The interaction of DNA with cationic gemini surfactant trimethylene-1

3-bis(dodecyl dimethyl-ammonium bromide)(12-3-12)and anionic surfactant sodium dodecyl sulfate(SDS) mixed system has been investigated by measuring the fluorescence

zeta potential

UV-V is spectrum

and circular dichroism.In the absence of SDS

owing to the electrostatic and hydrophobic interactions

12-3-12 forms micelle-like structure on the DNA chain before the micellization in bulk phase.For the mixed system of 12-3-12 and SDS

the negative charges on SDS can compete against DNA to bind with cationic 12-3-12 because of the stronger interaction between oppositely charged surfactants

and thus

the catanionic mixed micelles are formed before the formation of DNA/12-3-12 complexes.Thereafter

the positive charges on the mixed micelles bind with DNA

and thus

the change of the zeta potential from negative to positive is distinctly different from the system without SDS.Meanwhile

the existence of SDS postpones the exclusion of ethidium bromide(EB) from DNA/EB complexes.The conformation of DNA undergoes a change from native B-form to chiralψ-phase as binding with 12-3-12 process.Upon adding SDS to the DNA/12-3-12 complex solution

however

DNA is released to the bulk and theψ-phase returns to B-form again.