Syringyl units

It weis found that hardwood lignins yielded both guaiacyl and syringyl units on oxidation, whereas softwood lignins yielded only guaiacyl units. Hibbert and co-workers (20), (21) distinguished perennial plant lignins... 

Noguchi et al. (8) synthesized precursors of lignin biosynthesis (i.e., coniferyl and sinapyl alcohols) in which the side chain and methoxy groups were doubly labeled by and H, prepared the DHP (synthetic lignin) from them, and treated the DHP with the laccase. These results showed that about 25% of Hs of the a- and -positions were removed in a ratio of one to one, respectively, and that the methoxyl group of a syringyl unit was de-methylated easier than that of a guaiacyl. 

Using the assigned 8 ranges, the number of each type of aromatic unit may be estimated from the relative number of protons in the aromatic region of the NMR spectra alone. This has been done for several acetylated preparations, and the results are summarized in Table IV. The total number of condensed guaiacyl plus syringyl units is estimated at... 

The syringyl unit, derived from the frans-sinapyl alcohol, which is present with the guaiacyl unit in hardwoods... 

As briefly described above, proposed biosynthetic pathways to the monolignols follow a metabolic grid, with a number of parallel and redundant pathways for the synthesis of the major lignin components, namely guaiacyl and syringyl units. Currently, the methylation steps in the lignin biosynthetic pathway are believed to follow two main branches dependent on COMT and CCoAOMT,... 

It has been suggested that the major part of condensation processes occurs at the unoccupied C-5 position of phenolic units. Thus, in isolated MWL preparations about half of the C-5 positions are unsubstituted, while in isolated kraft lignins only about one third of these positions remain free. The syringyl units of hardwood lignins cannot, of course, undergo condensation reactions of this type. 

The lignin content can be estimated from the spectrum by measuring the fractional area, A, of a band of signals between chemical shifts of 141 and 159 parts per million (ppm) relative to the total area of the spectrum. This band is assigned to C-3 and C-4 of guaiacyl units or C-3 and C-5 of syringyl units the fact that it is associated with just two carbon atoms of each structural unit means that the method can be used for both softwoods and hardwoods. 

Fig. 4.7.11. Comparison of pyrograms from Py-GCFID experiments. H, G, and S indicate p-hydroxyphenyl, guaiacyl, and syringyl units. Compounds corresponding to numbered peaks are listed in Table 4.7.1...

For synthetic polymers composed of 4-hydroxylphenyl, guaiacyl, and syringyl units and for polyurethanes containing lignin, the Tg values are affected by factors such as structure, molecular flexibility, and molecular weight of the other components of the polymers (Hatakeyama and Kanetsuna 1989, Yoshida et al. 1990). 

Aromatic protons in syringyl units (certain vinyl protons)... 

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