螺旋结构与旋光性 Ⅰ.螺旋结构和旋光方向

本文分析了旋光性分子中的螺旋结构,由此得出结论:螺旋结构是引起旋光性的根本原因。右手螺旋一定为右旋的,左手螺旋一定为左旋的。当分子内存在螺旋结构,而这些螺旋结构的旋光性不能完全相互抵消时,这个分子一定有旋光性。从螺旋方向可以预测旋光方向,知道旋光方向以预测螺旋方向,进而预测化合物的构型。     

螺旋结构和旋光性

旋光性是由分子结构决定的。各种类型的光活性分子内都存在螺旋结构。左螺旋为左旋的，右螺旋为右旋的。分子内含有多少个螺旋时，则所有螺旋旋光度的代数和为正是右旋的，为负是左旋的。因此，由旋光方向可预测分子的构型和构象。     

螺旋结构与旋光性 4.螺旋理论对旋光度大小的预测

本文分析了影响旋光度的各种因素,并列举了各类型的大量化合物的旋光数据,证明螺旋理论可以预测旋光度大小。     

导致旋光性根本原因的探讨

旋光性是由结构决定的，手性是一种抽象概念，抽象概念不能决定旋光性，旋光性具有明确的方向性，它必然是由具有明确方向性的结构特征所决定。螺旋结构是具有明确方向性的结构，它是实体不是抽象概念，它与旋光方向和旋光度大小可以联系在一起，螺旋结构才是导致旋光性的根本原因。     

单糖结构及其旋光度的计算

根据螺旋理论的规律，对常见单糖及其甲甙的旋光度进行了计算。计算结果和测定值相当接近，在旋光方向上完全一致，旋光度数值也都在可接受的误差范围之内。证明单糖及其甙的立体结构和旋光性的关系是十分密切的。根据糖和糖甙的立体结构，可以推断旋光方向和旋光度的大小，反之，从旋光性也可以推断单糖及其甲甙的立体结构。     

旋光性与分子结构关系的新发展

本文以1860年Pasteur提出的旋光性与手性分子所含螺旋结构有关联的设想为模型,剖析了某些光学活性分子。表明Pasteur的设想是符合实际情况的,即右螺旋呈现右旋光性,左螺旋呈现左旋光性。同时,通过螺旋模型和旋光方向与Brewster的基团可极化性序列相结合,则可以推导出手性碳原子的构型。介绍了螺苯类化合物、联苯类化合物、聚积二烯类化合物及含手性碳原子的化合物所含螺旋结构的剖析和与旋光方向的关系。为阐明结构与旋光性的关系提出了一种可供参考的论点。     

甲基取代的手性环酯化合物的螺旋结构与旋光性分析

利用螺旋片段判定规则,结合X射线衍射结构图及模型结构,对4个大环酯类化合物及甲基取代产物的螺旋结构与旋光性进行了定量分析,检测结果证明了理论分析的正确性;对旋光值变化规律给予了合理解释.     

旋光性高分子的旋光能力（上）

本文分析总结了内在的结构因素和外的各种条件对旋光性高分子的旋光能力的影响。在这些影响因素中，构象起着重要的作用，也即高分子的链形态是否具有稳定的、单一螺旋方向的构象是决定高分子是否具有高度旋光能力的关键。     

邻苯二甲酸L-(+)-2,3-O-亚异丙基苏力糖酯的两种构象式的螺旋结构与旋光性

在晶 (固 )态下 ,化合物邻苯二甲酸 L-( +) -2 ,3 -O-亚异丙基苏力糖酯存在两种不同的构象形式 ,利用螺旋片段判定规则及ω值 ,结合 X射线衍射数据及扭转角数据 ,对这两种构象形式的所有螺旋片段进行了分析 ,尽管两种构象形式的净值螺旋性相反 ,但仍然得出总净值螺旋性与旋光方向一致的结论     

