Flory Theory of a Polyampholyte Chain

Overview & Synopsis

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 t=N/[u(N++N-)], where u is the ratio of the Bjerrum length lB 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 (N+-N-)2/(N+ +N-) independent of the temperature. The nearly symmetric chains with |N+-N-|≤(N+ +N-)1/2 directly cross over from regime (i) to regime (iii), but stay nearly spherical.