Coniferyl alcohol radical

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 2-8. Laccase-catalyzed formation of coniferyl alcohol radicals.

Figure 3-10. Dirigent-protein mediated formation of (+)-pinoresinol from two coniferyl alcohol radicals. The formation of the ring results from intramolecular cyclization.

Fig. 4-7. Endwise polymerization (Adler, 1977). A guaiacylglycerol-/3-aryl ether structure (1) is dehydrogenated and after resonance, radical c is coupled with a coniferyl alcohol radical b (cf. Fig. 4-4). The /3-5 coupling product (3) is tautomerized and undergoes intramolecular ring closure (a phenylcoumaran structure, 5).

Halls, S.C., Davin, L.B., Kramer, D.M. and Lewis, N.G. (2004) Kinetic study of coniferyl alcohol radical binding to the (-1-)—pinoresinol forming dirigent protein. Biochemistry, 43, 2587-95. 

Fig. 301. Formation of coniferyl alcohol radicals 1 / -Glucosidase 2 peroxidase...

Fig. 302. Addition of a molecule of coniferyl alcohol to a coniferyl alcohol radical...

Fig. 303. Products of the dimerization of coniferyl alcohol radicals The configuration of pinoresinol given is that found in higher plants...

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. 

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)-... 

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. 

When coniferyl alcohol is dehydrogenated, it loses its phenolic hydrogen atom to form first an aroxyl radical Ra (XI), which is in eflFect also present as the mesomeric radicals Rb (XII), Rc (XIII), and Rd (XIV). Of these limiting structures, Rb is the most favored. The existence of the radicals in these forms is recognized by their reaction products. In very dilute dioxane-water solution (1 1 vol.), the half-life of the radicals is about 45 seconds 13). 

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