Geometric effects in olefinic cation-π interactions with alkali metals: a computational study

Stable dispersion of carbon nanotubes in a molten salt of KNO3-NaNO3-NaNO2-LiNO3-LiOH

Carbon nanotubes (CNTs) have excellent mechanical and electrical properties; however, they suffer from dispersion problems in various applications. Traditional dispersing strategies of CNTs mostly use oxidation with strong acids or mechanical milling with high energy, which causes serious damage to the intrinsic structures and properties of CNTs. Therefore, it is important to develop new methods for dispersing CNTs without destroying their structures. This paper proposes to disperse CNTs in low-temperature molten salts composed of KNO3-NaNO3-NaNO2-LiNO3-LiOH. By adjusting the composition ratio of molten salts and alkaline, the interaction between charged ions and CNT electrons in the molten salt is studied. The alkaline molten salts can stably disperse CNTs and do not destroy their lengths, thereby offering better electric conductivity. This work will provide a new yet effective method for dispersing CNTs with high aspect ratios, which are important for the application of CNTs and other nanocarbons.

The disordered plant dehydrin Lti30 protects the membrane during water-related stress by cross-linking lipids

Dehydrins are intrinsically disordered proteins, generally expressed in plants as a response to embryogenesis and water-related stress. Their suggested functions are in membrane stabilization and cell protection. All dehydrins contain at least one copy of the highly conserved K-segment, proposed to be a membrane-binding motif. The dehydrin Lti30 (Arabidopsis thaliana) is up-regulated during cold and drought stress conditions and comprises six K-segments, each with two adjacent histidines. Lti30 interacts with the membrane electrostatically via pH-dependent protonation of the histidines. In this work, we seek a molecular understanding of the membrane interaction mechanism of Lti30 by determining the diffusion and molecular organization of Lti30 on model membrane systems by imaging total internal reflection- fluorescence correlation spectroscopy (ITIR-FCS) and molecular dynamics (MD) simulations. The dependence of the diffusion coefficient explored by ITIR-FCS together with MD simulations yields insights into Lti30 binding, domain partitioning, and aggregation. The effect of Lti30 on membrane lipid diffusion was studied on fluorescently labeled supported lipid bilayers of different lipid compositions at mechanistically important pH conditions. In parallel, we compared the mode of diffusion for short individual K-segment peptides. The results indicate that Lti30 binds the lipid bilayer via electrostatics, which restricts the mobility of lipids and bound protein molecules. At low pH, Lti30 binding induced lipid microdomain formation as well as protein aggregation, which could be correlated with one another. Moreover, at physiological pH, Lti30 forms nanoscale aggregates when proximal to the membrane suggesting that Lti30 may protect the cell by "cross-linking" the membrane lipids.

Interlocked benzenes in triangular π-architectures: anchoring groups dictate ion binding and transmission

Graphynes and graphenylenes – potential materials for lithium ion batteries.

Unique cation–cyclohexane interactions in tri- and hexa-fluorocyclohexane multidecker complexes in the gas phase: a DFT study

The formation of stable sandwich and multidecker complexes through electrostatic interaction in tri- and hexa-fluorocyclohexane has been analyzed in the light of density functional theory.

Structural, QTAIM, thermodynamic properties, bonding, aromaticity and NMR analyses of cation–π interactions of mono and divalent metal cations (Li+, Na+, K+, Be2+, Mg2+, and Ca2+) with substituted pyrazine derivatives

Theoretical investigation of 42 cation-π complexes formed by the alkali metal ( Li +, Na +, K +), alkaline-earth cations ( Be 2+, Mg 2+, Ca 2+) and π-system of the pyrazine and its derivatives have been performed at density functional theory (DFT) (B3LYP functional) and MP2 methods with 6-311++G** basis set in the gas phase and the polarized continuum model (PCM)-water solvation. The following substituents have been taken into consideration: Br , Cl , CH 3, OH , OCH 3 and SH . The interactions present in these complexes have been investigated by means of the natural bond orbital (NBO) and the Bader's quantum theory of atoms in molecules (QTAIMs) approaches. The effects of the interactions on NMR data have been probed using the GIAO-based method to extend investigation of the studied compounds. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies show that charge transfer occurs within each complex. Vibrational frequencies and physical properties such as dipole moment, chemical potential, chemical hardness and chemical electrophilicity of these compounds have been systematically explored. The aromaticity of aromatic rings has been measured using several well-established indices of aromaticity such as nucleus-independent chemical shift, harmonic oscillator models of the aromaticity, para-delocalization index, average two-center indices, aromatic fluctuation index and π-fluctuation aromatic index.

Dual role for alkali metal cations in enhancing the low-temperature radical polymerization of N,N-dimethylacrylamide

Alkali metal cations, including the Li⁺ cation, have been proposed to play a dual role in radical polymerization of N,N-dimethylacrylamide.

