A Monte Carlo Study of Weak Polyampholytes:  Stiffness and Primary Structure Influences on Titration Curves and Chain Conformations

The conformation and titrn. curves of weak polyampholytes are examd. using Monte Carlo simulations with screened Coulomb potentials in the Grand Canonical ensemble. Two different types of monomers are considered. Depending on the soln. pH, monomers A are weak acidic sites that can either be neg. charged or uncharged (as carboxylic groups), whereas monomers B are weak basic sites that can either be pos. charged or uncharged (as amino groups). The influence of the chain stiffness, primary structure, and ionic concn. on the acid/base properties of the polyampholyte chains are systematically investigated. By adjusting the pH values, titrn. curves and then the fractions of pos. and neg. ionized charged monomers are calcd. Stiffness influence is estd. by comparing two models of chain: a fully flexible and a rod-like polyampholyte. Different primary structures such as statistical (diblock, octablock, and alternating) and random polyampholytes are also considered. We demonstrate that the primary structure plays important roles in the acid/base properties as well as the charge distribution along the polymer backbone of a statistical rod-like polyampholyte. When flexible polyampholytes are considered, polyampholyte conformations promote the attractive electrostatic interactions between pos. and neg. charged monomers, hence leading to more or less compact conformations and acid/base properties relatively different in comparison to the rod-like polyampholytes. Various conformations such as extended, globular, and pearl-necklace conformations are found in good agreement with the literature by adjusting the interaction parameter between monomers and monomer stoichiometry.