Dihedral angle variations

Figure 37. Dihedral-angle variations for hinge mode (a) versus residue number (b) versus residue number (c) sidechain angles x1 versus residue number. Each bar corresponds to the change in the dihedral angle in going from the 1- A rms displaced closed structure to the 1-A rms displaced open structure.

As an example of analysis of side-chain dihedral angles, the Bayesian analysis of methionine side-chain dihedrals is given in Table 3 for the ri = rotamers. In cases where there are a large number of data—for example, the (3, 3, 3) rotamer—the data and posterior distributions are essentially identical. These are normal distributions with the averages and standard variations given in the table. But in cases where there are few data. 

In other cases, polymorphic forms are characterized by slightly different conformations. In other words, while the chain conformations packed in the different polymorphs are different, they correspond, however, to small variations in the sequences of the dihedral angles along the main chain. 

The main factor in determining the handedness of the cholesterics induced by bridged 1,1 -binaphtliyls is the helicity (P or M) of the solute, and this observation is the basis of many configurational studies of chiral binaphthyls. All the homochiral (aP)-binaphthyls 15-19 have an M helicity of the core, and all induce, in biphenyl nematics, M cholesterics.65,75 By systematic structural variations of the covalent bridge, it is possible to obtain I J -binaphthalenes with dihedral angles ranging from 60° to 96° (see series 20-24) the handedness of the cholesteric phase always matches the helicity... 

Table VI lists a number of dipeptide ester complexes prepared via aminolysis in Me2SO and isolated using ion-exchange chromatography many others have been obtained from similar syntheses. Crystal structures are available for A-[Co(en)2((S)-Ala-CR)-Phe)]Br3 H20 (26), obtained from reaction of A-[Co(en)2((S)-AlaOMe]3+ with (i )-PheOMe and acid hydrolysis (Fig. 1), and for A-[Co(en)2((S)-Leu-(S)-Leu OMe)]Cl3 4H20 (24), A-[Co(en)2((S,fl)-Ala-(S)-ValOMe)](C104)3 (24), and /3-[Co(trien)(Gly-GlyOEt)](C104)3 H20 (10). These show considerable variation in chelate 0-Ci-C2-N dihedral angles (0-35°) (10) and it remains to be seen whether this property is important to epimerization (at C2) in these species.

Single crystal analysis of polysilanes is rare, since the structural variations in polymer samples usually preclude the formation of crystals of sufficient quality and size, although there are a few reports. The structure of l,6-bis[(i )-2-phenylpropyl]dodecapropylhexasilane, 5(R), with terminal chiral groups has been reported 15 the conformation is M-screw sense all-transoid with backbone dihedral angles in the range —172° to —177°, and was described as a 157 helix the structure is shown in Figure 21. 

The corresponding Cu(I) complex, [Cu(dpa)2] +, has been isolated and its structure solved. It maintains a tetrahedral geometry, though with a few significant variations in the structural parameters (Cu-N = 2.02 A dihedral angle between the two CuN2 planes = 73.3°).176... 

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