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Hierarchical propagation of chirality through reversible polymerization: the cholesteric phase of DNA oligomers

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Although chiral nematic ordering has been repeatedly observed and studied in various lyotropic systems [1], quantitative account for the macroscopic chirality on the basis of the chiral structure of its microscopic constituents represents a challenge for theory and computations [2]. We consider here the chirality of the nematic ordering that develops in solutions of palindromic DNA dodecamers via their assembling into weakly bound linear chains. In this system the cholesteric pitch has a non-trivial dependence on both DNA concentration and temperature. In order to grasp a physical understanding of this complex behavior, we developed a novel theoretical approach that bridges the structure and chirality of the elementary building blocks to the helical organization of self-assembly-driven cholesteric phases. Our theoretical approach combines an Onsager-like theory for orientational order, extended to the elastic and chiral properties of the cholesteric phase [3] with a theory for self-assembly-driven nematic liquid crystals [4]. Noticeably, our theory contains no adjustable parameter other than those previously determined from the phase behavior [5].

Solutions of Dickerson dodecamer provide a suitable benchmark for our approach, since the concentration of cholesteric phases observed in this system is rather high. Indeed, under these conditions electrostatic interactions play a negligible role due to the high density of counterions present in solution, which fully screen the DNA charges. We performed new accurate measurements of the cholesteric pitch, which is shown in Figure 1 as a function of temperature in samples at different concentrations together with theoretical predictions. A right-handed cholesteric phase is formed (p > 0), with a pitch of few microns, which increases with lowering temperature and increasing concentration. The good quantitative agreement between theoretical and experimental results allows us to unveil the physical mechanisms at play in the propagation of chirality and to identify in the temperature and concentration dependence of the pitch an hallmark of self-assembly. Our theoretical framework and computational methodology, which connects the mesoscopic helical structure of the cholesteric phase formed by reversible polymers to the microscopic structural features and interactions of the their constituent building blocks, can be applied to other self-assembling cholesteric liquid crystals, to gain new insight into the microscopic origin of their helical ordering which is, at present, unexplained.

References [1] G. Zanchetta et al., PNAS -USA 107, 17497 (2010); S. Bonazzi et al., JACS 113 , 5809 (1991); G. Proni et al., Chem. Eur. J. 6, 3249 (2000); T. Sato et al, Macromolecules 26, 4551 (1993). [2] ] H. H. Wensink, Europhys. Lett. 107, 36001 (2014); D. Frenkel, Eur. Phys. J. Plus 128, 10 (2013). [3] F. Tombolato and A. Ferrarini, J. Chem. Phys. 122, 054908 (2005). [4] C. De Michele, T. Bellini, and F. Sciortino, Macromolecules 45, 1090 (2012). [5] C. De Michele, L. Rovigatti, T. Bellini, and F. Sciortino, Soft Matter 8, 8388 (2012); K. T. Nguyen, A. Battisti, D. Ancora, F. Sciortino, and C. De Michele, Soft Matter 11, 2934 (2015); K. T. Nguyen, F. Sciortino, and C. De Michele, Langmuir 30, 4814 (2014).

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