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

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)₁Ac₁₄, (ii) (XLG)₁Ac₁₇, (iii) (XXXG)₁Ac₁₉, (iv) (XLG)₁GA₁Ac₁₇, (v) (XXLG/XLXG)₁GA₁Ac₂₂, (vi) (XLLG)₁Ac₂₅, and (vii) (XLLG)₁GA₁Ac₂₅, 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.