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Description :
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Lignification: Random vs Template Directed. In terms of energy content, lignins
are thought to be the most abundant of all biopolymers. They are composed of
p-hydroxyphenylpropanoid units interconnected through 8-O-4, 8-5, 8-8, 8-1,
5-5, and 4-O-5 linkages. Corresponding substructures in the polymer include
alkyl aryl ethers, phenylcoumarans, resinols, tetrahydrofuran-spiro-cyclohexadienones,
biphenyls, dibenzodioxocins, and diaryl ethers (see Fig. A). The primary
precursors themselves—the three monolignols p-coumaryl, coniferyl, and sinapyl
alcohols—differ only according to their aromatic methoxy substitution patterns.
These monolignols are oxidized enzymatically through single-electron transfer
to generate the respective phenoxy radicals. The actual coupling of a monolignol
radical with the growing end of a lignin chain, however, may not fall under
direct enzymatic control. Accordingly, many investigators have assumed that
lignin primary structures must be “random” or combinatorial as far as sequences
of interunit
linkages are concerned. More recently, this theory has been reinforced by reports
that certain kinds of non-native monolignols can be incorporated into macromolecular
lignin structures. Lignins and lignin derivatives exhibit two fundamental characteristics
that traditionally have been viewed as evidence in favor of randomness in their
configurations: They are both noncrystalline and optically inactive.1 Nevertheless,
a number of observations are thought by some to point in the opposite direction.
The individual molecular components in (nonpolyionic) lignin preparations tend
to associate very strongly with one another in a well-defined way. These processes
are thought to be governed by vital structural motifs derived from corresponding
features disposed nonrandomly along the native biopolymer chain. Moreover,
dimeric pinoresinol moieties are linked predominantly to the macromolecular
lignin chain through at least one of their aromatic C-5 positions.We do not
know whether such features can be explained through combinatorial mechanisms under simple chemical control or if higher-level
control mechanisms are required. One hypothesis proposes a way to replicate
specific sequences of interunit linkages through a direct template polymerization
mechanism. According to this model, an antiparallel double-stranded lignin
template, maintained in a dynamic state at the leading edge of each lignifying
domain, determines the configuration of the daughter chain being assembled
on the proximal strand’s exposed face. Furthermore, replication fidelity could
be controlled by strong nonbonded orbital interactions between matching pairs
of aromatic rings in the parent and the growing daughter chains. The overall
process seems to be consistent with the lack of both crystallinity and optical
activity in macromolecular lignin domains.Finally, required sequence information
may be encoded in polypeptide chains that embody arrays of adjacent lignol-binding
sites analogous to those found in dirigent positioning proteins.2
Cited References:
1. J. Ralph et al. 2004. “Lignins: Natural
Polymers from Oxidative Coupling of 4-Hydroxyphenylpropanoids,”
Phytochemistry Rev. 3, 29–60.
2. S. Sarkanen.
1998. “Template Polymerization in Lignin Biosynthesis,” pp. 194–208 in Lignin
and Lignan Biosynthesis 697, ed. N. G. Lewis and S. Sarkanen, American Chemical
Society, Washington, D.C. Theory proposed by G. Brunow and coworkers in 1998
(reproduced with permission).
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