Effect of substituent and solvent on cation-π interactions in benzene and borazine: a computational study

A DFT and ab initio quantum chemical study has been carried out at different theoretical levels to delve into the role of the cation-π interaction within the main group metal cations (Li(+), Na(+) and K(+)), substituted benzene and borazine. The effects of electron withdrawing and electron donating groups on these non-covalent forces of interaction were also studied. The excellent correlation between Hammett constants and binding energy values indicates that the cation-π interaction is influenced by both inductive and resonance effects. Electron donating groups (EDG) such as -CH3 and -NH2 attached to benzene at the 1, 3 and 5 position and the three boron atoms of borazine were found to strengthen these interactions, while electron withdrawing groups (EWG) such as -NO2 did the reverse. These results were further substantiated by topological analysis using the quantum theory of atoms in molecules (QTAIM). The polarized continuum model (PCM) and the discrete solvation model were used to elucidate the effect of solvation on the cation-π interaction. The size of the cations and the nature of the substituents were found to influence the enthalpy and binding energy of the systems (or complex). In the gas phase, the cation-π interaction was found to be exothermic, whereas in the presence of a polar solvent the interaction was highly endothermic. Thermochemical analysis predicts the presence of thermodynamic driving forces for borazine and benzene substituted with EDG. DFT based reactivity descriptors, such as global hardness (η), chemical potential (μ) and the electrophilicity index (ω) were used to elucidate the effect of the substituent on the reactivity of the cation-π complexes.

Effects of the aromatic substitution pattern in cation-π sandwich complexes

A computational study investigating the effects of the aromatic substitution pattern on the structure and binding energies of cation-π sandwich complexes is reported. The correlation between the binding energies (Ebind) and Hammett substituent constants is approximately the same as what is observed for cation-π half-sandwich complexes. For cation-π sandwich complexes where both aromatics contain substituents the issue of relative conformation is a possible factor in the strength of the binding; however, the work presented here shows the Ebind values are approximately the same regardless of the relative conformation of the two substituted aromatics. Finally, recent computational work has shown conflicting results on whether cation-π sandwich Ebind values (Ebind,S) are approximately equal to twice the respective half-sandwich Ebind values (Ebind,HS), or if cation-π sandwich Ebind,S values are less than double the respective half-sandwich Ebind,HS values. The work presented here shows that for cation-π sandwich complexes involving substituted aromatics the Ebind,S values are less than twice the respective half-sandwich Ebind,HS values, and this is termed nonadditive. The extent to which the cation-π sandwich complexes investigated here are nonadditive is greater for B3LYP calculated values than for MP2 calculated values and for sandwich complexes with electron-donating substituents than those with electron-withdrawing groups.

The multiple roles of histidine in protein interactions

Background

Among the 20 natural amino acids histidine is the most active and versatile member that plays the multiple roles in protein interactions, often the key residue in enzyme catalytic reactions. A theoretical and comprehensive study on the structural features and interaction properties of histidine is certainly helpful.

Results

Four interaction types of histidine are quantitatively calculated, including: (1) Cation-π interactions, in which the histidine acts as the aromatic π-motif in neutral form (His), or plays the cation role in protonated form (His⁺); (2) π-π stacking interactions between histidine and other aromatic amino acids; (3) Hydrogen-π interactions between histidine and other aromatic amino acids; (4) Coordinate interactions between histidine and metallic cations. The energies of π-π stacking interactions and hydrogen-π interactions are calculated using CCSD/6-31+G(d,p). The energies of cation-π interactions and coordinate interactions are calculated using B3LYP/6-31+G(d,p) method and adjusted by empirical method for dispersion energy.

Conclusions

The coordinate interactions between histidine and metallic cations are the strongest one acting in broad range, followed by the cation-π, hydrogen-π, and π-π stacking interactions. When the histidine is in neutral form, the cation-π interactions are attractive; when it is protonated (His⁺), the interactions turn to repulsive. The two protonation forms (and pK_a values) of histidine are reversibly switched by the attractive and repulsive cation-π interactions. In proteins the π-π stacking interaction between neutral histidine and aromatic amino acids (Phe, Tyr, Trp) are in the range from -3.0 to -4.0 kcal/mol, significantly larger than the van der Waals energies.

