Conformational Behavior of Polyampholytes with Different Charge Statistics in a Good Solvent

We consider single-chain globally neutral polyampholytes under good solvent conditions. Theory predicts that for PAs, the transition from a swollen coil to a condensed globule is highly sequence-dependent. To test these predictions, molecular dynamics simulations are performed for three type of PAs: diblock, random and alternating. Diblock PAs exhibit a continuous contraction, and the gyration radius follows the theoretically expected scaling law of R_g ∼ u^-0.18 . Shrinking of random PAs is qualitatively similar, R_g ~ u^-0.62 , albeit sequence polydispersity and a narrower transition width make it harder to precisely confirm the theoretical scaling slope for the chain lengths considered. The behavior of alternating chains however, is drastically different. In them, charges appear in pairs which enables them to be viewed as chains of connected dipoles. Short-range Coulomb attractions between dipoles only renormalizes the second virial coefficient. This makes the coil-to-globule transition in alternating PAs analogous to that in neutral polymers. The sharpness of the transition increases with chain length, making it a phase transition in the limit of infinitely long chains. This is supported by collapsing curves for alternating PAs for different solvent qualities and chain lengths on each other and on top of that of neutral polymer chains. They all fall onto one universal master curve well described by the theory of the coil-to-globule transition in non-ionic polymers.