Coniferyl radicals

Combination of two Rb forms of coniferyl radicals (Fig. 5) gives rise to the transient double -quinone methide (III). Here nucleophilic attack on the y-carbon of the -quinone methide by the hydroxyl oxygen... 

The mixture of quinone methides initially formed by combination of the coniferyl radicals in their various mcsomeric forms, i.e. (I), (III), (V), (IX) and others, can be detected by means of their characteristic spectrum with a maximum at about 312 mp (52) the haU-hfe of the mixture in 70 % aqueous dioxan is 1 hour. Those quinone methides that can rearomatize by keto-enol tautomerism, e.g. (IX), or intramolecular additions, e.g. (I) or (III) may become stabilized faster than those of type (V) which rely on addition of a foreign molecule. The quinone methides that rearomatize intramolecularly appear to react exclusively in this way, probably by a concerted mechanism that represents collapse of the activated transition state. 

Fig. 7. Mechanism of the cross-linking of lignin macromolecules or of the grafting of lignin onto polysaccharides by single Rb-type coniferyl radicals...

Evidence for the participation in lignification of radicals in the form Rfl was not obtained from lignols extracted from simulated lignification experiments with coniferyl alcohol in vitro. Actually more direct proof was obtained here structures, e.g. (XII), that must have arisen from this form of the coniferyl radical were isolated from natural lignins by mild hydrolysis (see Section I). So far only structures derived from combinations of Rd type radicals with Rj, t5rpe radicals have been isolated 47, 705, 776, 777), but evidence for combinations of Rd radicals with Ra radicals has been secured by the detection of 40-(3,4-dimethox3q)henyl)-... 

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. 

However, the first four products identified 44), viz. dehydro-diconiferyl alcohol (II), DL-pinoresinol (IV), guaiacylglyccrol-p-coniferyl ether (VI) and conifcraldchydc (VII) ipOa) already revealed the nature of most of the secondary reactions taking place after formation of the free phenoxyl radicals by the enzymes, although this was not immediately... 

Combination of an Ri, radical with an Ra radical yields the single p-qninone methide dimer (V). Here the quinone methide cannot become stabilized by an intramolecnlar addition reaction. Instead, nucleophilic attack of its y-carbon atom occurs by a hydroxyl ion from the medium, for example aromatization and protonation of the phenoxido ion thus formed give rise to guaiacylglycerol- 3-coniferyl ether (VI), again in racemic form dc-spite its two asymmetric carbon atoms. Since attack by the extraneous hydroxyl ion can occur on either side of C-y of the p-quinone methide (V), complete equilibration of the specific hydrogens from the original conifcryl alcohol moiety in the lower half of (V) presumably occurs (sec formulae on p. 131). 

Lignans are phenylpropanoid dimers in which the monomers are linked by the central carbon (C8) atoms of the propyl side chains (Fig. 12.1) [10]. Many lignans are formed from coniferyl alcohol, a typical lignin monomer, and the coupling of two coniferyl alcohol radicals proceeds under the control of a unique asymmetric... 

Figure 1. Degradation (Q) of 17 gL dehydropolymerizate (0) from [2-l C]coniferyl alcohol by hydroxyl radicals generated from 1 M H2O2/IO mM FeS04. Sephadex LH20/DMF elution profiles adapted and redrawn fi om reference 12.

Attachment of Hydroxycinnamic Acids to Structural Cell Wall Polymers. Peroxidase mediation may also result in binding the hydroxycinnamic acids to the plant cell wall polymers (66,67). For example, it was reported that peroxidases isolated from the cell walls of Pinus elliottii catalyze the formation of alkali-stable linkages between [2-14C] ferulic acid 1 and pine cell walls (66). Presumably this is a consequence of free-radical coupling of the phenoxy radical species (from ferulic acid 1) with other free-radical moieties on the lignin polymer. There is some additional indirect support for this hypothesis, since we have established that E-ferulic acid 1 is a good substrate for horseradish peroxidase with an apparent Km (77 /tM), which is approximately one fifth of that for E-coniferyl alcohol (400 /iM) (unpublished data). 

Related research has been reported by Elder and Worley (39), in which MNDO was used to examine the structure of coniferyl alcohol, and its corresponding phenolate anion and free radical. This method represents an improvement over the PPP method, in that MNDO is an all-electron technique, and performs geometry optimizations. It was found that the calculated spin densities and charge values for the reactive sites did not correlate quantitatively with observed bond frequency, but it was observed that positions with partial negative charge and positive spin densities are the positions through which the polymerization has been found to occur. 

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