Polyelectrolytes in biology and soft matter

Many important biological macromole-cules, such as DNA, RNA, polypeptides, and polysaccharides, as well as large number of artificial macromolecules, such as poly(styrene sulfonate) (PSS) and pol-yacrylic acid (PAA), are highly charged polyelectrolytes. Commercial applica-tions of polyelectrolytes range from colloidal stabilization, flocculation and flow modification to superabsorbent gels (diapers being a prominent example). Electrostatic interactions, mediated by mobile ions and water, play crucial role in these systems, influencing a molecule’s structure, physical properties and function. These macromolecules often self-organize by electrostatic forces into large-scale superstructures. This is exem-plified by DNA compaction into chro-matin fiber in the nuclei of eukaryotic cells. Viral self-assembly, including proteins, DNA or RNA provides another salient example. The reason poly-electrolytes are so prevalent in life and their properties are so difficult to under-stand is related to long-range electrostatic interactions between charged groups on the chains as well as their interactions with mobile counterions and salt. Given the importance and ubiquity of polyelectrolytes both in artificial and biological systems, a growing number of theoretical and experimental investiga-tions have been addressing various aspects of polymer physics and biophysics in these systems. Among the major open problems is the question of how the ionic atmosphere around the polyelectrolyte couples to its internal modes. This, in turn, may explain how electrostatic effects contribute to determining the persistence length of polyelectrolytes. In the pio-neering work of Odijk, 1 Skolnick and Fixman 2 (OSF), a Debye–H€ uckel based approach to modeling electrostatic inter-actions was used to define an electrostatic persistence length of semiflexible polyelectrolytes proportional to the square of the Debye screening length. This approach was extended to flexible polyelectrolytes by Khokhlov and Kha-chaturian 3 with predictions similar to the OSF theory. Barrat and Joanny 4 and Dobrynin 5 (BJD) introduced alternative models in which the electrostatic persis-tence length of flexible polyelectrolytes is on the order of the Debye screening length, and is much shorter than the OSF prediction. 6 Experimental data on flexible polyelectrolytes seems to be more consis-tent with BJD predictions. 7,8 Manning recently suggested that the interplay between the electrostatic self-repulsion and the polymer chain buckling is important in determining the poly-electrolyte persistence length. 9 Atomistic and coarse-grained simulations by Pa-poian and coworkers showed that the electrostatic contribution to DNA’s large persistence length is significantly higher than OSF predictions at physiological salt concentrations. 10 Thus the effect of many-body interactions between ions in solu-tions and polyelectrolyte ions on overall chain rigidity is still poorly understood, and much more work is needed to achieve a quantitative agreement between theory and experiments. A number of major challenges remain in the field of biological polyelectrolytes. Hydration-mediated interactions are not well described at distances at and below 1 nm from the polyelectrolyte surfaces, which leads to significant difficulties in applying continuum electrostatics-based models. It has been known for over hundred years that different small counter-and co-ions may result in dramatically different overall behaviors of polyelectrolytes, even when their charges are the same. 11 Although many aspects of ion-specific interactions have been recently understood, 12 we are still far from a satisfactory simple conceptual model explaining the large amounts of experimental data. Interestingly, it was recently discovered from atomistic simu-lations that major biological poly-electrolytes, such as DNA and proteins preferentially interact with Na