Specific interactions versus counterion condensation. 1. Nongelling ions/polyuronate systems

Area of Science:

• Polymer Chemistry
• Biophysical Chemistry
• Materials Science

Background:

• Divalent cations, such as magnesium (Mg2+), are crucial in biological systems and interact with polyuronates.
• Understanding these interactions is key to fields like biomaterials and drug delivery.
• Classical counterion condensation (CC) theory often predicts these interactions, but experimental deviations occur.

Purpose of the Study:

• To investigate the interaction characteristics between nongelling divalent cations and specific polyuronates.
• To determine the enthalpy of mixing and explore deviations from the CC theory.
• To quantify the nonbonding affinity between divalent counterions and polyuronates.

Main Methods:

• Isothermal calorimetry was employed to measure the enthalpy of mixing.
• Three types of polyuronates were studied: polyguluronate (polyG), polymannuronate (polyM), and polyalternating (polyMG).
• Circular dichroism spectroscopy confirmed the absence of site-specific chemical bonding.

Main Results:

• A significant deviation from CC theory predictions was observed, attributed to a "generic" nonbonding affinity.
• The CC theory was extended to include counterion affinity, enabling parametric calculation of condensed ion fractions.
• Enthalpy of mixing analysis revealed distinct affinity contributions (desolvation vs. ion pairing) for polyM, polyG, and polyMG.

Conclusions:

• Polyuronate-divalent cation interactions exhibit a nonbonding affinity beyond classical CC theory.
• The study successfully estimated counterion affinity parameters for different polyuronates.
• Differences in desolvation and ion pairing mechanisms were identified among polyG, polyM, and polyMG.