Interaction of short-fragmented DNA with dipalmitoylphosphatidylcholine bilayers in presence of zinc

Abstract

The structure and temperature behaviour of the DNA+dipalmitoylphosphatidylcholine (DPPC) bilayer as a function of ZnCl2 concentration were examined using differential scanning calorimetry (DSC), small-angle neutron scattering (SANS) and small-angle X-ray diffraction (SAXD). Experiments revealed the coexistence of two lamellar phases in the mixture: the L(PC) phase, formed due to Zn(2+) binding to the DPPC bilayers, and the condensed lamellar phase L(DNA+PC) with DNA strands packed between the DPPC bilayers. With increasing concentration of zinc, the temperature of the gel - liquid-crystal phase transition of DPPC increases in both phases, and the volume fraction of L(DNA+PC) phase decreases. In the gel state (at 20 degrees C), the repeat distance of L(DNA+PC) phase is constant, d(DNA+PC) approximately 8.3 nm, up to 20 mmol/l of ZnCl2, and increases for higher concentrations of the salt. The periodicity of the L(PC) lamellar phase decreases substantially with the increasing concentration of the salt in the mixture. In the liquid-crystalline state, concentrations above 20 mmol/l ZnCl2 promote the dissolution of the L(DNA+PC) phase into DPPC + Zn(2+) unilamellar vesicles and DNA is neutralized by Zn(2+) ions. The screening of Zn(2+) charge and the formation of a diffuse double layer due to increasing ionic strength of solution are responsible for the observed changes.