Structural and thermal characterization of galactomannans from non-conventional sources

Abstract

Galactomannans of Gleditsia triacanthos, Caesalpinia pulcherrima and Adenanthera pavonina were structurally and thermally characterized. Methylation analyses confirmed that these three galactomannans are composed by a 4-linked mannose polymer with galactose side chains attached at the C6 position. G. triacanthos galactomannan had an estimated average degree of polymerization of 224 mannose residues, with a degree of branching of 0.24, C. pulcherrima galactomannan had a degree of polymerization of 252 and a degree of branching of 0.30, and A. pavonina galactomannan had a degree of polymerization of 475 and a degree of branching of 0.60. Enzymatic hydrolysis with endo-β-mannanase and the subsequent analyses of the low molecular weight fraction by ESI-MS/MS indicated the presence of characteristic structural features of galactomannans. Furthermore, acetyl and pentosyl residues were detected in the galactomannan of G. triacanthos and analyses of higher molecular weight fractions from G. triacanthos were performed, confirming that presence. Differential scanning calorimetry (DSC) showed the presence of two peaks related with water loss, confirmed by thermogravimetric analysis (TGA). The different compositions of galactomannans with different mannose content and different molecular weights influence their thermal behaviour. A higher content of mannose lead to higher values of the enthalpy change (first thermal transition) and glass transition temperature (Tg), while the enthalpy change of the second thermal transition was influenced by the viscosity average molecular weight (Mv) of the samples. This work provides understanding of structural and thermal properties of three galactomannans from non-conventional sources, thus contributing to a better insight on their possible food, pharmaceutical or biomedical applications.