Helix-sense reversal

Photoinduced Helix-sense Reversal in Azobenzene-containing Poly(L-aspartate)s... 

Fig. 1. Two ribbon representations of the crystal structure of the DNA decamer d(CCTCG -CTCTC/GAGAG CGAGG) containing a unique cisplatin interstrand cross-link at d(GpC)-d(GpC) site (asterisks indicate the chelated bases in the adduct). A front view (A) allows to see the structure with the lesion in the minor groove. A side view (B) shows the chicane of the backbone with the helix-sense reversal. Ptn atom, yellow ammine groups, navy blue sugars, pink guanines, navy blue adenines, red thymines, yellow cytosines, hght blue phosphodiester backbone, green.

In a similar manner, the coalescence temperature for the methyl groups of the tetramer was determined as 4°C. AG was calculated as 12.7 kcal/mol which was 3.7 kcal/mol smaller than that for the pentamer. The hexamer showed a total of seven signals with narrow linewidth due to two methyl groups and six methine protons even at 70°C, indicating that the rate of helix sense reversal is much slower than the rate for the pentamer. This suggests the possibility that the symmetrical oligomers over the pentamer level may be optically resolved at room temperature based entirely on conformational asymmetry. This was confirmed by the chiral HPLC technique using (+)poly(triphenylmethyl methacrylate) as a stationary phase.282... 

Large photoresponse effects could be observed in solvent mixtures, provided that the irradiation was carried out at appropriate solvent compositions. A copolypeptide composed of 33 mol% P-benzyl-L-aspartate and 67 mol% para-phenylazo-L-aspartate (X) was found to give different kinds of photoresponse, depending on the composition of the solvent in which irradiation was carried out. In dichloroethane (DCE)/ hexafluoropropanol (HFP) = 95/5, irradiation at 320-390 nm produced an increase in right-handed helix content in DCE/HFP = 54/26, a light-induced conformational change from left-handed helix to random coil was observed while, finally, reversal of the helix sense occurred in DCE/HFP = 65/35.150,511... 

More recently, Ueno et al. have prepared and investigated a new series of copolymers containing p-phenylazobenzyl-L-aspartate and n-octadecyl-L-aspartate residues (Scheme 7, Structure XII). 55 561 In the case of copolymers containing less than 50 mol% azo residues, the CD spectra at 25 °C were consistent with the presence of right-handed helical conformations, which were not affected by irradiation at 320 nm. In contrast, in the case of copolymers containing 68 and 89 mol% azobenzene groups, irradiation caused the reversal of helix sense from the left-handed to the right-handed form. 

Reversible control over the helix sense in polysilanes was achieved in the case of poly(diaryl-silanes)128 as well as in (co)polymers of ((S)-3,7-dimethyloctyl)(3-methylbutyl)silane,129 which both showed helix reversal upon heating. For the latter polymer (Figure 12a), it was calculated that the potential curve has a double-well ( W j shape (Figure 12b) with a slight preference for the M-helix over the P-helix. CD spectroscopy indeed revealed that above the transition temperature the ordered (low-entropy) M-helical conformation becomes less stable than the entropically more favored P-helical state (Figure 12c,d). 

DMPMA (32), an optically active monomer, also affords an optically active helical polymer whose helicity is mainly controlled not by the chirality of the ligand used for the polymerization but by the chirality of the monomer itself. Poly (DMPMA) exhibited reversible helix-helix transition where excess helix sense is determined by solvent properties. This is the first example of a reversible helix-helix transition of a vinyl polymer. 

Helix-sense-selective polymerization of achiral carbodii-mides 159 and 160 was performed using optically active titanium(IV) complex (161). " ° The anthracene-containing poly-159 slowly racemized in toluene at 80 °C. Below the racemization temperature, poly-159 exhibited reversible switching of chiroptical properties driven by temperature and solvent. ° The switching is due to a change in the orientation of pendent anthracene moieties, and the kinetically controlled helicity during the polymerization seems to remain unchanged. 

The versatility of the polymer for helix sense selection and chiral amphfication was demonstrated in a number of experiments performed in CHCI3. For instance, once the chiral amphlication was obtained with a divalent cation (i.e. cation to mm ratio of 0.1), the helicity could be reversed by addition of a monovalent cation at a higher concentration (i.e. cation to mm ratio of 1.0). The opposite addition sequence (first monovalent, second divalent) acted in a similar way the axial chirality induced by the cation in excess was predominant. CD spectra analogous to those obtained by step-by-step additions were recorded when mixtures of mono-and divalent cations were added simultaneously. 

Here, ry is the separation between the molecules resolved along the helix axis and is the angle between an appropriate molecular axis in the two chiral molecules. For this system the C axis closest to the symmetry axes of the constituent Gay-Berne molecules is used. In the chiral nematic phase G2(r ) is periodic with a periodicity equal to half the pitch of the helix. For this system, like that with a point chiral centre, the pitch of the helix is approximately twice the dimensions of the simulation box. This clearly shows the influence of the periodic boundary conditions on the structure of the phase formed [74]. As we would expect simulations using the atropisomer with the opposite helicity simply reverses the sense of the helix. 

It is possible that the helicity is a result of the chiral substitution itself and that the polymers with achiral substituents have, in fact, all-anti conformations. While this possibility cannot be directly ruled out, comparison of the spectroscopic data for the polymers with chiral substituents and achiral substituents, for example, 47 and 48, respectively, indicates similar main-chain dihedral angles, since the UV absorption maxima are so similar. Both polymers should therefore be latent helical, that is, contain segments of opposite screw sense separated by strong kinks (helix reversal points), with the difference being that in the case of 47 the overall numbers of P and M turns are equal, whereas for 48, one of the screw senses predominates, resulting in net helicity and optical activity. 

Chirality (or a lack of mirror symmetry) plays an important role in the LC field. Molecular chirality, due to one or more chiral carbon site(s), can lead to a reduction in the phase symmetry, and yield a large variety of novel mesophases that possess unique structures and optical properties. One important consequence of chirality is polar order when molecules contain lateral electric dipoles. Electric polarization is obtained in tilted smectic phases. The reduced symmetry in the phase yields an in-layer polarization and the tilt sense of each layer can change synclinically (chiral SmC ) or anticlinically (SmC)) to form a helical superstructure perpendicular to the layer planes. Hence helical distributions of the molecules in the superstructure can result in a ferro- (SmC ), antiferro- (SmC)), and ferri-electric phases. Other chiral subphases (e.g., Q) can also exist. In the SmC) phase, the directions of the tilt alternate from one layer to the next, and the in-plane spontaneous polarization reverses by 180° between two neighbouring layers. The structures of the C a and C phases are less certain. The ferrielectric C shows two interdigitated helices as in the SmC) phase, but here the molecules are rotated by an angle different from 180° w.r.t. the helix axis between two neighbouring layers. 

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