Propagation of chirality in mixtures of natural and enantiomeric DNA oligomers

Concentrated solutions of ultra-short duplex forming DNA oligomers may develop various forms of liquid crystal ordering among which is the chiral nematic phase, characterized by a macroscopic helical precession of molecular orientation [1]. The specifics of how chirality propagates from the molecular- to meso-scale is still unclear, both in general and in the case of DNAbased liquid crystals. We have here investigated the onset of nematic ordering and its chiral character in mixtures of natural DDNA oligomers forming right-handed duplex helices and of mirror symmetric (LDNA) molecules, forming left-handed helices [2]. Since the nematic ordering of DNA duplexes is mediated by their end-to-end aggregation into linear columns, by controlling the terminals of both enantiomers we could study the propagation of chirality in solutions where the D- and L- species form mixtures of homochiral columns, and in solutions of heterochiral columns. The two systems behave in markedly different fashion. By adopting a simple model based on nearest-neighbor interactions, we account for the different observed dependence of the chirality of these two systems on the enantiomeric ratio. The success of the simple models here employed indicates that all the difficulties in interpreting the nematic ordering of chiral molecules are actually nested in the determination of the torque resulting from pair interactions, while its effect on the phase can be satisfactorily accounted for by a simple approach.