Crystallographic observation of 'induced fit' in a cryptophane host–guest model system

Abstract

Cryptophane-A, comprised of two cyclotriguaiacylenes joined by three ethylene linkers, is a prototypal organic host molecule that binds reversibly to neutral small molecules via London forces. Of note are trifunctionalized, water-soluble cryptophane-A derivatives, which exhibit exceptional affinity for xenon in aqueous solution. In this paper, we report high-resolution X-ray structures of cryptophane-A and trifunctionalized derivatives in crown–crown and crown–saddle conformations, as well as in complexes with water, methanol, xenon or chloroform. Cryptophane internal volume varied by more than 20% across this series, which exemplifies 'induced fit' in a model host–guest system.

Strongly underestimated dispersion energy in cryptophanes and their complexes

“…This increase is explained by entropic (hydrophobic) effects that likely involve desolvation of xenon clathrates upon complexation. An increase of the binding constant can also be observed when the cavity is adjusted to the guest 16 . The D 3 -symmetric cryptophane-E (D 3 -1, see Fig.…”

“…The latter conformation has already been described for cryptophane-A derivatives and larger cryptophanes 16,28,29 . To determine the conformation of impl-1, a mixture of CHCl 3 @1 in 1,2-C 6 D 4 Cl 2 was heated at 423 K for 12 h to completely remove CHCl 3 from the solution.…”

“…For structural investigations of the imploded cryptophanes a C 1 -symmetric out-saddle conformation of imploded cryptophane-A (impl-2) 16 , a C 3 -symmetric in-out conformation of imploded cryptophane-E (impl-1) and a C 1 -symmetric out-saddle conformation of impl-1 were optimized using M05-2X/6-31G*,cc-pVTZ (see Fig. 6).…”

Measurement of radon and xenon binding to a cryptophane molecular host

“…90 Å 3 , assuming modest expansion from the average 4-Xe structure) (12) is approximately twice the radon atomic volume (35). Furthermore, in lieu of three ethylamine groups, tripropargylated 4 can be reacted with a wide variety of water-solubilizing azido linkers that may fine tune the host-noble gas interaction (12)(13)(14).…”

“…Cryptophane-A, consisting of two cyclotriguaiacylene caps joined by three ethylene linkers, has been shown to encapsulate xenon reversibly as a host-guest complex (11). Recent X-ray crystallographic studies have shown that the internal volume of trisubstituted cryptophane-A derivatives can vary by more than 20%, depending on the size of the encapsulated guest (12). Several cryptophane-A derivatives have been synthesized and shown by isothermal titration calorimetry (ITC), fluorescence quenching, and NMR studies to exhibit xenon association constants as high as 33;000 AE 3;000 M −1 at 293 K in phosphate-buffered water (13)(14)(15)(16).…”