Thermodynamics for the Interaction of PEG-PLL Copolymers with DNA

Abstract

One focus of our research is to select polycations to deliver oligonucleotides into the cell for the control of gene expression. In this work, we report on the interaction of poly(ethylene glycol)-b-poly-L-lysine (PEG-PLL) copolymer with a variety of DNA molecules. Specifically, three PEG-PLL copolymers with similar PEG segment but different length of the PLL chains were used to interact with DNA duplexes as a function of duplex length. A combination of spectroscopic and calorimetric techniques was used to investigate the unfolding of both DNA molecules and polycation-DNA complexes, and to determine their thermodynamic binding profiles. The resulting polycation-DNA complexes were stable in aqueous solution at room temperature. The binding of each copolymer to DNA stabilized the helix-coil transition of all DNA molecules, yielding binding affinities of ∼104 M-1, which were lowered by the increase in salt concentration. However, binding affinities of 105 were obtained with the ethhidium bromide displacement essay. Isothermal titration calorimetric experiments yielded negligible heats of interaction. Therefore, the favorable formation of the copolymer-DNA complexes is entropy driven which was rationalized in terms of the release of both counterions and water molecules upon complex formation. In summary, polycation binding to DNA was found to be electrostatic in nature, i.e., the positively charged lysine groups formed ion pairs with the negatively charged phosphate groups of DNA. Supported by Grant MCB-1122029 from the National Science Foundation.