Article,
Thermal depolymerization of alginate in the solid state
Carbohydr. Polym., 54 (4): 431 -- 438 (December 2003)
Abstract
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.
Tags
- ;
- ;;
- acid
- acid;
- activation
- beta-elimination;
- degradation;
- depolymerization;
- energy;
- hydrolysis;
- nmr
- nmr;
- ph
- polysaccharides;
- sequence;
- spectroscopy
- spectroscopy;
- thermal
- [isi:]
