The Influence of Substituents on Cation−π Interactions. 4. Absolute Binding Energies of Alkali Metal Cation−Phenol Complexes Determined by Threshold Collision-Induced Dissociation and Theoretical Studies,

Complexes of Alkaline and Ammonium Cations with Dopamine and Eumelanin Precursors: Dissecting the Role of Noncovalent Cation-π and Cation-Lone Pair (σ-Type) Interactions

Cation-π interactions and their possible competition with other noncovalent interactions (NCI) might play a key role in both dopamine- and eumelanin-based bioinspired materials. In this contribution, to unravel the delicate interplay between cation-π interactions and other possible competing forces, the configurational space of noncovalent complexes formed by dopamine or eumelanin precursors (o-benzoquinone, DHI and a semiquinone dimer) and three different cations (Na⁺, K⁺, and NH₄⁺) is sampled by means of accurate ab initio calculations. To this end, we resort to the mp2^mod method, recently validated by us for benzene-, phenol-, and catechol-cation complexes, whose computational convenience allows for an extensive exploration of the cation-molecule interaction energy surface, by sampling a total of more than 10⁴ arrangements. The mp2^mod interaction energy landscapes reveal that, besides the expected cation-π driven arrangements, for all considered molecule-cation pairs the most stable complexes are found when the cation lies within the plane containing the six-membered ring, thus maximizing the σ-type interaction with the oxygen's lone pairs. Due to the loss of aromaticity, the σ-type/cation-π strength ratio is remarkably large in o-benzoquinone, where cation-π complexes seem unlikely to be formed. The above features are shared among all considered cations but are significantly larger when considering the smaller Na⁺. Besides delivering a deeper insight onto the NCI network established by the considered precursors in the presence of ions, the present results can serve as a reference database to validate or refine lower level methods, as, for instance, the force fields employed in classical simulations.

17β-Hydroxysteroid Dehydrogenase Type 1 Inhibition: A Potential Treatment Option for Non-Small Cell Lung Cancer

In the face of the clinical challenge posed by non-small cell lung cancer (NSCLC), the present need for new therapeutic approaches is genuine. Up to now, no proof existed that 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1) is a viable target for treating this disease. Synthesis of a rationally designed library of 2,5-disubstituted furan derivatives followed by biological screening led to the discovery of 17β-HSD1 inhibitor 1, capable of fully inhibiting human NSCLC Calu-1 cell proliferation. Its pharmacological profile renders it eligible for further in vivo studies. The very high selectivity of 1 over 17β-HSD2 was investigated, revealing a rational approach for the design of selective inhibitors. 17β-HSD1 and 1 hold promise in fighting NSCLC.

Theoretical investigations into the intermolecular hydrogen-bonding interactions of N-(hydroxymethyl)acetamide dimers

The structures of the N-(hydroxymethyl)acetamide (model molecule of ceramide) dimers have been fully optimized at B3LYP/6-311++G** level. The intermolecular hydrogen bonding interaction energies have been calculated using the B3LYP/6-311++G**, B3LYP/6-311++G(2df,2p), MP2(full)/6-311++G** and MP2(full)/6-311++G(2df,2p) methods, respectively. The results show that the O-H···O, N-H···O, O-H···N, and C-H···O hydrogen bonding interactions could exist in N-(hydroxymethyl)acetamide dimers, and the O-H···O, N-H···O, and O-H···N hydrogen bonding interactions could be stronger than C-H···O. The three-dimensional network structure formed by ceramide molecules through intermolecular hydrogen bonding interactions may be the main reason why the stratum corneum of skin could prevent foreign substances from entering our body, as is in accordance with the experimental results. The stability of hydrogen-bonding interactions follow the order of (a) > (b) ≈ (c) > (d) > (e) ≈ (f) > (g) > (h). The analyses of the energy decomposition, frequency, atoms in molecules (AIM), natural bond orbital (NBO), and electron density shift are used to further reveal the nature of the complex formation. In the range of 263.0-328.0 K, the complex is formed via an exothermic reaction, and the solvent with lower temperature and dielectric constant is favorable to this process. Graphical abstract The structures and the O-H···O=C, N-H···O=C and C-H···O=C H-bonding interactions in the N-(hydroxymethyl)acetamide (model molecule of ceramide) dimers were investigated using the B3LYP and MP2(full) methods.

