Conformational states in solution

The fragments of macromolecules with ordered cholesterol group sequences, that are formed in bad solvents, may serve as nuclei of supermolecular order in films, obtained from these solvents. Structural and optical studies have shown that PChMA-11 films produced by solvent evaporation display different properties those obtained from chloroform and toluene solutions (small relaxation times, see Table 17) are optically isotropic, and those obtained from heptane solutions (large relaxation times, see Table 17) are optically anisotropic, what reflects the differences in conformational state of polymeric chains in these films. Contrary to the optically isotropic films, a high degree of side branch ordering characterizes optically anisotropic films, which is confirmed by X-ray studies. The observed difference of LC polymer structure in the bulk is thus the consequence of their different conformational state in solution this reveals some possibilities for the control of LC polymer structure at the initial steps of mesophase nucleation in solutions. 

As mentioned above, peptide 153 exists in two or three stable conformational states in solution. However, the proline analog 224 showed only one conformation in various solvents, and extensive NMR experiments revealed that this conformation is very similar to the major conformer of peptide 153. Since analog 224 showed potent cytotoxicity, it is concluded that the major conformation of peptide 153 is at least in part responsible for the activity. The pipecolic acid analog 225 showed similar conformational and biological tendencies. 

As mentioned in the previous section, peptide 166 exists in two or three stable conformational states in solution. However, the proline analog 238... 

Experimental Jc,h values for conformationally rigid carbohydrate derivatives allow to calculate the constant values of Aj and Bj in equations [12.2.1], The major practical use of these equations is their ability to estimate the glycosidic dihedral angles from experimental Jqh in combination with other complementary results, for example, from NOESY, X-ray and chiro-optical experiments. Since the experimental values are averaged over all the accessible conformational states in solution, they do not necessary reflect the property of only the most probable conformer but they nevertheless include contributions of all the conformers, each one taken with its proper statistical weight. Thus, the quantitative interpretation of experimental data in terms of accessible conformational states of flexible molecules requires the additional theoretical evaluation of the energy of the molecule as a function of internal coordinates. Since the dependence of the observed coupling constants on the conformation is non-linear, it derives that ... 

Conformational considerations in peptide synthesis remain one of the greatest challenges in the design of biologically active peptides. Peptides will usually be an ensemble of conformation states in solution. If biological activity involves only one discrete conformer, this conformational ensemble represents a dilution of the biologically active sp>ecies. 

From the data in Table VI and Figure 7, it is clear that the dominant conformer from the lowest energy two-state structural solution is quite similar to the lowest energy one-state structural solution, the chief difference being in the orientation of the terminal /3-D-GalNAc residue. We conclude, therefore, that structures which are similar to the lowest energy one-state structural solution A", represent the predominant conformer present in solution. The one-state procedure thus provides a reasonable and time efficient approach to structural analysis for this molecule. 

On the one hand, x-ray crystallography data could support this idea because the heterocycles are coplanar relative to the B-H bonds. Moreover, in the case of the compound in Strncture 5.5, this conformation remains in solution, according to NOE NMR measnrements. On the other hand, the solid-state CH- -HB distances are too long (< 2.65 A), and the HF/6-31G calculations carried ont for the componnd have shown, that where the heterocyclic rings are oriented orthogonally, the conformation is more stable. Thns, this question is still open. It is quite probable that the interaction between protonic hydrogens on the a-carbons and hydridic BH hydrogens is electrostatically attractive. 

Diphenyl-1,3-butadiene. The excited-state behavior of this diene differs significantly from stilbene and is the subject of a review. Unlike tS in which the lowest vertical excited singlet state is the 1 B state and S2 is the 2 Ag state in solution, these two excited states lie very close to each other in all-trans-1,4-diphenyl-1,3-butadiene (DPB). The additional carbon-carbon double bond introduces a new conformational equilibrium involving the s-trans and s-cis rota-mers. Most spectroscopic studies in solution have concluded that the l B state is S. The DPB compound has a low quantum yield for photoisomerization, so the use of DPB in time-resolved spectroscopic studies on photoisomerization, especially those that monitor only fluorescence decay, needs to be considered cautiously and critically. 

Simulation of spectra for (R)-acacpnH2 and its trifluoro derivative indicates that the syn conformation, predominant in solution, is that having the pn methyl group anti to the N atom, as found in the solid state.101 A similar conformer distribution is expected also for acacenH2 in view of the similarity of absorption spectra. In conclusion, it appears that these compounds exist in solution essentially in a unique conformation, maintained by strong intramolecular hydrogen bonds. 

X-ray crystallographic analysis revealed that methyl cA-tetrahydro-a- (36) and -ft-santoninate (37) have a nonsteroid decalone conformation in the solid state. Furthermore, positive Cotton effects shown by both cis -fused decalone analogues indicated the presence of such nonsteroid conformation (38) in solution, in accord with the octant rule121. 

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