′) | – | 0 | – | 1 |
| 5–5′ Dibenzodioxocins (D) | – | 5 | – | 4 |
| β–1′ Spirodienones (F) | – | 3 | – | 1 |
| Lignin end-groupsa | ||||
| Cinnamyl alcohol end-groups (I) | – | 6 | – | 6 |
| Cinnamaldehyde end-groups (J) | – | 5 | – | 5 |
| Lignin side-chain γ-acylation (%) | – | 10 | – | 12 |
| Lignin aromatic unitsb | ||||
| H (%) | 5 | 7 | 7 | 5 |
| G (%) | 85 | 81 | 71 | 71 |
| S (%) | 10 | 12 | 22 | 24 |
| S/G ratio | 0.12 | 0.15 | 0.31 | 0.34 |
| p-Hydroxycinnamatesc | ||||
| p-coumarates (pCA) | 28 | 12 | 39 | 16 |
| ferulates (FA) | 19 | 4 | 53 | 4 |
| pCA/FA ratio | 1.5 | 3.0 | 0.7 | 4.0 |
| Tricin (T)c | 5 | 7 | 18 | 27 |
aExpressed as a fraction of the total lignin inter-unit linkage types A–F.bRelative percentages (H + G + S = 100).cpCA, FA, and T molar contents as percentages of total lignin content (H + G + S = 100).