3.1. Characterization of carboxymethyl chitosan
(N/O-CM-chitosan) by solid state 13C NMR
Carboxymethylation of chitosan is achieved with
monochloroacetic acid and sodium hydroxide. According to
this reaction takes place preferentially either at C-6 hydroxyl
groups or at the NH2-group resulting in N/O-carboxymethyl
chitosan (N/O-CM-chitosan). The solid state 13C NMR spectrum
for a typical N-carboxymethyl chitosan shows signals attributed
to the N-carboxymethyl substituent, at 47.7 and 168.7 ppm, for
N-CH2 and COOH, respectively, but in case of our results, the
solid state 13C NMR described in Fig. 1 shows signals at 73 and
175 ppm which attributed to –O-CH2– and COOH carboxyl group,
respectively. This downfield shift of the carbon indicates the
formation of O-carboxymethyl chitosan. The formation of this
product agrees with the higher reactivity of hydroxyl group of C6
in this heterogeneous reaction. The N-carboxymethyl substituent
is not present because of the absence of peaks at 47 and 168 ppm
for N-CH2 and COOH, respectively.