Amplification of chirality in DNA liquid crystals

Concentrated solutions of duplex-forming DNA oligomers display various forms of liquid crystal ordering mediated by end-to-end assembly of helices into reversible aggregates. In the chiral nematic (N*) phase, the chirality of the DNA double strands gives rise to a macroscopic helical precession of molecular orientation, but it remains unclear how chirality propagates from the molecular- to the meso-scale. We have determined the handedness and pitch of the N* helix for a large number of sequences ranging from 8 to 20 bases. While long helices always form N* phases with left-handed pitch in the μm range, short duplexes show an extremely diverse behavior, with both left- and right-handed N* 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. The combination of these parameters determines the concentration of the N* phase and thus the balance between electrostatic and steric interactions, setting the preferred phase handedness. Furthermore, we have studied mixtures of natural D-DNA oligomers (forming right-handed duplex helices) and of mirror symmetric, L-DNA molecules, forming left-handed helices. By controlling the terminals of both enantiomers, it is possible to obtain solutions where the D- and L- species form mixed but homochiral columns, and solutions of heterochiral columns in which D- and L-DNA are mixed within each column. In the two systems, the resulting meso-scale chirality depends on the enantiomeric ratio in distinctly different ways.