In this research, we observe and rationalize theoretically the transition from hexagonal to cholesteric packing of double-stranded (ds) DNA in dispersion particles. The samples were obtained by phase exclusion of linear ds DNA molecules from water-salt solutions of poly(ethylene glycol)—PEG—with concentrations ranging from 120 mg ml−1 to 300 mg ml−1. In the range of PEG concentrations from 120 mg ml−1 to 220 mg ml−1 at room temperature, we find ds DNA molecule packing, typical of classical cholesterics. The corresponding parameters for dispersion particles obtained at concentrations greater than 220 mg ml−1 indicate hexagonal packing of the ds DNA molecules. However, slightly counter-intuitively, the cholesteric-like packing reappears upon the heating of dispersions with hexagonal packing of ds DNA molecules. This transition occurs when the PEG concentration is larger than 220 mg ml−1. The obtained new cholesteric structure differs from the classical cholesterics observed in the PEG concentration range 120–220 mg ml−1 (hence, the term ‘re-entrant’). Our conclusions are based on the measurements of circular dichroism spectra, X-ray scattering curves and textures of liquid-crystalline phases. We propose a qualitative (similar to the Lindemann criterion for melting of conventional crystals) explanation of this phenomenon in terms of partial melting of so-called quasinematic layers formed by the DNA molecules. The quasinematic layers change their spatial orientation as a result of the competition between the osmotic pressure of the solvent (favoring dense, unidirectional alignment of ds DNA molecules) and twist Frank orientation energy of adjacent layers (favoring cholesteric-like molecular packing).
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机译:在这项研究中,我们观察并从理论上合理化了分散颗粒中双链(ds)DNA从六角形到胆甾型填充的转变。通过从浓度为120 mg ml -1 sup>至300mg ml -的聚(乙二醇)-PEG的水盐溶液中进行线性ds DNA分子的相排斥而获得样品。 1 sup>。在室温下PEG浓度范围从120 mg ml -1 sup>到220 mg ml -1 sup>的范围内,我们发现ds DNA分子堆积,这是典型的胆甾醇型。浓度大于220 mg ml -1 sup>的分散体颗粒的相应参数表明ds DNA分子呈六边形堆积。然而,与直觉相反,胆甾型包装在以ds DNA分子的六方堆积加热分散体时再次出现。当PEG浓度大于220 mg ml -1 sup>时,发生这种转变。获得的新胆甾醇结构与在PEG浓度范围120–220 mg ml -1 sup>(因此称为“重入”)中观察到的经典胆甾醇有所不同。我们的结论基于对圆二色性光谱,X射线散射曲线和液晶相织构的测量。我们提出了这种现象的定性解释(类似于用于常规晶体融化的Lindemann准则),以部分熔化由DNA分子形成的所谓准运动层来解释这种现象。由于溶剂的渗透压(有利于ds DNA分子的致密单向排列)和相邻层的Frank取向能扭曲(有利于胆甾型分子堆积)之间的竞争,准运动层改变了其空间取向。
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