期刊
CARBOHYDRATE POLYMERS
卷 54, 期 4, 页码 431-438出版社
ELSEVIER SCI LTD
DOI: 10.1016/S0144-8617(03)00134-6
关键词
depolymerization; thermal degradation; acid hydrolysis; beta-elimination; activation energy; NMR spectroscopy
The mechanism of thermal depolymerization of alginate in the solid state has been investigated. Depolymerization at elevated temperatures of two commercial highly purified alginates, one with high content of guluronic acid (G) and another with high content of mannuronic acid (M) was followed by measuring the apparent viscosity and the intrinsic viscosity. The initial rate constants were determined from the intrinsic viscosity data, and no significant difference between the G-rich (F-G = 0.63) and M-rich (F-G = 0.43) alginate was found. The activation energies of the G-rich alginate and the M-rich alginate were determined from the initial rate constants to be 114 +/- 6 kJ/mol and 126 +/- 12 kJ/mol, respectively. The rate of depolymerization was not affected by the presence of oxygen, showing that the oxidative-reductive depolymerization mechanism is not responsible for the thermal depolymerization. The initial rate constants for alginates prepared by freeze-drying of solutions with pH between 3.8 and 9.5, were pH dependent. The depolymerization was found to be catalyzed simultaneously by protons and hydroxide ions. These catalytic effects were negligible between pH 5 and pH 8. The catalytic constants for the OH- were identical for the M-fich and G-rich alginate. However, the catalytic constant for the H+ was about 2 times greater for the M-fich alginate than for the G-rich alginate. This suggests that the M-fich alginate is more susceptible to acid hydrolysis than the G-rich alginate in the solid state, as also was found for alginate in solution when the mechanism of intramolecular acid catalysis prevailed. (1) H and C-13 NMR spectroscopy of the thermally degraded alginates were used to identify new non-reducing ends obtained from the depolymerization. New non-reducing ends from P-elimination caused by alkaline hydrolysis were clearly identified in the spectra obtained from alginates with pH between 4.3 and 9.5, when pH is defined as the pH of 1%, (w/w) alginate solution prepared from a given freeze dried sample. The results reported herein indicate that acid hydrolysis and alkaline hydrolysis are the primary mechanisms involved in the thermal depolymerization of highly purified alginate in the solid state. (C) 2003 Elsevier Ltd. All rights reserved.
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