Temperature and Sequence-Dependent Conformational Properties of Polyzwitterions

We have performed Langevin dynamics simulations of single polyzwitterion chains by accounting for the combined effects of the hydrophobic interactions from the nonpolar chain backbone and the dipolar interactions among all zwitterionic groups attached to the chain backbone. In the present coarse-grained united atom model of polyzwitterions, the solvent quality is represented as the hydrophobicity parameter and in terms of a change in temperature, and the dipolar interactions in terms of orientations of the zwitterionic groups and the dielectric constant of the medium, which is taken as a constant. By extracting the size exponent of a chain under different temperatures, we have deduced the Θ-temperature and its variation with respect to the dipole moment and distribution of dipolar units. The Θ-temperature of a polymer chain, whose every monomer is explicitly zwitterionic, shows a quadratic dependence on the dipole moment of the zwitterionic group, which is buttressed by theoretical considerations. These results from simulations show that the ansatz of randomly oriented zwitterionic groups determining polyzwitterion solution behaviors is inapplicable. Furthermore, for polymer chains with half zwitterionic monomers and the other half with nonpolar monomers, we find that the Θ-temperature is lower for a more homogeneous distribution of dipoles. The obtained results provide a rough estimate of the location of the critical temperature of polyzwitterion phase separation and its sequence-dependent modification.