pH Effects on Conformation and Ionization of Charge Sequence Varied Polyampholytes

Polyampholytes (PAs) are polymers containing both positively and negatively charged groups along their backbone. Previous studies have found pH and salt affect phase behavior and viscoelastic properties of PA solutions through electrostatic interactions. From weak polyelectrolyte (PE) pH studies, the increase in charge fraction has been shown to decrease ionization of charged groups due to electrostatic repulsion of neighboring like charges. Conformation has also been shown to affect ionization of charged groups in PEs. Electrostatic attraction between oppositely charged groups in weak PAs, in contrast to PEs, create a favorable ionization environment that counter electrostatic repulsion between like charges. Hence, using solid phase peptide synthesis, we synthesized a set of poly-L-(lysine, glutamic acid) peptides with neutral net charge consisting of 32 residues arranged in increasing block sizes to evaluate the effect of increasing neighboring electrostatic repulsion with increasing block size with oppositely charged groups. From pH titrations and small angle X-ray (SAXS) experiments, we find increasing buffering behavior with growing block length as well as extension of the molecule, approaching PE behavior at extreme pHs. The PAs do not demonstrate clear pKas, unlike expectations from the titration of individual monomers. We further discuss effects of pH on ionization states and hydrogen bonding of charge sequence varied PA systems and its implications in conformation and solution phase behavior using NMR and SAXS.