Flory Theory of a Polyampholyte Chain

(Received 8 August 1995, revised 12 January 1995, accepted 8 February 1995)

Abstract
The behavior of a polyampholyte chain is analyzed in the framework of the two-parameter Flory theory. We consider a N-mer containing N₊ positively charged monomers and N_- negatively charged ones. There are three different regimes for polyampholytes with charge asymmetry |N₊ -N_-|>(N₊ +N_-)^1/2 depending on the effective temperature , where u is the ratio of the Bjerrum length to the bond size b: (i) Unperturbed regime: at high effective temperatures t>t₁ the electrostatic interactions are unimportant and polyampholyte configuration is controlled by the solvent quality for the uncharged backbone, (ii) Polyelectrolyte regime: at intermediate effective temperatures t₂<t<t₁ the repulsion between uncompensated charges e(N₊-N_-) dominates over the charge-fluctuation-induced attraction. The chain is stretched into an array of electrostatic blobs, (iii) Polyampholyte regime (t<t₂): when the Debye screening length due to the charges on the chain becomes smaller than the electrostatic blob size, the charge-fluctuation-induced attraction collapses the polyampholyte into an elongated globule with aspect ratio independent of the temperature. The nearly symmetric chains with directly cross over from regime (i) to regime (iii), but stay nearly spherical.