Lignin units/precursors

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. 

Phenylalanine Ammonia-Lyase. The building units of lignin are formed from carbohydrate via the shikimic acid pathway to give aromatic amino acids. Once the aromatic amino acids are formed, a key enzyme for the control of lignin precursor synthesis is phenylalanine ammonia-lyase (PAL) (1). This enzyme catalyzes the production of cinnamic acid from phenylalanine. It is very active in those tissues of the plant that become lignified and it is also a central enzyme for the production of other phenylpropanoid-derived compounds such as flavonoids and coumarins, which can occur in many parts of the plant and in many different organs (35). Radioactive phenylalanine and cinnamic acid are directly incorporated into lignin in vascular tissue (36). 

Other factors such as temperature (15), plant hormone addition (16), light (17), gravity (14), and pH of the precursor solution (15), also have an effect on the D.S. value. This can best be illustrated by examples (i) growth of P. thunbergii at room temperature (25-30°C) gave a lignin with more condensed units (48-73%) than that obtained at 10°C (33-57%) (11) ... 

Lignin. Lignin is a phenolic substance consisting of an irregular array of variously bonded hydroxy- and methoxy-substituted phe-nylpropane units. The precursors of lignin biosynthesis are p-cou-maryl alcohol (I), coniferyl alcohol (II), and sinapyl alcohol (III). I... 

Typical organic precursors for COS, CS2 and the methylated sulfur gases include methionine and cysteine from proteins and isothiocyanates and thiocyanates from plant secondary metabolites. Methanethiol and DMS are also formed in anoxic freshwater sediments from reactions based on H2S and various methyl donors, for example, methoxylated aromatic acids, such as syringic acid from lignins. The rates of DMS emission per unit area are similar for both the oceans and Sphagnum-AommattA wetlands. Only the area of this peat land limited the relative importance of the latter source. 

Although P-l-units feature prominently in products from various acidolytic treatments, they have been extraordinarily difficult to find by NMR of lignins isolated by dioxane water extraction, but have been reported [28,30,34,59,134,275,276]. In part this was because of overlapping signals in ID and 2D experiments. Ede et al. initially estimated that the level was <2% in pine [28]. It was later found that P-l-units were more efficiently extracted into acetone-water fractions, or by direct derivatization and extraction into acetic anhydride [276]. TOCSY spectra clearly authenticated P-l-units for the first time. The structure is rather convincingly revealed in spectra of an acetylated poplar MWL [50,78,134]. It is likely that this acidic extraction converted other P-l-precursors in lignins to the conventionally described P-l-products, as discussed further below. The most extensive model data is from Miyakoshi and Chen [243,244]. Other data derives primarily from reports on the synthesis of such model compounds [250,277,278]. 

Sporopollenin, the polymeric material forming the protective coatings of spores and pollens, may contain carotenoid and/or carotenoid ester structures such as astaxanthin dipalmitate. However, the major part of sporopollenin is synthesized from unbranched aliphatic chains, probably derived from fatty acids. Polyhydroxy-benzene units similar to lignin precursors (see Section 2.5.1) have also been identified. Sporopollenin is found in the pollen grains of angiosperms and gymnosperms,... 

The pathways of carbon flow in eukaryotes are more complex than in prokaryotes, mainly because the C2 units produced within mitochondria can be exported (e.g. for lipid biosynthesis) only as part of a citrate unit produced in the citric acid cycle. The branch point in carbon flow to either isoprene or acetate can lead to additional isotopic fractionation. In general, it seems that n-alkyl hpids in a particular organism are depleted in 13C by c. 1.5%o relative to isoprenoids produced from the same substrate (Hayes 1993). In higher plants, the phenolic precursors of lignin derive from glucose (Fig. 2.29), so it is not surprising that the carbon isotopic composition of lignin reflects the major photosynthetic pathway involved (C3 or C4 Benner et al. 1987). 

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