Polyelectrolyte and polyampholyte effects in synthetic and biological macromolecules

摘要

The nature of electrostatic interactions involving polyanions modulate theproperties of both synthetic and biological macromolecules. Although intenselystudied for decades the interplay of many length scales has prevented acomplete resolution of some of the basic questions such as how theelectrostatic persistence length ($l_e$) varies with ionic strength ($I$). Inthis review we describe certain characteristics of polyelectrolytes (PAs) andpolyampholytes (PAs), which are polymers whose monomers have a randomdistribution of opposite charges. After reviewing the current theoreticalunderstanding of the dependence of $l_e$ on $I$ we present experimental datathat conform to two distinct behavior. For RNA and DNA it is found that that$l_e \sim I^{-1}$ whereas for some proteins and other polyelectrolytes $l_e\sim i^{-1/2}$. A scaling type theory, which delineates charge correlation andpure polyelectrolyte effects for the shape of PAs that is valid over a widerange of salt concentration is described. We also use theory and simulations toargue that the distinct stages in the kinetics of collapse of PAs (with a netcharge that is small enough to induce globule formation) and PEs (relevant forRNA folding) are similar. In both cases the major initial conformation changeinvolves formation of metastable pearl-necklace structures. In the coarseningprocess large clusters (pearls) grow at the expense of smaller ones by aprocess that is reminiscent of Lifshitz-Slyozov mechanism. Finally, recenttheories and single molecule experiments on stretching of single stranded DNAand PEs further sheds insights into the complex behavior of chargedmacromolecules. The survey, which is limited to very few topics, shows theimportance of polyelectrolyte effects in a wide range of disciplines.