On the distribution of the diastereomers of the structural elements in lignins: the steric course of reactions mimicking lignin biosynthesis

Abstract

Stereochemical studies on the formation of the diastereomers of arylglycerol-fl-aryl ether structures during lignin biosynthesis have been carried out with model compounds. The addition of water to quinone methides of the fl-syringyl ether type gives arylglycerol fl-syringyl ethers with a predominance of the erythro isomer when the pH of the medium is low. Since erythro forms of arylglycerol fl-syringyl ethers are prevalent in hardwood lignins, this indicates that the pH of the medium in which lignin biosynthesis occurs is lower than has been assumed until now. Equilibration studies with non-phenolic model compounds of the arylglycerol-fl-guaiacyl ether and fl-syringyl ether types under acidolysis conditions indicate that the erythro predominance observed in the syringyl ethers in lignins does not correspond to equilibrium conditions. A remarkable resistance to acidolysis is observed in the model compounds of etherified syringylglycerol fl-syringyl ether type.

NMR characterization of lignins in Arabidopsis altered in the activity of ferulate 5-hydroxylase

“…The major additional data revealed by the HSQC-TOCSY spectra relate to the stereochemistry of ␤-ether units A. Consistent with its syringyl-rich nature (22,36), ␤-ether units A in the C4H-F5H lignin (Fig. 2f ) are Ϸ80% of erythro-stereochemistry.…”

Characteristics of guaiacyl-syringyl lignin in reaction wood in the gymnosperm Gnetum gnemon L.

“…In gymnosperm lignins the erythro and threo forms are in a similar range, but in angiosperm lignins the erythro form dominates as demonstrated by NMR (Lundquist 1979;Nimz et al 1984;Bardet et al 1998) and ozonation studies (Matsumoto et al 1986;Akiyama et al 2005;Akiyama et al 2015). The predominance of the erythro form is the result of stereo-selective water addition to a quinone methide intermediate during biosynthesis, and the selectivity is different for a S-or a G-type aromatic ring ( Figure 6) (Brunow et al 1993). The proportion of the erythro form (erythro ratio) was highest at the usW (180°) of the leaning G. gnemon stem and lowest at the lsW (0°), as was the case for the syringyl ratio (Figure 5a1).…”

Profiling of Oligolignols Reveals Monolignol Coupling Conditions in Lignifying Poplar Xylem

“…Together, the oligolignol structures in the xylem extract are in agreement with coupling conditions favoring endwise coupling. Both threo-and erythro-b-aryl ethers are also found in lignin, but, in contrast to the xylem-extracted oligolignols, the erythro forms of the b-aryl ether linkages predominate in angiosperm lignins (Brunow et al, 1993;Akiyama et al, 2003). Because gymnosperm lignins, composed of G units, contain approximately equal amounts of threo and erythro configurations and because the threo/erythro ratios correlate inversely with the S:G ratios in dicots (Akiyama et al, 2003), the preponderance of the erythro form has been attributed to the presence of b-syringyl ether structures in angiosperm lignins.…”

“…Because gymnosperm lignins, composed of G units, contain approximately equal amounts of threo and erythro configurations and because the threo/erythro ratios correlate inversely with the S:G ratios in dicots (Akiyama et al, 2003), the preponderance of the erythro form has been attributed to the presence of b-syringyl ether structures in angiosperm lignins. Both in vivo and in vitro, 8-O-4-guaiacyl ethers and 8-O-4-syringyl ethers in lignin are produced with approximately 50:50 and approximately 25:75 threo/erythro ratios, respectively, whereas their equilibrium ratios are nearly equal (Brunow et al, 1993;Ralph et al, 2004b). The reason for the apparent threo-isomer predominance in the xylem oligolignol fraction is currently not clear, unless for some reason erythro-isomers couple (to higher oligomers) more rapidly.…”