Complexation of DNA with positive spheres: Phase diagram of charge inversion and reentrant condensation

The phase diagram of a water solution of DNA and oppositely charged spheric al macroions is studied. DNA winds around spheres to form beads-on-a-string complexes resembling the chromatin 10 nm fiber. At small enough concentrat ion of spheres these "artificial chromatin" complexes are negative, while a t large enough concentrations of spheres the charge of DNA is inverted by t he adsorbed spheres. Charges of complexes stabilize their solutions. In the plane of concentrations of DNA and spheres the phases with positive and ne gative complexes are separated by another phase, which contains the condens ate of neutral DNA-spheres complexes. Thus, when the concentration of spher es grows, DNA-spheres complexes experience condensation and resolubilizatio n (or reentrant condensation). Phenomenological theory of the phase diagram of reentrant condensation and charge inversion is suggested. Parameters of this theory are calculated by microscopic theory. It is shown that an impo rtant part of the effect of a monovalent salt on the phase diagram can be d escribed by the nontrivial renormalization of the effective linear charge d ensity of DNA wound around a sphere, due to the Onsager-Manning condensatio n. We argue that our phenomenological phase diagram or reentrant condensati on is generic to a large class of strongly asymmetric electrolytes. Possibl e implications of these results for the natural chromatin are discussed. (C ) 2001 American Institute of Physics.