Condensation of model chromatin in the form of fully saturated 12-mer nucleosome arrays, induced by addition of cationic ligands (epsilon-oligolysines with charge varied from +4 to +11), was studied in a range of KCl concentrations (10-500 mM) using light scattering and precipitation assay titrations. The depmdence of EC50 (ligand concentration at the midpoint of the array condensation) on C-kappa Cl displays two regimes, a salt-independent at low C-kappa Cl and a salt-dependent at higher salt concentrations. In the salt-dependent regime EC50 rises sharply with increase of C-kappa Cl. Increase of ligand charge shifts the transition from the salt-independent to salt-dependent regime to higher salt. In the nucleosome array system, due to the partial neutralization of the DNA charge by histones, a lower oligocation concentration is needed to provoke condensation in the salt-independent regime compared to the related case of DNA condensation by the same cation. In the physiological range of salt concentrations (C-kappa Cl = 50-300 mM), K+ ions assist array condensation by shifting EC50 of the epsilon-oligolysines to lower values. At higher C-kappa Cl, K+ competes with the cationic ligands, which leads to increase of EC50. Values of salt-dependent dissociation constant for the E-oligolysine-nucleosome array interaction were obtained, by fitting to a general equation developed earlier for DNA, describing the dependence of EC50 on dissociation constant, salt and polyelectrolyte concentrations. (C) 2011 Elsevier Inc. All rights reserved.