Conformation complexation

In order to explore systematically the main principles that guide peptide self-assembly, a useful approach is to study model self-assembling peptides. In this way, the namral chemical and conformational complexity of biological peptides... 

The next homolog, 1,5-hexadiene (1,5-HD), is of special chemical interest because the molecule is capable of undergoing the so-called Cope rearrangement. A GED study of 1,5-HD was also recently reported6. Because of the increased conformational complexity of this molecule compared to that of 1,4-PD, the structural details of the various con-formers could not be resolved and only averaged structure parameters were determined from the gas phase. Molecules in the solid state are frozen, mostly in only one conformation, which may but must not represent the conformational ground state. Therefore, conformational isomerization is usually not discussed with X-ray structures presented in the literature. 

Because of their structural and conformational complexity, polypeptides, proteins, and their feedstock contaminants thus represent an especially challenging case for the development of reliable adsorption models. Iterative simulation approaches, involving the application of several different isothermal representations8,367 369 enable an efficient strategy to be developed in terms of computational time and cost. Utilizing these iterative strategies, more reliable values of the relevant adsorption parameters, such as q, Kd, or the mass transfer coefficients (the latter often lumped into an apparent axial dispersion coefficient), can be derived, enabling the model simulations to more closely approximate the physical reality of the actual adsorption process. 

Considerable conformational complexity is found in disubstituted cycloalkanes,... 

HCl forms a hydrogen bond with Br atom of 2.605 A. Figures 11.6c and 11.6d show the pre- and post-reactive complexes predicted by the IRC calculation of TSlb. Both complexes describe the reaction being bromopropane in the gauche conformation, complexing Cl at the Br-o H-jSH side of bromopropane in the pre-complex... 

The energetically most stable structure is illustrated in Fig. 10(a). The NH2-group position of the N-alkyl chain is important since it is the most obvious structural difference between the first two conformations. Complex 6(a) possesses two additional hydrogen bonds as compared to conformation (b), thus stabilizing (a). In contrast to (a) and (b), structure (c) shows two trans-cis alterations and an imidazole ring flip. 

Even before any crystal structure was determined, examination of electrophoretic mobility had already shown that sugars which can present the aea sequence in one of their conformations complex well. This is the most favourable arrangement the 1,3,5-triaxial arrangement on a cyclohexane is even better but not possible with a pyranose. For a furanose, this aea sequence corresponds to the cis arrangement of three hydroxyl groups, as in the )3-D-mannofuranose discussed above. The flexible furanose can thus adopt a conformation which places the hydroxyl groups in a situation very closely related to the aea arrangement. 

In a later study, Grenier-Loustalot et al. [40] examined the reaction of monoacids and monoamines as models for the condensation reaction. Peaks due the acid and amide were resolved in the solid-state NMR spectra, however, the linewidths in the spectra of these model compounds were appreciably smaller than those expected for the conformationally-complex poly(amic acid). In this paper, the authors used HPLC and FTIR to determine the effect of size and basicity of the amine, as well as the removal of water on the kinetics of reaction. Full imidization was achieved only on the addition of catalysts to the reaction mixture. 

Scheme 5 illustrates the conformational complexity which is built into symmetrical IV compounds that have unsymmetrical M groups attached to symmetrical bridges by single bonds. Rotation about these bonds interconverts syn and anti conformations of the M groups. For these compounds, double nitrogen inversion also interconverts syn and anti ferf-butyl group conformations, so there are four diastereomeric conformations of 18. All are present in equal amounts for neutral 18,... 

Bis(oxazoline)-magnesium complex 20 (10mol%) catalyzes the indicated cycloaddition (Scheme 22) to give 19 (2R) in 82% yield and 91% ee endolexo= 97 3) [74]. The absolute stereochemistry of the product is consistent with biden-tate activation of the substrate through a tetrahedral metal geometry with reaction out of the s-cis conformer. Complex 21, derived from the opposite enantio-... 

Another factor that affects stereoselectivity of carbonyl addition reactions is chelation If an a or p substiment can form a chelate with a metal ion involving the carbonyl oxygen, the stereoselectivity is usually governed by the chelated conformation. Complexation between a donor substituent, the carbonyl oxygen, and the Lewis acid can establish a preferred conformation for the reactant, which then controls reduction. Usually hydride is delivered from the less sterically hindered face of the chelate. 

Alchemical approaches, however, generally allow for the larger range of conformational complexity typical of biochemical systems. 

Key words. Calixarenes, calixarene oxyanions, conformation, complex formation, catalysis. 

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