Molecular interactions of DNA with transfectants: a study based on infrared spectroscopy and quantum chemistry in aid to fluorescence spectroscopy and dynamic light scattering analyses

Cationic polymers are promising non-viral agents because of their easiness of use, inexpensiveness and favorable safety profile. Being inherently cationic, they do spontaneously assemble in water with nucleic acids to give polyplexes. The more popular techniques and methods routinely applied for the characterization of such nanosupensions, unfortunately, do not afford direct observation of anion-cation interactions. We thus tackle the issue of probing at the very molecular level the interplay occurring between the PO4- groups of double stranded DNA and the NH3+ groups of the simple and representative transfectant poly(L-Lysine). By means of IR spectroscopy and Density Functional Theory (DFT) calculations we assigned the red shift of the anti-symmetric PO stretching wavenumber to the intermolecular water- mediated NH3+:PO4- interaction. These changes observed in moving from ineffective, low nitrogen (N) to DNA phosphate (P) ratios (N/Ps) to more effective conditions are in overall good agreement with changes in polyplex behavior unveiled by means of other analytical techniques such as dynamic light scattering, laser Doppler micro-electrophoresis, and fluorescence spectroscopy, but provide direct insight into the basic intermolecular DNA:transfectant interplay. The application of IR spectroscopic analysis to other non-viral gene delivery vectors may become an appealing approach to disclose molecular details on PO4- :cationic polymer interaction, taking into account conformational constraints and steric hindrance effects that the other techniques actually overlook.