Right-handed double-helix ultrashort DNA yields chiral nematic phases with both right- and left-handed director twist

Abstract

Concentrated solutions of duplex-forming DNA oligomers organize into various mesophases among which is the nematic (N Ã ), which exhibits a macroscopic chiral helical precession of molecular orientation because of the chirality of the DNA molecule. Using a quantitative analysis of the transmission spectra in polarized optical microscopy, we have determined the handedness and pitch of this chiral nematic helix for a large number of sequences ranging from 8 to 20 bases. The B-DNA molecule exhibits a right-handed molecular double-helix structure that, for long molecules, always yields N Ã phases with left-handed pitch in the μm range. We report here that ultrashort oligomeric duplexes show an extremely diverse behavior, with both left-and right-handed N Ã helices and pitches ranging from macroscopic down to 0.3 μm. The behavior depends on the length and the sequence of the oligomers, and on the nature of the end-to-end interactions between helices. In particular, the N Ã handedness strongly correlates with the oligomer length and concentration. Right-handed phases are found only for oligomers shorter than 14 base pairs, and for the sequences having the transition to the N Ã phase at concentration larger than 620 mg∕mL. Our findings indicate that in short DNA, the intermolecular doublehelical interactions switch the preferred liquid crystal handedness when the columns of stacked duplexes are forced at high concentrations to separations comparable to the DNA double-helix pitch, a regime still to be theoretically described.cholesteric | RNA U nderstanding how the physicochemical details of macromolecules affect their interactions and their self-ordering properties represents one of the major challenges in the physics of soft and biological matter. In particular, because most of the biologically relevant molecules are chiral-i.e., they lack mirror symmetry-the relevant biointeractions are typically highly sensitive to the molecular handedness. One important effect of chiral interactions is the propagation of chirality from the molecular structure to the supramolecular assemblies such as aggregates or ordered phases. Often, in this phenomenon, minor molecular modifications are amplified to produce remarkable changes in the macroscopic ordering, as for DNA supercoiling and its biological role (1). The propagation of chirality has been studied in various systems, including thermotropic liquid crystals (LCs) and lyotropic assemblies of viruses, DNA, polymers, and dye aggregates. Despite the generality of the phenomenon, its understanding is still poor. Predicting, only on the basis of the molecular structure, the most basic parity of chiral LC ordering, the rightvs. left-handedness, is still a challenge (2-4). One would expect the problem to become easier when restricted to molecular structures with a clean cylindrical shape decorated with regular helical structures and charges, such as DNA, G-quartets, and filamentous viruses. However, even in this limited frame, experimental and theoretical results appear difficult to fi...

Self-assembly of hard helices: a rich and unconventional polymorphism

“…In principle, helical biopolymers such as DNA or helical colloidal particles, appear as good candidates for this investigation. [51][52][53] Indeed, a screw-nematic phase was already observed a few years ago in colloidal suspensions of helical flagella isolated from prokaryotic bacteria. [17] The helical pitch p of these particles is µm in size and hence the phase modulation is easily visible under polarized optical microscopy.…”

Liquid crystal self-assembly of random-sequence DNA oligomers

“…Selective interactions of DNA oligomers have been exploited in the past years in a variety of strategies for the construction of designed self-assembled nanostructures (1)(2)(3)(4). Selectivity combines with self-assembly in the recent observation that short oligomers of nucleic acids having complementary sequences exhibit liquid crystal (LC) ordering (5)(6)(7). In this article, we report LC ordering in solutions of DNA oligomers with random sequences where the large body of different competing sequences effectively reduces the selectivity of the interactions.…”

Density functional theory for chiral nematic liquid crystals

“…The wavelength associated to a full rotation of the nematic director around the chiral directorχ is known as the cholesteric pitch P . Cholesteric phases are commonly found in both thermotropic molecular compounds (e.g., derivatives of cholesterol [2][3][4][5]) and in lyotropic colloidal suspensions of, e.g., DNA [6,7] and filamentous viruses [8][9][10][11][12]. Their widespread occurrence explains why cholesterics were the first liquid-crystal phase experimentally observed [2].…”