Positively charged elastin-like polypeptides (ELPs) were synthesized for the compaction of genetic material. A recombinant ELP (VPGXG)40 with X = V,M (3:1) was post-modified in two steps to introduce chemoselectively either primary or secondary amine pendant groups at each methionine residue. Positively charged ELPs were characterized by SDS-PAGE, size exclusion chromatography, 1H NMR, potentiometric titrations and dynamic light scattering to assess their purity and determine their degree of functionalization, molecular weight, isoelectric point and thermo-responsive behaviour. Electrostatic complexation between the different ELP derivatives and nucleic acids was studied to determine the stoichiometry of ELPS/nucleic acids complex formation, and to find optimal conditions leading to stable nanoparticles with controlled size and surface potential. The stability of these complexes was investigated in the presence of salts at physiological concentrations and in the presence of surfactant. This study revealed that two regimes of stable nanoparticles in terms of size and charge can be obtained from the electrostatic complexation between the primary amine containing ELP derivative, ELP(-NH2), and plasmid DNA. Resulting complexes were found to be stable to dissociation for charge ratios up to 2.5 under physiological salt concentrations (154 mM NaCl), showing that plasmid DNA was completely condensed by the polycationic ELP and protected against electrolyte-mediated dissociation.Read less <