Water-based carbodiimide mediated synthesis of polysaccharide-amino acid conjugates: Deprotection, charge and structural analysis

We report here a one-step aqueous method for the synthesis of isolated and purified polysaccharide-amino acid conjugates. Two different types of amino acid esters: glycine methyl ester and L-tryptophan methyl ester, as model compounds for peptides, were conjugated to the polysaccharide carboxymethylcellulose (CMC) in water using carbodiimide at ambient conditions. Detailed and systematic pH-dependent charge titration and spectroscopy (infrared, nuclear magnetic resonance: ¹H, ¹³C- DEPT 135, ¹H- ¹³C HMBC/HSQC correlation), UV–vis, elemental and ninhydrin analysis provided solid and direct evidence for the successful conjugation of the amino acid esters to the CMC backbone via an amide bond. As the concentration of amino acid esters increased, a conjugation efficiency of 20–80% was achieved. Activated charcoal aided base-catalyzed deprotection of the methyl esters improved the solubility of the conjugates in water. The approach proposed in this work should have the potential to tailor the backbone of polysaccharides containing di- or tri-peptides.