螺旋结构与旋光性——3.螺旋理论能统一旋光性方面的经验规律

螺旋理论可以解释和统一旋光性方面的经验规律,并赋予它们以物理意义和理论依据。     

Helix formation in the polymer brush

This work considers the physics of a brush formed by polymers capable of undergoing a helix-coil transition. A self-consistent field approximation for strongly stretched polymer chains is used in combination with a lattice model of the interaction energy in helix-coil mixtures. Crowding-induced chain stretching stabilizes helix formation at moderate tethering densities while high tethering density causes sufficiently strong stretching to unravel segments of the helix, resulting in distinct layers of monomer density and helical content. Compared to a random-coil brush at low-to-moderate tethering density, a helicogenic brush is less resistant to compression in the direction perpendicular to stretching due to easy alignment of helices and fewer unfavorable interactions between helical segments. At higher tethering density, the abovementioned stretch-induced decrease in helical content resists further compression. The proposed model is useful for understanding an emerging class of biomaterials that utilize helix-forming polymer brushes to induce shape changes or to stabilize biofunctional helical peptide sequences.     

The bacterial flagella and the flagellar protein flagellin

In comparison with the relatively complicated flagella of all nucleated organisms, most of the actively motile bacteria have very much simpler locomotion organelles. These flagella are long thin filaments with a helical superstructure, anchored at one end in the cell membrane. Helical waves pass along the flagella in the distal direction during locomotion. In the flagella, identical structural units of a single protein species, flagellin, are linked to one another by non-covalent bonds. The flagellin subunits can be separated from one another by mild methods, and their biochemistry as well as their structure and morphology can be investigated in the isolated state. Under certain conditions they can reaggregate in vitro to form polymorphous helical filaments, which are practically indistinguishable from intact flagella. In the flagellum, the elongated flagellin molecules have a layered arrangement in about ten parallel strands, which in turn form a hollow cylinder. Since this cylinder is deformed into a large helix, the individual chemically identical flagellin protomers and the protomer strands in the helical flagella are only quasi-equivalent. The conformation and bonding pattern of the individual longitudinal flagellin lines can presumably be controlled in vitro by the basal structures, so that movement within the flagellum could conceivably occur as follows. A cyclically induced change in the length of the individual strands could result in an apparent rotation of the helical flagellum. According to hydrodynamic calculations, the resulting forward thrust is sufficient to propel the bacteria at the observed speeds of up to 50 μm/s.     

Trismaleimide Dendrimers: Helix‐to‐Superhelix Supramolecular Transition Accompanied by White‐Light Emission

Reported here are unprecedented fluorescent superhelices composed of primary, supramolecular polymers of the opposite helical twist. A new class of functional dendrimers was synthesized by amino‐ene click reactions, and they demonstrate an alternating OFF/ON fluorescence with generation growth. A peripherally alkyl‐modified dendrimer displays helix‐sense‐selective supramolecular polymerization, which predominantly forms right‐handed (or left‐handed) helical supramolecular polymers in the solution containing chiral solvents. With increasing the concentration, these primary helical supramolecular polymers spontaneously twist around themselves in the opposite direction to form superhelical structures. Atomic force microscopy and circular dichroism measurements were used to directly observe the helix‐to‐superhelix transition occurring with a reversal in the helical direction. Exceptional white‐light emission was observed during superhelix formation.     

ORD and CD studies of poly[(S)(−)-N-α-methylbenzylethylenimine]

The advantages of chiral N-substituted polyethylenimines for the chiroptical study of macromolecular conformations in solution have been considered. Poly[(S)(?)-N-α-methylbenzylethylenimine] (poly-3) and five model compounds have been obtained and their chiroptical properties studied. The cyclic dimer and tetramer of imine 3 are suitable models for the chiroptical study of configurational and conformational behaviour of randomly coiled and helical poly-3 in solution. Unlike the disordered poly-3 in acidified solvents and the configurational models in various solvents, helical poly-3 in hydrocarbon solvents has positive optical rotation and positive CD-band of the tertiary amine chromophore at 223 nm. The appearance of this inherently chiral chromophore of the helix explains the observed changes of the sign and values of the optical rotation of poly-3 in various solvents which are mainly caused by conformational transformations of the main chain. The presence of helical conformations of poly-3 in solution are also confirmed by optical rotation data of stereocopolymers and the concentration dependence data of ORD curves of poly-3. This conclusion agrees with NMR and i.r.-data.     

