deactylation is 95%. The peaks at 7.3 and 7.6 ppm refer
to the H-b and H-a benzene protons of HOBt, respectively.
Comparing Figure 1A to Figure 1B, the change in chemical
shift of the HOBt benzene protons indicate the formation
of a complex with chitosan as an organic salt, 1, as
demonstrated in Scheme 1.
The pH of chitosan-HOBt in Figure 1B is 4.0. In order to
confirm the complexation and observe its stability, the 1H
NMRspectra of HOBt and chitosan-HOBt at differing mole
ratios have been determined at various pH values. A series
of 1H NMR spectra of HOBt with changing pH (varied by
adding NaOD in D2O) is shown in Figure 1C. A solution of
HOBt in D2O has a pH of 2.8 (Figure 1A). At this pH
the peaks of the aromatic protons appear as triplet peaks at
7.41 and 7.51 ppm, and doublet peaks at 7.60 and 7.66 ppm.
After adding NaOD to adjust the pH to a) 4.9, b) 4.4, and c)
3.8, the peaks of the aromatic protons are significantly
shifted and split into different patterns as shown in
Figure 1C. This implies that the HOBt and NaOD undergo
an acid/base interaction, which induces a change in
chemical shift of the H-a and H-b signals. Figure 1D shows
1H NMR spectra of mixtures of chitosan and HOBt in D2O
with a mole ratio of chitosan/HOBt of a) 1:1, b) 1:1.5, and c)
1:2. For each mixture the pH was adjusted to a) 4.9, b) 4.3,
and c) 4.0, respectively. It should be noted that the benzene
protons (H-a and H-b) of the chitosan-HOBt complex in
aqueous solution (Figure 1D (a) to (c)) show a similar