Coumaryl alcohols

According to a widely accepted concept, lignin [8068-00-6] may be defined as an amorphous, polyphenoHc material arising from enzymatic dehydrogenative polymerization of three phenylpropanoid monomers, namely, coniferyl alcohol [485-35-5] (2), sinapyl alcohol [537-35-7] (3), and /)-coumaryl alcohol (1). 

Tanahashi M. Takeuchi H. Higuchi T. Dehydrogenative polymerization of 3,5-disubstituted p-coumaryl alcohols. Wood Res. 1976, 61, 44—53. 

Nakatsubo, F. Higuchi, T. Enzymic dehydrogenation ofp-coumaryl alcohol. III. Analysis of dilignols by gas chromatography and NMR spectrometry. Wood Res. 1975, 58, 12-19. 

Kobayashi, T. Taguchi, H. Shigematsu, M. Tanahashi, M. Substituent effects of 3,5-disubstituted p-coumaryl alcohols on their oxidation using horseradish peroxidase-H202 as the oxidant. J. Wood Sci. 2005, 51, 607-614. 

Recent work by Atalla(H) supports the idea that lignin is at least a semi-ordered substance in wood with the plane of the aromatic ring parallel to the cell wall surface. Woody plants synthesize lignin from trans-coniferyl alcohol (pines), trans-sinapyl alcohol 2 (deciduous), and trans-4-coumaryl alcohol 3 by free radical crosslinking initiated by enzymatic dehydrogenation(l2). Structures of these alcohols are given in Figure 1. 

Whilst it is not possible to give a completely detailed structure for lignin, a great deal is known about the molecule. All lignins appear to be polymers of 4-hydroxycinnamyl alcohol (/>-coumaryl alcohol) or its 3- and/or 3,5-methoxylated derivatives, respectively coniferyl and sinapyl alcohol (Figure 3.1). 

The Canadian school has recently shown that the production of the higher lignin precursors, i.e. the /i-coumaryl alcohols or their glucosides, does not proceed via the simple acids shown in Fig. 2 but actually via insoluble esters of same. The esters arc probably activated esters of coenzyme A, but esters of quinic acid analogous to chlorogenic acid arc also feasible in this role 35). 

Nevertheless, it had to be known beforehand that lignin is in fact derived from the -coumaryl alcohols and this information was not in fact accrued from the biochemical work on lignin precursors. Actually... 

Since numerous degradations of hgnin, e.g. ethanolysis according to Hibberi (77) or hydrogenation with a variety of catalysts 72, 77, 119, 135), had shown that it is in fact made up largely of phenylpro-panoid units, it seemed that the theory of its origin from the three -p-coumaryl alcohols could be considered reliable. It remained therefore tO establish the nature of the processes involved in the polymerization. 

The question is, therefore, what happens to the phenoxyl radicals produced from the -coumaryl alcohols when they di-sappear. 

Comparisons made between unpurified spruce milled-wood lignin fractions and unfractionated poljrmers made from coniferjd alcohol alone already revealed the great qualitative similarity between natural and biosynthetically duplicated lignins [see, for example, K. Freudenberg in 94), pp, 125—126]. When a purified lignin fraction is compared tvith an identical fraction made from a mixture of all three -coumaryl alcohols, the resemblance is of course much greater. 

The similarities in the structure and 3neld of the acids obtained by this degradation from natural spruce lignin and a biosynthetic "spruce lignin copolymer (mixed DHP) made in vitro from a mixture of the three -coumaryl alcohols established the general identity of the two preparations 46, 47). 

Figure 49 The precursors of lignin biosynthesis, p-coumaryl alcohol (I), coniferyl alcohol (II) and sinapyl alcohol (III). [65]...

The mechanism was confirmed by enzymatic experiments [20, 21]. A crude enzyme preparation from A. officinalis cultured cells catalyzed the conversion of j9-coumaryl alcohol and j9-coumaroyl CoA to (Z)-hinokiresinol [20], while a crude enzyme preparation from Cryptomeria japonica cultured cells mediated the formation of ( )-hinokiresinol from the same substrates [21]. In addition, both enzyme preparations converted j9-coumaryl j9-coumarate into (Z)-hinokiresinol and ( )-hinokiresinol [20, 21]. Thus, the biosynthesis of hinokiresinol originating from phenylpropanoid monomers was established. 

In gymnosperms, the primary lignin precursors are the two monolignols coniferyl and /7-coumaryl alcohols, while in angiosperms sinapyl alcohol is also a participant in... 

Moreover, no NMR spectral changes were detected as a consequence of treating dehydropolymerizates from [1- C], [2- C] and [3- C]coniferyl alcohol, respectively, or a dehydrogenative copolymer of /7-[/ing-4 -i C]coumaryl alcohol and coniferyl alcohol, at pH 3.0 with racellular P, chrysosporium culture fluid, or purifled lignin peroxidase, in the presence of H2O2 nor was the outcome affected by prior methylation of the substrates (52). Thus the result originally encountered with the purified spruce wood extract (13) is not representative of polymeric lignin-like preparations at all. 

Condensed Structures in p-Hydroxyphenyl-Guaiacyl Type DHP. It has been difficult to determine the exact amount of p-hydroxyphenylpropane units in lignin. This can best be illustrated by an example nitrobenzene oxidation of a DHP prepared by the Zutropfverfahren method from a mixture of p-coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, gave no detectable p-hydroxybenzaldehyde on alkaline nitrobenzene oxidation... 

To determine the reasons for this, DHP s were prepared from a mixture of [ring-2-3H] p-coumaryl alcohol and [or-14C] coniferyl alcohol in the presence of carbohydrates by the procedure described above (35). The LCC fraction so obtained was subjected to combustion, and the exact p-hydroxyphenylpropane guaiacylpropane ratio was determined from the activity of 3H20 and 14C02 released (Figure 2). A portion of the same LCC fraction was also oxidized with nitrobenzene and alkali, and the resulting liberated aromatic aldehydes were then analyzed by HPLC. Results are shown in Table V. 

Figure 2. Dehydrogenative polymerization of a mixture of p-coumaryl alcohol-[ring-2-3H] and coniferyl alcohol-[U-14C], and nitrobenzene oxidation of the DHP to give p-hydroxybenzaldehyde-[ring-2-3H] and vanillin-[formyl-14C].

As can be seen, the molar ratios of p-hydroxyphenyl to guaiacyi units were slightly lower for the DHP s when compared to the original mixtures. This implies that coniferyl alcohol tends to be incorporated into the polymer slightly more readily than p-coumaryl alcohol. On the other hand, the much-reduced ratio of p-hydroxybenzaldehyde to vanillin, liberated during alkaline nitrobenzene oxidation, proved that this DHP contained a larger amount of condensed p-hydroxyphenylpropane units than condensed guaiacyi units. 

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