Helical inversion

Very recently, however, two papers were published by the group of Fujiki which report successful solid-state CD studies of chiral polysilanes. In the first, a helix-coil transition was described for film samples of poly[(A)-3,7-dimethyloctyl- -propylsilylene)], 113.327 This polymer has a relatively low glass transition temperature, T, which was considered critical for the observation of a helix-helix transition in the solid state, since helical inversion would be precluded if the inversion temperature, Tc, were below Ts as the segmental motion of the chain,... 

A similar chiral stimuli-responsive, macromolecular helicity inversion was also observed for (R)- or (S)-68 bearing an optically active (1-(1-naphthyl)ethyl)carbamoyl group by interacting with optically active small molecules, such as (R)- and (S)-39 (Fig. 15) [126]. The ICD of (R)-68 or (S)-68 in DMF changed to almost a mirror image in the presence of an excess of (R)-or (S)-39, respectively. Thus, optically active 39 can be used to regulate the helix-sense of 68. 

Cotton effect sign in the presence of (S)-37 in DMSO/alkaline water (3/7, v/v), while the solution color remained yellow with a positive first Cotton effect sign in the presence of (R)-37 (Fig. 24). The chirality of 37 might induce the macromolecular helicity inversion of 69, which can be readily visible with the naked eye. The color change may be due to a change in the twist angle of... 

The photoresponsive helicity inversion process of 4 was applied to the photoregulation of permeability across a membrane from a graft copolymer containing the photoresponsive peptide (7) as a branch (Fig. 2) [44]. The graft polymer membrane casted from a 1,2-dichloroethane solution, followed by immersion in trimethylphosphate, exhibited a positive CD band at 215 nm, characteristic to... 

Yashima et al. [88] investigated the interaction of poly(phenylacetylene) modified with fi-CD moieties with guest molecules, including 1-adamantanol, (-)-borneol, and R)- and (S)-l-phenylethylamine. They observed a color change upon complex-ation because of a change in the helicity pitch arising from helicity inversion. 

H. Hayasaka, T. Miyashita, M. Nakayama, K. Kuwada, K. Akagi, Dynamic photoswitching of helical inversion in liquid crystals containing photoresponsive axially chiral dopants. 

Montgomery CP, New EJ, Parker D et al. (2008) Enantioselective regulation of a metal complex In reversible binding to serum albumin dynamic helicity inversion signalled by circularly polarised luminescence. Chem Commun 4261 263... 

Upon benzoate binding intramolecular hydrogen bonds are disrupted and this binding event leads to helix uncoiling. This behavior was in contrast to most of the foldamers described, where anion binding induces helicity or causes helicity inversion. ... 

Type C2b has two (or more) mesogenic groups that want to be parallel to the director, but they are twisted by a chiral spacer. These compounds can cause very strong chiral effects, but the molecules are often not mesogenic themselves. If the mesogenic axes are almost parallel, then helical inversions can be induced by small conformational changes. 

Most studies on helicity inversion have been based on single-mode regulation, i.e., interconversion between two heUcaUy different species. Sequential metal addition results in three-step helicity inversion. This strategy should open new paths for creating different functional compounds with multi-response properties toward different effectors and combinations of effectors. 

K. Okoshi, S.I. Sakurai, S. Ohsawa, J. Kumaki, E. Yashima, Control of main-chain stif iess of a helical poly(phenylacetylene) by switching on and off the intramolecular hydrogen bonding through macromolecular helicity inversion. Angew. Chem. Int. Ed. 48, 8173-8176 (2006)... 

Another particularity of knotted molecules is that the double-helical multimetallic complex that has been used as a templating core will be more or less frozen. In other words, the stability of the double helix will be insured by tying the molecular string used to make the knot in such a way as to make ligand exchange, helicity inversion, etc... 

Abstract New helical poly(phenylacetylene)s have been successfully designed and synthesised and their properties checked. The new polymers behave as sensors of metal cation valences and/or the polar and draior character of solvents. In the presence of metal salts, poly(phenylacetylene)s form helical polymer-metal complexes (HPMCs) that, in the case of a-methoxyphenylacetic acid (MPA)-containing poly(phenylacetylene), has led to a new family of nanospheres made by complex-ation between the polymer and divalent metal ions. These HPMC nanostructures present properties such as (1) their diameter can be tuned to different sizes, (2) the helicity of the polymeric material can be tuned to either of the two helical senses, and (3) they can encapsulate a number of inorganic and organic substances. These polymers also display phenomena such as helical inversion, chiral amplification and axial chirality selection, making them versatile materials. 

Keywords Chiral amplification Helical inversion Helical polymer-metal complexes Metal cations Methoxyphenylacetic acid Nanospheres Poly(phenylacetylene)s... 

Fig. 2 Reversible helical inversion by metal itai complexation on poly-(.S)-l with Ba acac acetyiacetone...

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