Spacer length controlled oblique-columnar to Lamello-Columnar mesophase transition in liquid crystalline DNA-discotic cationic lipid complexes

Asymmetric triphenylene imidaolium salts with spacer lengths of C-5, C-8, and C-11 were successfully synthesized. DNA complexation with these cationic discotic lipids was achieved by mixing DNA and triphenylene imidazolium salts in aqueous solutions. The molecular composition of the complexes in the bulk was quantitatively determined using Fourier transform infrared (FTIR) spectroscopy: the ratio of negative phosphate groups to positive imidazolium salt ranged from 1.4 to 2.0 for different discotic cationic lipids. The columnar liquid crystalline morphology was characterized by polarized light microscopy, X-ray diffraction (XRD), and transmission electron microscopy. When the spacer length of the triphenylene imidazolium salts increased from C-5 to C-8 and C-11, a morphological transition from an oblique-columnar (Col(o)) phase to lamello-columnar (COIL) phases was observed. From detailed XRD results, the face-to-face interdisk spacing in a triphenylene column (0.35 nm) was similar to the period of hydrogen-bonded pairs in DNA (0.34 nm), suggesting a 1:1 cornplexation between the phosphate group and the imidazolium salt. The higher phosphate/imidazolium salt ratio (1.4-2.0) determined by FTIR suggested that the DNA chains were partially complexed due to rapid precipitation of the complexes upon mixing the two components in aqueous solutions. The columnar stacking of the triphenylene discotic molecules destroyed the helical conformation in the double-strand DNA, as evidenced by a circular dichroism study. When the semirigid DNA molecules were replaced with flexible total RNA, columnar liquid crystalline self-assembly of the triphenylene molecules in the complexes disappeared, and only a lamellar morphology with random discotic triphenylenes sandwiched between neighboring RNA layers was observed.