Phase diagrams of DNA and poly(styrene-sulfonate) condensed by a poly-cationic protein, the salmon protamine

Protamines are important biomacromolecules in many respects: they compact DNA most efficiently in the sperm head; they are commonly used in the formulation of non-toxic and efficient gene carriers and they have a very interesting structural charge in-between the small multivalent ions and the large liposomes, colloids or long polycations. In this experimental study, we examine in detail how these small basic proteins induce DNA condensation. A study of the phase separation of a synthetic polyelectrolyte, Na-poly(styrene-sulfonate) (PSS), in aqueous solution with protamines also explores the salt-free range domain and completes this work in order to get a general and broad view of the most representative phase diagram. From the two sets of data, phase diagrams and unique representations are proposed. The solubility of DNA or PSS seems to depend on the ionic conditions through two parameters: C(+)/C(-) and L/r(s). L/r(s) denotes the length ratio between the polyanion size and the Debye screening length and C(+)/C(-) denotes the charge concentration ratio of protamine versus DNA or PSS. Protamines bind to polyanions and condense them concomitantly. The presence of soluble condensed and uncondensed polyanions delimits the different domains of the diagram defined by the axes C(+)/C(-) and L/r(s). Another representation emerges only when the molar amount of salt prevents the protein binding up to a certain threshold of protein concentration.