Lamellar to inverted hexagonal mesophase transition in DNA complexes with calamitic, discotic, and cubic shaped cationic lipids

In this study, we report on the lipid tail molecular shape/size effect on the mesophase self-assembly behaviors of various cationic lipids complexed with double-stranded DNA. The molecular shape of the cationic lipids was tailored from rodlike (a cyanobiphenyl imidazolium salt) to discotic (a triphenylene imidazolium salt), and finally to cubic a polyhedral oligomeric silsesquioxane (POSS) imidazolium salt]. An increase in the cross-sectional area of the hydrophobic tails with respect to the hydrophilic imidazolium head induced a negative spontaneous curvature of the cationic lipids. As a result, a morphological change from lamello-columnar (L(C)(alpha)) phase for the DNA-cyanobiphenyl imidazolium salt (DNA-rod) and DNA-triphenylene imidazolium salt (DNA-disk) complexes to an inverted hexagonal columnar (H(C)(II)) phase for the DNA-POSS imidazolium salt (DNA-cube) complex was observed. The DNA-rod complex had a typical smectic A (SmA) L(C)(alpha) morphology, whereas the DNA-disk complex had a double lamello-columnar liquid crystalline phase. However, when the lipid tail changed to POSS, an H(C)(II) morphology was achieved. These morphological changes were successfully characterized by X-ray diffraction and transmission electron microscopy. We expect that these liquid crystalline and crystalline DNA hybrid materials may become potential functional materials for various applications such as organic microelectronics and gene transfection.