M-Helicity

FIGURE 6.39 Relative frequencies of occurrence of amino acid residues in m-helices, /3-sheets, and /S-turns in proteins of known structure. (Adapted from Belt, J E., and Belt, E. T, 1988, Proteins and. Enzymes, Englewood Cliffs, NJ Prentice-Hall.)... 

Recently, a variety of natural peptides that form transmembrane channels have been identified and characterized. Melittin (Figure 10.35) is a bee venom toxin peptide of 26 residues. The cecropins are peptides induced in Hyalophora cecropia (Figure 10.36) and other related silkworms when challenged by bacterial infections. These peptides are thought to form m-helical aggregates in mem-... 

As shown in Scheme 2, positive angles define P-helicity while negative 6 values define M-helicity. 

To quantitatively characterize the PM populations using chiroptical characteristics, it may be useful to use the gabs values of 16 at each temperature with reference to the regression curve of these gabs values in 17, which is assumed to adopt a purely P 73-helical structure, instead of the Ae value. The analysis is based on the assumption that the weak temperature dependence of the ymax for 17 is due to a minute modification in the screw pitch of the P helix, rather than any formation of the M-helical motif. 

The idea that the most stable conformation of 42 and 43 may be helical is supported by a molecular mechanics calculation using Discover III with a PCFF force held (MSI, ver. 3.00). Figure 4.31a shows the total energy of a 42 oligomer with 21 Si repeat units as a function of the Si-Si-Si-Si dihedral angle. The respective P- and M-helical conformations of 42 near dihedral angles of 160° and 200° are more stable than a trans planar conformation of... 

Figure 5.27 Scanning electron micrograph of copper-coated DCg PC (38) tubules. Tubules are hollow with diameter of approximately 0.5 p.m (bar = 0.5 p,m). Helical wrappings evident in some tubules are all right handed. Reprinted with permission from Ref. 125. Copyright 1998 by the American Chemical Society. 

The main factor in determining the handedness of the cholesterics induced by bridged 1,1 -binaphtliyls is the helicity (P or M) of the solute, and this observation is the basis of many configurational studies of chiral binaphthyls. All the homochiral (aP)-binaphthyls 15-19 have an M helicity of the core, and all induce, in biphenyl nematics, M cholesterics.65,75 By systematic structural variations of the covalent bridge, it is possible to obtain I J -binaphthalenes with dihedral angles ranging from 60° to 96° (see series 20-24) the handedness of the cholesteric phase always matches the helicity... 

Figure 47 Structural models of P and M helical 117 above and below 7C.314 Reprinted with permission from Fujiki, M. J. Am. Chem. Soc. 2000, 122, 3336-3343, 2000 American Chemical Society.

Koe, J. R. Fujiki, M. Nakashima, H. Motonaga, M. Helical Poly(diarylsilylene)s Effects of Higher Order Structure on Optical Activity. In Synthetic Macromolecules with Higher Order Structure, Khan, I. M., Ed. ACS Symposium Series 812 Washington DC, 2002 pp 67-86. 

Unlike CO, it is possible to polymerize isocyanides (R—N=C), isoelectronic analogs to CO. When R is a bulky group, such as tert-Bu, the polymer forms a stable helical structure. Asymmetric catalytic polymerization has been reported for t-Bu-NC using [Ni(T 3-allyl)(iV-trifluoroacetyl-proline)]2 providing (M)-helical polymer with 69% ee. The more stable helical polymer was prepared from 1,2-diisocya-nobenzene derivative initiated by a chiral Pd complex. (See Scheme 4.19.)... 

Isocyanide Polymers Bulky isocyanides give polymers having a 4 1 helical conformation (115) [154]. An optically active polyisocyanide was first obtained by chromatographic resolution of poly(f-butyl isocyanide) (poly-116) using optically active poly((S)-sec-butyl isocyanide) as a stationary phase and the polymer showing positive rotation was found to possess an M-helical conformation on the basis of CD spectral analysis [155,156]. Polymerization of bulky isocyanides with chiral catalysts also leads to optically active polymers. 

Isocyanide Polymers Optically active polyisocyanides with excess helix sense are obtained from optically active monomers by polymerization with NiCI-,. The polymers obtained from (R)-(CH3)2CHCH(CH,)NC, (fl)-(CH3)2CHCH2CH(CH3)NC, and (R)-n-C6H13CH(CH3)NC have M helical sense with screw-sense excesses of 62%. 56%, and 20%, respectively [189]. The copolymerization of achiral phenyl isocyanide with optically active... 

Diarylethene switches are present in two, rapidly exchanging conformations (P and M helicity), which upon photochemical ring-closure lead to the RR or SS enantiomers in equal amounts. The authors showed that aggregation of a chiral diarylethene switch in the open form 283 leads to selection of only one of the helical forms of 283 in the gel state. Subsequent photochemical ring-closure of 283 to 284 in the gel state proceeds with 96% diastereoisomeric excess and a photosta-tionary state (PSS) of 40%, whereas in solution, no stereoselection is observed. 

All the 311 molecules adopt a P-helical conformation in the central hexatriene part (this crystal is named (P)-311 NpF). Single crystals with the opposite chirality, (M)-311 NpF, in which 311 is in an M-helical conformation, were also obtained from the same sample batch. The chiral cocrystals underwent photochromism. Upon irradiation with 365 nm light, the colorless crystal turned blue. 

Figure 10.72 (a) Helical screw about a defined axis, (b) M helicity and (c) P hel icily, courtesy of the... 

Schmitt, J. L., Stadler, A. M., Kyritsakas, N., Lehn, J. M., Helicity-encoded molecular strands Efficient access by the hydra-zone route and structural features. Helv. Chim. Acta 2003, 86, 1598-1624. 

FIGURE 7.19. (P)-helices (D-Glu-8, 28a) and (M)-helices (L-Glu-12, L-28d) first separate (a bar, 100 nm) and then (b bar 300 nm) unite to form elongated racemic platelets. Reproduced with permission of the American Chemical Society. 

Scheme 24 Chiral photoswitchable polyisocyanates A) schematic representation of the shift in equilibrium between P and M helices upon irradiation. B) illustration ofPto M helix transition in polyisocyanates upon photoisomerization ofthe azobenzene unit (adapted from references 75-78).

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