We describe the novel structure and behavior of a DNA-DDAB complex film cast from an organic solvent which exhibits a structural switching transition as it is dried or wetted with water. The film can be easily prepared by forming a complex between the negatively charged phosphate groups of DNA and the positively charged headgroup of the surfactant DDAB. This complex is then purified, dried, dissolved in isopropanol and cast onto a glass slide to form a self-standing film by means of slow evaporation. While the structure of the dried film was found to be composed of single-stranded DNA and a monolayer of DDAB, upon hydration of the film the structure switched to double stranded DNA complexed to a bilayer of DDAB. We expect that this phenomenon would serve as a useful model for the design of new responsive materials and programmable self-assembly.

The structure of a DNA-dimethyldidodecylammonium bromide (DDAB) film was recently described to undergo a distinctive transition in response to the water content in the surrounding environment. 1 The existence, preparation, and basic properties of DNA-surfactant films have been known in the literature for some time.2 , 3 Here, we describe the structural response of DNA-DDAB films to environmental changes, particularly temperature and humidity, in greater detail revealing new structural states. We can direct the lamellar structure of the film into three distinct states -doublestranded DNA (dsDNA) paired with an interdigitated bilayer of DDAB (bDDAB), single-stranded DNA (ssDNA) with monolayer of DDAB (mDDAB), and ssDNA with bDDAB. Both temperature and humidity cause the molecules composing the lamellar structure to change reversibly from ssDNA to dsDNA and/or from mDDAB to bDDAB. We found that the structural transition from dsDNA to ssDNA and bDDAB to mDDAB is concerted and follows apparent first order kinetics. We also found that the double-stranded conformation of DNA in the film can be stabilized with the inclusion of cholesterol even while the DDAB in the film is able to form either a monolayer or bilayer depending on the environmental conditions. Films treated with ethidium bromide prompt switching of dsDNA to ssDNA before bDDAB transitions to mDDAB. Swelling experiments have determined that there is a direct proportionality between the macroscopic increase in volume and the nanoscopic increase in lamellar spacing when a film is allowed to swell in water. Finally, experiments with phosphatebuffered saline (PBS) indicate that the films can disassemble in a simulated biological environment due to screening of their charges by buffer salt. We conclude that the structure of DNA in the film depends on the water content (as measured by the relative humidity) and temperature of the environment, while the state of DDAB depends essentially only on the water content. The structure of the film is quite flexible, and can be altered by changing environmental conditions as well as the chemical ingredients. These films will have useful, new applications as responsive materials, e.g. in drug and gene delivery.

We detail a method originally described by Okahata et al. (Macromol. Rapid Commun. 2002, 23, 252-255) to prepare noncovalent self-assembling films by exchanging the counter-ions of the nucleic acid phosphate moieties with those of cationic lipid amphiphiles. We are able to control the strength and surface properties of these films by varying the composition between blends of DNA of high molecular weight and RNA of low molecular weight. X-ray and AFM results indicate that these films have a lamellar multilayered structure with layers of nucleic acid separated by layers of cationic amphiphile. The tensile strength of the blended films between DNA and RNA increases elastically with DNA content. The length as well as the molecular structure of nucleic acids can affect the topology and mechanical properties of these films. We suspect that the permeability properties of these films make them good candidates for further biological applications in vivo.

While most nucleic acid (NA)-lipid or NA-polymer complexes are studied in solution, there is growing interest in understanding their properties as naturally derived, biodegradable, biocompatible, solid-state materials with tailorable properties influenced by environmental parameters. Therapeutic and cell programming applications comprise an important new research field, particularly in gene transfection and silencing using plasmid DNA and siRNA with targeted local delivery for use in cell culture. Dried solid films have lower nuclease degradation, fewer barriers to long term storage, and allow localized delivery by direct implantation in combination with controlled release and dosage adjustment. In contrast to particulate complexes or other methods of drug delivery which are prepared and must remain in solution, films can regain their biological activity once wetted. However our understanding of the types of cationic agents that predictably form self-standing films with NA is still limited. The self-assembly and structural, physical, and chemical properties of these materials are of key importance to maintaining their activity. We therefore discuss the material properties of NA-lipid and NA-polymer films as the focus of this article. Recent studies have indicated that there is also growing interest in NA films beyond bioengineering and medical applications in the fields of nano- and optoelectronics. We survey the self-assembly of solid-state materials composed of NA complexed with lipids, surfactants, or polymers, and summarize investigations of nanoscopic assembly, structure, optical, and macroscopic material properties. We further evaluate the current and future applications of NA-lipid and NA-polymer films and the benefits and drawbacks of each type.

Here we report the nucleic acid/cationic amphiphile based-materials in which we exchange the counter-ions of the polyanionic backbone of the nucleic acids with the cationic amphiphiles to form self-assembled transparent films with the thickness of several microns. Predominantly, single stranded poly(A), poly(U) and double stranded poly(AU) were employed for these studies. Small-angle X-ray scattering (SAXS) experiments suggested lamellar-like structure for all the film samples. However, the molecule length as well as the molecular structure of nucleic acids can affect the topology and mechanical properties of these films. Complementary base-paring of poly(AU) is reported here with comparison to poly(A) and poly(U) complexes.