Optimization of guar gum galactomannan sulfation process with sulfamic acid

Galactomannans are plant polysaccharides with the beneficial properties, which have a wide range of industrial application. The most important galactomannan derivatives are sulfates, which exhibit the anticoagulant and other biological activities. Here, we report on the results of investigations of the effect of urea-based solvents and activators on the sulfation of galactomannan guar gum with sulfamic acid. It has been shown that 1,4-dioxane is the most effective solvent, while urea is the most effective activator of the sulfation process. The numerical optimization (the Box‒Behnken design) of sulfation of galactomannan guar gum with sulfamic acid in the presence of the most effective activator has been carried out. It has been established that the optimal conditions for obtaining the galactomannan sulfates are a sulfamic acid amount of 34 mmol per 1 g of galactomannan, a temperature of 85 °C, and a time of 2.6 h. The introduction of a sulfate group into a galactomannan molecule has been proven by Fourier transform infra-red spectroscopy. It has been found that the Fourier transform infra-red spectrum of sulfated galactomannan contains absorption bands at 1249 and 817 cm‒1, which correspond to vibrations of the sulfate group. It has been demonstrated using gel permeation chromatography that, during sulfation of guar gum galactomannan by a complex of sulfamic acid and 1,4-dioxane, the molecular weight decreases from 8.5 × 105 to 3.0 × 105 g/mol and the polydispersity value increases from 1.816 to 2.049.