Helix inversion, chirality sensing

Several biopolymers and synthetic optically active polymers are known to exhibit an inversion of helicity (helix-helix transition) between right- and left-handed helical conformations when changing the external conditions, such as solvent, temperature, or by light irradiation. However, switching of the macromolecular helicity by chiral stimuli is rare, and can be used to sense the chirality of specific chiral guests. The helicity of optically active helical poly(phenylacetylene)s 67-69 can be switched by external chiral and achiral stimuli [123-126]. The first example of such a helix inversion induced by... 

We published a review paper in this journal entitled Asymmetric Polymerization in 1994 which encompassed this aspect of helical polymer synthesis in addition to the other types of polymerization in which chirality is introduced during the polymerization process.34 There have been several other review papers on asymmetric polymerization and chiral polymers.35-40 On the other hand, if the energy barrier is low enough to allow rapid helix inversion at room temperature, one cannot expect to obtain a stable one-handed helical polymer but may expect to induce a prevailing helical sense with a small amount of chiral residue or stimulant. The existence of this type of polymer was most clearly demonstrated with poly(alkyl isocyanate)s.23,41... 

Several procedures have been established in the past for the helix-sense-selective polymerization of isocyanides.169 For the introduction of a chiral bias, required for the discrimination of the otherwise enantiomeric helical senses, three distinct approaches can be applied. One is the use of a chiral monomer, which makes the left- and right-handed helices diastereotopic, and as a result one of the two will be energetically favorable (Figure 5). A second approach involves the use of a chiral catalyst or a chiral initiator, in which case during the initiation of the polymerization there will be a preference for the formation of one particular helical sense that will be retained upon propagation due to the helix inversion barrier. The former approach has been employed in numerous cases (vide infra) the accounts of the use of optically active Ni catalysts are limited.189-192 The... 

Molecular models are much more difficult for type C3, where the chiral centers are located inside the mesogenic axis. Small changes of the orientation of the main axis of the molecule can cause large changes of the chiral properties, thus, they are very sensitive to small changes of pressure, temperature, or concentration. Inversion of the helix [33] or of the spontaneous polarization [34] is quite common. Even cholesterol chloride and iodide have a different sense of the cholesteric helical pitch [36], and the first observation of helix inversion was done with a steroid compound [37]. 

A theoretical approach to the observed temperature effects is achieved from the macroscopic aspects given by Osipov [13] he stated that the two proposed chiral interactions which are possibly the basic mechanisms for lyotropic phase chirality have different signs. Their action leads to opposite twist senses of the macroscopic helix. In principle, temperature-induced helix inversions could be explained. 

Since the pioneering work of researchers such as Percec [4, 5] and others [1, 2], the possibility of achieving helix-sense bias (i.e. helix inversion) on PPAs has stimulated the development of a number of tools for their manipulation for instance, through specific non-covalent interactions of achiral pendants with chiral molecules [1, 2],... 

A knowledge and understanding on the nature of the helix switching may be helpful in designing PM-transition capability and characteristics in diverse screw-sense inversion related applications, such as chiroptical switches and memory, switchable chiral separation columns, molecular recognition and molecular motor applications. 

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. 

In a former investigation, an inversion of the helix sense with dopant concentration has been observed which is unusual. The phenomenon was explained again by a change of the cisitrans ratio of the surfactant (the chloride of the decyl ester of L-proline). NMR gave at least qualitative evidence for the existence of the two rotamers [28]. An inversion of the twist sense has been found with a-hydroxy carbon acids and some corresponding potassium salts [23]. For all of the investigated couples the respective opposite handedness was proven by a twist compensation seen in contact preparations. The result is unexpected because the molecular chirality of the acid and the salt are nearly identical. It was supposed that the presumably different locations with respect to the micelles due to the ionic character of the salt and/or a dimerization of the acid could be the reason for the observed behavior. 

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