Interactions between oligonucleotides and cationic polymers investigated by fluorescence correlation spectroscopy

Purpose. To evaluate whether fluorescence correlation spectroscopy (FCS) can be used to characterize the complexation between oligonucleotides and cationic polymers. Methods. The features of the complexes between rhodamine labeled oligonucleotides (Rh-ONs) and poly(2-dimethylamino)ethyl methacrylate (pDMAEMA), poly(ethylene glycol)-poly(ethyleneimine) (pEG-pEI), and diaminobutane-dendrimer-(NH₂)₆₄ (DAB₆₄) were characterized by light scattering, electrophoretic mobility, electrophoresis, 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. This 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 fluorescence peaks in the fluorescence fluctuation profile as measured by FCS. For pDMAEMA and DAB₆₄, at higher polymer/Rh-ON ratios the fluorescence of the polyplexes increased, caused by the formation of monomolecular complexes, which consist of only one Rh-ON per polymer. In the case of pEG-pEI, the fluorescence stayed constant when the polymer/Rh-ON ratio increased, so multimolecular polyplexes remained. FCS confirmed these results as the high fluorescence peaks disappeared in case of pDMAEMA/Rh-ON and DAB₆₄/Rh-ON dispersions, 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.