Endocyclic torsion angles

Another experimental verification of theoretical data may be provided by the analysis of a range of variation of endocyclic torsion angles in crystal structures containing uracil and cytosine fragments. It was demonstrated [32] that range of variation of the N1-C2-N3-C4 torsion angle in uracil and the C6-N1-C2-N3 torsion angle in cytosine amounts to 12-13°. For example, it was found that the value of the C6-N1-C2-N3 in the crystal of 5-bromo-2 -deoxycytidine is —7.1°, —12.4°, and — 13.9°, respectively, for three molecules in asymmetrical part of unit cell [39],... 

Table 21.4 Population of planar conformation of rings (with endocyclic torsion angles less than 10° ) in nucleic acid bases according to Carr-Parinello molecular dynamics simulations [45]...

Fig. 15. a) Stereoview, along the c axis, of the encaged conformations of chlorocyclohexane. Dotted line crystallographic twofold axis. For clarity the guest molecules are shown single-positioned. b) Endocyclic torsion angles of chlorocyclohexane in various conformations (a) major observed (b) major calculated, symmetry (c) minor observed (d) minor calculated. The axial Cl atom. is linked to atom C(l). Reproduced with permission from J. Inch Phenom. 3, 335 (1985)... 

Bond lengths in A, angles in degrees. The T are endocyclic torsional angles, e.g. ... 

Bond lengths in A, angles in degrees. In the l,3-i omer, t is the endocyclic torsional angle around the =C-C= bond in the 1,4-isomer a is the dihedral angle between the planes of the moieties. 

Table 2 Endocyclic torsion angles for piperidine derivatives with their conformer description...

In addition, the program incorporates the concept of pseudorotation developed by Altona and Sundaralingam3i in which the five torsion angles of the ribose ring (0o-04, Figure 1) are mathematically related to the phase angle of pseudorotation P. In the most elementary form, the relationship between the five endocyclic torsion angles (0i, i = 0 -4) can be expressed as a simple cosine function. 

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