Supramolecular self-assembled chaos: polyphenolic lignin's barrier to cost-effective lignocellulosic biofuels

Abstract

Phenylpropanoid metabolism yields a mixture of monolignols that undergo chaotic, non-enzymatic reactions such as free radical polymerization and spontaneous self-assembly in order to form the polyphenolic lignin which is a barrier to cost-effective lignocellulosic biofuels. Post-synthesis lignin integration into the plant cell wall is unclear, including how the hydrophobic lignin incorporates into the wall in an initially hydrophilic milieu. Self-assembly, self-organization and aggregation give rise to a complex, 3D network of lignin that displays randomly branched topology and fractal properties. Attempts at isolating lignin, analogous to archaeology, are instantly destructive and non-representative of in planta. Lack of plant ligninases or enzymes that hydrolyze specific bonds in lignin-carbohydrate complexes (LCCs) also frustrate a better grasp of lignin. Supramolecular self-assembly, nano-mechanical properties of lignin-lignin, lignin-polysaccharide interactions and association-dissociation kinetics affect biomass deconstruction and thereby cost-effective biofuels production.

Figure 1

Structures of phenols relevant to lignin. 1, guaiacol; 2, vanillin; 3, thymol; 4, eugenol; 5, isoeugenol; 6, phenylalanine; 7, tyrosine; 8, veratryl alcohol; 9, veratraldehyde; 10, vanillyl alcohol; 11, cinnamyl alcohol; 12, cinnamaldehyde; 13, cinnamic acid; 14, p-coumaric acid; 15, caffeic acid; 16, ferulic acid; 17, sinapic acid; 18, 5-hydroxyferulic acid; 19, p-coumaraldehyde; 20, caffeoyl aldehyde; 21, coniferaldehyde; 22, p-coumaryl alcohol; 23, 5-hydroxyconiferaldehye; 24, caffeoyl alcohol; 25, sinapaldehyde; 26, coniferyl alcohol; 27, 5-hydroxyconiferyl alcohol; 28, sinapyl alcohol; 29, (+)-pinoresinol; 30, matairesinol; 31, (-)-secoisolariciresinol. Phenols 29 to 31 are lignans.

Figure 2

Overview of the shikimate pathway for the biosynthesis of phenylalanine and tyrosine.

Figure 3

Outline of enzymes/pathways involved in monolignol biosynthesis. Enzymes are enclosed in ellipses while substrates and products are boxed. Phenylpropanoid pathway is shown in blue and monolignols biosynthesis in red. Arrows indicate sequential enzymatic steps. Abbreviations are (order of appearance): PAL, phenylalanine ammonia lyase; C4H, cinnamate 4-hydroxylase; TAL, tyrosine ammonia lyase; 4CL, 4-coumarate-CoA ligase; CCR, hydroxycinnamoyl-CoA reductase; CAD, coniferyl alcohol dehydrogenase; SAD, sinapyl alcohol dehydrogenase; HCT, p-hydroxycinnamoyl-CoA:quinate shikimate p-hydroxycinnamoyl-CoA transferase; C3H, p-coumarate 3-hydroxylase; CCoAOMT, caffeoyl-CoA O-methyltransferase; F5H, ferulate 5-hydroxylase (coniferaldehyde 5-hydroxylase); COMT, caffeic acid/5-hydroxyferulic acid O-methyltransferase.

Figure 4

Coniferyl alcohol and its radicals. 1, coniferyl alcohol; 2 and 4 are radicals; 3 and 5 are quinone methide radicals. First step is coniferyl alcohol radicalization by enzymatic dehydrogenation.

Figure 5

Lignin crosslinks. 1, 8-O-4/β-O-4 phenylpropane β-arylether; arylglycerol-β-arylether; ~50% softwood lignin; 2, dibenzodioxocin; ~20% softwood lignin along with 5-5 links; 3, phenylpropane α-arylether; ~7% softwood lignin; 4, phenylcoumaran; ~10% softwood lignin; 5, 5-5, biphenyl; ~20% along with dibenzodioxocin; 6, 5-O-4 biphenylether; ~6% softwood lignin; 7, 4-O-5 diarylether; 8, β-β, pinoresinol; ~3% softwood lignin; 9, 1,2-diarylpropane-1,3-diol; ~8% softwood lignin.

Figure 6

Important plant polysaccharides. 1, cellulose, β-1,4-D-glucose (linear); 2, softwood galactoglucomannan, β-D-1,4-glucose-mannose (linear) and α-D-1,6-galactose (branch); 3, softwood xylan, arabinoglucuronoxylan, β-1,4-xylose (linear) and C₂-4-O-methyl-α-D-glucuronic acid, C₃-α-L-arabinose (branches); 4, hardwood xylan, glucuronoxylan, 1,4-β-D-xylose (linear) and α-1,2-4-O-methyl-α-D-glucuronic acid (branch); 5, hardwood glucomannan, β-D-glucose and β-D-mannose are alternately in β-1,4 linkages (linear); 6, pectin, poly-α-1,4-D-galacturonic acid.

Figure 7

LCC Bonds. 1, benzyl ether; 2, benzyl ester; 3, phenyl glycoside; 4, acetal.

Figure 8

Illustration of a plant cell wall. The various features of the plant cell wall described above are shown including the relative thickness of the various layers and the relative abundance and specific localization of the various cell wall components, such as pectin, cellulose, hemicellulose, lignin and protein. The relative contributions of the three major monlignols to the lignin in the various layers are also indicated. The cell wall-bound enzymes might participate in the various steps of lignification.

Figure 9

Schematic of supramolecular self-assembly of lignin and lignin-carbohydrates interactions.

Figure 10

A. Schematic of a coastline demonstrating the concept of a natural fractal’s self-similarity. Note the similarity of contours at various levels of magnification. B. Schematic of a dimensional fractal. Notice the increase in Df with increasing levels of magnification (shown inside the circled areas).