The Interaction of a Gemini Surfactant with a DNA Quadruplex

Abstract

DNA secondary structures are stabilized by mono- and divalent cations. To examine the stability of the DNA quadruplex formed from (TTAGGG)₄, its interaction with a dicationic Gemini surfactant in standard phosphate buffer was investigated. The Gemini surfactant begins to form micelles in buffer at a cmc (critical micelle concentration) of 1.5 mM. In this study, solutions of DNA were prepared in buffer with surfactant concentrations ranging from 0.0 to 3.0 mM, i.e., above and below the cmc of the surfactant. In all samples of DNA and surfactant, a precipitate formed. The fraction of DNA precipitated depends upon both the initial DNA concentration and the initial concentration of the surfactant. In those samples where the DNA did not totally precipitate, the residual DNA assumed a quadruplex conformation. It was determined that two surfactant molecules per DNA phosphate are needed to completely precipitate all of the DNA in a particular sample. An estimated apparent K _sp for the DNA:surfactant complex was determined.

Figure 1

Gemini surfactant (I) and the simple diammonium compound (II). The surfactant is often designated as 12–4–12.

Figure 2

Plots of conductivity vs concentration of diammonium compound at 25 °C for the 12–4–12 surfactant I (red squares) and the simple diammonium compound II (black circles) in standard 10 mM phosphate buffer, 15 mM K⁺ or 115 mM K⁺, pH 7.0 with linear regression lines.

Figure 3

CD spectra at 25 °C for (TTAGGG)₄ after treatment with the 12–4–12 surfactant I. For these spectra, DNA samples were prepared in standard phosphate buffer, pH 7.0, and 115 mM K⁺ with increasing concentrations of the 12–4–12 surfactant from 0.00 to 3.00 mM. In all samples with the surfactant, a precipitate was formed, which was separated from the supernatant by centrifugation. The CD spectra for these supernatants are shown with the corresponding concentration of the surfactant.

Figure 4

UV spectra at 25 °C of supernatants from samples prepared with concentrations of 12–4–12 of 0.00 mM (black), 0.025 mM (pink), 0.050 mM (blue), and 0.075 mM (yellow). 0.10 mM (green) and 0.15 mM (red) and charged with (TTAGGG)₄ at 1.4 μm, 4.5 μM, and 11.0 μM. All samples were prepared in standard phosphate buffer with 115 mM K⁺.

Figure 5

Plots of percent precipitated DNA as a function of the concentration of surfactant. The concentration of DNA was 11.0 μM (black), 6.1 μM (red), 4.5 μM (green), 3.3 μM (blue), and 1.4 μM (pink). The least-squares fits are for data from 0 to 85% precipitation.

Figure 6

Plot of the minimum concentration of surfactant needed to precipitate all of the DNA in a sample of known DNA concentration.

Scheme 1. Possible Mechanism for the Precipitation of the DNA in the Presence of the Surfactant. DNA = (TTAGGG)₄; SUR = 12-4-12 Surfactant