The three-dimensional model for helical columns on type-J synchronous counter-current chromatography

Unlike the existing 2-D pseudo-ring model for helical columns undergoing synchronous type-J planetary motion of counter-current chromatograph (CCC), the 3-D "helix" model developed in this work shows that there is a second normal force (i.e. the binormal force) applied virtually in the axial direction of the helical column. This force alternates in the two opposite directions and intensifies phase mixing with increasing the helix angle. On the contrary, the 2-D spiral column operated on the same CCC device lacks this third-dimensional mixing force. The (principal) normal force quantified by this "helix" model has been the same as that by the pseudo-ring model. With β>0.25, this normal centrifugal force has been one-directional and fluctuates cyclically. Different to the spiral column, this "helix" model shows that the centrifugal force (i.e. the hydrostatic force) does not contribute to stationary phase retention in the helical column. Between the popular helical columns and the emerging spiral columns for type-J synchronous CCC, this work has thus illustrated that the former is associated with better phase mixing yet poor retention for the stationary phase whereas the latter has potential for better retention for the stationary phase yet poor phase mixing. The methodology developed in this work may be regarded as a new platform for designing optimised CCC columns for analytical and engineering applications.     

Remote Control of the Planar Chirality in Peptide‐Bound Metallomacrocycles and Dynamic‐to‐Static Planar Chirality Control Triggered by Solvent‐Induced 310‐to‐α‐Helix Transitions

The dynamic planar chirality in a peptide‐bound Ni^II‐salphen‐based macrocycle can be remotely controlled. First, a right‐handed (P)‐3₁₀‐helix is induced in the dynamic helical oligopeptides by a chiral amino acid residue far from the macrocyclic framework. The induced planar chirality remains dynamic in chloroform and acetonitrile, but is almost completely locked in fluoroalcohols as a result of the solvent‐induced transition of the peptide chains from a 3₁₀‐helix to a wider α‐helix, which freezes the rotation of the pendant peptide units around the macrocycle.     

Current-induced rotation of helical molecular wires

We show that electric current running through a nanojunction with a biased helical molecule can induce unidirectional rotation of the molecular component. In an electric field, conduction electrons injected into the molecule are accelerated along the helical path going through its body, thereby gaining directed angular momentum. Conservation laws require that an angular momentum of the same size but opposite sense is imparted to the rigid-body rotation of the helix. We describe the angular momentum exchange processes that underlie the operation of the nanorotor, discuss factors limiting its efficiency, and propose potential applications.     

Study of low-frequency molecular motions in polydimethylsiloxane polymers by neutron inelastic scattering

The intrachain and interchain vibrations below 900 cm^?1 of polydimethylsiloxane (PDMS) have been studied by slow neutron inelastic scattering. A composite motion observed at +25°C for the methyl groups corresponds to nearly free rotation about the threefold axis of symmetry together with a large-amplitude rotation of the entire methyl group. At ?123°C, rotation about the threefold axis evolves to a torsional oscillation. The large-amplitude rotation evolves to the skeletal vibrations of a helical conformation. Vestiges of the cooperative skeletal vibrations of the conformation at ?123°C persist into the 25°C spectrum. The results indicate the presence of interrupted helical conformations at 25°C, which result from thermal disordering of the low temperature helices. The effects of crosslinking, low molecular-weight oils, and silica filler on the freedom of the methyl group motions and on skeletal vibrations have been determined. The effects of different crosslinking agents and different relative amounts of filler and oil on both the macroscopic physical properties and the observed molecular motions of PDMS can also be interpreted in terms of an interrupted helix.     

Conformational Studies on Polytripeptides as a Model of Collagen

To study the role of glycine residues in stabilizing the collagen triple helix, the glycine residues in the polytripeptide (Pro-Ala-Gly)_n were partially replaced by alanine. The proline content was kept constant. The stability of the helical conformation of these polypeptides was studied by IR- and X-ray measurements in the solid state and by ORD, CD and viscometry in solution. The renaturation was followed as a function of time. All the polytripeptides studied, with the exception of (-Pro-Ala-Ala-)_n attained the polyproline II conformation. However the stability decreased with increasing alanine content. Obviously the molecules of (-Pro-Ala-Gly-) are highly associated by intermolecular hydrogen bonds and one may therefore suppose that a triple-stranded helix aggregation occurs. The results of the refolding process show that the stability of the helices seems to also affect the refolding rate in terms of the optical rotation, Two transitions appeared: the first one is responsible for a rapid reversible change in conformation and the second one for a further slow and irreversible change in the hydrodynamic shape. The latter seems to be due to the partial helical nature, leading to higher chain mobility.