Electrostatics, distortion mechanism

The available data support a mechanism involving catalysis by distortion in which the enzyme binds and stabilizes a transition state that is characterized by partial rotation about the C-N amide bond. The energy that is required to distort this bond out of planarity with the C=0 bond, thereby destroying the resonance stabilization of the amide linkage, is supplied by favorable transition state binding interactions between enzyme and substrate. As Lumry states (1986), mechanical distortion as a source of small-molecule reactivity is attractive as a basis for enzymatic catalysis. It is quite realistic to assume that a distorted substrate will have enhanced reactivity, either because its ground state or the activated complex for its chemical reaction or both are altered by strain and stress in the protein conformation. However, as mentioned previously, this distortion need not be the result of mechanical deformation but could also be the result of desolvation or electrostatic destabilization. In fact, the current data support contributions from all three mechanisms for distortion. 

More recently there have been some modifications in the model, and the mathematical treatments have been considerably improved. Kirkwood (1111) used the Bernal-Fowler model and devised a statistical method to Eillow for near neighbors. Pople (1659) allowed for bending H bonds of the type shown in Fig. 8-1. Harris and Alder applied to Pople s model a statistical mechanical treatment which attempts a precise evaluation of distortion polarization (877). The calculated results are shown in Table 2-III, and in Fig. 2-1 they are compared with experimental values. These changes correct for failures of the simplest dipole picture, and as more of them are added the principal appeal of the electrostatic model—simplicity—tends to be lost. 

The exposure of an entire wafer necessarily involves the movement of the beam over the entire area. The deflection of the beam by the electromagnetic or electrostatic scan coils is not without errors because of the inherent limits to how far these coils can accurately and precisely deflect the beam. Good pattern quality requires the edge gradient of the electron-beam profile, the distortion of the exposed pattern, and the positional stability of the beam to be held below a small fraction of the minimum feature size. These considerations mandate that the size of the scan field be limited to a few millimeters at most and necessitate mechanical exposure stages to move the substrate through the deflection field of the electron-beam column. ... 

In the cyclopentyl series, 5y/i-elimination occurs from a planar transition state, but a/in -elimination is slightly distorted from a dihedral angle of 180°. Consequently, the preference for n/m -elimination is less marked in the five-than in the six-membered ring systems (Table 12). With the small neutral base trimethylamine, electrostatic forces of attraction between the sulphonate ester and the partially neutralised base balance the normal preference for n/iti-elimination and the reaction is almost non-stereospecific. None of the reactions follows the carbanion mechanism, as general base catalysis is observed for the 2-p-tolylsulphonylcyclopentyl tosylate elimination and the studies on 2-phenylcyclopentyl tosylates revealed large isotope effects (A h/A d) and p values smaller than in the 2-phenylethyl series (see Table 9, p. 209) " . 

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