Ether LCC linkages

FIGURE 1 Major structural moieties of hemicelluloses, lignin, and LCC linkages. (A) Phenyl glycoside, (B) benzyl ether LCC linkages, (D) p-O-4/a-OH, (O ) P-O /a-OH... 

Multidimensional NMR techniques can potentially provide valuable information on the carbohydrate sites involved in ether LCC linkages. However, these efforts were so far unsuccessful, as the region where these signals should be located (ca. 65-75/3.0-4.0 ppm) is heavily overlapped and a very accurate NMR database is required to unambiguously assign the signals in this region. 

MWEL 5 2 + 3 Analysis of all original ether LCC linkages with wet chemistry methods only, not appropriate for ester and PhGly linkages... 

The MWEL is a very representative preparation to study ether lignin-carbohydrate linkages in wood since it would appear that it contains all of the original ether linkages [20]. Therefore, it is very appropriate to study the ether LCC linkages with the help of wet chemistry methods, such as meth-ylation and DDQ degradative techniques. However, in contrast to the CEL, the MWEL is not completely soluble in NMR solvents, and therefore its analysis by high-resolution spectroscopic methods is not viable. 

There is numerous evidence that lignin and polysaccharides are covalently linked, forming manifestations of the so-called lignin-carbohydrate complex (LCC) [2,7,17-19]. In spite of there being relatively low amounts of these linkages in wood [20,21], they play a very important role, as almost all wood lignin is covalently linked to polysaccharides, mainly to hemicelluloses [22]. There are three types of LCC linkages benzyl ether, phenyl... 

An elegant method based on selective enrichment of specific positions of the lignin side chain with C followed by C NMR studies [79] was applied to the study of LCC isolated from labeled wood [80]. The author claimed the presence in this preparation of LCC linkages of acetal, ether, and ester types at the a-position of the side chain and the absence of LCC bonds at the p- and y-positions of the side chain. However, a comprehensive discussirai revealed that these conclusions were not properly supported [29]. These same problems did not allow reliable NMR characterization of LCC linkages in a model Xylan-DHP substrate [58]. The main conclusion drawn from these studies is that ID C NMR is not a reliable tool to investigate LCC linkages even when using labeled preparations. 

Apphcation of 2D NMR methods overcame this obstacle and elucidated various LCC linkages in preparations isolated from softwoods, HWs, and pulps [21,25,29,48,50,51]. A HSQC correlation 2D NMR technique allowed for the first time direct detection of phenyl glycoside (Structure A, Fig. 1) and benzyl ether (Structure C, Fig. 1) LCC linkages [25,29]. In contrast to the common belief [2,14,17,18], no benzyl ester (a-ester) LCC linkages were detected in these studies. However, y-ester LCC moieties (Structure B, Fig. 1)... 

Signals of LCC benzyl ether of the C2 type are overlapped with a signal of the spirodienone structure (E, Fig. 1). The latter can be quantified from another individual signal and the amount of C2-type LCC linkages can be determined then by the difference [21]. However, considering the relatively low amounts of these structures, this type of quantification should be considered semiquantitative. 

The numbers (5 is the highest, 1 is the lowest) are relative and approximate rather than quantitative. BE, benzyl ether, PhGly, phenyl glycoside LCC linkages. ... 

The DDQ oxidation technique identifies only the linkage sites of carbohydrates involved in benzyl ether-type LCC bonds (Fig. 5) [18,73-76]. Briefly, an LCC preparation is thoroughly acetylated, then subjected to DDQ oxidation to induce the oxidative cleavage of LCC bonds of the benzyl ether type into the corresponding a-carbonyl in acetylated lignin and terminal hydroxyl groups in acetylated carbohydrate moieties, respectively. The resulting carbohydrate mixture is then methylated by Prehm s procedure and hydrolyzed... 

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