Interactions between oligonucleotides and cationic polymers investigated by fluorescence correlation spectroscopy

Purpose, To evaluate whether fluorescence correlation spectroscopy (FCS) ca n be used to characterize the complexation between oligonucleotides and cat ionic polymers. Methods. The features of the complexes between rhodamine labeled oligonucle otides (Rh-ONs) and poly(2-dimethylamino)ethyl methacrylate (pDMAEMA), poly (ethylene glycol)-poly(ethyleneimine) (pEG-pEI), and diaminobutane-dendrime r-(NH2)(64) (DAB(64)) were characterized by light scattering, electrophoret ic mobility, elcctrophoresis, and FCS. Results. At low polymer/Rh-ON ratios, a decrease of the fluorescence of the Rh-ONs was observed on binding of the Rh-ONs to all cationic polymers. Thi s was explained by the creation of "multimolecular complexes" in which the Rh-labels quench each other. The multimolecular complexes, which are highly fluorescent as they carry a number of Rh-ONs, resulted in high fluorescenc e peaks in the fluorescence fluctuation profile as measured by FCS. For pDM AEMA and DAB(64), at higher polymer/Rh-ON ratios the fluorescence of the po lyplexes increased, caused by the formation of "mono-molecular complexes." which consist of only one Ph-ON per poly mer. In the case of pEG-pEI, the f luorescence stayed constant when the polymer/Rh-ON ratio increased, so mult imolecular polyplexes remained. FCS confirmed these results as the high flu orescence peaks disappeared in case of pDMAEMA/Rh-ON and DAB(64)/Rh-ON disp ersions, but remained present for pEG-pEI/Rh-ON dispersions. Conclusions. FCS seems applicable for study of the interactions between ONs and different types of cationic polymers.