7t-stacking interactions

Figure 7-45. Two views of the solid state structure of the [3]-catenane 7.64. The large ring formed from the coupling of the 7.65 ligands has been coloured black and the two macrocyclic ligands 7.63 white. The second view, along the Cu—Cu axis, emphasises the folded structure of the large, central macrocyclic ring and shows some of the 7t-stacking interactions that are responsible for the adoption of this conformation.

These amino acids form hydrophobic (water-repelling) nonpolar regions in proteins. There are three more of this kind with special roles. Phenylalanine and tryptophan have aromatic rings and, though they are still hydrophobic, they can form attractive 7t-stacking interactions with other aromatic molecules. Enzyme-catalysed hydrolysis of proteins often happens next to one of these residues. Proline is very special. It has its amino group inside a ring and has a different shape from all the other amino acids. It appears in proteins where a bend or a twist in the structure is needed. 

Indeed, this is a case of complex additivity of binding energies, a phenomenon commonly seen in biological systems [35, 36]. Jencks has attributed the complex additivity in many systems to changes in translational and rotational entropy [36]. For example, the two complexing chromophores of the molecular tweezers are covalently connected. This means that the enthalpy of the second 7t-stacking interaction comes without paying the translational and rotation entropy price a second time. 

In 1994, the first enantioselective trifluoromethylation reaction was achieved with the Ruppert-Prakash reagent, TMSCF3, in the presence of the cinchona-based quaternary ammonium fluoride 140 [65]. The chiral induction can arise from the dual activation mode of the catalyst, that is, the fluoride anion acts as the nucleophilic activator of (TMS)CF3 and the chiral ammonium cation activates the carbonyl group of 141. However, the observed ee values of the obtained carbinols 142 do not exceed 51 % and decrease considerably when nonaromatic carbonyl compounds (15% ee for R1 = n-C7H15 R2 = H) are used, which implies that 7t-7t stacking interactions between the carbonyl compound and cinchoninium occur (Scheme 8.54). 

Figure 23 Hypothesized structure of the [Zn"(16)(trp)] adduct. Tryptophane (trp) is recognized by the Zn" tetramine receptor through (i) the formation of a metal-carboxylate coordinative bond (ii) the establishing of 7t-stacking interactions between the aromatic part of the amino acid and one of the facing polyaromatic substituents of the tripodal tetramine framework.

Figure 6.33 X-ray crystal structure of the p-nitrophenol complex of 6.66. Note the C—H ttand, OH N hydrogen bonds and the 7t-7t stacking interactions." ...

Fig. 1 Multiple molecular recognition Free metallocycle 1 is formed in dilute solution because of the enthalpy of formation of the Pd—N coordination bonds. When the solution is concentrated from 2-50 mM, the lability of these bonds allows the in situ formation of the interlocked catenane 2. This is driven by hydrophobic interactions to minimize the contact of the metallocycle cavities with the water solvent, the formation of 7t-stacking interactions between two such rings, and an entropic effect due to a decline in the number of species present in solution after interlocking.

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