Helix screw sense

Another useful property of APhe is its strong absorption in the near-UV region (=280 nm). This characteristic has been exploited in several studies to probe peptide conformational transitions and helix screw sense changes of chiral switches (26-28). 

Exciton coupling can also occur between the benzamide chromophore and other jt-electron systems, and (S)-Af-benzoyl-a -(2-furfuryl)alkylamines [(5)-139j show positive ECCE spectra with a bisignate CE couplet centered at about 228 nm. For the enantiomers the ECCE spectra are negative. Similarly the p-bromobenzoyl group linked to the Af-terminus of a helical peptide chain is useful as a CD probe in the determination of the relationship between the C(a )-configuration of coded and noncoded a-amino acids and peptide helix screw sense in solution, the bisignate CE center around 238 nm the result of the interaction between the p-bromobenzamido chromophore and the peptide chromophores ". ... 

Switching Preferential Screw Sense by Helix-Helix Transition... 

Preferential screw sense helix induced by chiral side groups and/or chiral chemical influence... 

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. 

Single crystal analysis of polysilanes is rare, since the structural variations in polymer samples usually preclude the formation of crystals of sufficient quality and size, although there are a few reports. The structure of l,6-bis[(i )-2-phenylpropyl]dodecapropylhexasilane, 5(R), with terminal chiral groups has been reported 15 the conformation is M-screw sense all-transoid with backbone dihedral angles in the range —172° to —177°, and was described as a 157 helix the structure is shown in Figure 21. 

A concomitant shift in the UV absorption wavelength to 310 nm characteristic of a random coil conformation is apparent.333 The report is not explicit concerning the screw sense of the PTrMA block, and thus the screw sense of the silicon helix is not certain. 

From the positive Cotton effect observed in the amylose case and the negative Cotton effect in the SPG case, it is clear that the oligosilane adopts opposite screw sense helical conformations in the two cases.337 It now appears possible to associate an d/-screw sense helix with a positive sign Cotton effect (and vice versa), although other experimental confirmations of this are desirable. 

Homopolymers such as poly[(V)-3,7-dimethyloctyl-2-methylpropylsilylene], 117, were initially studied, and the helix-helix transition was discussed in terms of an entropically driven phenomenon in which at temperatures below Tc the side chains of the helical polymer are in a very ordered state and enforce a particular screw sense, whereas above Tc, the side chains become disordered such that the main chain can relax into the opposite screw sense.314 This concept is expressed in Figure 47. 

The twist angles between the terminal rungs of 141, 142 and 143 are 44.0, 63.0, and 80.3°, respectively. The double helix structures crystallize with equal amounts of left- and right-handed screw sense forms, as expected in the absence of any chiral field. Some ladder polysilanes crystallize with both helical forms in the same centrosymmetric unit cell for others, the two forms can crystallize separately. It will be interesting to carry out the crystallization in a chiral field. 

Note 3 Some asymmetric chirogenic polymerizations give helical polymer molecules of only one screw sense that usually show optical activity due to the helicity (see Examples 2.5, 2.6 and 2.7). These polymerizations are termed asymmetric helix-chirogenic polymerizations . 

These achiral poly(A -propargylamides) form helices with an equivalent amount of right- and left-handed screw senses. Addition of chiral alcohols induces predominantly one-handed screw sense in polyl7a and polyl7d. NMR spectroscopic analysis has revealed that the amide side chains interact with optically active alcohols by hydrogen bonding. Terpenes also induce a one-handed helix. In this case, hydrophobic interaction plays an important role for helix induction. 

The helix-inducing effect of Aib has been compared to that of a number of other C -disubstituted glycines (Scheme 2) when incorporated in peptides. The helix-inducing ability of isovaline (Iva, 2) compares well to that of Aib and despite its chirality, it similarly exhibits a screw-sense indifference. 39,40 C -Methylvaline [(aMe)Val, 3], 40,41 C -methylleucine [(aMe)Leu], and C -methylphenylalanine [(aMe)Phe, 4]t40-42 aiso have helix-inducing abilities similar to that of Aib. Their helical propensities are higher than their unmethylated counterparts with L-(aMe)Val showing a clear-cut preference for the right-handed screw sense. 

Short peptides three to five residues in length that contain one or two APhe residues adopt a fJ-bend 45"48 and 310-helical folding. For example, crystal structure studies of a three-residue peptide containing two APhe residues, Ac-APhe-Ala-APhe-NHMe, 49 and a five-residue peptide with one APhe residue, Boc-Leu-Phe-Ala-APhe-Leu-OMe, 50 show the presence of two consecutive (3-bends, corresponding to an incipient 310-helix in both peptides. Such didehydropeptides exist in both screw senses and the spacing between APhe residues does not seem to affect the type of helix formed. 51,52 ... 

According to the above data and to preliminary determinations of the polymers optical activity in the solid state (112) it was suggested (105) that the principal chain of optically active poly-a-olefins, which in general posses a helix type conformation in the solid state, consists of sections having helix conformation also in the molten state or in dilute solution. Most sections of the principal chain are spiraled in one screw sense only, depending on the absolute configuration of the asymmetric carbon atom of the lateral chains. 

According to the above model, the left- and the right-handed helix sections in an ideal isotactic macromolecule are supposed to change continuously their length with time, the percent of spiraled principal chain in each screw sense being constant at a given temperature (116). 

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