BEGIN:VCALENDAR VERSION:2.0 PRODID:-//Talks.cam//talks.cam.ac.uk// X-WR-CALNAME:Talks.cam BEGIN:VEVENT SUMMARY:Hierarchical propagation of chirality through reversible polymeriz ation: the cholesteric phase of DNA oligomers - Cristiano De Michele\, Uni versity of Rome "\;La Sapienza"\; DTSTART:20151014T131500Z DTEND:20151014T141500Z UID:TALK61833@talks.cam.ac.uk CONTACT:Lucy Colwell DESCRIPTION:Although chiral nematic ordering has been repeatedly observed and studied in various lyotropic systems [1]\, quantitative account for th e macroscopic chirality on the basis of the chiral structure of its micros copic constituents represents a challenge for theory and computations [2]. We consider here the chirality of the\nnematic ordering that develops in solutions of palindromic DNA dodecamers via their assembling into weakly b ound 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 th eoretical approach that bridges the structure and chirality of the element ary building blocks to the helical organization of self-assembly-driven ch olesteric phases. Our theoretical approach combines an Onsager-like theory for orientational order\, extended to the elastic and chiral properties o f the cholesteric phase [3] with a theory for self-assembly-driven nematic liquid crystals [4]. Noticeably\, our theory contains no adjustable param eter other than those previously determined from the phase behavior [5].\n \nSolutions of Dickerson dodecamer provide a suitable benchmark for our ap proach\, since the concentration of cholesteric phases observed in this sy stem is rather high. Indeed\, under these conditions electrostatic interac tions play a negligible role due to the high\ndensity of counterions prese nt in solution\, which fully screen the DNA charges. We performed new accu rate 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 temp erature and increasing concentration. The good quantitative agreement betw een 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 interact ions of the their constituent building blocks\, can be applied to other se lf-assembling cholesteric liquid crystals\, to gain new insight into the m icroscopic origin of their helical ordering which is\, at present\, unexpl ained.\n\nReferences\n[1] G. Zanchetta et al.\, PNAS-USA 107\, 17497 (2010 )\; S. Bonazzi et al.\, JACS 113\, 5809 (1991)\; G. Proni et al.\, Chem. E ur. J. 6\, 3249 (2000)\; T. Sato et al\, Macromolecules 26\, 4551 (1993).\ n[2] ] H. H. Wensink\, Europhys. Lett. 107\, 36001 (2014)\; D. Frenkel\, E ur. Phys. J. Plus 128\, 10 (2013).\n[3] F. Tombolato and A. Ferrarini\, J. Chem. Phys. 122\, 054908 (2005).\n[4] C. De Michele\, T. Bellini\, and F. Sciortino\, Macromolecules 45\, 1090 (2012).\n[5] C. De Michele\, L. Rovi gatti\, T. Bellini\, and F. Sciortino\, Soft Matter 8\, 8388 (2012)\; K. T . Nguyen\,\nA. Battisti\, D. Ancora\, F. Sciortino\, and C. De Michele\, S oft Matter 11\, 2934 (2015)\; K. T. Nguyen\,\nF. Sciortino\, and C. De Mic hele\, Langmuir 30\, 4814 (2014). LOCATION:Department of Chemistry\, Cambridge\, Unilever lecture theatre END:VEVENT END:VCALENDAR