Thermo-reversible capture and release of DNA by zwitterionic surfactants

Abstract

The thermo-reversible capture and release of DNA were studied by the protonation and deprotonation of alkyldimethylamine oxide (C_nDMAO, n = 10, 12 and 14) in Tris–HCl buffer solution. DNA/C₁₄DMAO in Tris–HCl buffer solution with pH = 7.2 is transparent at 25 °C, indicating that DNA molecules exist mainly in individuals and the binding of C₁₄DMAO is weak. With the increase of temperature, the pH of the buffer solution continuously decreases, which leads to protonation of C₁₄DMAO (C₁₄DMAO + H⁺ → C₁₄DMAOH⁺) and an obvious increase of the turbidity of the samples. This indicates a stronger binding of the protonated C₁₄DMAOH⁺ to DNA. Further investigations demonstrated the formation of DNA/C₁₄DMAOH⁺ complexes, in which the stretched DNA molecules are effectively compacted as evidenced from UV-vis absorptions, circular dichroism (CD) measurements, atomic force microscopy (AFM) observations, dynamic light scattering (DLS) measurements and agarose gel electrophoresis (AGE). Interestingly, when the temperature is turned back to 25 °C, the compacted DNA molecules can fully recover to the stretched conformation. This cycle can be repeated several times without obvious loss of efficiency. The effect of the chain length of C_nDMAO has also been investigated. When C₁₄DMAO was replaced by C₁₂DMAO, similar phenomena can be observed with a slightly higher critical surfactant concentration for DNA compaction and a slightly lower pH of Tris–HCl buffer solution with pH = 6.8. For the DNA/C₁₀DMAO system, however, no DNA compaction was observed even in Tris–HCl buffer solution with a much lower pH and a much higher C₁₀DMAO concentration. The negative charges of DNA molecules can easily be neutralized by positive charges of cationic C_nDMAOH⁺ (n = 12 and 14) micelles. DNA was compacted and then insoluble DNA/C_nDMAOH⁺ complexes were formed. Because of the much higher critical micelle concentration (cmc) of the shorter chain length C₁₀DMAOH⁺, cationic C₁₀DMAOH⁺ micelles cannot form under the studied condition to compact DNA. The strategy may provide an efficient and alternative approach for stimuli-responsive gene therapy and drug release.