Effect of Charged Block Sequence on Dilute Polyampholyte Solution Behavior

Polyampholytes (PAs) are polymers containing both positively and negatively charged groups along their backbone. Previous studies have found pH and salt greatly affect phase behavior and viscoelastic properties of PA solutions through electrostatic interactions. However, limitations in PA synthesis prevented further exploration of charge arrangement on these systems. Nonetheless, simulations have shown larger oppositely charged blocks lead to greater collapse due to attractive Coulombic interactions. 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. Solubility of these PAs decrease as block size increases, and the addition of NaCl increases solubility. We observed charge block size may play a more complex role in the collapse of the PAs especially relative to ionization state and counterion interactions, from pH titration and scattering experiments, respectively. PAs at high salt concentrations behave more similarly to neutral polymers due to electrostatic shielding, and PAs at extreme pH's behave more consistent with polyelectrolytes. This study demonstrates charge sequence is as notable as charge density and symmetry in tuning PA materials.