Formation of Complex Langmuir and Langmuir-Blodgett Films of DNA with a Dichain Cationic Surfactant

Phase behaviour and adsorption of deoxyribonucleic acid onto an azobenzene liquid crystalline ligand at the interfaces

Azobenzene liquid crystalline (ALC) ligand contains a cholesteryl group linked to an azobenzene moiety through a carbonyl dioxy spacer (C7) and terminated with an amine group as a polar head. The phase behaviour of the C7 ALC ligand at the air-water (A-W) interface is investigated employing surface manometry. The surface pressure-area per molecule isotherm shows that C7 ALC ligand exhibit two different phases following the phase sequence viz., liquid expanded (LE₁ and LE₂) and then collapse to three-dimensional crystallites. Further, our investigations under different pH conditions and in the presence of DNA reveal the following. Compared to the bulk, the acid dissociation constant (pK_a) of an individual amine reduces to 5 at the interfaces. For pH (3.5) < pK_a, the protonation of amine groups of C7 ALC ligand facilitates the condensation of the film and enhances the stability. For pH values > pK_a, the phase behaviour of the ligand remains the same due to the partial dissociation of the amine groups. The presence of DNA in the sub-phase result in the expansion of isotherm to the higher area per molecule and the compressional modulus extracted reveals the phase sequence; liquid expanded, liquid condensed, followed by a collapse. Further, the kinetics of adsorption of DNA to the amine groups of the ligand is investigated, suggesting the interactions are influenced by surface pressure corresponding to different phases and pH of the sub-phase. Brewster angle microscope studies are carried out at different surface densities of the ligand as well as in the presence of DNA also supports this inference. Atomic force microscope is employed to acquire the surface topography and height profile of C7 ALC ligand (1 layer) after transferring on onto a silicon substrate using Langmuir Blodgett deposition. The difference in the surface topography and thickness of the film indicates the adsorption of DNA onto the amine groups of the ligand. The characteristic UV-visible absorption bands of the ligand films (10 layers) at the air-solid interface are tracked and the hypsochromic shift of these bands is also attributed to these DNA interactions.

Kelvin probe force microscopy to study electrostatic interactions of DNA with lipid-gemini surfactant monolayers for gene delivery

In novel gene therapy mechanisms utilising gemini surfactants, electrostatic interactions of the surfactant molecules with the DNA strands is a primary mechanism by which the two components of the delivery vehicle bind. In this work, we show for the first time direct evidence of electrostatic interactions of these compounds visualised with Kelvin probe force microscopy (KPFM) and correlated to their topography from atomic force microscopy (AFM). We construct monolayers of lipids and gemini surfactant to simulate interactions on a cellular level, using lipids commonly found in cell membranes, and allow DNA to bind to the monolayer as it is formed on a Langmuir-Blodgett trough. The difference in topography and electrical surface potential between monolayers with and without DNA is striking. In fact, KPFM reveals a strongly positive relative electrical surface potential in between where we identify a background lipid and the DNA strands, evidenced by the height profiles of the domains. Such identification is not possible without KPFM. We conclude that it is likely we are seeing cationic surfactant molecules surrounding DNA strands within a sea of background lipid.

The interaction between the outer layer of a mixed ion pair amphiphile/double-chained cationic surfactant vesicle and DNA: a Langmuir monolayer study

The charge density of vesicular bilayers plays an important role in the structure characteristic of the vesicle-DNA complex for gene delivery. In this work, the charge density effect of catanionic vesicle surfaces on the association behavior of the vesicle with DNA was explored with the model Langmuir monolayer approach. The interaction of negatively charged DNA with positively charged Langmuir monolayers composed of catanionic vesicle-forming materials, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS) and dihexadecyldimethylammonium bromide (DHDAB), was investigated with surface pressure-area isotherms, area-time relaxation curves and Brewster angle microscope images. The results showed that the adsorption of DNA molecules onto the monolayers was enhanced with an increased DHDAB molar fraction (XDHDAB), which was apparently related to the increased charge density of the monolayers. With XDHDAB being increased up to 0.5, the mixed monolayers with a higher XDHDAB, or higher charge density, possessed a more stable characteristic at high surface pressures, at which the molecular status was close to that in a corresponding vesicular bilayer, due to the DHDAB-improved molecular packing/interaction. It was found that the composition of the mixed HTMA-DS-DHDAB monolayers at high surface pressures would be affected by the adsorbed DNA with the extent depending on XDHDAB. For the formation of stable HTMA-DS-DHDAB monolayer-DNA complexes, a strong electrostatic interaction of DNA with a monolayer of high charge density and a high monolayer stability characteristic resulting from DHDAB-improved molecular packing/interaction were thus required. The finding has an implication for the formulation of catanionic vesicles composed of an ion pair amphiphile, HTMA-DS, with DHDAB in gene delivery applications.