Flapping equilibrium

A number of l,3-dioxan-2-ylium ions have been studied with respect to the preferred conformation. From a detailed H and C NMR study (790MR616) and the X-ray crystal structural analysis (94CJC2084), it was concluded that the 1,3-dioxan-2-ylium ions adopt the envelope conformation with the C —O —C —O—moiety in one plane (due to mesomerism) and C —C —forming the flap (Scheme 16). Ring interconversion 19a 19b and a 1 1 conformational equilibrium was assumed. 

Data on the apparent dipole moment and H NMR spectra of thianthrene-5-oxide 24 under different conditions of solvent and temperature support the rapid conformational equilibrium of 24, which flaps between two limit boat conformations with the sulfoxide group in a pseudoaxial or pseudoequatorial position, respectively. Variable-temperature... 

A widely accepted two-step mechanism of HIV-1 protease binding implies the creation of a loose complex with the open form of the enzyme, followed by the conformational change involving the closure of the flap region over the active site and formation of the final bound complex. Consequently, binding affinity differences between the HIV-1 protease and its mutants may also result from the changes in the internal equilibrium between the bound form of the protease with closed flaps conformation and the unbound open form... 

FIGURE 24. Transition state structure for rc-bond rotation and equilibrium triplet structure (in parentheses) of disilene at MCSCF/3-21G. Bond lengths are in A, bond angles in deg. (j> is the flap angle between the Si-Si bond and the SiH2 plane147. 

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