Synthesis of tamarind seed xyloglucan-gallic acid conjugate: Characterization, properties, and assessment of antioxidant activity.

The tamarind seed-derived xyloglucan (TXyG) was chemically modified via an esterification reaction with gallic acid (GA), a molecule with known antioxidant activity, using a 1,1'-carbonyldiimidazole-mediated process. Thus, four novel water-soluble gallic acid xyloglucan conjugates (GA-TXyGs), comprising various degrees of substitution (0.05 - 0.4), were reproducibly synthesized. The successful grafting of GA onto xyloglucan was revealed by UV-vis, infrared, nuclear magnetic resonance spectroscopy, and electrospray ionization mass spectrometry (ESI-MS). A linkage was formed between the Gal hydroxyl group of xyloglucan and the carboxyl group of GA. Various series of oligosaccharides, including (i) (XXG)Ac, (ii) (XLG)Ac, (iii) (XXXG)Ac, (iv) (XLG)GAAc, (v) (XXLG/XLXG)GAAc, (vi) (XLLG)Ac, and (vii) (XLLG)GAAc, epitomizing glycoconjugate structure, were generated and characterised. The grafting reaction induced antioxidant activity in the product glycoconjugates (scavenged ≤89% DPPH radicals), and their activity varied with the degree of esterification. These conjugates had lower intrinsic viscosities ([η] = 260-330 mL/g) than the xyloglucan precursor ([η] = 375 mL/g). Fluorescence spectroscopic investigation showed that they formed hydrophobic microdomains in dilute solutions. Thus, novel polysaccharide systems based on naturally occurring organic substances possessing antioxidant and viscosifying qualities were synthesized.