Influence of Linkage Stereochemistry and Protecting Groups on Glycosidic Bond Stability of Sodium Cationized Glycosyl Phosphates

Energy-resolved collision-induced dissociation (ER-CID) experiments of sodium cationized glycosyl phosphate complexes, [GP _x +Na]⁺, are performed to elucidate the effects of linkage stereochemistry (α versus β), the geometry of the leaving groups (1,2-cis versus 1,2-trans), and protecting groups (cyclic versus non-cyclic) on the stability of the glycosyl phosphate linkage via survival yield analyses. A four parameter logistic dynamic fitting model is used to determine CID_50% values, which correspond to the level of rf excitation required to produce 50% dissociation of the precursor ion complexes. Present results suggest that dissociation of 1,2-trans [GP _x +Na]⁺ occurs via a McLafferty-type rearrangement that is facilitated by a syn orientation of the leaving groups, whereas dissociation of 1,2-cis [GP_x+Na]⁺ is more energetic as it involves the formation of an oxocarbenium ion intermediate. Thus, the C1-C2 configuration plays a major role in determining the stability/reactivity of glycosyl phosphate stereoisomers. For 1,2-cis anomers, the cyclic protecting groups at the C4 and C6 positions stabilize the glycosidic bond, whereas for 1,2-trans anomers, the cyclic protecting groups at the C4 and C6 positions tend to activate the glycosidic bond. The C3 O-benzyl (3 BnO) substituent is key to determining whether the sugar or phosphate moiety retains the sodium cation upon CID. For 1,2-cis anomers, the 3 BnO substituent weakens the glycosidic bond, whereas for 1,2-trans anomers, the 3 BnO substituent stabilizes the glycosidic bond. The C2 O-benzyl substituent does not significantly impact the glycosidic bond stability regardless of its orientation. Graphical abstract ᅟ.

Cationic Noncovalent Interactions: Energetics and Periodic Trends

In this review, noncovalent interactions of ions with neutral molecules are discussed. After defining the scope of the article, which excludes anionic and most protonated systems, methods associated with measuring thermodynamic information for such systems are briefly recounted. An extensive set of tables detailing available thermodynamic information for the noncovalent interactions of metal cations with a host of ligands is provided. Ligands include small molecules (H2, NH3, CO, CS, H2O, CH3CN, and others), organic ligands (O- and N-donors, crown ethers and related molecules, MALDI matrix molecules), π-ligands (alkenes, alkynes, benzene, and substituted benzenes), miscellaneous inorganic ligands, and biological systems (amino acids, peptides, sugars, nucleobases, nucleosides, and nucleotides). Hydration of metalated biological systems is also included along with selected proton-based systems: 18-crown-6 polyether with protonated peptides and base-pairing energies of nucleobases. In all cases, the literature thermochemistry is evaluated and, in many cases, reanchored or adjusted to 0 K bond dissociation energies. Trends in these values are discussed and related to a variety of simple molecular concepts.

Diarylferrocene tweezers for cation binding

Diarylferrocenes can act as molecular tweezers of cations. Their unique molecular shape and low torsional potentials allow for strong binding of small cations in the gas phase.

Silver cation affinities of monomeric building blocks of polyethers and polyphenols determined by guided ion beam tandem mass spectrometry

Energy-resolved collision-induced dissociation (CID) of seven silver cation-ligand complexes, Ag(+)(L), with Xe is studied using guided ion beam tandem mass spectrometry techniques. The ligands, L, investigated are monomeric building blocks of polyethers and polyphenols including phenol, 2-hydroxyphenol, 3-hydroxyphenol, 4-hydroxyphenol, 2-hydroxymethyl phenol, 3-hydroxymethyl phenol, and 4-hydroxymethyl phenol. In all cases, Ag(+) is observed as the primary CID product, corresponding to endothermic loss of the intact neutral ligand. The kinetic-energy-dependent cross sections for CID of these Ag(+)(L) complexes are analyzed using an empirical threshold law to extract absolute 0 and 298 K Ag(+)-L bond dissociation energies (BDEs). Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of the neutral ligands and their complexes to Ag(+) using either the Stuttgart RSC 1997 valence basis set and effective core potential (SRSC ECP) or DZVP-DFT to describe Ag(+). Theoretical BDEs are determined at the B3LYP/6-311+G(2d,2p) level of theory again using the SRSC ECP or DZVP-DFT for Ag(+). For all systems, the most stable binding conformations found involve cation-π interactions when the SRSC ECP is used to describe Ag(+). When DZVP-DFT is employed, the most stable binding geometries remain cation-π complexes except for the complex to 2HP, where the ground-state conformer involves bidentate binding of Ag(+) to the hydroxyl oxygen atoms of both substituents. The agreement between the measured and calculated BDEs is excellent with a MAD of 2.9 ± 1.7 kJ/mol when the SRSC ECP is used to describe Ag(+) and less satisfactory for DZVP-DFT, which underestimates the strength of binding in these systems by ~14% or 26.0 ± 6.7 kJ/mol.

