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Monitoring of the Formation and Dissociation of Polyethylenimine/DNA Complexes by Two Photon Fluorescence Correlation Spectroscopy

Laboratoire Pharmacologie et Physico-Chimie des Interactions Cellulaires et Moléculaires, UMR 7034 du CNRS, Faculté de Pharmacie, Université Louis Pasteur de Strasbourg, 74, Route du Rhin, 67401 Illkirch Cedex, France

Correspondence: Address reprint requests to Yves Mély, Tel.: +33-(0)3-90-24-42-63; Fax: +33-(0)3-90-24-43-12; E-mail: mely{at}pharma.u-strasbg.fr.

Polyethylenimines (PEI) constitute efficient nonviral vectors for gene transfer. However, because free PEI shows some cytotoxicity and because intracellular dissociation of PEI/DNA complexes seems to be required for efficient transfection, it is important to monitor the concentrations of free and bound partners in the mixtures of DNA and PEI used for transfection. To reach this objective, we used fluorescence correlation spectroscopy with two-photon excitation to characterize the complexes formed with either rhodamine-labeled 25 kDa PEI or DNA plasmid molecules. At the molar ratios of PEI nitrogen atoms to DNA phosphate usually used for transfection, we found that 86% of the PEI molecules were in a free form. The PEI/DNA complexes are composed on the average by 3.5 (±1) DNA plasmids and 30 PEI molecules. From this composition and the pK_a of PEI, it could be inferred that in contrast to DNA condensation by small multivalent cations, only a limited neutralization of the DNA phosphate groups is required for DNA condensation by PEI. Moreover, DNA appears only poorly compacted in the PEI/DNA complexes. As an application, fluorescence correlation spectroscopy was used to monitor the purification of PEI/DNA complexes by ultrafiltration as well as the heparin-induced dissociation of the complexes.

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