A new radiochemical method to investigate ion binding with polyelectrolytes

Area of Science:

• Biophysical chemistry
• Polymer science
• Solution chemistry

Background:

• Understanding ion binding to polyelectrolytes is crucial for various applications.
• Existing methods may not fully distinguish between different binding modes.
• Polyelectrolyte behavior is significantly influenced by ionic strength and ion interactions.

Purpose of the Study:

• To develop and validate a new method for investigating ion binding with polyelectrolytes.
• To differentiate between territorial and specific ion binding.
• To quantify ion binding fractions in multicomponent solutions under varying ionic strengths.

Main Methods:

• Utilized Donnan equilibrium principles.
• Employed isotope exchange between electrolyte and polyelectrolyte.
• Used radioactive tracers (22Na+, 36Cl-) and heparin sodium salt.
• Investigated Na+ binding to heparin at 310 K across a range of ionic strengths.

Main Results:

• At zero ionic strength, all Na+ ions bind to heparin, with 45% being exchangeable, indicating both territorial and specific binding.
• The overall fraction of bound Na+ ions decreases sharply as ionic strength increases.
• Specifically bound Na+ ions become negligible above 0.01 M ionic strength.
• Territorially bound Na+ ions become negligible above 0.45 M ionic strength.

Conclusions:

• The developed method successfully distinguishes and quantifies territorial and specific ion binding.
• Ionic strength plays a critical role in modulating Na+ binding modes with heparin.
• The findings provide insights into the complex interactions between ions and polyelectrolytes.