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.

Infrared spectrum of the Ag(+)-(pyridine)2 ionic complex: probing interactions in artificial metal-mediated base pairing

The isolated pyridine-Ag(+)-pyridine unit (Py-Ag(+)-Py) is employed as a model system to characterize the recently observed Ag(+)-mediated base pairing in DNA oligonucleotides at the molecular level. The structure and infrared (IR) spectrum of the Ag(+)-Py(2) cationic complex are investigated in the gas phase by IR multiple-photon dissociation (IRMPD) spectroscopy and quantum chemical calculations to determine the preferred metal-ion binding site and other salient properties of the potential-energy surface. The IRMPD spectrum has been obtained in the 840-1720 cm(-1) fingerprint region by coupling the IR free electron laser at the Centre Laser Infrarouge d'Orsay (CLIO) with a Fourier-transform ion cyclotron resonance (FT-ICR) mass spectrometer equipped with an electrospray ionization source. The spectroscopic results are interpreted with quantum chemical calculations conducted at the B3LYP/aug-cc-pVDZ level. The analysis of the IRMPD spectrum is consistent with a σ complex, in which the Ag(+) ion binds to the nitrogen lone pairs of the two Py ligands in a linear configuration. The binding motif of Py-Ag(+)-Py in the gas phase is the same as that observed in Ag(+)-mediated base pairing in solution. Ag(+) bonding to the π-electron system of the aromatic ring is predicted to be a substantially less-favorable binding motif.

Vibrational spectroscopy of intermediates in benzene-to-pheno conversion by FeO+

Gas-phase FeO(+) can convert benzene to phenol under thermal conditions. Two key intermediates of this reaction are the [HO-Fe-C(6)H(5)](+) insertion intermediate and Fe(+)(C(6)H(5)OH) exit channel complex. These intermediates are selectively formed by reaction of laser ablated Fe(+) with specific organic precursors and are cooled in a supersonic expansion. Vibrational spectra of the sextet and quartet states of the intermediates in the O-H stretching region are measured by infrared multiphoton dissociation (IRMPD). For Fe(+)(C(6)H(5)OH), the O-H stretch is observed at 3598 cm(-1). Photodissociation primarily produces Fe(+) + C(6)H(5)OH; Fe(+)(C(6)H(4)) + H(2)O is also observed. IRMPD of [HO-Fe-C(6)H(5)](+) mainly produces FeOH(+) + C(6)H(5) and the O-H stretch spectrum consists of a peak at approximately 3700 cm(-1) with a shoulder at approximately 3670 cm(-1). Analysis of the experimental results is aided by comparison with hybrid density functional theory computed frequencies. Also, an improved potential energy surface for the FeO(+) + C(6)H(6) reaction is developed based on CBS-QB3 calculations for the reactants, intermediates, transition states, and products.

Interaction of the cesium cation with mono-, di-, and tricarboxylic acids in the gas phase. A Cs+ affinity scale for cesium carboxylates ion pairs

Humic substances (HS), including humic and fulvic acids, play a significant role in the fate of metals in soils. The interaction of metal cations with HS occurs predominantly through the ionized (anionic) acidic functions. In the context of the effect of HS on transport of radioactive cesium isotopes in soils, a study of the interaction between the cesium cation and model carboxylic acids was undertaken. Structure and energetics of the adducts formed between Cs+ and cesium carboxylate salts [Cs+RCOO-] were studied by the kinetic method and density functional theory (DFT). Clusters generated by electrospray ionization mass spectrometry from mixtures of a cesium salt (nitrate, iodide, trifluoroacetate) and carboxylic acids were quantitatively studied by CID. By combining the results of the kinetic method and the energetic data from DFT calculations, a scale of cesium cation affinity, CsCA, was built for 33 cesium carboxylates representing the first scale of cation affinity of molecular salts. The structural effects on the CsCA values are discussed.

