Lignin carbohydrate bonds

It is extremely difficult to extract lignin from wood. The reason for this is not merely that it is so intimately mixed with cellulose or even encased in cellulose membranes like starch granules. The reverse is truer the cellulose is embedded in a paste of lignin. The lignin is actually attached to the polysaccharides in plants by chemical bonds. Evidence for such lignin-carbohydrate bonds up to 1960 has been reviewed by Mere-... 

Bolker, H. I. A Lignin-Carbohydrate Bond ns Revealed by Infrared Spectroscopy. Nature 197, 489—490 (1963). 

We should note, also, that the lignin in the S2 layers is chemically bonded to the polysaccharide moiety (87-92). Such bonds occur not only in wood but may be formed during chemical pulping (93,94). Even if the lignin-carbohydrate bonds are restricted to the hemicelluloses (95), the regularity of these chain molecules will probably impose some non-randomness on the lignin structure. 

Possibilities include (a) the hydrolysis of the lignin-carbohydrate bond by small amounts of water present in the wood despite all precautions to make the system anhydrous, (b) the dehydration of carbohydrates under these conditions to produce water and hence hydrolytic conditions, (c) transacetalization of the extracting acetal with carbohydrates to liberate alcohol and create conditions of alcoholysis, and (d) transacetalization of the extracting acetal with free or bound keto groups in the wood to liberate acetone and create conditions of acetonolysis. 

O Karlsson, T Ikeda, T Kishimoto, K Magara, Y Matsumoto, S Hosoya. Isolation of lignin-carbohydrate bonds in wood Model experiments and preliminary application to pine wood. J Wood Sci 50 142-150, 2004. 

JR Obst. Frequency and alkah resistance of lignin-carbohydrate bonds in wood. Tappi 65 109-112, 1982. 

T Iversen, S Wannstrom. Lignin-Carbohydrate Bonds in a Residual Lignin Isolated from Pine Kraft Pulp. Holzforschung 40 19-22, 1986. 

Different degradation techniques are commonly used for the analysis of LCC isolated from plant tissues. This approach includes cleavage of lignin-carbohydrate bonds and identification of the resulting products by way of alkaline hydrolysis (saponification) [20], acid hydrolysis [36,66], Smith degradation [67], ozonolysis [68], methylation analysis [69-72], and DDQ (2,3-dichloro-5,6-dicyano-l,4-benzoquinone) oxidation [18,73-76]. The two last techniques are the most common for the analysis of carbohydrate linkage sites in LCC preparations. 

Several substituted benzyl ethers have been examined. As part of a study of lignin-carbohydrate bonding, vanillyl alcohol was treated with glucose in aqueous acetate buffer at 60 °C and gave 48% of an anomeric mixture of the 6-ethers (1). In work on the same theme l,2 5,6-di-0-isopropylidene-a-D-... 

In general the results indicate that an effective extraction of oat spelt xylan requires the application of elevated temperatures. It can be assumed that even at the high alkaline concentration applied, elevated temperatures are needed to effectively cleave lignin carbohydrate bonds in the limited time of the alkaline treatment. 

Nelson, R. Factors Influencing the Removal of Pentosans from Wood By Alkali Evidence of the Lignin-Carbohydrate Bond, Ph.D. Thesis, SUNY-College of Environmental Science and Forestry at Syracuse, 1956. 

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