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Equilibrium Dynamics of Spermine-Induced Plasmid DNA Condensation Revealed by Fluorescence Lifetime Correlation Spectroscopy

Jana Humpolíková ^*, Ale Benda ^*, Jan Skora ^*, Radek Machá ^*, Teresa Kral ^* , Barbara Gasinska ^*, Joerg Enderlein  and Martin Hof ^*

^* J. Heyrovsk Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague, Czech Republic; Agricultural University, Department of Physics and Biophysics, Wrocaw, Poland; and Institut für Physikalische und Theoretische Chemie, Eberhard Karls Universität Tübingen, Tübingen, Germany

Correspondence: Address reprint requests and inquiries to Martin Hof, E-mail: hof{at}jh-inst.cas.cz.

The spermine-induced DNA condensation is a first-order phase transition. Here, we apply a novel technique fluorescence lifetime correlation spectroscopy to analyze this transition in a greater detail. We show that the method allows for the observation of the condensed and uncondensed molecules simultaneously based solely on different fluorescence lifetimes of the intercalating fluorophore PicoGreen in the folded und unfolded domains of DNA. The auto- and cross-correlation functions reveal that a small fraction of the DNA molecules is involved in the dynamic intramolecular equilibrium. Careful inspection of the cross-correlation curves suggests that folding occurs gradually within milliseconds.