DNA condensation with spermine dendrimers: interactions in solution, charge inversion, and morphology control

Abstract

In this report, we characterize the formation of structured condensates of 884 base pair, double stranded DNA and spin-labeled, second generation dendrons (SL-G2) that are spermine-based and cationic using continuous wave electron paramagnetic resonance spectroscopy (CW EPR) and transmission electron microscopy (TEM). The electrostatic interaction between DNA and SL-G2 in solution leads to condensation of DNA into densely packed structures. At a particular charge ratio of 2.3 (cationic charges/anionic charges), the structures appear as thick rod-like condensates of parallelly ordered, stretched DNA and SL-G2. Depending on the concentration of DNA, the charge ratio and the ionic strength, which has been adjusted with monovalent salts, a large variety of structures were observed by TEM. By adding manganese(II) salts charge inversion of DNA could be observed by CW EPR. This could be achieved, because paramagnetic Mn2+ ions are usually electrostatically bound to the strong DNA polyions immediately after adding the salt and only give rise to an EPR signal when expelled from the DNA molecules. At a charge ratio of 2.3 the Mn(II) ions are expelled from the formed DNA dendriplexes, which indicates an inverted charge of the DNA. Since CW EPR spectra of the nitroxide-based SL-G2 and Mn2+-ions are spectroscopically distinct, the interaction of the condensation agent with DNA and the interaction of DNA with Mn2+ counterions could be observed simultaneously. While the interaction between condensation agent and DNA did not change, irrespective of the conditions present in the solution, condensate/aggregate morphology changed drastically when the conditions were varied, as could be inferred from the Mn2+ signal and TEM data.