Simulations and Potentiometric Titrations Enable Reliable Determination of Effective pK(a) Values of Various Polyzwitterions
Abstract
We synthesized three different polyzwitterions-poly(N,N-diallyl glutamate) (PDAGA), poly(dehydroalanine) (PDha), and poly(2-(imidazol-1-yl)acrylic acid) (PImAA)-and investigated how their ionization states respond to changes in solution pH. We used molecular simulations to determine how the net charge per monomer and the ionization states of individual acidic and basic groups differ from the ideal (Henderson-Hasselbalch) behavior.
To complement the theoretical predictions, we performed potentiometric titrations and zeta-potential measurements of all studied polyzwitterions. By comparing these experiments with theoretical predictions, we could show that molecular simulations can predict and explain the origin of the differences between the effective and bare pK(a) values of individual titratable groups.
Furthermore, we have shown that it is not possible to obtain these effective pK(a) values directly from the equivalence point recognition criterion (ERC), commonly used in potentiometric titrations. However, the effective pK(a) values can be reliably obtained by calculating the net charge per monomer from the potentiometric titration curves and validating these results against theoretical predictions.
The approach we propose works reliably for polyzwitterions in which the ionization response is dominated by electrostatic interactions, such as PDAGA or PDha; however, it fails if other specific interactions contribute significantly, such as in the case of PImAA.