Polyelectrolyte and polyampholyte effects in synthetic and biological macromolecules

Abstract

The nature of electrostatic interactions involving polyanions modulate the properties of both synthetic and biological macromolecules. Although intensely studied for decades the interplay of many length scales has prevented a complete resolution of some of the basic questions such as how the electrostatic persistence length ($l_e$) varies with ionic strength ($I$). In this review we describe certain characteristics of polyelectrolytes (PAs) and polyampholytes (PAs), which are polymers whose monomers have a random distribution of opposite charges. After reviewing the current theoretical understanding of the dependence of $l_e$ on $I$ we present experimental data that 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 and pure polyelectrolyte effects for the shape of PAs that is valid over a wide range of salt concentration is described. We also use theory and simulations to argue that the distinct stages in the kinetics of collapse of PAs (with a net charge that is small enough to induce globule formation) and PEs (relevant for RNA folding) are similar. In both cases the major initial conformation change involves formation of metastable pearl-necklace structures. In the coarsening process large clusters (pearls) grow at the expense of smaller ones by a process that is reminiscent of Lifshitz-Slyozov mechanism. Finally, recent theories and single molecule experiments on stretching of single stranded DNA and PEs further sheds insights into the complex behavior of charged macromolecules. The survey, which is limited to very few topics, shows the importance of polyelectrolyte effects in a wide range of disciplines.