Cation-tr interactions

Alkali-earth cation-Tr interactions, 154 Alkene oxides, 245... 

Figure 1.14 NMR-structure of the Grb2-SH2 domain in complex with inhibitor 39. A central antiparallel P-sheet in the protein is flanked by two a-helices, with peptide binding mediated by the sheet, intervening loops, and one of the helices. The specific recognition of phospho-tyrosine involves cation-tr interactions between...

In factor Xa, the D pocket is lined by aromatic residues in addition to the negatively charged Glu97, therefore, the finding that hydrophobic or cation-Tr interactions are important again underlines the strength of our x-variable weighting procedure. 

Noncovalent interactions of different types may be responsible for complex formation. Particularly important classes are (1) hydrogen bonds (2) ion-ion ( salt ) interactions (3) ion-dipole interactions and, probably, not identical to them (4) cation-tr interactions (5) coordination bonds with metals (6) aromatic stacking interactions and (7) so-called hydrophobic interactions. 

There are two main Tr-interactions that can be found in supramolecular systems, namely (i) cations interactions and (ii) tt-t interactions. Cation-Tr interactions are well known in the field of organometallic chemistry, whereby olefinic groups are bound to transition metal centres, for example, ferrocene and Zeise s salt ([PtCl3(Ti -C2H4)] ), but these are not regarded as non-covalent interactions. However, alkaline- and alkaline-earth metals also form interactions with doublebond systems, typically between 5 and 80 kj mor For example, the interaction of potassium ions with benzene has a similar energy to the K+-OH2 interaction. The potassium cation is more soluble in water than in benzene, however, as it is not sterically possible to fit as many benzene molecules around the metal ion as water molecules (Figure 1.18). 

The interactions between tetramethylammonium (TMA) and tt systems may be described as strong CH/tt interactions, as these systems have short contacts between the C - H bonds and the tt system. However, the physical origin of the attraction in the TMA complex is completely different from that in the CH/tt interaction between neutral molecules [124,125). The MP2-level interaction energies of TMA complexes with benzene, pyrrole, furan and imidazole are -8.45, - 10.02, -6.98 and - 16.37 kcal/mol, respectively. The calculations show that the electrostatic interaction is the major somce of the attraction in these complexes, while dispersion is mainly responsible for the attraction in the CH/tt interaction between neutral molecules. The physical origin of the attraction between the TMA and tt systems is the same as that of other cation/Tr complexes therefore, the interaction between TMA and tt systems is essentially a cation/Tr interaction (see the next section). 

The noncovalent interactions or van der Waals forces involved in supramolecular entities may be a combination of several interactions, e.g., ion-pairing, hydrophobic, hydrogen bonding, cation-Tr, tt-tt interactions, etc. They comprise interactions between permanent multipoles, between a permanent multipole and an induced multipole, and between a time-variable multipole and an induced multipole. 

Schwabacher et al. have studied the directionality of the cation-tr effect by measuring the association constants of guests of various sizes with cationic and anionic water-soluble macrocycles (Figure 103) [133]. These studies have provided support for ion-quadrupole interactions contributing to the binding in the anionic host. 

This force field and ab initio evidence of strong 1,3 tt-overlap in the ally system confirm predictions more than 30 years old. Such 1,3 Tr-interactions in alkylated allyl cations are discussed in Section 4.4. 

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