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Simple procedure for preparing zwitterionic cellulose using N-protected aspartic anhydrides under a green and click chemistry approach
Cellulose is a natural polymer composed exclusively of glucopyranose units, easy to obtain, economically profitable, sustainable and with good mechanical properties. As polysaccharide, its physicochemical properties are associated to OH’s groups and how they can interact through hydrogen bonds. They can be chemically modified by different reactions like addition, substitution and oxidation. Some of these reactions are focused in conferring ionic charge to cellulose. Unfortunately the majority of these modifications occur in a homogeneous phase, and this biopolymer is only soluble under concentrated acids, alkalis or ionic liquids than can be harmful for the environment. According to this, the present work proposes a simple procedure to synthesize zwitterionic cellulose by means of N-protected aspartic anhydride under a green and click chemistry approach. Modified cellulose samples were obtained under mild aqueous conditions, from the reaction of NH4OH activated cellulose with aspartic anhydrides N-protected with trifluoroacetyl (TFAc) and carbobenzyloxy (Cbz). The resulted samples Cel-Asp-N-TFAc and Cel-Asp-N-Cbz were characterized by FTIR and 13C solid state NMR techniques. The functionalization degree of each cellulose sample was determined by the 13C NMR signal area values corresponding to cellulose C1 vs Cα of the aspartate residue and corroborated by Elemental Analysis. In agreement, both analytical methods averaged a degree of functionalization of 20% for Cel-Asp-N-TFAc and 16% for Cel-Asp-N-Cbz. Conveniently, Cel-Asp-N-TFAc was concomitantly partially N-deprotected (65%) as determined by the ninhydrin method. The total synthesis procedure reported in the present work agrees with the green chemistry principles and the click chemistry concept.