title: Right-handed double-helix ultrashort DNA yields chiral nematic phases with both right- and left-handed director twist.
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.
authors.affiliation: Dipartimento di Chimica, Biochimica e Biotecnologie per la Medicina, Università degli studi di Milano, Via Saldini 50, Milan, Italy 20133.
authors.forename: Giuliano
authors.initials: G
authors.lastname: Zanchetta
authors.forename: Fabio
authors.initials: F
authors.lastname: Giavazzi
authors.forename: Michi
authors.initials: M
authors.lastname: Nakata
authors.forename: Marco
authors.initials: M
authors.lastname: Buscaglia
authors.forename: Roberto
authors.initials: R
authors.lastname: Cerbino
authors.forename: Noel A
authors.initials: NA
authors.lastname: Clark
authors.forename: Tommaso
authors.initials: T
authors.lastname: Bellini
mesh_terms: D001483:Base Sequence; D002851:Chromatography, High Pressure Liquid; D004247:DNA / Q000737:chemistry* / Q000648:ultrastructure; D009690:Nucleic Acid Conformation; D009838:Oligodeoxyribonucleotides / Q000235:genetics; D013056:Spectrophotometry, Ultraviolet
chemical_list: D009838:Oligodeoxyribonucleotides; D004247:DNA