Protolignin

The molecular weight of lignin in the wood, ie, of protolignin, is unknown. In addition to difficulties of isolation and purification, the polymer exhibits strong solvent, ionic, and associative effects in solution. An unequivocal method of measurement has not been developed. The polymer properties of lignin and its derivatives have been discussed (10,16). 

Smith, D. G., and A. C. Neish Alkaline Oxidation of C-Labellcd Protolignin Formed from Cinnamic Acid in Spruce and Aspen Twigs. Phytochem. 3, 609—616 (1964). 

In summary, we conclude that laccase m can degrade a portion of protolignin through depolymerization and solubilization. 

Finally, it should be noted that the structure of the DHP varies greatly depending on the polymerization conditions employed. Additionally, the yield and chemical and physical properties of these DHP preparations differ substantially from protolignin. Further improvements in simulation of the lignification process are therefore needed, and the radiotracer method can be employed as one approach to solve such problems. 

The specific labeling of specific moieties in protolignin can be achieved by administration of an appropriate labeled precursor to a growing plant. 

It is well known that the structure, distribution and properties of protolignin in cell walls vary according to cell type and morphological location. This is based upon extensive studies on topochemical properties of lignin using various methods such as ultraviolet microscopic photometry (1,2), bromination-SEM-EDXA (3) and other physical or chemical analyses of isolated tissue fractions (4). 

In woody gymnosperms, there are significant differences in the distribution, reactivity and physical properties of protolignins found in the compound middle lamella and the secondary wall (1-3). Additionally, variations between lignins in vessels and fibers have also been noted (3). All of these... 

The monolignol utilized varies with type and age of the cell. Indeed, incorporation of monolignols into protolignin occurs in order of in-... 

Figure 5. A schematic representation of the process of deposition of cell wall components and the heterogeneous formation of protolignin macromolecule. ML, middle lamella CC, cell corner P, primary wall CML, compound middle lamella S1 S2, and S3, outer, middle, and inner layer of secondary wall H, G, and S,p-hydroxy-, guaiacyl-, and syringylpropane units.

From the discussion in the previous sections we see that the behavior of the protolignin in wood and the pattern of its subsequent degradation and solution are compatible, in a broad sense, with the paradigm of a random three-dimensional network polymer. There emerge, however, several aspects of the problem which do not fit easily with the simple paradigm given above. In the following sections some of these anomalies are discussed. 

If lignin is heated with phenol, the phenol condenses with lignin in the a-position of the side chain. Phenol generally couples in its para- position (15). Under optimum conditions, from 2.5 to 3 moles of phenol or phenolic derivative per phenylpropane unit are added to the protolignin (18). 

A chromatogram illustrating the separation of a number of authentic methyl ethers is presented in Figure 3. The particular methyl ethers shown were selected on the assumption that the corresponding phenols may be present in the oxidation products of protolignin in wood or lignin-containing products derived from wood. 

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