Helical conformation effects

Derivatized amylose is the basis for the Chiralpak AD CSP. This CSP has been utilized for the resolution of ibuprofen and flurbiprofen, as well as other members of the family of nonsteroidal inflammatory drugs (NSAIDs) [39, 61]. Ibuprofen was not resolved on the Chiralpak AD CSP in LC. Pressure-related effects on stereoselectivity were observed by Bargmann-Leyder et al. on a Chiralpak AD CSP [58]. No corresponding effect of pressure on selectivity was observed with a Chiralcel OD CSP. The authors speculated that the helical conformation of the amylose-based CSP is more flexible than that of the cellulose-based CSP. 

Fig. 12. Schematic representation of the a-helix-inducing effect of specific bound anions like CIO4 or SCN" in the case of N-co-trialkylaled BPAA the extended helical conformation contracts by the insertion of the anions. The water molecules of the hydration shells are omitted 1091...

Fig. 2.16 Effect of electrostatic interactions on 3i4-helix formation in an aqueous environment [1 75 a, 175 b, 176]. y -Peptides 86 and 87 adopt a stable helical conformation mediated by salt bridges near neutral pH. While the propensity of these peptides to adopt a helical conformation is strongly de-...

These y9 -peptides are not expected to adopt a 3i4-helical conformation in an aqueous environment because of the destabihzing effect of cationic charges. The circular dichroism spectrum of a non-labeled analog of 165 does not display the characteristic signature of the 3i4-helix in aqueous solution however it is highly hehcoidal in MeOH. 

Heparin has been reported to complex with a variety of basic species, including biogenic amines and drugs for reviews, see Refs. 10 and 391. For its possible relevance to the pharmacological properties of heparin and complexed species, mention is made here of complexes with histamine392,393 and anthracycline antibiotics.394 C.d. studies on the interaction of basic homopolypeptides with heparin and other glycosaminogly-cans have shown that heparin is able to induce an ordered, helical conformation in the polypeptide.395 397 Similar, and even more dramatic, effects were observed with mixed basic polypeptides, presumed to represent better models for the biologically relevant interactions with plasma proteins.368... 

In this part, two series of 44 copolymers with coiled main-chain structures and 45 copolymers with stiff main-chain conformations were described. It was concluded that both optically inactive 42 and 43 adopt helical conformations with an equal proportion of P and M screw senses by means of UV and CD spectra as well as molecular mechanics calculations. A marked positive cooperative induction effect of the preferential screw sense in 44 and 45 copolymers was found. However, there is a marked difference in the helical cooperativity between 44 and 45, probably because of the differences in their global and local conformations. This difference can be related to the persistence of the helical conformation against defects allowing change of... 

In contrast, the CD spectra of 50, 51, and 52 exhibit negative Cotton effects, as shown in Figure 4.36 for 50. This implies a helical conformation, but of the opposite preferential screw sense to 48 and 49, even though the enantiopure chiral moiety in all cases is the (5 )-2-methylbutyl group. Similarly, the Cotton effect is of greatest magnitude (negative) at low temperature and almost very much reduced at 80°C. 

It was found that these showed UV absorption maxima at the same wavelength (ca. 395 nm) as their non-chiral-substituted analog, poly(bis- -butylphenylsilylene), as shown in Figure 37, from which it was concluded that the dihedral angles of these polymers were the same.282 The CD spectra showed Cotton effects, thus indicating that the polymers adopted PSS helical conformations and that they do not take all-anti conformations. 

Recently, the first example of chiral solvation of a polysilane was demonstrated dissolution of the inherently optically inactive poly(methylphenylsilyene), PMPS, and poly(hexylmethylsilylene), PHMS, in the optically active solvents (V)-2-methyl-l-propoxybutane and (V)-(2-methylbutoxymethyl)benzene induced the polymer chains to adopt PSS helical conformations as evidenced by (positive-signed) Cotton effects almost coincident with the UV a-a transition at 340 and 305 nm, respectively.332... 

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. 

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