title: Two-dimensional packing of short DNA with nonpairing overhangs in cationic liposome-DNA complexes: from Onsager nematics to columnar nematics with finite-length columns.
abstract: We report the formation of liquid crystalline (LC) phases of short double-stranded DNA with nonpairing (nonsticky) overhangs, confined between two-dimensional (2D) lipid bilayers of cationic liposome-DNA complexes. In a landmark study (Science2007, 318, 1276), Nakata et al. reported on the discovery of strong end-to-end stacking interactions between short DNAs (sDNAs) with blunt ends, leading to the formation of 3D nematic (N) and columnar LC phases. Employing synchrotron small-angle X-ray scattering, we have studied the interplay between shape anisotropy-induced and DNA end-to-end interaction-induced N ordering for 11, 24, and 48 bp sDNA rods with single-stranded oligo-thymine (T) overhangs modulating the end-to-end interactions. For suppressed stacking interactions with 10-T overhangs, the volume fraction of sDNA at which the 2D isotropic (I)-to-N transition occurs for 24 and 48 bp sDNA rods depended on their length-to-width (L/D) shape anisotropy, qualitatively consistent with Onsager's theory for the entropic alignment of rigid rods. As the overhang length is reduced from 10 to 5 and 2 T for 24 and 48 bp sDNA, the N-to-I transition occurs at lower volume fractions, indicating the onset of some degree of end-to-end stacking interactions. The 11 bp sDNA rods with 5- and 10-T overhangs remain in the I phase, consistent with their small shape anisotropy (L/D ≈ 1.9) below the limit for Onsager LC ordering. Unexpectedly, in contrast to the behavior of 24 and 48 bp sDNA, the end-to-end interactions between 11 bp sDNA rods with 2-T overhangs set in dramatically, and a novel 2D columnar N phase (N(C)) with finite-length columns formed. The building blocks of this phase are comprised of 1D stacks of (on average) four 11 bp DNA-2T rods with an effective L(stacked)/D ≈ 8.2. Our findings have implications for the DNA-directed assembly of nanoparticles on 2D platforms via end-to-end interactions and in designing optimally packed LC phases of short anisotropic biomolecules (such as peptides and short-interfering RNAs) on nanoparticle membranes, which are used in gene silencing and chemical delivery.
authors.affiliation: Materials Department, University of California, Santa Barbara, California 93106, USA.
authors.forename: Nathan F
authors.initials: NF
authors.lastname: Bouxsein
authors.forename: Cecília
authors.initials: C
authors.lastname: Leal
authors.forename: Christopher S
authors.initials: CS
authors.lastname: McAllister
authors.forename: Kai K
authors.initials: KK
authors.lastname: Ewert
authors.forename: Youli
authors.initials: Y
authors.lastname: Li
authors.forename: Charles E
authors.initials: CE
authors.lastname: Samuel
authors.forename: Cyrus R
authors.initials: CR
authors.lastname: Safinya
mesh_terms: D002412:Cations; D004247:DNA / Q000737:chemistry*; D008081:Liposomes / Q000737:chemistry*; D050866:Liquid Crystals / Q000737:chemistry*; D008954:Models, Biological
chemical_list: D002412:Cations; D008081:Liposomes; D004247:DNA