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

Concentrated solutions of duplex-forming DNA oligomers organize into various mesophases among which is the chiral nematic (N*), whose macroscopic chiral helical precession reflects the chirality of the DNA molecule [1]. Using a quantitative analysis of the transmission spectra in polarized optical microscopy, we have determined the handedness and pitch of the N* 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* phases with left-handed pitch in the μm range [2]. We report here that ultrashort oligomeric duplexes show an extremely diverse behavior, with both left- and right-handed N* helices and pitches ranging from macroscopic down to 0.3 μm, as shown in the figure for a dodecamer [3]. Such 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* handedness strongly correlates with the oligomer length - right-handed phases are only found for oligomers shorter than 14 base pairs - and concentration – right-handed phases are found at concentration larger than 620 mg⁄mL. Our findings indicate that in short DNA, the intermolecular 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.