Interaction of DNA Oligomers with Cationic Lipidic Monolayers: Complexation and Splitting Svetlana Erokhina, ² Tatiana Berzina, ²,‡ Luigi Cristofolini, ²,‡ Oleg Konovalov, § Victor Erokhin,* ,²,‡, | and Marco P. Fontana ²,‡ Department of Physics, UniVersity of Parma, Viale Usberti 7 A, 43100 Parma, Italy, Centro SOFT CNR-INFM, Rome, Italy, European Synchrotron Radiation Facility, 38043 Grenoble Cedex, France, and Institute of Crystallography, Russian Academy of Sciences, Leninsky pr. 59, Moscow, Russia ReceiVed January 10, 2007. In Final Form: February 12, 2007 Interactions of native DNA with octadecylamine (ODA) and hexadecymdimethylammonium bromide (HTAB) monolayers at the air/water interface were studied by π-A isotherms, ellipsometry, and X-ray reflectivity. We show that the microscopic structure of ODA-DNA complexes is definitely consistent with a single-stranded form for DNA. On the contrary, with HTAB, DNA complexes in its native form. The crucial difference in the behavior of these two fairly similar lipids is due to the presence of the amine group in ODA. These results should be relevant to applications such as DNA chips and sensors. 1. Introduction In the present study, we have used synchrotron radiation X-ray reflectivity (XRR) measurements and ellipsometry together with surface pressure-area isotherm studies to investigate the structure of complex monolayers of DNA with octadecylamine (ODA) and hexadecymdimethylammonium bromide (HTAB) at the air/ water interface. These measurements were carried out to clarify whether there is a double-to-single helix transformation of DNA molecules during their interactions with some cationic lipid molecules in monolayers at the air/water interface, and to define what are the specific molecular features and mechanisms responsible for the eventual denaturation. We report evidence that strongly supports the proposal that DNA complexes with ODA in single-stranded form, whereas this does not take place for the similar cationic lipid HTAB. Besides clarifying an ongoing debate in the literature, this result could have important implications in the fabrication of DNA sequencers, biosensors, etc. The capability of DNA to form layers 1 and complexes with charged cationic lipid-like monolayers at the air/water interface has recently attracted much attention as it can be used for the assembly of sensitive layers for DNA chips and as it can clarify fundamental aspects of the DNA-biological membranes inter- actions. X-ray analysis of multilayers, fabricated from complexes of DNA with hexadecylamine and octadecylamine, formed at the air/water interface and transferred onto solid supports by the Langmuir-Blodgett (LB) technique, revealed that interlayer spacing was decreased with respect to the expected value. 2,3 This difference was first explained by the tilting of hydrocarbon chains of aliphatic amines in the film. Later, on the basis of a combination of X-ray and IR spectroscopic evidence, it was suggested that the chains are practically not tilted with respect to the normal to the film plane, but DNA is in a single-stranded form in the layer. 4,5 It was also found that such DNA splitting is not a general phenomenon for the DNA-cationic lipid complexes. In the case of HTAB, for example, the splitting was not observed. 6,7 Therefore, it was concluded that the transformation of the DNA in its single-stranded form takes place only at amine headgroups of amphiphilic molecules. Despite the rather large number of recent publications, several questions are still open. One of the main open questions is whether DNA is actually split into the single-stranded form. For example, it was confirmed that spacing of DNA-containing LB films strongly supports DNA splitting. 8 On the other hand, an attempt to observe this splitting at the air/water interface by X-ray reflectivity measurements was not successful; 9 the authors interpreted their experimental results as the attachment of a double DNA layer to the cationic lipid headgroups of the monolayer at the air/water interface: a double stranded DNA layer and an additional sublayer of DNA. However, these results do not clarify the question mentioned above for two reasons. First, the lipid layer was constructed from ammonium bromide-ending lipids, and, therefore, no splitting is supposed even to occur on it. Second, utilization of the rotating anode as the X-ray source does not provide enough intensity for an adequate signal-to-noise ratio, resulting, as mentioned by authors, in the rather unsatisfactory agreement of the fitting curves to the experimental data. Other important results in the literature are connected to the hybridization of DNA in complex ODA-DNA layers to complementary counterparts. 10-12 Here, the authors claimed the * Corresponding author. Phone: +39 0521 905276. Fax: +39 0521 905223. E-mail: erokhin@fis.unipr.it. ² University of Parma. ‡ Centro SOFT CNR-INFM. § European Synchrotron Radiation Facility. | Russian Academy of Sciences. (1) Frommer, M. A.; Miller, I. R. J. Phys. Chem. 1968, 72, 2862-2866. (2) Erokhin, V.; Popov, B.; Samori, B.; Yakovlev, A. Mol. Cryst. Liq. Cryst. 1992, 215, 213-220. (3) Sukhorukov, G.; Erokhin, V.; Tronin, A. Biofizika 1993, 38, 257-262. (4) Sukhorukov, G. B.; Feigin, L. A.; Montrel, M. M.; Sukhorukov, B. I. 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