Organometallic Polymers Assembled from Cation–π Interactions: Use of Ferrocene as a Ditopic Linker Within the Homologous Series [{(Me3Si)2NM}2⋅(Cp2Fe)]∞ (M=Na, K, Rb, Cs; Cp=cyclopentadienyl)

Addition of ferrocene to solutions of alkali metal hexamethyldisilazides M(HMDS) in arenes (in which M=Na, K, Rb, Cs) allows the subsequent crystallization of the homologous series of compounds [{(Me(3)Si)(2)NM}(2) (Cp(2)Fe)](infinity) (1-4). Similar reactions using LiHMDS led to the recrystallization of the starting materials. The crystal structures of 1-4 reveal the formation of one-dimensional chains composed of dimeric [{M(HMDS)}(2)] aggregates, which are bridged through neutral ferrocene molecules by eta(5)-cation-pi interactions. In addition, compounds 3 and 4 also contain interchain agostic M--C interactions, producing two-dimensional 4(4)-nets. Whereas 1 and 2 were prepared from toluene, the syntheses of 3 and 4 required the use of tert-butylbenzene as the reaction media. The attempted crystallization of 3 and 4 from toluene resulted in formation of the mixed toluene/ferrocene solvated complexes [{(Me(3)Si)(2)NM)(2)}(2) (Cp(2)Fe)(x)(Tol)(y)](infinity) (in which M=Rb, x=0.6, y=0.8, 5; M=Cs, x=0.5, y=1, 6). The extended solid-state structures of 5 and 6 are closely related to the 4(4)-sheets 3 and 4, but are now assembled from a combination of cation-pi, agostic, and pi-pi interactions. The charge-separated complex [K{(C(6)H(6))(2)Cr}(1.5)(Mes)][Mg(HMDS)(3)] (15) was also structurally characterized and found to adopt an anionic two-dimensional 6(3)-network through doubly eta(3)-coordinated bis(benzene)chromium molecules. DFT calculations at the B3 LYP/6-31G* level of theory indicate that the binding energies of both ferrocene and toluene to the M(HMDS) dimers increases in the sequence Li<Na<K. This pattern is a consequence of the larger metals allowing more open coordination spheres to support cation-pi contacts. By comparison, binding of the isolated metal cations to the aromatic groups follow the reverse order K<Na<Li. A combined analysis of theoretical and experimental data suggest that ferrocene is a stronger cation-pi donor than toluene for the lighter metals, but that this difference is eliminated on descending the group.

Cation−π Interactions and Oxidative Effects on Cu+ and Cu2+ Binding to Phe, Tyr, Trp, and His Amino Acids in the Gas Phase. Insights from First-Principles Calculations

The coordination properties of the four natural aromatic amino acids (AA(arom) = Phe, Tyr, Trp, and His) to Cu+ and Cu2+ have been exhaustively studied by means of ab initio calculations. For Cu+-Phe, Cu+-Tyr and Cu+-Trp, the two charge solvated tridentate N/O/ring and bidentate N/ring structures, with the metal cation interacting with the pi system of the ring, were found to be the lowest ones, relative DeltaG(298K) energies being less than 0.5 kcal/mol. The Cu+-His ground-state structure has the metal cation interacting with the NH2 group and the imidazole N. For these low-lying structures vibrational features are also discussed. Unlike Cu+ complexes, the ground-state structure of Cu2+-Phe, Cu2+-Tyr, and Cu2+-Trp does not present cation-pi interactions due to the oxidation of the aromatic ring induced by the metal cation. The ground-state structure of Cu2+-His does not present oxidation of the amino acid, the coordination to Cu2+ being tridentate with the oxygen of the carbonyl group, the nitrogen of the amine, and the N of the imidazole. Other less stable isomers, however, show oxidation of His, particularly of the imidazole ring, which can induce spontaneous proton-transfer reactions from the NH of the imidazole to the NH2 of the backbone. Finally, the computed binding energies for Cu+-AA(arom) and Cu2+-AA(arom) systems have been computed, the order found for the single charged systems being Cu+-His > Cu+-Trp > Cu+-Tyr > Cu+-Phe, in very good agreement with the experimental data.

Competition between cation-π interactions and intermolecular hydrogen bonds in alkali metal ion-phenol clusters. II. Phenol trimer

The competition between ion-molecule and molecule-molecule interactions was investigated in M+(phenol)3 cluster ions for M=Li, Na, K, and Cs. Infrared predissociation spectroscopy in the O-H stretch region was used to characterize the structure of the cluster ions. By adjusting the experimental conditions, it was possible to generate species where argon was additionally bound in order to investigate cold cluster ions. From a comparison of the M+(phenol)3 spectra with the M+(phenol)3Ar spectra, it is clear that the relative populations of hydrogen-bonded configurations are significantly higher in the colder (argon-bearing) species. For the cold species, the IR spectra were compared with minimum energy ab initio calculations to elucidate the hydrogen-bonded structures. The experimental spectra are most consistent with a cyclic hydrogen-bonded configuration for Cs+(phenol)3 in which the ion binds to the phenol molecules via cation-pi interactions, and noncyclic configurations for Li+, Na+, and K+.

