O-methylation-free TMC. As a result, DQ increased from 29.4% for
TMC 1 to 46.6% for TMC 2. This increased quaternization was also
related to an improvement of solubility and is considered to be
advantageous for complexation with anionic polymers in order to
form nanoparticles, or to carry drug or nucleic acids for pharmaceutical
applications.
3.1.1. CMTMC characterization
Concerning carboxymethylation, Hansson et al. used
chloroacetic acid in NMP under basic conditions (pH 10). By
varying reaction time, they substituted up to 27% carboxymethyl
groups (Hansson et al., 2012b). This relatively low value may be
due to previous O-methylation at C-3 and C-6, resulting in a low
availability of OH groups for carboxymethylation (Fig. 4A). Using
NMP, we also observed a very low degree of substitution (data not
shown). Switching reaction medium to isopropanol led to better
results, achieving degrees of O-carboxymethylation in extent of
85% (Fig. 4B) at an overall yield of 74%. This might be related
to a better conformation of TMC in isopropanol offering higher
accessibility to the reaction sites.
Thehighdegree of O-carboxymethylation was obtained by using
a strong basic medium, pointing out the importance of alkaline
conditions for O-carboxymethylation. A sufficiently strong base is
needed to allow chloroacetate penetration on the whole TMC chain,
avoiding side reactions between NaOH and chloroacetate (Barros
et al., 2013).
Successful O-carboxymethylation of TMC is further demonstrated
by heteronuclear single quantum correlation (1H–13C
HSQC) map (Fig. 5). The 13C NMR signals that belong to the COOH
group substituted on OH-3 and OH-6 are present at f1 = 71.1 ppm
and 71.5 ppm, respectively, and also appear on 1H NMR spectra
(Fig. 4A) at 4.15–4.5 ppm (peak 8). Their positions on HSQC map
indicate that there are two possible sites for O-carboxymethylation
on the TMC backbone, but the major active site is on C-6 as shown