A thermodynamic characterization of the interaction of a cationic copolymer with DNA

In recent years, the design of non-viral artificial gene delivery systems h as been an important trend in the field of gene therapy. Such systems inclu de the use of copolymer-DNA complexes due to the ionic interactions among t he participating species. The resulting complexes are stable in aqueous dis persion, despite complete charge neutralization. To optimize the biological activity of these complexes, it is important to have a complete knowledge of their physicochemical properties. In this work, we report on the interac tion of a cationic graft copolymer, poly(ethylene oxide) -g-polyethylenimin e (PEO-g-PEI) with poly[d(AT)]. poly[d(AT)] (DNA). A combination of gel ele ctrophoresis, optical, and calorimetric techniques is used to obtain a comp lete thermodynamic description for both the unfolding of the free and polyc ation bound DNA, and the interaction of the polycation with DNA. The copoly mer-DNA complexes are produced spontaneously resulting from the formation o f ion pairs between ionized amino groups of PEI segments of the copolymer a nd the phosphate groups of DNA. Polycation binding reduces the cooperative unfolding of the DNA without changing the overall conformation of the polyn ucleotide. The complete thermodynamic profiles show that the interaction of this particular polycation with DNA is generally electrostatic in nature b ecause it exhibits the typical effects induced by increasing the salt conce ntration. The favorable formation of the polycation-DNA complex is entropy driven and consistent with the observed removal of counterions. The thermod ynamic approach taken for this investigation is appropriate, but in order t o improve conditions for better DNA delivery systems further investigations of other systems are needed. These systems will have to include variation of the copolymer length, changes in the hydrophilic-hydrophobic balance of the copolymer, as well as the sequence, length, and conformation of DNA.