Highly charged macroion surfaces in solutions of multivalent electrolytes attract each other electrostatically through correlations in their counterion environment. We show that significant correlation occurs when the counterion distribution has a pseudo-two-dimensional character. This allows us to treat the electrostatic correlation attraction semianalytically by reducing the problem to interaction between layers of adsorbed but mobile counterions neutralizing surfaces of similar charge density. Both when the counterion distribution is in the two-dimensional limit, and when it has a more realistic three-dimensional character, Coulomb repulsion between counterions produces an alternation of positive and negative charges at the surface. Two such apposing patterns adjust complementarily to each other, resulting in electrostatic attraction of the surfaces. The magnitude of this attraction depends solely on the surface charge density and the solution dielectric constant, while its range is defined by the size of the planar correlation hole around each ion. The attraction is stable with respect to the disruptive influence of planar thermal motion of the ions. The theory enables construction of a universal function which, after being scaled with the appropriate parameters of the system, yields the attractive electrostatic correlation pressure.

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As double-stranded DNA is stretched to its B-form contour length, models of polymer elasticity can describe the dramatic increase in measured force. When the molecule is stretched beyond this contour length, it shows a highly cooperative overstretching transition. We have measured the elasticity and overstretching transition as a function of monovalent salt concentration by stretching single DNA molecules in an optical tweezers apparatus. As the sodium ion concentration was decreased from 1000 to 2.57 mM, the persistence length of DNA increased from 46 to 59 nm, while the elastic stretch modulus remained approximately constant. These results are consistent with the model of Podgornik, et al. (2000, J. Chem. Phys. 113:9343-9350) using an effective DNA length per charge of 0.67 nm. As the monovalent salt concentration was decreased over the same range, the overstretching transition force decreased from 68 to 52 pN. This reduction in force is attributed to a decrease in the stability of the DNA double helix with decreasing salt concentration. Although, as was shown previously, the hydrogen bonds holding DNA strands in a helical conformation break as DNA is overstretched, these data indicate that both DNA strands remain close together during the transition.

The highly cooperative elongation of a single B-DNA molecule to almost twice its contour length upon application of a stretching force is interpreted as force-induced DNA melting. This interpretation is based on the similarity between experimental and calculated stretching profiles, when the force-dependent free energy of melting is obtained directly from the experimental force versus extension curves of double- and single-stranded DNA. The high cooperativity of the overstretching transition is consistent with a melting interpretation. The ability of nicked DNA to withstand forces greater than that at the transition midpoint is explained as a result of the one-dimensional nature of the melting transition, which leads to alternating zones of melted and unmelted DNA even substantially above the melting midpoint. We discuss the relationship between force-induced melting and the B-to-S transition suggested by other authors. The recently measured effect on T7 DNA polymerase activity of the force applied to a ssDNA template is interpreted in terms of preferential stabilization of dsDNA by weak forces approximately equal to 7 pN.

A theory of condensation and resolubilization of a dilute DNA solution with growing concentration of multivalent cations, N is suggested. It is based on a new theory of screening of a macroion by multivalent cations, which shows that due to strong cation correlations at the surface of DNA the net charge of DNA changes sign at some small concentration of cations N0. DNA condensation takes place in the vicinity of N0, where absolute value of the DNA net charge is small and the correlation induced short range attraction dominates the Coulomb repulsion. At N > N0 positive DNA should move in the oppisite direction in an electrophoresis experiment. From comparison of our theory with experimental values of condensation and resolubilization thresholds for DNA solution containing Spe 4+ , we obtain that N0 = 3.2 mM and that the energy of DNA condensation per nucleotide is 0.07 kBT .