MALDI-TOF/MS analyses of decomposition behavior of beech xylan as treated by semi-flow hot-compressed water

Japanese beech (Fagus crenata) was treated with semi-flow hot-compressed water at various temperatures of 150–230 °C under 10 MPa. The obtained various products were then analyzed with matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF/MS). In a temperature range of 150 °C up to 210 °C, however, no hydrolyzed products were found, and at 210 °C/10 MPa, O-acetyl-4-O-methylglucuronoxylo-oligosaccharides (X ₙ Ac ₘ MG ᵢ ) and O-acetyl-xylo-oligosaccharides (X ₙ Ac ₘ ) were obtained, indicating the first cleavage of native xylan β-1,4-glycosidic linkages followed by a cleavage of α-1,2-glycosidic linkage in 4-O-methylglucuronic acid (MG) residue at mainly 220–230 °C under 10 MPa. At subsequent stage of 230 °C/10 MPa, X ₙ Ac ₘ were predominantly recovered. As the treatment was prolonged at 230 °C, X ₙ Ac ₘ were reduced, but remained to some extent, indicating that the acetyl group which is hydrolyzed to be acetic acid is more resistant than MG residue. In such a stage of treatment, cellulose started to hydrolyze to cello-oligosaccharides. These lines of evidence can clearly indicate the hydrolysis pathway of native O-acetyl-4-O-methylglucuronoxylan as treated by hot-compressed water. Thus, xylo-oligosaccharides recovered in a very early stage of the semi-flow hot-compressed water treatment preserve native O-acetyl-4-O-methylglucuronoxylan.