Coupling of amines with polyglucuronic acid: Evidence for amide bond formation

Cellulose III was reacted with catalytic amounts of 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO), sodium hypochlorite and sodium bromide in order to selectively oxidize the cellulose primary hydroxyl groups to yield polyglucuronic acid. This product was further modified by a coupling of the carboxyl groups with a series of amines in the presence of carbodiimide. The samples were characterized by NMR and FTIR. These techniques allowed us to determine the degree of conversion (DC), which was deduced from the decrease of carboxyl and the increase in nitrogen content. In addition, the formation of an amide bond between both linear or cyclic amines and polyglucuronic acid was systematically observed. For this, the results of NMR spectroscopy experiments are of particular interest as they indicate clearly that a bond between the amines and polyglucuronic acid has been formed during the reaction. This bond was revealed by the presence of an extra signal near 170.5 ppm in the 13C NMR spectra, characteristic of the acetamide moiety. In the case of coupling with 2-methoxy ethyl amine, the 2D 13C–1H HMBC spectrum revealed a set of cross-peaks between the C6 of the glucuronic residues and the methylenic protons Ha, Hb, thus confirming further the formation of the amide linkage. When the amidation was achieved with 4-amino TEMPO, the degree of conversion was further confirmed by electron paramagnetic resonance (EPR) spectroscopy.