Bonding energetics in clusters formed by cesium salts: a study by collision-induced dissociation and density functional theory

In relation to the interaction between (137)Cs and soil organic matter, electrospray mass spectrometry experiments and density functional theory (DFT) calculations were carried out on the dissociation of positively charged adducts formed by cesium nitrate and cesium organic salts attached to a cesium cation [Cs(CsNO(3))(CsA)](+) (A = benzoate, salicylate, hydrogen phthalate, hydrogen maleate, hydrogen fumarate, hydrogen oxalate, and hydrogen malonate ion). These mixed clusters were generated by electrospray from methanol solutions containing cesium nitrate and an organic acid. Collision-induced dissociation of [Cs(CsNO(3))(CsA)](+) in a quadrupole ion trap gave [Cs(CsNO(3))](+) and [Cs(CsA)](+) as major product ions. Loss of HNO(3) was observed, and also CO(2) loss in the case of A = hydrogen malonate. Branching ratios for the dissociation into [Cs(CsNO(3))](+) and [Cs(CsA)](+) were treated by the Cooks' kinetic method to obtain a quantitative order of bonding energetics (enthalpies and Gibbs free energies) between Cs(+) and the molecular salt (ion pair) CsA, and were correlated with the corresponding values calculated using DFT. The kinetic method leads to relative scales of Cs(+) affinities and basicities that are consistent with the DFT-calculated values. This study brings new data on the strong interaction between the cesium cation and molecular salts CsA.

Cation−π Interactions with a Model for the Side Chain of Tryptophan: Structures and Absolute Binding Energies of Alkali Metal Cation−Indole Complexes†

Threshold collision-induced dissociation techniques are employed to determine bond dissociation energies (BDEs) of mono- and bis-complexes of alkali metal cations, Li+, Na+, K+, Rb+, and Cs+, with indole, C8H7N. The primary and lowest energy dissociation pathway in all cases is endothermic loss of an intact indole ligand. Sequential loss of a second indole ligand is observed at elevated energies for the bis-complexes. Density functional theory calculations at the B3LYP/6-31G level of theory are used to determine the structures, vibrational frequencies, and rotational constants of these complexes. Theoretical BDEs are determined from single point energy calculations at the MP2(full)/6-311+G(2d,2p) level using the B3LYP/6-31G* geometries. The agreement between theory and experiment is very good for all complexes except Li+ (C8H7N), where theory underestimates the strength of the binding. The trends in the BDEs of these alkali metal cation-indole complexes are compared with the analogous benzene and naphthalene complexes to examine the influence of the extended pi network and heteroatom on the strength of the cation-pi interaction. The Na+ and K+ binding affinities of benzene, phenol, and indole are also compared to those of the aromatic amino acids, phenylalanine, tyrosine, and tryptophan to elucidate the factors that contribute to the binding in complexes to the aromatic amino acids. The nature of the binding and trends in the BDEs of cation-pi complexes between alkali metal cations and benzene, phenol, and indole are examined to help understand nature's preference for engaging tryptophan over phenylalanine and tyrosine in cation-pi interactions in biological systems.

Competition between cation-π interactions and intermolecular hydrogen bonds in alkali metal ion-phenol clusters. I. Phenol dimer

The competition between ion-molecule and molecule-molecule interactions was investigated in M+(phenol)2 cluster ions for M=Li, Na, K, and Cs. Infrared predissociation spectroscopy in the O-H stretch region was used to characterize the structure of the cluster ions. By adjusting the experimental conditions, it was possible to generate species where argon was additionally bound in order to investigate cold cluster ions. The spectra showed the presence of hydrogen bonding in the colder M+(phenol)2Ar cluster ions but the absence of hydrogen bonding in the warmer M+(phenol)2 species. For the cold species, the IR spectra were compared with minimum-energy ab initio calculations to elucidate the hydrogen-bonded structures. In the dominant hydrogen-bonded configurations observed experimentally, the phenol molecules form hydrogen-bonded dimers and the alkali-metal ions bind to the phenol via a cation-pi interaction with the aromatic ring. Increasing the strength of the cation-pi interaction by decreasing the ion size forces the distance between the phenol O-H groups to increase, thus weakening the intermolecular hydrogen bond. Free-energy differences of different configurations relative to the ground state demonstrate that hydrogen-bonded structures are enthalpically favored, while non-hydrogen-bonded structures are entropically favored and are thus observed in the warm cluster ions.