Osmotic-pressure-induced phase transition of a surfactant-DNA complex

We have studied the effect of osmotic pressure on complexes formed by DNA with the cationic surfactant cetyltrimethylammonium tosylate using small-angle x-ray scattering. Earlier studies have shown that these complexes exhibit three different phases depending on the DNA and surfactant concentrations in the solution. The hexagonal superlattice phase ($H_{I,s}^c$) is found to be corralled into the hexagonal phase ($H_I^c$) above a threshold osmotic pressure. We have also estimated the DNA to surfactant micelle stoichiometry of the complexes in the three phases using elemental analysis. Our results provide further support for the structures of these complexes proposed earlier based on small-angle x-ray scattering data.

Figure 1

Schematic of different two-dimensional structures observed in CTAT-DNA complexes. The smaller disks correspond to DNA strands, and the larger ones correspond to cylindrical surfactant micelles with an inner hydrocarbon core surrounded by the headgroup region. (a) Close-packed square lattice ($S_I^c$), (b) close-packed hexagonal lattice ($H_I^c$), and (c) swollen hexagonal superlattice ($H_{I,s}^c$). The DNA to micelle stoichiometry in (a) is 1:1, whereas it is 2:1 in both (b) and (c). Parallel arrangement of the DNA strand and micelle along the direction normal to the plane of the lattice is depicted in (d).

Figure 2

Typical SAXS patterns of the hexagonal ($H_I^c$), superhexagonal ($H_{I,s}^c$), and square ($S_I^c$) phases of CTAT-DNA complexes. Peaks in the scattering patterns of $H_I^c$ and $H_{I,s}^c$ can be indexed following the scheme $q_{hk}\propto \sqrt{h^2+k^2+hk}$, whereas those in the scattering pattern of $S_I^c$ can be indexed following the scheme $q_{hk}\propto \sqrt{h^2+k^2}$. Note that the (1,0) peak is always absent in the case of $H_{I,s}^c$.

Figure 3

SAXS patterns showing the effect of osmotic pressure on $H_{I,s}^c$. Numbers against the curves indicate the concentration of PEG8000 in the solution. (a) $\rho =0.5,\left[\mathrm{CTAT}\right]=100\mathrm{mM}$. (b) $\rho =1.0,\left[\mathrm{CTAT}\right]=150\mathrm{mM}$.

Figure 4

SAXS patterns showing the effect of osmotic pressure on $H_{I,s}^c$. Numbers against curves indicate the concentration of PVP10 000 in the solution. (a) $\rho =0.5,\left[\mathrm{CTAT}\right]=100\mathrm{mM}$. (b) $\rho =1.0,\left[\mathrm{CTAT}\right]=150\mathrm{mM}$.

Figure 5

SAXS patterns showing the effect of osmotic pressure on $H_I^c$; $\rho =0.5,\left[\mathrm{CTAT}\right]=50\mathrm{mM}$. Numbers against the curves indicate the concentration of PEG8000 in the solution. Peaks in the scattering patterns can be indexed following the scheme $q_{hk}\propto \sqrt{h^2+k^2+hk}$.

Figure 6

SAXS patterns showing the effect of osmotic pressure on $H_I^c$; $\rho =0.5,\left[\mathrm{CTAT}\right]=50\mathrm{mM}$. Numbers against the curves indicate the concentration of PVP10 000 in the solution. Peaks in the scattering patterns can be indexed following the scheme $q_{hk}\propto \sqrt{h^2+k^2+hk}$.

Figure 7

SAXS patterns showing the effect of osmotic pressure on $S_I^c$; $\rho =2,\left[\mathrm{CTAT}\right]=100\mathrm{mM}$. Numbers against the curves indicate the concentration of PEG8000 in the solution. Peaks in the scattering patterns can be indexed following the scheme $q_{hk}\propto \sqrt{h^2+k^2}$.

Figure 8

SAXS patterns showing the effect of osmotic pressure on $S_I^c$; $\rho =2,\left[\mathrm{CTAT}\right]=100\mathrm{mM}$. Numbers against the curves indicate the concentration of PVP10 000 in the solution. Peaks in the scattering patterns can be indexed following the scheme $q_{hk}\propto \sqrt{h^2+k^2}$.