Synthetic supramolecular zipper complexes have been used to quantify substituent effects on the free energies of aromatic stacking interactions. The conformational properties of the complexes have been characterised using NMR spectroscopy in CDCl(3), and by comparison with the solid state structures of model compounds. The structural similarity of the complexes makes it possible to apply the double mutant cycle method to evaluate the magnitudes of 24 different aromatic stacking interactions. The major trends in the interaction energy can be rationalised using a simple model based on electrostatic interactions between the pi-faces of the two aromatic rings. However, electrostatic interactions between the substituents of one ring and the pi-face of the other make an additional contribution, due to the slight offset in the stacking geometry. This property makes aromatic stacking interactions particularly sensitive to changes in orientation as well as the nature and location of substituents.

A synthetic supramolecular complex has been adapted to quantify cation-interactions in chloroform by using chemical doublemutant cycles. The interaction of a pyridinium cation with the -face of an aromatic ring is found to be very sensitive to the -electron density. Electron-donating substituents lead to a strong attractive interaction (؊8 kJ͞mol ؊1 ), but electron-withdrawing groups lead to a repulsive interaction (؉2 kJ͞mol ؊1 ).T he interactions of cations with aromatic rings play an important role in a range of biological processes, including ion channels, membrane receptors, and enzyme substrate interactions (1-9). Supramolecular chemical model systems have been instrumental in establishing the basic properties of this important class of noncovalent interactions, but cation-interactions are still poorly understood at a quantitative level, and it is difficult to predict substituent effects. Dougherty and coworkers (10) used the interaction between a synthetic aromatic host and a pyridinium guest to estimate a value of Ϫ10 kJ͞mol Ϫ1 for the interaction of a cation with four -systems in water. This value agrees well with the value of Ϫ11 kJ͞mol Ϫ1 measured by using protein engineering for the interaction of S-methylmethionine with a cavity lined by three -systems (11). Schneider et al. (12) obtained a value of Ϫ3 kJ͞mol Ϫ1 for a single cation-interaction by using a positively charged lipophilic host and an aromatic guest in water.We have developed an approach to the quantitative measurement of noncovalent functional group interactions based on chemical double-mutant cycles. This approach has proved particularly valuable for investigating structure-activity relationships in edge-to-face aromatic interactions, providing new insight into the physical basis for substituent effects on the strengths of these interactions (13,14). Here, we apply this approach to the cation-interaction, or more specifically, to the interaction of a pyridinium cation with the -face of functionalized aromatic rings. The double-mutant cycle is illustrated in Fig. 1. The difference between the stabilities of complexes A and B (⌬G A -⌬G B ) provides an indication of the magnitude of the cation-interaction in complex A, but the value is perturbed by changes in H-bond strength and other secondary interactions associated with the A3B mutation. The secondary effects can be quantified by using complexes C and D where there are no cation-interactions, but the same chemical mutation is made. Thus, the difference ⌬G C -⌬G D provides a direct measure of the changes in H-bond strength and secondary interactions associated with the A3B mutation, and it is possible to dissect out the thermodynamic contribution of the pyridinium-interaction from all of the other interactions present in complex A (⌬⌬G in Eq.

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The C 3 vanadium(V) amine triphenolate complex 1f has been characterized as a structural and functional model of vanadium haloperoxidases. The complex catalyzes efficiently sulfoxidations at room temperature using hydrogen peroxide as the terminal oxidant, yielding the corresponding sulfoxides in quantitative yields and high selectivities (catalyst loading down to 0.01%, TONs up to 9900, and TOFs up to 8000 h (-1)) as well as bromination of 1,3,5-trimethoxybenzene (catalyst loading down to 0.05%, TONs up to 1260, and TOFs up to 220 h (-1)).