Study The Properties Of Gemini/DNA Complex Monolayers At The Air-Water Interface

Compared with the conventional surfactant, Gemini surfactant shows lower critical micelle concentration (CMC), higher surface activity, lower C2o value, better wettability, solubility, foamability and lime soap dispersing power. As a kind of anionic polyelectrolyte, DNA could produce the electrostatic attraction with cationic Gemini surfactant. Therefore, in this dissertation, the interaction between cationic Gemini surfactant(18-s-18) and double-strand DNA/single-strand DNA(dsDNA/ssDNA) was investigated by means of LB technology, atom force microscope, FT-IR spectra, IRRAS srectra and Circular Dichroism srectra ect.The liquid-expanded films are formed on ssDNA subphase at the air/water interface. In this paper, the impact of spacers onπ-A isotherms was investigated. Results show that molecule area increases with the spacer and the maximum area appears at s=10. This is due to that the spacer is rigid and lies on the air/water interface when s<6, while when the spacer is flexible(s≥6), the increasing flexibility and hydrophobility of spacer leads to form the arch shape and even reverse U-shape conformation at the air/water interface.The comformation of DNA in 18-s-18/ssDNA and 18-s-18/ssDNA system are studied by infrared spectrum. The FT-IR spectra imply that the comformation of DNA in 18-s-18/ssDNA and 18-s-18/ssDNA monolayers is respective single-strand DNA and double helix DNA. However, the regular bases stacking are destroyed in double helix DNA after complex monolayers transferred onto substrates. The distortion and deformation of Double helix could lead to the red shift of C=O absorption. Nevertheless, it still has a double-stranded structure in 18-s-18/ssDNA complex system. We investigated the impact of the humidity on the comformation of DNA in Gemini/dsDNA complex using CD srectra. The result shows that humidity has a significant effect on the conformation of DNA. The DNA is like-B-form conformation at 85%r.h. in 18-3-18/dsDNA and 18-6-18/dsDNA complex. However, at 15%r.h., only a negative peak appears in CD srectra. This implies that Gemini/DNA complex is a typical chiralψ-phase, in which the DNA molecules are tightly packed together to fabricate highly condensed structure with a left-handed tertiary conformation. Because the surface morphology of 18-8-18/dsDNA is flat and not easy to from the tertiary structure, DNA remains the like-B-form conformation in the case of the lower relative humidity.We studied the impact of ssDNA and dsDNA on the properties of Gemini/DNA complex. LS films are available to get the information of on the surface and lower surface, while LB films would explore the influence of the liquidstream on the structure. In 18-s-18/ssDNA and 18-s-18/ssDNA systems, dsDNA and ssDNA do not flat lie at the air/water interface, but show the "┓or┎"-like arrangement. After transferred to LB films, DNA molecules appear curve and aggregation, the height of the fiber structure is 7-10 nm.