Persistence Length of Intrinsically Stiff Polyampholyte Chains

We calculate the contribution, l pA , to the persistence length arising from charge fluctuations of an intrinsically stiff polyampholyte (PA) chain. The interaction between charges along the PA backbone is taken to be given by the Debye-Huckel potential. When the charges along the chain are uncorrelated the contribution to l pA comes from two sources. One of them is due to the overall charge on the PA chain for which the contribution to l pA OC K -2 where K -1 is the Debye screening length. Surprisingly the contribution to l pA from charge fluctuations (δσ) 2 , namely due to the polyampholyte effect, is proportional to -(δσ) 4 k -1 so that there is a reduction in the total persistence length when PA chain is overall neutral. We also show that the shape of a PA chain is cylindrical with length l p = l 0 + l pA with l 0 being the bare persistence length. The diameter of an overall neutral chain can become of the order of a monomer size which is considerably smaller than that of the corresponding polyelectrolyte. As a consequence, we argue that the interaction between two neutral stiff polyampholyte chains is attractive. The implication of the effective attractive interaction is that a dilute dispersion of neutral polyampholyte PA chains would phase separate into dense (with possible nematic order) and a rare phase. When correlations between the charges are included one gets a polyelectrolyte like behavior with l pA α k -2 even when the chain is neutral. If the range of correlation, λ, is large compared to the screening length (usually difficult to obtain in experiments) there is a large negative contribution to l pA which scales as l pA α -(δσ) 4 λ/k 2 where in is the Bjerrum length.