Editorial

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Special Issue: Intramolecular Hydrogen Bonding

by Ronald K. Castellano

Molecules 2014, 19(10), 15783-15785; https://doi.org/10.3390/molecules191015783 - 29 Sep 2014

Cited by 16 | Viewed by 4961

Intramolecular hydrogen bonds play critical structure- and function-serving roles in biological and synthetic molecular systems. This special issue, through eight contributions, showcases the prominence of these non-covalent interactions within several scientific disciplines, and in various structural contexts and environments. Reported, for example, are [...] Read more.

Intramolecular hydrogen bonds play critical structure- and function-serving roles in biological and synthetic molecular systems. This special issue, through eight contributions, showcases the prominence of these non-covalent interactions within several scientific disciplines, and in various structural contexts and environments. Reported, for example, are the consequences of intramolecular hydrogen bonds on the structures of molecules that show biological activity, for biological mechanisms, and for the conformational switching of functional synthetic molecules. Also showcased in the contributions are the state-of-the-art experimental and theoretical methods available for the characterization of intramolecular hydrogen bonds, which critically report on their strengths, geometries, and spectroscopic signatures in the gas, solid, and solution phases. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

Research

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Solid State Structure and Solution Thermodynamics of Three-Centered Hydrogen Bonds (O∙∙∙H∙∙∙O) Using N-(2-Benzoyl-phenyl) Oxalyl Derivatives as Model Compounds

by Carlos Z. Gómez-Castro, Itzia I. Padilla-Martínez, Efrén V. García-Báez, José L. Castrejón-Flores, Ana L. Peraza-Campos and Francisco J. Martínez-Martínez

Molecules 2014, 19(9), 14446-14460; https://doi.org/10.3390/molecules190914446 - 12 Sep 2014

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Abstract

Intramolecular hydrogen bond (HB) formation was analyzed in the model compounds N-(2-benzoylphenyl)acetamide, N-(2-benzoylphenyl)oxalamate and N¹,N²-bis(2-benzoylphenyl)oxalamide. The formation of three-center hydrogen bonds in oxalyl derivatives was demonstrated in the solid state by the X-ray diffraction analysis of [...] Read more.

Intramolecular hydrogen bond (HB) formation was analyzed in the model compounds N-(2-benzoylphenyl)acetamide, N-(2-benzoylphenyl)oxalamate and N¹,N²-bis(2-benzoylphenyl)oxalamide. The formation of three-center hydrogen bonds in oxalyl derivatives was demonstrated in the solid state by the X-ray diffraction analysis of the geometric parameters associated with the molecular structures. The solvent effect on the chemical shift of H6 [δH6(DMSO-d₆)–δH6(CDCl₃)] and Δδ(ΝΗ)/ΔT measurements, in DMSO-d₆ as solvent, have been used to establish the energetics associated with intramolecular hydrogen bonding. Two center intramolecular HB is not allowed in N-(2-benzoylphenyl)acetamide either in the solid state or in DMSO-d₆ solution because of the unfavorable steric effects of the o-benzoyl group. The estimated ΔHº and ΔSº values for the hydrogen bonding disruption by DMSO-d₆ of 28.3(0.1) kJ·mol⁻¹ and 69.1(0.4) J·mol⁻¹·K⁻¹ for oxalamide, are in agreement with intramolecular three-center hydrogen bonding in solution. In the solid, the benzoyl group contributes to develop 1-D and 2-D crystal networks, through C–H∙∙∙A (A = O, π) and dipolar C=O∙∙∙A (A = CO, π) interactions, in oxalyl derivatives. To the best of our knowledge, this is the first example where three-center hydrogen bond is claimed to overcome steric constraints. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

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On the Nature of the Transition State Characterizing Gated Molecular Encapsulations

by Xiaoyong Lu, Bao-Yu Wang, Shigui Chen and Jovica D. Badjić

Molecules 2014, 19(9), 14292-14303; https://doi.org/10.3390/molecules190914292 - 11 Sep 2014

Cited by 3 | Viewed by 6409

Abstract

Gated molecular encapsulations, with baskets of type 1, are postulated to occur by the mechanism in which solvent molecule penetrates the inner space of 1, through one of its apertures, while the residing guest simultaneously departs the cavity. In the transition [...] Read more.

