Cation-7t Interactions

Caldwell, J.W. and Kollman, P.A. (1995). Cation-7t interactions nonadditive effects are critical in their accurate representation. J. Am. Chem. Soc. 117, 4177-4178... 

The cation-7t interactions were evidenced by Prodi, who studied the photophysical properties of calix[4]arene-crown and their complexes with alkali metal ions. The presence of these cation ions usually caused weak effects on the absorption spectra, but sometimes caused marked changes in the intensity and wavelength maxima of the fluorescence bands of the calixarenes. The fluorescence quantum yields of complexes with alkali metal follows a precise trend for both MC46 and MC7, decreasing from potassium to cesium. These changes were explained by cation-Tt interactions between the metal ion and the two aromatic rings pointing toward it.30... 

Organic cations are especially good guests, since their inclusion is favored by cation-7t interactions with the aromatic walls. This enables the encapsulation to be studied by mass spectrometry also [36]. If cations such as tetraethylammonium or cobaltocenium are included [37], the directionality of the hydrogen-bonded belt... 

Cation-7T interactions play a crucial role in molecular recognition.64 For example, Na+/K+ cation interactions with aromatic rings of some amino acids are implicated in the biological functions of specific enzymes and in the selectivity of Na+/K+ channels.74-78 The same factors are responsible of the relative stability of the charge-solvated vs. zwitterionic structure of amino acids.70,79-81... 

Kim and co-workers explored synthetic receptors for ammonium ions. They utilized cation-7t interactions and ethereal H-bonding <2001AGE2116>. They synthesized cryptand 29, which they found binds ammonium ions (Figure 6) more strongly than nonactin, a nature antibiotic and a strong host for ammonium ions. 

Figure 1 Typical interactions found in protein-ligand complexes. Usually, the lipophilic part of the ligand is in contact with the lipophilic parts of the protein (side chains of He, Val, Leu, Phe, Trp, perpendicular contact to amide bonds). In addition, hydrogen bonds are often involved. Some interactions can be charge assisted. Cation-7t interactions and metal complexation can also play a significant role in individual cases.

Amal-Herault C, Barboiu M, Petit E, Michau M, van der Lee A Cation-7t Interaction a case for Macrocycle-Cation fi-Interaction by its ureidoarene counteranion. New J. Chem. 2005 29 1535-1539. 

As seen in Fig. 10, in aromatic hydrocarbons the Jt-electron-rich regions can interact with cations through so-called cation-7t interactions [40], In mixtures with ionic liquids, the aromatic molecules may be solvated in the nonpolar domains but they may have sufficient affinity for the cationic head-groups to be found among the polar domain as well. When looking at the structure of such mixtures, it is useful to study the local environments of the ions and of the aromatic compound. 

These studies support the idea that the bilayer is actually somewhat permeable to macromolecules under certain conditions. The exact mechanism of this permeability is not known, but the arginine specificity of nonclassical vectors (see Section 13.4.1.1) suggests a reasonable pathway. Although these side chains are not expected to interact with the apolar interior of the bilayer, they may interact with the numerous transmembrane proteins that span biological membranes. The helices of membrane-spanning proteins are often found to be studded with aromatic residues,158 160 providing an ideal environment for the formation of cation-7t interactions between arginine (and potentially lysine) residues of a vector and the... 

Figure 102 A water-soluble receptor for studying cation-7t interactions [131J...

Intramolecular cation-7t interaction in organic synthesis (particularly, S3mthesis of chiral dihydrop37ridines) 070BC2903. 

FIGURE 7.9 Cation-7t interaction-directed supramolecular polymerization. 

---