Interactions between\nHyaluronic Acid and Chitosan\nby Isothermal Titration Calorimetry: The Effect of Ionic Strength,\npH, and Polymer Molecular Weight

Hyaluronic acid (HA)/chitosan (CHI) complex coacervates\nhave recently\ngained interest due to the pH-dependent ionization and semiflexibility\nof the polymers as well as their applicability in tissue engineering.\nHere, we apply isothermal titration calorimetry (ITC) to understand\nthe apparent thermodynamics of coacervation for HA/CHI as a function\nof the pH, ionic strength, and chain length. We couple these ITC experiments\nwith the knowledge of the charge states of HA and CHI from potentiometric\ntitration to understand the mechanistic aspects of complex formation.\nOur data demonstrate that the driving force for the complex coacervation\nof HA and CHI is entropic in nature and this driving force decreased\nwith increasing ionic strength. We also observed a decrease in the\nstoichiometry for ion-pairing with increasing ionic strength, which\nwe suggest is a consequence of the changing degree of ionization for\nHA at higher ionic strengths. An increase in the strength of interactions\nwith pH was hypothesized to also be a result of changes in the degree\nof ionization of HA, though stronger interactions were observed at\nthe lowest pH tested, likely due to contributions from hydrogen bonding\nbetween HA and CHI.