Network Formation of DNA/Polyelectrolyte Fibrous Aggregates Adsorbed at the Water–Air Interface

Abstract

It is discovered that complexes of DNA and hydrophobically modified polyelectrolytes form a rigid network of threadlike or fibrous aggregates at the liquidgas interface whose morphology can dramatically affect the mechanical properties. While mixed solutions of DNA and poly(N,N-diallyl-N,N-dimethylammonium chloride) (PDADMAC) exhibit no notable surface activity, the complexes formed from DNA with poly(N,N-diallyl-N-butyl-N-methylammonium chloride) are surface active, in contrast to either of the separate components. Further, complexes of DNA and poly(N,N-diallyl-N-hexyl-N-methylammonium chloride) (PDAHMAC) with its longer hydrophobic side chains exhibit pronounced surface activity with values of the surface pressures up to 16 mN/m and dynamic surface elasticity up to 58mN/m. If the PDAHMAC nitrogen to DNA phosphate molar ratio, N/P, is between 0.6 and 3, abrupt compression of the adsorption layer leads unexpectedly to a noticeable decrease of the surface elasticity. Application of imaging techniques reveals that this effect is a consequence of the destruction of a rigid network of threadlike DNA/polyelectrolyte aggregates at the interface. The toroidal aggregates, which are typical for the bulk phase of DNA/PDADMAC solutions in this range of N/P ratios, are not observed in the surface layer. The observed link between the mechanical properties and interfacial morphology of surface active complexes formed from DNA with hydrophobically modified polyelectrolytes indicates that tuning the polyelectrolyte hydrophobicity in these systems may be a means to develop their use in applications ranging from nonviral gene delivery vehicles to conductive nanowires.

DNA Interaction with a Polyelectrolyte Monolayer at Solution—Air Interface

“…In this case, the formation of a network of fibrous aggregates at the liquid surface at certain DNA/surfactant ratios was observed. A similar behavior was also reported in our recent work on solutions of the mixture of DNA and synthetic polyelectrolytes [ 38 ]. Indeed, while previous works have addressed the conformation of DNA with synthetic polyelectrolytes at the solution–solid interface [ 39 , 40 ], far fewer studies have addressed such interactions at the solution—air interface.…”

“…The surface properties start to change only when the size and/or surface concentration of the aggregates increase and they start to interact. The increase of the dynamic surface elasticity of polyelectrolyte solutions upon DNA addition has been attributed recently to the formation of a rigid network of DNA/polyelectrolyte aggregates at the solution-air interface [ 38 ]. The same process presumably results in a strong increase of the dynamic surface elasticity after the induction period in the system under investigation but it becomes more complicated in the latter case and consists of two main steps at least.…”

“…The main distinctions of the DNA/PDAHMAC networks in the given study from the surface networks, which are formed as a result of adsorption of the both components [ 38 ], or from DNA/surfactant networks [ 27 ], consist in their more regular shape and in a narrower distribution of the cell size in the first case ( Figure 4 and Figure S4 ). Note that the micromorphology of DNA/PDAHMAC network is not always reproducible, and a denser network ( Figure S4e ), as well as separate patches of the network ( Figure S4f ) can be observed occasionally.…”

“…Calf thymus DNA was purchased from Sigma-Aldrich and used as received. PDAHMAC was synthesized as described previously by the radical polymerization of N , N -diallyl- N -hexyl- N -methylammonium chloride [ 38 , 41 ]. The monomer (2.225 g, 9.6 mmol), 4,4′-azobis(4-cyanovaleric acid) (84 mg, 0.3 mmol) and deionized water (0.690 g) were vortexed in a glass ampoule until dissolution, the mixture was subjected to three freeze-pump-thaw cycles in order to remove oxygen, sealed and heated at 70 °C for 13 h. The highly viscous product was dissolved in water and dialyzed against deionized water for three days using Spectra/Por MWCO 6-8 000 (Serva, Heidelberg, Germany).…”

“…Indeed, while previous works have addressed the conformation of DNA with synthetic polyelectrolytes at the solution–solid interface [ 39 , 40 ], far fewer studies have addressed such interactions at the solution—air interface. Our recent work on the interactions of DNA with synthetic polyelectrolytes revealed that fibrous aggregates formed at the solution surface depend both on the hydrophobicity of the synthetic polyelectrolyte and the bulk molar mixing ratio [ 38 ].…”