Propeller twist angle

The occurrence of structural/dynamic deviations from the ideal base pair in real base pairs of DNA is not random. Certain sequences of the DNA bases and base pairs are more predisposed than others to adopt certain propeller twist angles, are more fluctional in base-pair opening, and so forth. This is the fundamental basis for the sequence-directed structure and dynamics that are the subject of this chapter. 

The twist angles of the aryl groups from coplanarity in the propeller conformation of a,a-diarylethyl cation [31C ] are comparable with those of [29C (X,Y)]. The dependence of the selectivity parameters upon conformation therefore should be as significant as in the above [29C (X,Y)] system. The substituent effects on the protonation equilibrium of diarylethylenes [31(X,Y)] can be interpreted by assigning a preferred conformation to the cations. It also seems evident in this system that the p value for the P-conformers does not vary so significantly with a series of Y substituents. Any processes involving a.a-diarylethyl cation [31C ] as an intermediate should reflect the dependence of selectivity upon conformations. Small p values for the solvolyses of 1,1-diphenylethyl-OPNB [13(X,Y)] cannot be interpreted by the tool of increasing electron demand . 

Propeller twist (DNA) The dihedral angle between the planes of the pyrimidine and the purine in a base pair of a nucleic acid,... 

Propeller twist refers to the angle between the planes of two bps. A bp is rarely a perfect flat plane with each aromatic base in the same plane. Rather, each base has a slightly different roll angle with respect to the other base. 

Each Li atom is co-ordinated to two N atoms of the bidentate chelate and to one ir-carbanion. Each Li-N distance is 2.10 A. The Li atom is not located directly over any one carbon atom but has four close contacts to the carbanion 2.49 and 2.51 A to two C atoms on one phenyl group, one close contact of 2.54 A to one C atom of a second phenyl group, and one 2.23 A distance to the central carbon atom. The twist angles of the phenyl rings depend on their interaction with the Li atom, giving an overall propeller geometry to the triphenylmethyl carbanion. It is not possible to explain the stereochemistry by two-centre a or ionic interactions alone, and a delocalized bonding mechanism which involves all the 2s and 2p orbitals of the Li... 

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