Molecular Modeling cation complexation

Fig. 4.13. Space-lilling molecular model depicting a metal cation complexed by 1 R-crown-6.

In what follows, the phenomenology of carrier transport will be briefly reviewed along with the mechanism of the Valinomycin model of carrier transport. The development of the molecular structure of Valinomycin will be considered in some detail, since the key to the dramatic selectivity of Valinomycin is thought to reside in the energetics of the molecular structure. Confidence in an understanding of the molecular structure of the Valinomycin-cation complex becomes tantamount to confidence in the presented basis of ion selectivity. 

It is clear that such calculation are very approximate. Nevertheless they appear to provide reasonable s values as judged from comparison with measurements on space-filling molecular models. The procedure is best suited for ligands which completely embed the cation in a tridimensional cavity. In the case of complexes having regions where the cation is accessible to solvent molecules (e. g. complexes with macrocycles of type D in figure 2, which have open top and bottom sides) one may hope that the effect of a large s in one direction and a small s in another is taken... 

The details of protonation of several alkyl-substituted phenanthrenes by superacids have been reported.73 The observed mono- and di-cations are usually in agreement with those predicted by AMI MO calculations. Molecular modelling studies have suggested a multi-step pathway for the sulfonation of toluene with sulfur trioxide.74 Intermediate 7r-complex, Wheland intermediate and pyrosulfonate species (34) are suggested, the product (/Molucncsulfonic acid) arising from an exothermic reaction between toluene and the acid (35) formed by a facile prototropic rearrangement of (34). The sulfur trioxide mono sulfonation of isopyrene and some derivatives leads usually to sulfonated... 

These qualitative assays show that one-armed cationic guanidiniocarbonyl pyrrole receptors can indeed effectively bind tetrapeptides even in water. Molecular modeling studies suggest a complex structure as shown for one specific example, the receptor Val-Val-Val-CBS, in Figure 2.3.11. Receptor and substrate form a hydrogen bonded //-sheet which is further stabilized by additional hydrophobic interactions between the apolar groups in the side-chains. Recognition of the tetrapep-tide thus seems to be controlled by a fine balanced interplay between electrostatic and hydrophobic interactions. 

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