Hexenuronic acids

AhlStage [Ahlstrom stage] A pulp-bleaching process that economizes on oxidizing agents by first destroying hexenuronic acid derivatives that would otherwise consume them. They are destroyed by hydrolysis with dilute sulfuric acid. Developed by Ahlstrom Machinery Corporation, Finland, in 1996. 

Hexenuronic acid (i.e., 4-deoxy-L-threo-hex-4-enopyranosyl-uronic acid) is formed under alkaline conditions by elimination of methanol from side chain residues in xylans [65] (Scheme 10). The reaction is promoted by both increasing alkali concentration and temperature [66,67]. After kraft pulping, only... 

The following gives a brief compilation of procedures to determine hexenuronic acids in cellulosic pulp samples. The common methods are based on hydrolysis of HexA moieties from pulp, either enzymatically or chemically, with subsequent quantification of the hydrolysis products either directly or after chemical conversion into UV active compounds. A comparison of these three methods is given by Tenkanen et al. [136]. For comparison rather than exact determination of HexA, e.g., during bleaching stages, the diffuse reflection UV VIS method can be applied [137]. A photoacoustic FTIR procedure based on chemometric analysis has been described as well [138]. In Table 3, the available methods to analyze HexA moieties in cellulosic material are summarized. 

Table 3 Methods for the determination of hexenuronic acids in cellulose...

M Ragnar. A Novel Spectrophotometric Tool for Bleaching Smdies and Determination of Hexenuronic Acid Removal. Nordic Pulp Paper Res. J. 16 68-71, 2001. 

E Sjoholm, F Lunqvist, T Liljenberg, and S Backa. Prediction of Kappa Number, Hexenuronic Acid and Klason Lignon in Pulps by NIR Spectroscopy. 11th International Symposium on Wood and Pulping Chemistry, Nice, 2001, IB. 

Other non lignin []]] Hexenuronic acid Klason lignin... 

FIGURE 10.5 Contribution from lignin, hexenuronic acid, and non lignin structures to the kappa number in various unbleached alkaline pine and birch pulps. (Li, J. and Gellerstedt, G., Nordic Pulp Pap Res J, 13(2),153-158, 1998.)... 

M Tenkanen, G Gellerstedt, T Vuorinen, A Teleman, M Perttula, J Li, J Buchert. Determination of Hexenuronic Acid in Softwood Kraft Pulps by Three Different Methods. J Pulp Pap Sci 25 306-311, 1999. 

FIGURE 11.1 Contribution to the kappa number in unbleached alkaline pulps from lignin, hexenuronic acid, and non lignin structures. 

Table I Blasticidin S and Other Peptidyl Nucleoside Antibiotics with a A -3-Hexenuronic Acid Moiety...

Under alkaline cooking conditions some of the 4-0-MeGlcA residues present in hardwood and softwood xylans are converted into unsaturated hexenuronic acid residues (HexA) by -elimination of methanol. In an early study (4) employing MALDI-MS, we were able to identify a number of different HexA and 4-0-MeGlcA containing xylooligosaccharides in the hydrolysate obtained after endoxylanase treatment of an unbleached hardwood kraft pulp. 

Figure 8, MALDI mass spectrum (negative-ion mode) of the acidic xylooligosaccharides obtained by endoxylanase treatment of a hexenuronoxylan isolated from alkaline peroxide bleached hardwood kraft pulp. The spectrum consists of two series of peaks assigned to the molecular ions [M-Hf of xylooligosaccharides with varying chain lengths and containing one hexenuronic acid (HexA) residue or one 4-O-methylglucuronic acid ( 4-0-...

The xylans used were non-substituted linear xylan from beech (Lenzing AG, Austria), 4-0-methylglucuronoxylan from birch (MeGlc-xylan) (Roth, Germany). Hexenuronic acid xylan (HexA-xylan) was alkali treated 4-0-... 

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