Asymmetric conformation

A typical example of steric control over spin delocalization has been described for the cation-radical of 3,4-bis(thioisopropyl)-2,5-dimethyl-l-phenylpyrrole (Domingo et al. 2001). Scheme 3.15 depicts this sitnation. In this cation-radical, one thioisopropyl group is almost coplanar with the pyrrole ring, whereas the other one occupies an orthogonal position. Accordingly, the ESR spectra established an eqnilibrinm between the symmetrical and asymmetrical conformations of the cation-radical. This equi-librinm is shifted toward the asymmetrical form at low temperatmes. The main feature of the equilibrium is the widening of spin delocalization, which includes not only the pyrrole ring but also one donor sulfur atom at the expense of the other sulfur atom. The steric control predetermines the discrimination of the other sulfur atom in the spin-delocalization process. 

For CH3SC50 four local conformational minima are obtained by molecular mechanics calculations using the MMX force field (Figure 6.4) [7]. Thereby, as shown in Figure 6.4, the asymmetric conformation for this radical is preferred. 

Electrochemical reduction of 2,8-dinitro- and 2-nitrophenoxatellurines in DMSO produces the corresponding radical anions [77JCS(P2)529]. In the former case, a presence of two interconverting asymmetric conformers in solution was detected by ESR. When 2-nitrophenoxatellurine was re-... 

Cydo- Sai-5ar-Gay)2 DMSO 1 C2-syininetric confonnation phis 4 asymmetric conformations or 3 C2- mmetric conformations plus 3 a mmetric conformatwns C2-symmetric conformation with two intra H-bond<41%... 

Cydo-(Pro-Sai>-Gly)2 CDQ3 CD3OD DMSO 1 C2-symmetric conformation two -tums with two Gly-NH intra H-bond all trans peptide bonds 92% in CDQs, 88% in CD3OD, 76% in DMSO 1 asymmetric conformation one CIS and one trots Pro-Sai peptide bonds one intra and one inter Qy-NH H-boid... 

The optical rotation of such imprinted polymers after template removal is clearly not caused by individual chiral centres, as is usually the case, but by the boundaries of the empty cavities as a whole. Their chiral construction is stabilised by means of the cross-linking of the polymer chains. This type of chirality can arise from the asymmetric configuration of the cross-linking points as well as from asymmetric conformations of the polymer chains which are stabilised by cross-linking. The extent to which these two factors contribute is not known. 

Three pairs of doublets are found for calix[6]arenes at low temperature, indicating a conformation with three different methylene bridges , while an asymmetric conformation with seven different methylene bridges is found for calix[7]arenes. For the larger calix[n]arenes the AG values determined from the coalescence temperature of the methylene proton signals show slight maxima for n = 12, 16 and. ... 

Studies of optical rotation at a single wavelength do not permit us to distinguish between effects due to asymmetric carbon atoms and effects due to helical or other asymmetric conformations. Such a distinction can be made by studying the dependence of optical rotation on the wavelength of the light employed. 

Cyclo-(Pro-Gly)3 has been demonstrated to take C3-symmetric conformation [S] in CH2CI2, CHCI3, and dioxane asymmetric conformation [A] in H2O, CH3OH, and DMSO mixed S] (40%) and [A] (W)%) conformation in CH3CN and mixed [S] (50%) and [A] (50%) conformation in CH3CN-CHQ3 (3 1 v/v) mixed solvent. These are summarized in Table 10. 

Pentikainen, U., Pentikainen, O.T., MulhoUand, A.J. Cooperative symmetric to asymmetric conformational transition of the apo-form of scavenger decapping enzyme revealed by simulations. Proteins 2008, 70(2), 498-508. 

Stmctural analyses (including X-ray, IR, and Raman spectra) of bis-arylazoles 203 related to the scintillant POPOP (l,4-bis-(5-phenyloxazol-2-yl)benzene 204) have been published (Fig. 2.22). No useful information on intermolecular hydrogen bonding in such compounds could be obtained from the vibrational spectra. The ring-stretch IR bands mentioned above confirmed that the nonplanar and asymmetric conformations seen in the X-ray crystal structures were present in solution as well. 

The constitution of TOT implies trigonal symmetry for the individual molecule. In all of its clathrates the three-propeller-shaped TOT molecule (Fig. 1) is constrained to adopt an asymmetrical conformation owing to the action of crystal... 

A different conformation is assumed by the complex between Mg and two molecules of cyclic(Gly-LPro-LPro-Gly-LPro-LPro), in which the two cyclic hexapeptide molecules have nearly identical, but asymmetric conformations. Each peptide has two cis Pro-Pro linkages and lacks any intra-ring hydrogen bonds. The Mg ion forms six ligands in a regular octahedral... 

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