Pretreatment with laccase and a phenolic mediator degrades lignin and enhances saccharification of Eucalyptus feedstock

Table 5 Lignin units and side-chains forming different inter-unit linkages (and end-groups) from the heteronuclear single quantum correlation spectra of cellulolytic enzyme lignin preparations from treated wood and controls

Lignin structure	Wood	Control	Laccase only	Laccase and methyl syringate
*Lignin units*
Syringyl (S) (% total)	77	81	83	93
Guaiacyl (G) (% total)	23	19	17	7
C_α-oxidized S units (Sʹ) (% S)	6	10	18	35
S/G ratio	3.4	4.3	4.9	14.1
*Chains forming inter-unit linkages and end-groups*
β-O-4ʹ Alkyl-aryl ethers (A) (% S + G)	58 (82)	61 (84)	56 (85)	49 (92)
Phenylcoumarans (B) (% S + G)	2 (3)	1 (1)	1 (2)	0
Resinols (C) (% S + G)	9 (13)	8 (11)	6 (9)	2 (4)
Spirodienones (D) (% S + G)	2 (3)	2 (3)	2 (3)	1 (2)
Cinnamyl end-groups (I) (% S + G)	1 (1)	1 (1)	1 (1)	0 (1)
Total (% S + G)	71 (100)	73 (100)	66 (100)	53 (100)
C_α-oxidized β-O-4ʹ ethers ( A_ox) (% A)	2	2	5	14

The lignin composition (S and G units), the amount of C_α-oxidized S units (with respect to total S units), the S/G ratio, the abundance of side chains forming different inter-unit linkages (A- D) and cinnamyl end-groups (I) per 100 phenylpropane units, and the relative abundance of C_α-oxidized β-O-4ʹ ethers (with respect to total β-O-4ʹ ethers) were determined from the heteronuclear single quantum correlation spectra of cellulolytic enzyme lignins isolated from the eucalypt wood treated with laccase (50 U · g^-1) and 3% methyl syringate and laccase alone, compared with a control without enzyme and the initial wood. The percentage of side chains forming each linkage type (or end-group) are also indicated (parentheses).

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