On the Continuity of States between DNA-Ordering Transitions in Mono- and Multi-Valent Salt Solutions

Abstract

At the increasing density imposed by external osmotic pressure, DNA in monovalent salt solutions (e.g., NaCl) to goes through a set of ordered mesophases, finally crystallizing into an orthorhombic crystal. While the transition from the (chiral) nematic to the line-hexatic phase has been observed before, it is still unclear whether the transition is of second order or weak first order. In multivalent salt solutions (e.g., CoHex = Co(NH3)6 Cl3), DNA is known to collapse into an ordered aggregate under osmotic pressure, or even at zero osmotic pressure at large enough concentrations of the multivalent salt. It has remained unclear what is the connection between these two ordering transitions -- with vs. without CoHex. By using a novel method of temperature-regulating the osmotic stress imposed on the DNA subphase, we show that there exists a continuity of states between these two ordering transitions and that they are of the same type. We use the small but accurately measurable temperature dependence of the osmotic pressure of a PEG solution to fine-regulate the osmotic stress with which it acts on the DNA subphase. This allows us to set the osmotic pressure to an accuracy never achieved before. This advance in experimental methodology allows us then to detect small but nevertheless finite changes in the density of DNA as it goes through the ordering transitions. In this way, we first determine experimentally the small density change that occurs at the (chiral) nematic to line-hexatic phase transition. For Na-DNA, this density change can be translated into a ∼1.5 Angstrom change in the interaxial spacing. This density discontinuity at the phase transition either does not depend on the NaCl concentration or varies by < 0.1 Angstrom as the salt concentration is varied.