Coil-Globule Transition of DNA Molecules Induced by Cationic Surfactants: A Dynamic Light Scattering Study Rita S. Dias,* ,†,‡ Josef Innerlohinger, § Otto Glatter, § Maria G. Miguel, † and Bjo 1 rn Lindman ‡ Departamento de Quı ´mica, UniVersidade de Coimbra, 3004-535 Coimbra, Portugal, Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund UniVersity, P.O. Box 124, 221 00 Lund, Sweden, and Institut fu ¨r Chemie, Karl-Franzens UniVersita ¨t Graz, 8010 Graz, Austria ReceiVed: December 7, 2004; In Final Form: February 25, 2005 The compaction and aggregation of DNA induced by cationic surfactants was studied by dynamic light scattering (DLS). Furthermore, the effect on surfactant-compacted DNA of the addition of nonionic amphiphiles and salt was studied. When using sufficiently low amounts of DNA and cetyltrimethylammonium bromide (CTAB), compacted DNA molecules could be monitored by the appearance of a band characterized by lower hydrodynamic radius and by the decrease in the intensity of the peak corresponding to extended DNA molecules. Notably, we observed a region where compacted molecules coexist with extended ones; these two populations were found to be stable with time. For higher concentrations of CTAB, only compacted molecules were observed and the size of the particles increased with time indicating aggregation. The number of globules present in the coexistence region increased linearly with the surfactant concentrations, as given by the area of the band corresponding to this population, which indicates a double-cooperativity of the binding. The DLS experiments were in good agreement with previous fluorescence microscopy studies, with certain advantages over this technique since there is no need to add fluorescence dyes and antioxidants. Furthermore, it allows the study of molecules which are too small to be visualized by fluorescence microscopy. Introduction The interaction between DNA and cationic surfactants has received, since early times, a great interest from the biomedical sciences. Recently, physical chemists have devoted particular attention to these systems in an attempt to better understand the driving forces behind the molecular interactions; this is also expected to increase the efficiency and number of uses for these systems. The strong associative behavior displayed by DNA and cationic surfactant systems is well-known and is related to most of its applications such as extraction, purification, and counting. Also gene delivery and transfection constitutes a potential use of these systems. However, synthetic surfactants, like CTAB (cetyltrimethylammonium bromide), cannot be used alone for this purpose because of toxicity and since the complexes between DNA and cationic micelles do not result in effective transfection; but these amphiphiles can be used, in small amounts, for positive charging of neutral liposomes, thus improving their efficiency. 1,2 Fluorescence microscopy (FM) is one of the most interesting techniques used for the study of these systems, since this technique allows for the direct visualization of single DNA molecules in solution and the study of their conformational behavior in the presence of cosolutes such as surfactants or polyamines. Several studies have been published based on this technique. 3-7 There are a number of interesting aspects concern- ing these systems. One is the fact that compaction of DNA is very drastic, that is, the DNA molecule appears either in an extended coil conformation, moving freely in solution with a slow wormlike motion, or in a compacted state, presenting a faster movement and higher fluorescence intensity. Intermediate states are usually not found. Instead, for intermediate concentra- tions of the cationic surfactants, the two populations coexist in solution. This is a very interesting phenomenon established for DNA molecules on the addition of not only cationic amphiphiles but also flexible polycations, 7 multivalent ions, 6,8,9 and organic solvents. 10 The coil-globule transition of DNA is described as a discrete (quasi-) first-order transition for individual chains but continuous for their ensemble average. 9,10 Compaction of DNA is driven by attractive interactions between different parts of the molecule due to ion correlation effects arising from a low solvent dielectric constant, 10 or by the presence of multivalent ions, for example. 11-13 The compaction is more efficient the higher charged the condensing agent, the maximum degree of compac- tion being obtained for the interaction with polycations. 14 Even though surfactant molecules are singly charged they self-assemble in the vicinity of DNA thus acting as multivalent ions. 15-17 The fact that the compaction of DNA depends on the hydrophobicity of the surfactant is a strong indication for this. 4 The coexistence of DNA molecules with different conforma- tions in the bulk has only been observed, to our knowledge, by fluorescence microscopy. It has been suggested that conventional techniques, like circular dichroism and light scattering, do not provide a discrete coil-globule transition of the DNA molecules since they monitor the characteristics of the ensemble of chains. 18 In fact, the conditions at which the coexistence can be observed are very limited in concentration. While for very dilute solutions there are few techniques available which are sensitive enough, when using larger concentrations of DNA and surfactant, for example, a fast aggregation and precipitation of the complexes is obtained, making the study of single molecule compaction impossible. * Corresponding author. E-mail: rita.dias@fkem1.lu.se. † Universidade de Coimbra. ‡ Lund University. § Karl-Franzens Universita ¨t Graz. 10458 J. Phys. Chem. B 2005, 109, 10458-10463 10.1021/jp0444464 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/04/2005