Polyelectrolyte charge corrected molecular weight and effective charge by sedimentation

Area of Science:

• Biophysical Chemistry
• Macromolecular Science
• Analytical Chemistry

Background:

• Macromolecular ionic charge complicates molecular weight determination via the Donnan effect.
• Accurate molecular weight measurement is crucial for understanding macromolecule behavior and structure.
• Traditional methods for assessing charge effects can be limited by membrane properties.

Purpose of the Study:

• To develop and validate a method for determining the effective charge of macromolecules in solution.
• To correct apparent molecular weight measurements affected by the Donnan effect.
• To explore macromolecular structural features by relating effective charge to titratable charge.

Main Methods:

• Utilizing a moving boundary of sedimenting molecules in a centrifugal field as a pseudo-membrane.
• Employing interference optics to monitor salt gradients caused by the Donnan effect.
• Performing sedimentation equilibrium experiments to obtain apparent molecular weights.

Main Results:

• The method successfully determined the effective charge of heparin salts (Na, Cs, Ca).
• Corrected molecular weights aligned with expected values, validating the approach.
• Analysis of effective charge ratio and axial charge spacing provided insights into ion binding, with Cs heparin showing excess binding.

Conclusions:

• The sedimentation-based method offers a robust way to determine effective charge and true molecular weight of charged macromolecules.
• The findings support the use of effective charge in conjunction with Manning counter-ion condensation theory for structural analysis.
• The study highlights differential ion binding behaviors for different counter-ions with heparin.