Do surfactants form micelles on the surface of DNA?

Abstract

The complex formation between the cationic surfactant, dodecyltrimethylammonium bromide (C₁₂TAB), and short fragments of Calf Thymus DNA, 220 base pairs, has been studied by using a combination of high-precision ultrasonic velocity and density measurements. The compressibility and the volume effects of the binding of C₁₂ TAB with DNA are 139 cm³ mol⁻¹ Gpa⁻¹ and 9.0 cm³ mol⁻¹, respectively. In addition to that we obtained the compressibility effect in the micelle formation for three surfactant molecules C₁₂TAB, C₁₄TAB and C₁₆TAB. These effects are 127.5, 148.9 and 169.2 cm³ mol⁻¹ Gpa⁻¹ for C₁₂TAB, C₁₄TAB and C₁₆TAB, respectively. The high value of the compressibility effect in the binding of the C₁₂TAB surfactant to DNA cannot be explained by hydration changes only and indicates the formation of a structure with a highly compressible core. This conclusion, and the close values of the compressibility effect in the formation of the C₁₂TAB micelles in solution, and in the binding of C₁₂TAB to DNA, demonstrates the formation of micelle-like aggregates of C₁₂TAB molecules on the DNA surface.