Structural characterization of aerogels derived from enzymatically oxidized galactomannans of fenugreek, sesbania and guar gums.

Aerogels are obtained by laccase/2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-oxidation of galactomannans (GMs) from the leguminous plants fenugreek (Trigonella foenum-graecum), sesbania (Sesbania bispinosa) and guar (Cyamopsis tetragonolobus). GM oxidation in aqueous solutions causes a viscosity increase, resulting in structured and stable hydrogels. Upon lyophilization, water-insoluble aerogels are obtained, capable of water uptake up to several times their own weight. The materials derived from the three gums have been analyzed and compared. Chemical modifications have been studied by electrospray-ionization mass spectrometry (ESI-MS) and Fourier-transform infrared spectroscopy (FTIR). Polymer structure has been determined by liquid-state and semi-quantitative solid-state nuclear magnetic resonance (NMR) spectroscopy and chemical stability by incubation under variable conditions of pH, ionic strength and solvent nature. The results show that hydrogel formation is due to oxidation of primary hydroxyl groups to carbonyl and carboxyl groups and subsequent formation of hemiacetal and ester bonds. Fenugreek displays the highest stability, compared to guar and sesbania rehydrated aerogels. This feature could be interpreted by its higher degree of substitution (Gal:Man = 1:1) and consequently higher amount of galactose primary alcohols, leading to more extensive crosslinking.