Potentiometric behavior of polyampholytes

The influence of nearest‐neighbor interaction on the potentiometric behavior of polyampholyte is discussed. It is shown how this effect superposes itself on the normal random electrostatic interactions between segments of the chain. Only interactions between oppositely charged nearest neighbors are considered. Other specific interactions, such as ion binding and nearest‐neighbor interaction between like charges, are neglected as being of interest only in special cases. A simplified model is treated in which one of the groups is present in large excess while the other exists only in singlemembered sequences in the chain. It is shown that positive‐negative nearest‐neighbor interaction always has the effect of increasing the strength of the group, an acidic group becoming a stronger acid, a basic group, a stronger base. The effect is most pronounced for groups which are already strong, and, in this case, the titration (proton binding) curve of the polyampholyte is considerably changed; on the other hand, if both groups are weak, the titration curve is hardly affected, even when the interaction is strong. In this case, however, the pH dependence of the degree of ionization of the group present in minority shows peculiarities. The isoelectric point of polyampholytes can be considerably affected by (+ −) nearest‐neighbor interaction, that of molecules containing an excess of basic groups being raised by the interaction, and that of polyampholyte containing an excess of acidic groups being lowered. These shifts are again far more pronounced when the groups involved are strong. It is shown how use can be made of these predictions to analyze and understand the behavior in practical cases. The data for two types of synthetic and one naturally occurring polyampholyte are interpreted in this way, and satisfactory results are obtained