Derivatives of N-Benzyl-2-phenylpyridinium Bromide, Minimalist Models for Face-to-Face, Center-to-Edge π-Stacking in Water

Interactions of Substituted Nitroaromatics with Model Graphene Systems: Applicability of Hammett Substituent Constants To Predict Binding Energies

Applicability of Hammett parameters (σ _m and σ _p ) was tested in extended π-systems in gas phase. Three different model graphene systems, viz. 5,5-graphene (GR), 3-B-5,5-graphene (3BGR), and 3-N-5,5-graphene (3NGR), were designed as extended π-systems, and interactions of various nitrobenzene derivatives (mainly m- and p-substituted together with some multiple substitutions) on such platforms were monitored using density functional theory (M06/cc-pVDZ, M06/cc-pVTZ, M06/sp-aug-cc-pVTZ) and Møller-Plesset second-order perturbation (MP2/cc-pV-DZ) theory. Offset face to face (OSFF) stackings were found to be the favored orientations, and reasonable correlations were found between binding energies (ΔE _B) and the ∑|σ _m | values of the substituted nitrobenzenes. It was proposed previously that |σ _m | contains information about the substituents' polarizability and controls electrostatic and dispersion interactions. The combination of ∑|σ _m | and molar refractivity (as ∑M _r) or change in polarizability (Δα: with respect to benzene) of nitrobenzene derivatives generated statistically significant correlation with respect to ΔE _B, thereby supporting the hypothesis related to the validity of |σ _m | correlations. The |σ _p | parameters also maintain similar correlations for the various p-substituted nitrobenzene derivatives together with several multiply-substituted nitrobenzene derivatives. The correlation properties in such cases are similar to the |σ _m | cases, and the energy partition analysis for both the situations reveled importance of electrostatic and dispersion contributions in such interactions. The applicability of Hammett parameters was observed previously on the restricted parallel face to face orientation of benzene···substituted benzene systems, and the present results show that such an idea could be used to predict ΔE _B values in OSFF orientations, if the scaffolds are designed in such a way that substituted benzene systems cannot escape their π-clouds.

Amino acids defining the acyl pocket of an invertebrate cholinesterase

Amphioxus (Branchiostoma floridae) cholinesterase 2 (ChE2) hydrolyzes acetylthiocholine (AsCh) almost exclusively. We constructed a homology model of ChE2 on the basis of Torpedo californica acetylcholinesterase (AChE) and found that the acyl pocket of the enzyme resembles that of Drosophila melanogaster AChE, which is proposed to be comprised of Phe330 (Phe290 in T. californica AChE) and Phe440 (Val400), rather than Leu328 (Phe288) and Phe330 (Phe290), as in vertebrate AChE. In ChE2, the homologous amino acids are Phe312 (Phe290) and Phe422 (Val400). To determine if these amino acids define the acyl pocket of ChE2 and its substrate specificity, and to obtain information about the hydrophobic subsite, partially comprised of Tyr352 (Phe330) and Phe353 (Phe331), we performed site-directed mutagenesis and in vitro expression. The aliphatic substitution mutant F312I ChE2 hydrolyzes AsCh preferentially but also butyrylthiocholine (BsCh), and the change in substrate specificity is due primarily to an increase in k(cat) for BsCh; K(m) and K(ss) are also altered. F422L and F422V produce enzymes that hydrolyze BsCh and AsCh equally due to an increase in k(cat) for BsCh and a decrease in k(cat) for AsCh. Our data suggest that Phe312 and Phe422 define the acyl pocket. We also screened mutants for changes in sensitivity to various inhibitors. Y352A increases the sensitivity of ChE2 to the bulky inhibitor ethopropazine. Y352A decreases inhibition by BW284c51, consistent with its role as part of the choline-binding site. Aliphatic replacement mutations produce enzymes that are more sensitive to inhibition by iso-OMPA, presumably by increasing access to the active site serine. Y352A, F353A and F353V make ChE2 considerably more resistant to inhibition by eserine and neostigmine, suggesting that binding of these aromatic inhibitors is mediated by pi-pi or cation-pi interactions at the hydrophobic site. Our results also provide information about the aromatic trapping of the active site histidine and the inactivation of ChE2 by sulfhydryl reagents.

Origin of Enantioselectivity in the Ru(arene)(amino alcohol)-Catalyzed Transfer Hydrogenation of Ketones

The origin of the enantioselectivity in the ruthenium-catalyzed transfer hydrogenation has been studied by means of experiment and density functional theory calculations. The results clearly show that electrostatic effects are of importance, not only in the T-shaped arene-aryl interaction in the favored transition state but also between the aryl of the substrate and the amine ligand in the disfavored TS. In addition, the electrostatic interaction between the alkyl substituent of the substrate and the catalyst is of importance to the enantioselectivity. The major cause of enantioselection is found to be of nonelectrostatic origin. This inherent property of the catalytic system is discussed in terms of dispersion forces and solvent effects. Finally, a minor but well-characterized steric effect was identified. The success of this class of catalysts in the reduction of alkyl aryl ketones is based on the fact that all factors work in the same direction.

Through-space interactions between face-to-face, center-to-edge oriented arenes: importance of polar-pi effects

Two series of conformationally restricted polycyclic compounds (1-3 and 4-7) have been synthesized as model systems for studying the through-space interactions between face-to-face, center-to-edge (parallel-offset) oriented arenes. These compounds feature different X substituents on one of the interacting rings. By monitoring the variation of the delta Gz for the rotation around the aryl-aryl bond in 1-7 as a function of X by 2D [1H,1H] EXSY NMR spectroscopy, it was found that the barriers increase on passing from electron-donating to electron-withdrawing substituted derivatives. Quantum mechanical calculations [MP2/DVZ (2d,p)//B3LYP/DVZ(2d,p)] gave barrier values and variations in agreement with the experimental data. The results are consistent with a repulsive arene-arene interaction dominated by electrostatic effects.

Analysis of the pi-pi stacking interactions between the aminoglycoside antibiotic kinase APH(3')-IIIa and its nucleotide ligands

A key contact in the active site of an aminoglycoside phosphotransferase enzyme (APH(3')-IIIa) is a pi-pi stacking interaction between Tyr42 and the adenine ring of bound nucleotides. We investigated the prevalence of similar Tyr-adenine contacts and found that many different protein systems employ Tyr residues in the recognition of the adenine ring. The geometry of these stacking interactions suggests that electrostatics play a role in the attraction between these aromatic systems. Kinetic and calorimetric experiments on wild-type and mutant forms of APH(3')-IIIa yielded further experimental evidence of the importance of electrostatics in the adenine binding region and suggested that the stacking interaction contributes approximately 2 kcal/mol of binding energy. This type of information concerning the forces that govern nucleotide binding in APH(3')-IIIa will facilitate inhibitor design strategies that target the nucleotide binding site of APH-type enzymes.