Host conformation

Host conformational rearrange mem. Enlhaipically and cnlradically unlavuur>tb c Unnecessary For reorganised hosts. 

The effects of host conformation cannot be ignored and these are particularly evident with the cleft-binding pyridine-spaced host R3P. In consequence of relative access to the appropriate number of NH+ donors, this host is less effective than cavity binding analogues R3Bm and R3F at hexaprotonated levels, but more effective at protonation levels less than four. 

The thermochemistry is given by Eq. 2, where AH now represents the binding enthalpy and AS, the differences in entropy between the complex and the gable conformation 2 (C2v). Only the stable conformations in equilibrium with the host conformation contribute to the term Aa. Thus, in the case of DPM and DPC, Aa=2, and a loss of mixing entropy by R71n2=0.41 kcallmol (at 300 K) must be taken into account. 

Pseudo-rotaxane complex formation was used to reversibly trigger the conformational preference of the bis-cholesterol derivative 58 (Scheme 22). Compound 58 was observed to gel cycloalkane and diphenyl ether at 19 mM but failed to gelate aromatic solvents under the same conditions [82]. However, addition of a diammonium guest into a benzene sol of 58 induced gelation to occur. This effect was attributed to a change in host conformation. Indeed pseudo-rotaxane formation forced the molecule to adopt the extended geometry, which resulted in a more favorable situation for one-dimensional aggregation and, in turn, gel formation [82],... 

This means that the host conformation changes so as to include a substrate tightly, the behavior being similar to the induced-fit binding of substrates proposed for enz3rmes. In this system, the appended naphthyl moiety acts as a spacer which narrows the large Y CD cavity and allows the inclusion of the guest molecules. 

Fig. 38.16 Flexibility of the /)ora-sulph(Hiatocalix[5]arene host (a) host ctmfonnation (top) and 1 1 inclusion complex formation with 1,10-phenantrolinium cation [34] (b) host conformatiMi (top) and 2 2 inclusion complex formation with 1,10-phenantrolinium cation [34] (c) host conformation (top) and 2 2 inclusion complex framation with l,4-diazabicyclo(2.2.2)octanium cation [35] (d) host conformation (top) and 1 1 inclusion complex formation with ethane-1,2-bis (4,4 -bipyridinium) cation [36]...

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