Lignin phenolic units

In general, the data demonstrated that this technique provides relatively good preservation of the original carboxyl and hydroxyl groups in lignin phenols units present in the macromolecular structure of HS owing to protection of the functional groups from thermal reactions. 

Since lignins are polymers of phenolics and are major plant constituents with resistance to microbial decomposition, they are the primary source of phenolic units for humic acid synthesis (178, 179). Once transformed, these humic acids become further resistant to microbial attack and can become bound to soils (180) form interactions with other high molecular weight phenolic compounds (ex. lignins, fulvic acids) and with clays (181) and influence the biodegradation of other organic substrates in soils (182, 183). 

The literature is devoid of any studies on the electrophoretic behavior of lignins. However, due to the presence of at least one phenolic hydroxyl group per lignin building unit, it appeared that such an investigation would be feasible. Moreover, due to the success of applying electrophoretic methods to the characterization and fractionation of proteins, a study as to the possible availability of the method to lignins was undertaken in this laboratory. 

The presence of lignin structural units was confirmed by retention time data and by comparisons of mass spectra to library spectra, to spectra of authentic standards, and to published mass spectra of lignin phenols Fill. 

The analysis of lignitic woods by analytical pyrolysis has shown that lignin structural units can be preserved as biomaikers in samples as old as Carboniferous age, or approximately 300 million years. At least half or more of the pyrolysis products in lignitic wood of Cretaceous age are methoxyphenols characteristic of lignin. The pn uct distributions in these Cretaceous samples indicate that the lignin is mainly altered in the 3-carbon side chains. Phenols, cresols, catechols, and other methylated phenols account for most of the remaining pyrolysis products. It is likely diat these products are also derived frx>m li in, especially lignin that has been altered by coalification reactions. 

P. chrysosporium and abstracts electrons from lignin substructures with lower redox potentials such as phenolic units. The enzyme directly oxidised Mn II to Mn III as the initial step in lignin attack. Other enzymes... 

The acid condensation reaction of the aromatic and phenolic units is a typical reaction of lignin. The presence of acids results in resonance stabilized carbonium ion structures formed in the lignin macromolecule. These car-bonium ion structures react further, e.g., with unsubstituted positions in the lignin macromolecule. Thus, thermal treatment of powdered wood in acidic conditions causes condensation, the coniferyl aldehyde and coniferyl alcohol groups being especially reactive. In addition, other inter- and/or intramolecular condensations may occur. 

A typical lignin C6C3 unit see reaction scheme below) contains a side chain at position 1, a methoxyl at position 3, and a phenolic hydroxyl or phenol ether group at position 4. Another methoxyl group or a condensed... 

By means of Fremy salt oxidation, Adler and Lundquist (5) estimated that about half of the phenolic units contain free S-positions in M.W.L. Mikawa (J/), by Mannich reaction, obtained evidence that about 25-40% of the phenolic units in his sulfate lignin preparation contained no substituent in 5-position. In this investigation we also conclude that at least one-third of the phenolic units in our kraft lignin preparation has free 5-positions. 

In the latter preparation, about 0.05 a-CO groups per C6-C3 were found in phenolic units (26). It is known that the phenolic hydroxyl in acetovanillone or in similar phenolic ketones is quite acidic (21) (pK = 8) reducing these ketones would decrease the acidity to the usual level of phenols (pK = 10, or still higher in sterically hindered phenols). This observation is a further indication that our present concept concerning the presence of a-CO groups in kraft lignin (24) is right. 

The network structure of lignin, which is made of phenol units, coagulates the cell wall in wood tissue, which is composed of cellulose and hemicellulose. Lignin is currently a waste product because of its complicated structure [1-4], It is produced by an oxidative polymerization of coniferyl alcohol, sinapil alcohol, and cumarol alcohol (Figure 1) catalyzed by metalloenzymes such as laccase and peroxidases. Laccase is a protein whose active center contains four coppers per one subunit [5-20],... 

The actual mechanisms of Kraft delignification are highly complex, revolving around the ionization of acid phenolic units in lignin by OH and nucleophilic displacement of lignin units with HS. The chemistry of delignification is reviewed in detail elsewhere. It is sufficient to note here the conditions specified above and the pulp yields typically 45-55% of the dry weight of wood furnish is produced as Kraft pulp. 

It has been suggested that the major part of condensation processes occurs at the unoccupied C-5 position of phenolic units. Thus, in isolated MWL preparations about half of the C-5 positions are unsubstituted, while in isolated kraft lignins only about one third of these positions remain free. The syringyl units of hardwood lignins cannot, of course, undergo condensation reactions of this type. 

In alkaline media, phenolic units may react with formaldehyde, forming methylol derivatives that condense with themselves or with another phenol (J, Fig. 1.4). This formaldehyde condensation reaction forms the basis for using technical lignins in the production of adhesives. 

Uncondensed phenolic units in lignin have been estimated by a color reaction based on oxidation with potassium nitrosodisulfonate (Fremy s salt) (Adler and Lundquist 1961) as illustrated below ... 

To eliminate interfering side reactions caused by units having carbonyl and hydroxyl groups at the a-position, the lignin is reduced with borohydride and alkylated with methanolic hydrochloric acid prior to oxidation to convert these groups to o-quinones. Following a procedure based on the above reaction, spruce milled-wood lignin was found to contain 15-18% of uncondensed phenolic units (Adler and Lundquist 1961). 

Adler E, Lundquist K (1961) Estimation of uncondensed phenolic units in spruce lignin Acta Chem Scand 15 223-224... 

Of the methods for determining lignin in solution based on a specific chemical reaction, that involving nitrosation, the so-called Pearl-Benson method, has found the widest application. In this procedure, reaction of the phenolic units in lignin with acidified sodium nitrite leads to the formation of a nitrosophenol which, upon addition of alkali, is tautomerized to an intensely colored quinone mono-oxime. The absorbance of the latter structure is measured at 430 nm and related to lignin concentration by calibration with a standard lignin. The procedure described below is essentially that developed by Barnes et al. (1963), who modified the original Pearl-Benson method (Pearl and Benson 1940) to improve its sensitivity. 

Irrespective of whether the starting material is a wood, plant, or pulp sample, an isolated lignin sample, or a lignin model compound, it is essential that all free phenolic hydroxyl groups be protected by alkylation prior to oxidation to prevent degradation of the phenolic units. This is usually accomplished by methylation or ethylation in an alkaline solution with dimethyl sulfate or diethyl sulfate, respectively. The reaction step should be performed in a well-ventilated hood. 

Another factor which may influence the yield of acids obtained from lignin in wood is the presence of easily hydrolyzable aryl ether linkages such as those represented by phenylpropane-a-aryl ether structures In phenolic units, such structures are easily cleaved even under very mild alkaline conditions (Adler et al 1968) Therefore, it can be assumed that, in the alkylation step of the oxidative degradation procedure, a partial hydrolysis of such structures may occur, thereby creating new phenolic end groups that may affect the analysis... 

Figure 2. Condensation of formaldehyde with lignin free-phenolic units.

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