Tetrahedral Recognition by Macrotricyclic Cryptands

The macrobicycle 9 also binds NH4+, forming cryptate 24. The dynamic properties of 24 compared with 23 reflect the receptor-substrate binding complemen- 

The remarkable protonation features of 21 led to the formulation of the diproto-nated species as the water cryptate, [H20 cr 21, 2H+] 25, in which the water molecule accepts two +N-H---0 bonds from the protonated nitrogens and donates two 0-H---N bonds to the unprotonated ones [2.17, 2.96], The second protonation of 21 is facilitated by the substrate it represents a positive cooperativity effect, mediated by H20, in which the first proton and the effector molecule water set the stage both structurally and energetically for the fixation of a second proton. When 21 is tetraprotonated it forms the chloride cryptate cryptate [Cl c 21,4H+] 26, in which the included anion is bound by four +N-H---X- hydrogen bonds [2.97] (see also Chapt. 3). 

Molecular mechanics and dynamics calculations have shown that the preorganization of 21 for cation or anion binding and recognition results from its high connectivity and its preformed cavity with converging binding sites and fairly fixed size [1.45,2.36b]. 

Unsymmetrical derivatives of 21 display notably perturbed NH4+ binding, with a marked loss in both stability and selectivity [2.94]. Replacing the oxygen atoms in 

21 by pyridine or phenyl units yields cryptands that generate stable and kinetically inert metal ion and proton cryptates [2.98]. 

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