Right-handed double-helix ultrashort DNA yields chiral nematic phases with both right- and left-handed director twist |
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Authors: | Giuliano Zanchetta Fabio Giavazzi Michi Nakata Marco Buscaglia Roberto Cerbino Noel A. Clark Tommaso Bellini |
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Affiliation: | aDipartimento di Chimica, Biochimica e Biotecnologie per la Medicina, Università degli studi di Milano, Via Saldini 50, Milan, Italy 20133; and ;bDepartment of Physics and Liquid Crystal Materials Research Center, University of Colorado, Boulder, CO 80309-0390 |
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Abstract: | Concentrated solutions of duplex-forming DNA oligomers organize into various mesophases among which is the nematic (N∗), which exhibits a macroscopic chiral helical precession of molecular orientation because of the chirality of the DNA molecule. Using a quantitative analysis of the transmission spectra in polarized optical microscopy, we have determined the handedness and pitch of this chiral nematic 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. 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. The 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 and concentration. Right-handed phases are found only for oligomers shorter than 14 base pairs, and for the sequences having the transition to the N∗ phase at concentration larger than 620 mg/mL. Our findings indicate that in short DNA, the intermolecular double-helical 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, a regime still to be theoretically described. |
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Keywords: | cholesteric RNA |
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