Accurate prediction of cation-π interaction energy using substituent effects

Substituent effects on cation-π interactions have been quantified using a variety of Φ-X···M(+) complexes where Φ, X, and M(+) are the π-system, substituent, and cation, respectively. The cation-π interaction energy, E(M(+)), showed a strong linear correlation with the molecular electrostatic potential (MESP) based measure of the substituent effect, ΔV(min) (the difference between the MESP minimum (V(min)) on the π-region of a substituted system and the corresponding unsubstituted system). This linear relationship is E(M(+)) = C(M(+))(ΔV(min)) + E(M(+))' where C(M(+)) is the reaction constant and E(M(+))' is the cation-π interaction energy of the unsubstituted complex. This relationship is similar to the Hammett equation and its first term yields the substituent contribution of the cation-π interaction energy. Further, a linear correlation between C(M(+))() and E(M(+))()' has been established, which facilitates the prediction of C(M(+)) for unknown cations. Thus, a prediction of E(M(+)) for any Φ-X···M(+) complex is achieved by knowing the values of E(M(+))' and ΔV(min). The generality of the equation is tested for a variety of cations (Li(+), Na(+), K(+), Mg(+), BeCl(+), MgCl(+), CaCl(+), TiCl(3)(+), CrCl(2)(+), NiCl(+), Cu(+), ZnCl(+), NH(4)(+), CH(3)NH(3)(+), N(CH(3))(4)(+), C(NH(2))(3)(+)), substituents (N(CH(3))(2), NH(2), OCH(3), CH(3), OH, H, SCH(3), SH, CCH, F, Cl, COOH, CHO, CF(3), CN, NO(2)), and a large number of π-systems. The tested systems also include multiple substituted π-systems, viz. ethylene, acetylene, hexa-1,3,5-triene, benzene, naphthalene, indole, pyrrole, phenylalanine, tryptophan, tyrosine, azulene, pyrene, [6]-cyclacene, and corannulene and found that E(M)(+) follows the additivity of substituent effects. Further, the substituent effects on cationic sandwich complexes of the type C(6)H(6)···M(+)···C(6)H(5)X have been assessed and found that E(M(+)) can be predicted with 97.7% accuracy using the values of E(M(+))' and ΔV(min). All the Φ-X···M(+) systems showed good agreement between the calculated and predicted E(M(+))() values, suggesting that the ΔV(min) approach to substituent effect is accurate and useful for predicting the interactive behavior of substituted π-systems with cations.

Energies and physicochemical properties of cation-π interactions in biological structures

The cation-π interactions occur frequently within or between proteins due to six (Phe, Tyr, Trp, Arg, Lys, and His) of the twenty natural amino acids potentially interacting with metallic cations via these interactions. In this study, quantum chemical calculations and molecular orbital (MO) theory are used to study the energies and properties of cation-π interactions in biological structures. The cation-π interactions of H⁺ and Li⁺ are similar to hydrogen bonds and lithium bonds, respectively, in which the small, naked cations H⁺ and Li⁺ are buried deep within the π-electron density of aromatic molecules, forming stable cation-π bonds that are much stronger than the cation-π interactions of other alkali metal cations. The cation-π interactions of metallic cations with atomic masses greater than that of Li⁺ arise mainly from the coordinate bond comprising empty valence atomic orbitals (AOs) of metallic cations and π-MOs of aromatic molecules, though electrostatic interactions may also contribute to the cation-π interaction. The binding strength of cation-π interactions is determined by the charge and types of AOs in the metallic cations. Cation-π interaction energies are distance- and orientation-dependent; energies decrease with the distance (r) and the orientation angle (θ). In solution, the cation-π energies decrease with the increase of the dielectric constant (ɛ) of the solvent; however, solvation has less influence on the H⁺-π and H₃O⁺-π interactions than on interactions with other cations. The conclusions from this study provide useful theoretical insights into the nature of cation-π interactions and may contribute to the development of better force field parameters for describing the molecular dynamics of cation-π interactions within and between proteins.

Quantitative assessment of substituent effects on cation-π interactions using molecular electrostatic potential topography

A molecular electrostatic potential (MESP) topography based approach has been proposed to quantify the substituent effects on cation-π interactions in complexes of mono-, di-, tri-, and hexasubstituted benzenes with Li(+), Na(+), K(+), and NH(4)(+). The MESP minimum (V(min)) on the π-region of C(6)H(5)X showed strong linear dependency to the cation-π interaction energy, E(M(+)). Further, cation-π distance correlated well with V(min)-π distance. The difference between V(min) of C(6)H(5)X and C(6)H(6) (ΔV(min)) is proposed as a good parameter to quantify the substituent effect on cation-π interaction. Compared to benzene, electron-donating groups stabilize the di-, tri-, and hexasubstituted cation-π complexes while electron-withdrawing groups destabilize them. In multiple substituted complexes, E(M(+)) is almost equal (∼95%) to the sum of the individual substituent contributions (E(M(+)) ≈ Σ(ΔE(M(+)))), suggesting that substituent effect on cation-π interactions is largely additive. The ΔV(min) of C(6)H(5)X systems and additivity feature have been used to make predictions on the interaction energies of 80 multiple substituted cation-π complexes with above 97% accuracy. The average mean absolute deviation of the V(min)-predicted interaction energy, E(M(+))(V) from the calculated E(M(+)) is -0.18 kcal/mol for Li(+), -0.09 kcal/mol for Na(+), -0.43 kcal/mol for K(+), and -0.67 kcal/mol for NH(4)(+), which emphasize the predictive power of V(min) as well as the additive feature of the substituent effect.