COUNTERION CONDENSATION - EFFECTS OF SITE BINDING, FLUCTUATIONS IN NEAREST-NEIGHBOR INTERACTIONS, AND BENDING

A direct binding approach to counterion condensation is proposed that incorporates both site binding and fluctuations in nearest-neighbor interactions as well as mean-field interactions between more distant neighbors. The predicted counterion binding ratio is identical to Manning's limiting law at zero salt concentration and departs only slightly from Manning's mean-field result up to 4 mM salt. The present theory is generalized to incorporate the effects of bending, especially the increase in configurational entropy due to the decrease in electrostatic bending rigidity caused by counterion binding. With this modification, the present theory predicts a much more rapid onset of counterion condensation as xi increases beyond 1.0 than is exhibited by the straight-chain theories. Comparison with experiment suggests that this predicted enhancement of counterion binding due to bending may be responsible for much or all of the discrepancy between the observations of Klein and Ware and the results of straight-chain theories. The present results also suggest a mechanism whereby the rapid onset of counterion condensation as xi increases past 1.0 might trigger a structural transition from a more compact or folded state to a more extended conformation.