Molecular helicity

Available experimental data for translational friction coefficients of polypeptides are still meager, but those obtained so far in helicogenic solvents display one feature in common plots of s0 or D0M against InM are virtually linear over a wide range of M, as illustrated in Fig. 25 (80, 81). Thus, from such data alone we may conclude that the molecular helices are essentially rigid up... 

The molecular helices and propellers discussed above contain no center of chirality, and the P and M nomenclature is thus the only way of describing their absolute configuration. This nomenclature, however, is also applicable to some series of chiral compounds which display several centers of chirality. As will be discussed in Section 6, the presence in a molecule of two or more centers of chirality usually implies the existence of several stereoisomers, but steric reasons may reduce down to two the possible number of stereoisomeric forms. Thus, 2,3-epoxycyclohexanone contains two asymmetric carbon atoms, but for steric reasons only two stereoisomers, namely the (2S 3S)-(—)- and the (2/ 3/J)-( + )-enantiomer, exist the former is depicted in diagram XL [49]. 

Figure 2.39 Fragment of the polymeric chain in [Tb2(L )3(dppe02)]n. [53b]. (Reproduced from S.N. Semenov, A.Y. Rogachev, S.V. Eliseeva, C. Pettinari, F. Marchetti, A.A. Drozdov and S.I. Troyanov, First direct assembly of molecular helical complexes into a coordination polymer, Chemical Communications, 17, 1992-1994, 2008, by permission of the Royal Society of Chemistry.)...

The axial chirality of the atropisomeric oxalic acid derivatives 57a, Z) according to Brewster also can be classified as helicity. (/ )-()-N,N,N, N -Tetramethyl-dithiooxamide 31a then constitutes one of the smallest molecular helices, the enantiomers of which have been separated semipreparatively using liquid chromatography... 

Figure 10.72 (a) Helical screw abont a defined axis, (b) M-helicity and (c) F-helicity, conrtesy of the coils of the Mexican golden tree snake and North American glass snake, respectively. (Both are entirely harmless ) (Serpents from Johnson s Natnral History, illustrated by S. G. Goodrich, Glidersleeve Press NewYork, 1888.) (d) More conventional molecular helices. 

Describing the Structures Molecular Helices and Pleated Sheets (Figure 6.5, Figure... 

As early as in 1993, Ohkubo and co-workers reported a photocatalytic system consisting of a chiral Ru complex A-[Ru(menbpy)3] [menbpy = 4,4 -di-(lR,2S,5R)-(-)-memthoxycarbonyl-2,2 -bipyridine] possessing a C3 molecular helicity as the photosensitizer and Co(acac)3 (acac = acetylacetonato) as the oxidant which could catalyze the oxidative homo-coupling of 2-naphthols (Scheme 3.13). Under the radiation of visible light, A-[Ru(menbpy)3] " can... 

Overcrowded alkenes are a special class of olefins with an inherently helical shape, resulting from extensive steric crowding around the central carbon-carbon double bond [20]. These types of molecules were first reported by Feringa and coworkers who continue to champion these materials for applications such as molecular switches, molecular motors, and enablers to photogenerate dynamic optical effects in CLCs [21]. The merit of these molecules as chiral dopants is that photoisomerization often leads to an inversion of molecular helicity, which typically results in the handedness inversion of the CLCs (Fig. 5.3b). 

Polypeptides that are characterized by helical structures are, at the molecular level, noncentrosymmetric. However, in randomly oriented polypeptide films such as might be obtained by solvent evaporation, noncentrosymmetry is lost and the sample does not exhibit nonlinear optical (NLO) properties such second harmonic generation (SHG). Alignment of the molecular helices by application of an electric field across the solution with simultaneous evaporation of the solvent yields SHG-active films with relaxation times reported to be at least 6 months [1]. 

Figure 6 Different helical conformations adopted by macromolecules in the crystalline state. The molecular helices are shown for PE, isotactic polypropylene, syndiotactic polypropylene, and polyethylene oxide from left to right, respectively. The type of the helix is denoted by the number of monomers per integer number of turns shown in subscript.

Molecular chirality has profoimd effect when it is introduced into LC polymers. The chiral LC phases having helical morphology are finding potential applications in optics and electrooptics [155]. Polymers having molecular asymmetry in the backbones are commonly seen in natural life forms. The DNA, collagen, and myosin, the well known biopolymers, display minute control over their molecular helicity. The configuration of the chiral center in the nucleotide or peptide backbone decides the twist senses of the chiral structures [156]. 

The contour flexibility of molecular helices, such as in the case of PBLG, is best described as persistent flexibility or homogeneous bend-elastidty. The measure of this type of flexibility is the persistence length, P, defined via... 

Fig. 5 Evolving self-assembly of compound 3 (10 M, wateriTHF = 4 1). Cryo-TEM images of (a) short curved fibers (b) longer curved fibers and (c) long thicker tube-like fibers, where the molecular helical 3-nm-thick fibers (a, b) are converted into straight tubular4.5-nm fibers. In good agreement with the molecular models (3.2 and 4.7 nm fiber widths). The assembly development can be stopped by the addition of water ( wateriTHF = 95 5, v/v) [38]...

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