Interactions between amphiphiles and DNA: A physico-chemical and morphologic study

The feasibility of gene therapy and the selectivity and effectiveness of nucleic acid transfection in vitro depend basically on the knowledge of the chemical processes underlying these aspects of molecular biology. Until recently, the introduction of a foreign nucleic acid into a cell in vitro or in vivo has been considered as a step which could essentially be performed by using commercially available transfection kits of undisclosed composition. However, the need of obtaining increasingly efficient, selective and safe transfections has led researchers to address the physico-chemical aspects of transfection. Particularly, attention has been focused on the kind and magnitude of the interactions between carrier molecules and nucleic acids, and the morphological changes induced by said interactions into the nucleic acid polymer such as, e.g., its condensation into a globular form which may pass more easily the cell membrane.[1] Our research group is involved in the study of the interactions among amphiphilic compounds and biological macromolecules,[2,3] and the present work reports some results of a multi-technique investigation on the morphological and physico-chemical modifications caused by cationic surfactants on a DNA duplex. The techniques employed in the present study were: 1) surface tension measurements; 2) conductometry; 3) UV-Vis spectrophotometry; 4) DNA melting; 5) Atomic Force Microscopy (AFM) topography; and 6) electron transmission microscopy (TEM). The surfactants studied were cetyltrimethylammonium bromide (CTAB) and cetyltributylammonium bromide (CTBAB), as compared with the corresponding non-amphiphile compounds tetramethylammonium bromide (TMAB) and tetrabutylammonium bromide (TBAB). Moreover, we have extended the comparison to a commercially available, efficient transfection carrier, ExGen 500 (Fermentas; lineare polyethyleneimine, ca. 22 kDa). The results show that the interactions of the above amphiphiles with a double-stranded DNA occur between surfactant monomers and nucleic acid, and micellar aggregates do not appear to have a role in this interaction. Spectrophotometry, conductometry and surface tension measurements allowed to determine a critical aggregation concentration (cac), at which the DNA molecules in solution change their folding morphology, presumably due to the surfactant "monomers" coating the macro-anion by ionic bonds with phosphate groups and hydrophobic interactions of alkyl tails. By DNA melting measurements, we obtained a quantitative estimate of the contribution of such diverse interactions to the total interaction energy of surfactants with DNA. Finally, AFM and TEM measurements supported the above physico-chemical changes with morphological evidence of a change in the folding patterns.