Effect of divalent cations on DNA condensed on the surface of rigid cationic membrane

It has been indicated in previous literature reports that divalent cations cannot directly condense DNA in aqueous solution. However, several studies have shown that divalent metal ions can induce a further condensation of DNA originally adsorbed on the surfaces of fluid cationic membranes. In this article, we examine the effect of divalent metal ions (Ca2+ and Ni2+) on DNA condensed on the surfaces of a rigid cationic membrane formed by the mixture of cholesteryl 3-N-(dimethyl-aminoethyl) carbamate (DC-Chol) and 1,2-di(cis-9-octadecenoyl)-sn-glycero-3-phosphocholine (DOPC). Our results demonstrate that the divalent cations exert an electrostatic screening effect between DNA, where the in-plane spacing between DNA (dDNA) confined between the lipid bilayers gradually decreases with increasing cation concentration when the concentration exceeds a threshold value. The collapse transition, where dDNA drops discontinuously at a critical cation concentration, observed previously for the fluid membrane is not identified here, indicating the critical role of membrane flexibility in the two-dimensional condensation of DNA.