Structure of DNA-CTAB-hexanol complexes

We have probed structures of the complexes formed by DNA with the cationic surfactant cetyltrimethylammonium bromide in the presence of the cosurfactant hexanol, using small angle x-ray diffraction techniques. They are found to exhibit a $\text{hexagonal}\to \text{lamellar}\to \text{hexagonal}$ transition with increasing hexanol content. Quantitative analysis of the diffraction data shows that the complexes formed at low hexanol concentrations have an intercalated hexagonal $\left(H_{\mathit{I}}\right)$ structure, whereas those formed at higher hexanol content have an inverted hexagonal $\left(H_{\mathit{II}}\right)$ structure.

Figure 1

Schematic of (a) the lamellar structure of DNA-lipid complexes, where the DNA strands, represented by the filled circles, are sandwiched between the lipid bilayers; (b) the inverted hexagonal $\left(H_{\mathit{II}}\right)$ structure, where the lipid-covered DNA strands are packed in a two-dimensional hexagonal lattice; and (c) the intercalated hexagonal $\left(H_{\mathit{I}}\right)$ structure, where each DNA strand is surrounded by three cylindrical micelles. Note that each unit cell in (b) contains one DNA strand surrounded by an inverted cylindrical micelle, whereas in (c) it contains one cylindrical micelle and two DNA strands.

Figure 2

Diffraction patterns of (a) the low-$\beta$ hexagonal phase ($\beta =3.5$; $\Gamma =36$), (b) the lamellar complex obtained at intermediate values of $\beta$, and (c) the high-$\beta$ hexagonal phase ($\beta =9$; $\Gamma =36$). The broader intermediate peak in (b) gives the average distance $d_{\mathit{DNA}}$ between the DNA strands. $\beta$ is the hexanol to CTAB molar ratio and $\Gamma$ is the CTAB to DNA weight ratio.

Figure 3

Variation of the average in-plane distance between DNA strands with DNA concentration in the lamellar complex. Note the abrupt change in $d_{\mathit{DNA}}$ near ${\Gamma }_{\mathit{iso}}(=1.1)$. Typical error in $d_{\mathit{DNA}}$ is $\pm 0.05\mathrm{nm}$. $\beta =8$.

Figure 4

The electron density models used in the calculation of the diffraction data. (a) $H_{\mathit{I}}$ (b) $H_{\mathit{II}}$. The shaded disk represents the DNA strand and the annular rings the head-group region. The disk of radius $r_c$ is the hydrocarbon region of the micelle. In the $H_{\mathit{I}}$ structure the micelles and DNA strands are surrounded by water, whose electron density is, therefore, taken as the reference value. In the $H_{\mathit{II}}$ structure water is intercalated with the DNA strands in the central pores of the inverted cylindrical micelles. In this case the reference value of the electron density is that of the background hydrocarbon region, which is, therefore, not shown in (b).

Figure 5

Variation of the form factor of the $H_{\mathit{I}}$ structure along ${\widehat{q}}_{10}$ (solid line); ${\widehat{q}}_{11}$ (dotted line); and ${\widehat{q}}_{21}$ (dashed line) for $a=5.6\mathrm{nm}$, ${\rho }_h=340\mathrm{e}∕{\mathrm{nm}}^3$, and $\delta =0.6\mathrm{nm}$, corresponding to the data given in Table for $\beta =0$. Values of the remaining parameters are $r_d=1.25\mathrm{nm}$, ${\rho }_d=400\mathrm{e}∕{\mathrm{nm}}^3$, ${\rho }_c=280\mathrm{e}∕{\mathrm{nm}}^3$, and ${\rho }_w=332\mathrm{e}∕{\mathrm{nm}}^3$. The positions of the peaks are indicated by the arrowheads.

Figure 6

Variation of the form factor of the $H_{\mathit{I}}$ structure along ${\widehat{q}}_{10}$ (solid line); ${\widehat{q}}_{11}$ (dotted line); and ${\widehat{q}}_{21}$ (dashed line) for $a=5.5\mathrm{nm}$, ${\rho }_h=325\mathrm{e}∕{\mathrm{nm}}^3$, and $\delta =0.7\mathrm{nm}$, corresponding to the data given in Table for $\beta =3.5$. Values of the remaining parameters are the same as those given in the caption of Fig. 5. The positions of the peaks are indicated by the arrowheads.

Figure 7

Form factor of the $H_{\mathit{II}}$ structure for ${\rho }_h=315\mathrm{e}∕{\mathrm{nm}}^3$, and $\delta =0.75\mathrm{nm}$, corresponding to the data given in Table for $\beta =9$. The rest of the parameters have the values given in the caption of Fig. 5. The positions of the peaks are indicated by the arrowheads.

Figure 8

Electron density map of the $H^{\mathit{I}}$ phase corresponding to the data given in Table for $\beta =0$, calculated using the phases obtained from the modeling procedure (Fig. 5). The electron density is higher in the lighter regions.