Heat-induced transfer of protons from chitosan to glycerol phosphate produces chitosan precipitation and gelation. Biomacromolecules 2008;9:640e50

Recently, chitosan dissolved in solutions containing glycerol phosphate (GP) were found to undergo a sol–gel transition when heated and the proposed gelling mechanism was based on increasing hydrophobic interactions with temperature. Subsequently, an investigation of ionization and precipitation behavior of chitosan, including dependencies on temperature, added salt, and fraction of deacetylated monomers (fD) was performed. This latter study revealed important differences in the temperature dependence of pKa of chitosan versus GP and led us to propose an alternative hypothesis for the mechanism of gelation in chitosan-GP systems whereby heat induces transfer of protons from chitosan to glycerol phosphate thereby neutralizing chitosan and allowing attractive interchain forces to form a physical gel. To investigate this specific molecular thermogelling mechanism, temperature ramp experiments on dilute chitosan-GP solutions were performed. Chitosans with fD of 0.72 and 0.98 were used to prepare solutions with a range of molar ratios of GP to chitosan glucosamine monomer of 1.25 to 10 and with 0 or 150 mM added monovalent salt. Light transmittance measurements were performed simultaneously to indicate precipitation in these dilute systems as a surrogate for gelation in concentrated systems. Measured temperatures of precipitation ranged from 15 to 85 °C, where solutions with less GP (used in a disodium salt form) had lower precipitation temperatures. A theoretical model using acid–base equilibria with temperature dependent pKa’s, including the electrostatic contribution from the polyelectrolytre nature of chitosan, was used t