Conformational frustration

An instructive case of conformational frustration of a carbonyl-forming reaction has been noted in the 5, 6 -epoxy-steroid series. The normal mode of reaction, formation of a 5a-6 ketone [ >21], is prevented in 5,6 S-epoxy-4,4 dimethyl-5)9-cholestan"3-one 9), although the products (ii) and (12) [iih,c,d] arise via a C(S) carbonium ion (10), despite the polar... 

There is an optimum level of crosslinking with typical values to be around 70-95 mol %. Conformational flexibiUty of the crossUnker has to be balanced with the overall level of crosslinks. The chemical nature of the crosslinker determines the useful solvent range for a MIP. The size of the crosslinker ideally matches the dimensions of the template, to avoid conformational frustrations within the polymer matrix as a result of a structural misfit, which can lead to a loss in capacity and/or selectivity. 

Keywords Smectic liquid crystals, Molecular conformations, polar and steric frustrations, Polyphilic and perfluorinated mesogens. X-ray diffraction... 

Unlike the bulk morphology, block copolymer thin films are often characterized by thickness-dependent highly oriented domains, as a result of surface and interfacial energy minimization [115,116]. For example, in the simplest composition-symmetric (ID lamellae) coil-coil thin films, the overall trend when t>Lo is for the lamellae to be oriented parallel to the plane of the film [115]. Under symmetric boundary conditions, frustration cannot be avoided if t is not commensurate with L0 in a confined film and the lamellar period deviates from the bulk value by compressing the chain conformation [117]. Under asymmetric boundary conditions, an incomplete top layer composed of islands and holes of height Lo forms as in the incommensurate case [118]. However, it has also been observed that microdomains can reorient such that they are perpendicular to the surface [ 119], or they can take mixed orientations to relieve the constraint [66]. 

Dong H, Paramonov SE, Aulisa L, Bakota EL, Hartgerink JD. Self-assembly of multi-domain peptides balancing molecular frustration controls conformation and nanostructure. J Am Chem Soc 2007 129 12468-12472. 

Figure 7 shows a plot of x as a function of cp. A rough correlation can be seen if cp is positive then x is positive and if cp is negative then x is negative. The crystal structure of XAGWUX contains two independent molecules with opposite conformations. One of the molecules is syn-syn the other is anti-anti. This occurrence frustrates any attempts to correlate the conformation of the cation with the acidic strength of the anion or other counter ion effects. 

One of the present authors has extensively used the dipole-moment method to calculate conformational equilibria of saturated heterocycles. In hindsight this has been a frustrating experience not so much because of the assumptions and approximations that must be made, but because the results in some cases are in good agreement with those derived from other methods, whereas for other groups of compounds the dipole-moment conclusions are clearly incorrect. In this discussion we first discuss the method, using piperidines as an example, and then attempt to assess its areas of applicability and causes of failure. 

For polycatenar hydrogen bonded complexes with fluorinated chains at both ends (e.g., 138,139, see Fig. 36) formation of columnar phases was observed [246]. However, compound 137, having a branched Rp-chain at one end and three RH-chains at the other has a sequence of three distinct phases in the unusual sequence Cub-Col-SmA-Iso. For the SmA phase of compound 137 a structure with intercalated aromatic cores and RF-chains and separated layers of the hydrocarbon chains was proposed. At lower temperature, when incompatibility rises and the aromatics and Rp-chains disintegrate, all three components form their own layers. However, this produces interface curvature and a columnar phase with square lattice is formed. On further cooling a transition to a cubic phase with Im3m lattice takes place which is most likely of the bicontinuous type [262]. This leads to the unusual phase sequence Cubv-Col-SmA where the positions of the Cubv and Col phases are exchanged with respect to the usually observed phase sequences. The Col-Cub transition at lower temperature could be the result of the decreased conformational disorder of the terminal chains which reduces the steric frustration and hence reduces the interface curvature. 

The crystal polymorphism of the chiral but racemic P5MH1 is, to some extent, very reminiscent of that of isotactic polypropylene. It exists in two crystal modifications. One crystal modification is stable at high temperature, and was observed early on by Corradini et al [39]. Its structure has been redefined as a chiral, frustrated one based on a trigonal cell with three threefold helices per cell. We have also discovered a second crystal modification produced from solution. It has an orthorhombic unit cell that contains four chains in - again - three-fold helical conformation, for which one must assume coexistence of two right- and two left-handed helices. Contrary to the a and ft phases of iPP, the frustrated structure of poly( 5-methyl-hexene-1) is the more stable one [40]. 

Polymers grafted at the surface at a density below the brush regime (see Figure 1) do not frustrate subsequent particle deposition. Still, the surface will dynamically respond to the indwelling particles. For instance, the conformation and orientation and, hence, the biological activity of adsorbed protein molecules may be manipulated. Two cases are discussed below. 

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