Gated molecular encapsulations, with baskets of type 1, are postulated to occur by the mechanism in which solvent molecule penetrates the inner space of 1, through one of its apertures, while the residing guest simultaneously departs the cavity. In the transition state of the exchange, three pyridine-based gates are proposed to assume an open position with both incoming solvent and departing guest molecules interacting with the concave surface of the host. The More O’Ferrall-Jencks diagram and linear free energy relationships (LFERs) suggest a more advanced departure of the guest when bigger solvents partake in the displacement. Full article

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Elucidation of the Relationships between H-Bonding Patterns and Excited State Dynamics in Cyclovalone

by Marco Lamperti, Angelo Maspero, Hanne H. Tønnesen, Maria Bondani and Luca Nardo

Molecules 2014, 19(9), 13282-13304; https://doi.org/10.3390/molecules190913282 - 28 Aug 2014

Cited by 14 | Viewed by 6738

Abstract

Cyclovalone is a synthetic curcumin derivative in which the keto-enolic system is replaced by a cyclohexanone ring. This modification of the chemical structure might in principle result in an excited state that is more stable than that of curcumin, which in turn should [...] Read more.

Cyclovalone is a synthetic curcumin derivative in which the keto-enolic system is replaced by a cyclohexanone ring. This modification of the chemical structure might in principle result in an excited state that is more stable than that of curcumin, which in turn should produce an enhanced phototoxicity. Indeed, although curcumin exhibits photosensitized antibacterial activity, this compound is characterized by very fast excited-state dynamics which limit its efficacy as a photosensitizer. In previous works we showed that the main non-radiative decay pathway of keto-enolic curcuminoids is through excited-state transfer of the enolic proton to the keto-oxygen. Another effective deactivation pathway involves an intermolecular charge transfer mechanism occurring at the phenyl rings, made possible by intramolecular H-bonding between the methoxy and the hydroxyl substituent. In this paper we present UV-Vis and IR absorption spectra data with the aim of elucidating the intramolecular charge distribution of this compound and its solvation patterns in different environments, with particular focus on solute-solvent H-bonding features. Moreover, we discuss steady state and time-resolved fluorescence data that aim at characterizing the excited-state dynamics of cyclovalone, and we compare its decay photophysics to that of curcumin. Finally, because during the characterization procedures we found evidence of very fast photodegradation of cyclovalone, its photostability in four organic solvents was studied by HPLC and the corresponding relative degradation rates were calculated. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

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Conjugates of 1'-Aminoferrocene-1-carboxylic Acid and Proline: Synthesis, Conformational Analysis and Biological Evaluation

by Monika Kovačević, Krešimir Molčanov, Kristina Radošević, Višnja Gaurina Srček, Sunčica Roca, Alan Čače and Lidija Barišić

Molecules 2014, 19(8), 12852-12880; https://doi.org/10.3390/molecules190812852 - 21 Aug 2014

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Abstract

Our previous studies showed that alteration of dipeptides Y-Fca-Ala-OMe (III) into Y-Ala-Fca-OMe (IV) (Y = Ac, Boc; Fca = 1'-aminoferrocene-1-carboxylic acid) significantly influenced their conformational space. The novel bioconjugates Y-Fca-Pro-OMe (1, Y = Ac; 2, Y [...] Read more.

Our previous studies showed that alteration of dipeptides Y-Fca-Ala-OMe (III) into Y-Ala-Fca-OMe (IV) (Y = Ac, Boc; Fca = 1'-aminoferrocene-1-carboxylic acid) significantly influenced their conformational space. The novel bioconjugates Y-Fca-Pro-OMe (1, Y = Ac; 2, Y = Boc) and Y-Pro-Fca-OMe (3, Y = Boc; 4, Y = Ac) have been prepared in order to investigate the influence of proline, a well-known turn-inducer, on the conformational properties of small organometallic peptides with an exchanged constituent amino acid sequences. For this purpose, peptides 1–4 were subjected to detailed spectroscopic analysis (IR, NMR, CD spectroscopy) in solution. The conformation of peptide 3 in the solid state was determined. Furthermore, the ability of the prepared conjugates to inhibit the growth of estrogen receptor-responsive MCF-7 mammary carcinoma cells and HeLa cervical carcinoma cells was tested. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

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Roles of Intramolecular and Intermolecular Hydrogen Bonding in a Three-Water-Assisted Mechanism of Succinimide Formation from Aspartic Acid Residues

by Ohgi Takahashi, Ryota Kirikoshi and Noriyoshi Manabe

Molecules 2014, 19(8), 11440-11452; https://doi.org/10.3390/molecules190811440 - 04 Aug 2014

Cited by 28 | Viewed by 9459

Abstract

Aspartic acid (Asp) residues in peptides and proteins are prone to isomerization to the β-form and racemization via a five-membered succinimide intermediate. These nonenzymatic reactions have relevance to aging and age-related diseases. In this paper, we report a three water molecule-assisted, six-step mechanism [...] Read more.

