Water-Soluble Polyion Complex Associates of DNA and Poly(ethylene glycol)−Poly(l-lysine) Block Copolymer

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Abstract
Complex formation of poly(ethylene glycol)−poly(l-lysine) (PEG−PLL) AB type block copolymer with salmon testes DNA or Col E1 plasmid DNA in aqueous milieu was studied. The PLL segment of PEG−PLL interacts with nucleic acid through an electrostatic force to form a water-soluble complex associate with a diameter of ca. 50 nm. PEG segments surrounding the core of the polyion complex prevented the complex from precipitation even under stoichiometric conditions, at which the unit ratio of l-lysine in PEG−PLL and phosphate in the DNA is equal. The profile of the thermal melting curve revealed a higher stabilization of DNA structure in PEG−PLL/DNA complexes compared to that in the complex made from DNA and PLL homopolymer with the same molecular weight as the PLL segment in PEG−PLL. This stabilizing effect on the DNA structure may be due to the compartmentalization of DNA into the microenvironment of PEG with low permittivity. The reversible nature of the PEG−PLL/DNA complex was further verified through the addition of polyanion [poly(l-aspartic acid)]: Poly(l-aspartic acid) replaced DNA in the complex with PEG−PLL, resulting in the release of free DNA in the medium. Furthermore, the PEG−PLL/DNA complex showed high resistance against DNase I attack, suggesting DNA protection through the segregation into the core of the associate having PEG palisade.