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Chiral liquid crystal antiferroelectric phase is undistinguishable from the ferroelectric phase by the classical non-resonant X-ray scattering. We report a very different behavior of the 2D high-resolution X-ray diffraction spectra (SAXS) for a phase with antiferroelectric properties. The new antiferroelectric phase is characterized by the appearing of additional peaks in the neighborhood of the smectic Bragg peak. We investigated the dimesogenic chiral liquid crystal siloxane DSI3-MR11 [1]. The phase sequence was determined by optical textural observation. The existence of different smectic phases was confirmed by dielectric, electrooptic and polarization reversal current measurements. We have focused on X-ray diffraction in the vicinity of the transition from SmC* to the antiferroelectric phase. We investigated both powder sample and material aligned under magnetic field. On cooling, the liquid crystal undergoes a rather rapid phase transition from the SmC* phase to a state displaying typical antiferroelectric properties. The transition is then followed by a slow structural evolution. We propose a structure model for the antiferroelectric phase and we develop a simple explanation for the observed evolution. Note that another peculiar property of DSI3-MR11 was previously reported [1, 2]: in the high temperature region it exhibits a de Vries SmA* phase. Resuming both phenomena, we stress that dimesogens with a soft junction can display physical and structural properties quite different with respect to those of simple mesogenic compounds.

Commentaires: Proceedings 21st Intern.Liquid Crystal Conference, Keystone, Colorado (USA), July 2-7, 2006

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We describe the new type of liquid-crystalline state which is stabilized in condensed solutions of DNA fragments in a biologically relevant range of densities. The analysis of strong correlations which take place between DNA at rather short interaxial distances is performed. It is shown that in a system of finite-size helical DNA molecules the rotational and positional fluctuations are coupled and lead to the appearance of the macroscopic helical order. We present a model which accounts for the helical structure of the molecules and for the correlation between the helices. Starting from a (liquid-crystalline) state in which the long (helical) axes of the molecules are already correlated (nematic phase; columnar hexatic phase; columnar 2D hexagonal phase) we study the transition to a correlated helical phase. In this phase, molecules cannot freely rotate in the plane perpendicular to the director and cannot freely shift in the director axis direction, but they can freely perform screw-like helical fluctuations. The model permits to explain a whole series of unusual experimental facts revealed by X-ray diffraction experiments performed both on DNA solutions and on hydrated DNA fibers. It contributes also to the understanding of the mechanisms of chiral recognition of biomolecules during crucial biological processes. The thermodynamics of the phase transition shows the effect of the macroscopic chirality on the correlated helical order. The elastic distortion induced by the chirality of the medium does not induce the macroscopic twisting of the director (as it is the case in the media of rod-like molecules) but induces a variation in the helical repeat unit of individual molecules themselves. The overwinding of the molecule which is due to the macroscopic chirality, is rather small. X-ray diffraction experiments performed on the condensed DNA solutions and in hydrated DNA fibers have shown no director twist but a gradual decrease of the helical repeat unit when increasing the DNA density in the solution. The measured decrease was up to ten percent in the solutions and of the order of few percent in hydrated fibers. The proposed model gives also a natural explanation of the specific crystallographic structure of the hexagonal phase in high-density DNA solutions and to the peculiarities of its X-ray diffraction spectra. To relate our results to the single-molecule experiments, namely to DNA elastic behavior in optical tweezers, we estimate the elastic energy transmitted to, and the elastic torque acting on an individual in the overwinding process. We compare the results with typical energies and torques involved in basic biological processes and discuss the probability of this scenario to take place in the crowded cell environment.

DOI: 10.1080/00150190600962176
WoS: 000242347100012
6 Citations
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We present the X-ray diffraction, polarizing optical microscopy and electrooptic response studies of the chiral liquid crystal siloxane DSi3-MR11 [1] with a de Vries-type behavior. In a wide temperature range (b.Delta T sim 50°C) the material shows no layer shrinkage but a weak nearly linear layer spacing variation with temperature. Neither discontinuity nor slope change occurs at the SmA*-SmC* transition. In addition, using the X-ray data, the unusual electrooptic response in both 1st and 2nd harmonics together with known thermal behavior of birefringence, we evidence the cross-over from the classical orthogonal to the de Vries-like SmA* regime in DSi3-MR11 siloxane.

Ref HAL: in2p3-00118727_v1
PMID 16907283
Ref Arxiv: physics/0602046
DOI: 10.1103/PhysRevLett.96.248102
WoS: 000238487900064
Ref. & Cit.: NASA ADS
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We argue that the paradoxal softness of the red blood cells (RBC) in fluctuation experiments is apparent. We show that the effective surface shear modulus µs of the RBC obtained from fluctuation data and that measured in static deformation experiments have the same order of magnitude. In the RBC model developed for this purpose the spectrin network cytoskeleton with the bulk shear modulus estimated as µ[approximate]105–165 Pa contributes to both normal and tangent fluctuations of the system and confines the membrane fluctuations. The calculated ratio of the mean-square amplitudes n²>/t²> is 2–3 orders of magnitude smaller than it is in the free membrane with the same bending and shear moduli.

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DOI: 10.1007/s00249-005-0502-z
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unknown

Commentaires: poster numéro 495, p.706