205
11
11.2.2 Binding Pattern of Lignin
The nomenclature of the different carbon atoms
in the monolignols allows an exact specification
of the linkages within the polymeric lignin. However, before we discuss the exact types of bonds,
we will briefly explain how a bond is formed
between the monolignols: A monolignol is enzymatically converted into a radical. Due to its
aromatic structure, this radical is mesomerically
stabilized over the entire molecule and thus particularly stable (. Fig. 11.4).
This radical formation can be regarded as
the start of a radical polymerization. Using the
example of coniferyl alcohol in . Fig. 11.5 shows
how initial dimerization can be considered a precursor to lignin formation. In this case, a radical
in position O4 attacks the β-position of another
coniferyl alcohol molecule, and with the simultaneous addition of an OH-radical, a β-O-4 bond
is formed.
Cumaryl alcohol has the basic structure without further substituents on the aromatic ring.
Coniferyl alcohol has one methoxy substituent,
and sinapyl alcohol has two methoxy substituents in the ortho position to the hydroxyl group.
In principle, all three alcohols are derived
from cinnamic acid, which is formed in the plant
metabolism from the amino acid phenylalanine.
In their monomeric form, however, these alcohols hardly occur in nature and therefore have no
significance.
For the exact identification of the carbon
atoms in the monolignols, a nomenclature has
established which does not correspond to that
of IUPAC: The carbon atom with the hydroxyl
group on the aromatic ring is given the number
C4, according to which the carbon atom C1 carries the propylene side chain. The carbon atoms
of the side chain are named with Greek letters,
beginning with α, β and finally γ, which carries a
primary hydroxy group.
OH
OCH 3
OH
O
OCH 3
OH
O
OCH 3
OH
O
OCH 3
OH
1/2 O 2
2 H 2 O
2
H 2 O
H 2 O 2
2
. Fig. 11.4 Enzymatic radical formation in monolignoles using the example of coniferyl alcohol
OH
OCH 3
OH
O
OCH 3
OH
HO
Dimerization
of two
coniferyl
alcohols
OH
OCH 3
HO
O
HO
OCH 3
OH
"Polymerization"
Lignin
4
-O-4-Bond
further links
β
β
. Fig. 11.5 Dimerization of two molecules of coniferyl alcohol
11.2 · Structure of Lignin
11
11.2.2 Binding Pattern of Lignin
The nomenclature of the different carbon atoms
in the monolignols allows an exact specification
of the linkages within the polymeric lignin. However, before we discuss the exact types of bonds,
we will briefly explain how a bond is formed
between the monolignols: A monolignol is enzymatically converted into a radical. Due to its
aromatic structure, this radical is mesomerically
stabilized over the entire molecule and thus particularly stable (. Fig. 11.4).
This radical formation can be regarded as
the start of a radical polymerization. Using the
example of coniferyl alcohol in . Fig. 11.5 shows
how initial dimerization can be considered a precursor to lignin formation. In this case, a radical
in position O4 attacks the β-position of another
coniferyl alcohol molecule, and with the simultaneous addition of an OH-radical, a β-O-4 bond
is formed.
Cumaryl alcohol has the basic structure without further substituents on the aromatic ring.
Coniferyl alcohol has one methoxy substituent,
and sinapyl alcohol has two methoxy substituents in the ortho position to the hydroxyl group.
In principle, all three alcohols are derived
from cinnamic acid, which is formed in the plant
metabolism from the amino acid phenylalanine.
In their monomeric form, however, these alcohols hardly occur in nature and therefore have no
significance.
For the exact identification of the carbon
atoms in the monolignols, a nomenclature has
established which does not correspond to that
of IUPAC: The carbon atom with the hydroxyl
group on the aromatic ring is given the number
C4, according to which the carbon atom C1 carries the propylene side chain. The carbon atoms
of the side chain are named with Greek letters,
beginning with α, β and finally γ, which carries a
primary hydroxy group.
OH
OCH 3
OH
O
OCH 3
OH
O
OCH 3
OH
O
OCH 3
OH
1/2 O 2
2 H 2 O
2
H 2 O
H 2 O 2
2
. Fig. 11.4 Enzymatic radical formation in monolignoles using the example of coniferyl alcohol
OH
OCH 3
OH
O
OCH 3
OH
HO
Dimerization
of two
coniferyl
alcohols
OH
OCH 3
HO
O
HO
OCH 3
OH
"Polymerization"
Lignin
4
-O-4-Bond
further links
β
β
. Fig. 11.5 Dimerization of two molecules of coniferyl alcohol
11.2 · Structure of Lignin