Disclosing an NMR-Invisible Fraction in Chitosan and
PEGylated Copolymers and Its Role on the Determination of Degrees
of Substitution
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Abstract
An unexpected <sup>1</sup>H NMR invisible
fraction (IF) for chitosan
(CS) and CS<i>-g-</i>PEG is reported. The presence of this
IF is remarkable considering that solution NMR is recognized as the
method of choice for studying structural modifications in CS, including
the degrees of acetylation (DA) and substitution (DS). In spite of
IF figures as high as 50%, this IF does not interfere in the correct
determination of the DA by <sup>1</sup>H NMR, pointing to a homogeneous
distribution of acetyl groups along the visible and invisible fractions.
Quite in contrast, the IF negatively biases the determination of the
DS in CS<i>-g-</i>PEG, with relative errors as high as 150%
in a broad range of temperatures, pH values, and concentrations. This
fact raises concerns about the accuracy of previously reported DS
data for CS<i>-g-</i>PEG and many other CS copolymers. Efficient
user-friendly conditions have been developed for the correct determination
of the DS of CS<i>-g-</i>PEG by depolymerization by nitrous
acid