Direct in Situ Observation of a Lipid Monolayer−DNA Complex at the Air−Water Interface by X-ray Reflectometry

Abstract

Direct in situ observation of the fine structure of the cationic lipid monolayer−DNA complex on a water surface has been carried out by using an air−water interface X-ray reflectivity (XR) instrument. Interestingly, the thickness of the DNA layer was markedly thinner than the geometry of the cylindrical DNA molecule when the complex was deposited on the solid substrates; the thickness was determined to be 11 Å by XR measurement in a dried state, while the diameter of the DNA molecule is about 24 Å. However, the thickness in the complex on a water surface was estimated by in situ XR measurement to be about 25−28 Å, which is comparable to the geometry of the DNA molecule. Thus, the anomalously thin thickness was due to some experimental treatments, such as deposition on a solid substrate and/or drying. The structure of the monolayer and monolayer−polymer complex on the solid substrates in a dried state is not the same as that on a water surface. The possibility of some dynamic fluctuation of its structure was also suggested. These results strongly indicate the importance of a direct in situ study such as by the XR technique for the structural study of the monolayer and monolayer complex on a water surface.

Influence of organic ions on DNA damage induced by 1 eV to 60 keV electrons

“…The deposit was frozen at −70 °C for 5 min, and then lyophilized with a hydrocarbon-free sorption pump under a pressure of 1 mTorr for 2 h. The lyophilized pure DNA formed a film with a diameter of 3.5±0.5 mm and thickness of 15 nm (5 ML), assuming a density of DNA of 1.7 g/cm 3 , and a monolayer (ML) corresponding to a thickness of 3 nm. 26,27 With the added salt in ratio of 6 per nucleotide, the film had a final thickness of 9 ML. The samples were directly transferred to an ultrahigh vacuum chamber (UHV).…”

“…This procedure produced films of 2900 nm thickness estimated from the density of DNA of 1.7 g cm −3 . 26,27 This thickness was chosen to absorb sufficient energy from the electron beam, while avoiding significant damage from secondary electrons emitted from the metal substrate.…”

Probing peptide–membrane interactions using AFM

“…However, because the fusion approach is much simpler and permits lipid diffusion as in free-standing bilayers, it is the most widely used method in supported lipid bilayer research. A variety of approaches are available to probe the structure, composition and properties of lipid films, including fluorescence [7] and Brewster angle microscopy [8] ; X-ray reflection [9] and diffraction [10] methods; neutron reflectivity [11] and fluorescence recovery after photobleaching techniques. [12] Transferring lipid films onto solid substrata offers the possibility to apply a range of surface analytical techniques that could not be used to study real biological membranes, such as ellipsometry, [13] X-ray photoelectron spectrometry, [14] and time-of-flight secondary ion mass spectrometry.…”

X-ray reflectivity studies on the deoxyribonucleic acid adsorption by 3-β-[N-(N',N'-dimethylaminoethane)carbamoyl]cholesterol monolayer with divalent ions

“…Although many studies have made of the structure and interaction of cationic lipid-DNA complexes in bulk and in solution (Koltover et al, 1998(Koltover et al, , 2000McManus et al, 2003;Wetzer et al, 2001), there are few studies on their interactions at the air-water interface. Recently, there have been some studies on using the Langmuir-Blodgett (LB) technique to study the cationic lipid-DNA interaction at the air-water interface (Kago et al, 1999;Okahata et al, 1996;Chen, Wang, Shen et al, 2002;Sastry et al, 2000;Symietz et al, 2004;McLoughlin et al, 2005;Cá rdenas et al, 2005). The DNA molecules adsorbed onto supported cationic bilayers could form an aligned two-dimensional structure with regular spacing as revealed by atomic force microscopy studies (Fang & Yang, 1997).…”

“…Furthermore, DNA adsorption on supported bilayers was shown to depend on the surface charge density (Clausen-Schaumann & Gaub, 1999). Kago et al (1999) used in-situ X-ray reflectivity to study the adsorption of DNA by the cationic dimethyldioctadecylammonium lipid, 2C 18 -glu-N + 2C 1 at the air-water interface and found that the reflectivity profiles can only be fitted with a two-layer model, with a first layer of about 25 Å containing more DNA and a second layer of about 11 Å containing less DNA. They also found that when the complex was transferred onto the solid substrate, the thickness of the DNA layer as determined by reflectivity was found to be about 11 Å .…”