Dihedral angles deviations from

For acyclic allylic alcohols, very little a,p-unsaturated enone formation was observed besides epoxidation. Chemoselectivity was much less for cyclic allylic alcohols, for which oxidation of fhe allylic alcohol group competed significantly with epoxidation. In the case of 2-cyclohexenol as the substrate, the enone was even found to be the main product. A comparative sandwich POM-catalyzed epoxidation study of various (subsfifufed) cycloalkenols revealed that the enone versus epoxide chemoselectivity is controlled by the C=C-C-OH dihedral angle Ma in the allylic alcohol substrate. The more this dihedral angle deviates from fhe optimum C=C-C-OW dihedral angle otw for allylic acohol epoxidation, the more enone is formed (Fig. 16.5). 

ERRAT contains a database of acceptable nonbonded atom-atom distances. It classifies the atom-atom distances in a proposed structure and then does a statistical evaluation of those atom-atom distance interactions. The parameters used to construct the database were derived from known protein structures of varying fold classifications. The known structures used for the database were required to have a maximum resolution of 2.5 A, an R-factor less than 25%, be monomeric or homo-oligomeric, be native structures, and contain peptide bond dihedral angles 15° from ideal values based on the secondary structure. A total of 96 solved X-ray protein structures from the PDB were used as the dataset of correct protein structures. The six types of atom-atom distances were restricted to be atoms from different amino acid residues or to be atoms that interact with each other with a through-space distance no greater than 3.5 The average and standard deviation of each atom-atom distance interaction fraction (the fraction of specific atom-atom pairwise distances in a protein structure), as illustrated in Eq. [4] for CC distances... 

Figure 15.6 Various representations of the molecular structure of ryclc-Si2 showing S atoms in three parallel planes. I he idealized point group symmetry is and the mean dihedral angle is 86.1 5.5 . In the crystal the symmetry is slightly distorted to C21, and the central group of 6 S atoms deviate from eoplanarily by 14pm.

As a third shape descriptor of 1, the dihedral angle between the two —COOH groups is considered. The mean value [83.9(7.7)°] (Table 20) reflects moderate deviation of these moieties from a roughly perpendicular steric positioning with respect to each other. 

The conformation A is a half-chair, in which all atoms, except for the C-6 atom, are virtually in a single plane, from which the C-6 atom deviates due to rotation about the N-O(l) bond. The C(3)-N(2)-0(l)-C(6) dihedral angle 01 characterizes the degree of rotation. The N(2)-C(3)-C(4)-C(5) dihedral angle 02 corresponds to rotation about the C(3)-C(4) bond, which can occur independently of the first process and leads to a deviation of the C-5 atom from the plane of the nitronate fragment in the direction opposite to the C-6 atom. For the conformation A, 01=24.3° and 02 = 6.5°,that is, A = /01 / - /02/, which characterizes the difference in the degree of rotation about two bonds, is 17.8° (for cyclohexene, A=0). It should be noted that the results of these calculations are... 

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