Aspartic acid (Asp) residues in peptides and proteins are prone to isomerization to the β-form and racemization via a five-membered succinimide intermediate. These nonenzymatic reactions have relevance to aging and age-related diseases. In this paper, we report a three water molecule-assisted, six-step mechanism for the formation of succinimide from Asp residues found by density functional theory calculations. The first two steps constitute a stepwise iminolization of the C-terminal amide group. This iminolization involves a quintuple proton transfer along intramolecular and intermolecular hydrogen bonds formed by the C-terminal amide group, the side-chain carboxyl group, and the three water molecules. After a conformational change (which breaks the intramolecular hydrogen bond involving the iminol nitrogen) and a reorganization of water molecules, the iminol nitrogen nucleophilically attacks the carboxyl carbon of the Asp side chain to form a five-membered ring. This cyclization is accompanied by a triple proton transfer involving two water molecules, so that a gem-diol tetrahedral intermediate is formed. The last step is dehydration of the gem-diol group catalyzed by one water molecule, and this is the rate-determining step. The calculated overall activation barrier (26.7 kcal mol⁻¹) agrees well with an experimental activation energy. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

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Redox-Dependent Conformational Switching of Diphenylacetylenes

by Ian M. Jones, Peter C. Knipe, Thoe Michaelos, Sam Thompson and Andrew D. Hamilton

Molecules 2014, 19(8), 11316-11332; https://doi.org/10.3390/molecules190811316 - 31 Jul 2014

Cited by 10 | Viewed by 8867

Abstract

Herein we describe the design and synthesis of a redox-dependent single-molecule switch. Appending a ferrocene unit to a diphenylacetylene scaffold gives a redox-sensitive handle, which undergoes reversible one-electron oxidation, as demonstrated by cyclic voltammetry analysis. ¹H-NMR spectroscopy of the partially oxidized switch [...] Read more.

Herein we describe the design and synthesis of a redox-dependent single-molecule switch. Appending a ferrocene unit to a diphenylacetylene scaffold gives a redox-sensitive handle, which undergoes reversible one-electron oxidation, as demonstrated by cyclic voltammetry analysis. ¹H-NMR spectroscopy of the partially oxidized switch and control compounds suggests that oxidation to the ferrocenium cation induces a change in hydrogen bonding interactions that results in a conformational switch. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

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Intramolecular Hydrogen Bond in Biologically Active o-Carbonyl Hydroquinones

by Maximiliano Martínez-Cifuentes, Boris E. Weiss-López, Leonardo S. Santos and Ramiro Araya-Maturana

Molecules 2014, 19(7), 9354-9368; https://doi.org/10.3390/molecules19079354 - 03 Jul 2014

Cited by 32 | Viewed by 8287

Abstract

Intramolecular hydrogen bonds (IHBs) play a central role in the molecular structure, chemical reactivity and interactions of biologically active molecules. Here, we study the IHBs of seven related o-carbonyl hydroquinones and one structurally-related aromatic lactone, some of which have shown anticancer and [...] Read more.

Intramolecular hydrogen bonds (IHBs) play a central role in the molecular structure, chemical reactivity and interactions of biologically active molecules. Here, we study the IHBs of seven related o-carbonyl hydroquinones and one structurally-related aromatic lactone, some of which have shown anticancer and antioxidant activity. Experimental NMR data were correlated with theoretical calculations at the DFT and ab initio levels. Natural bond orbital (NBO) and molecular electrostatic potential (MEP) calculations were used to study the electronic characteristics of these IHB. As expected, our results show that NBO calculations are better than MEP to describe the strength of the IHBs. NBO energies (∆E_ij⁽²⁾) show that the main contributions to energy stabilization correspond to LPàσ* interactions for IHBs, O₁^…O₂-H₂and the delocalization LPàπ* for O₂-C₂ = C_α(β). For the O₁^…O₂-H₂ interaction, the values of ∆E_ij⁽²⁾can be attributed to the difference in the overlap ability between orbitals i and j (F_ij), instead of the energy difference between them. The large energy for the LP O₂àπ* C₂ = C_α(_β)interaction in the compounds 9-Hydroxy-5-oxo-4,8, 8-trimethyl-l,9(8H)-anthracenecarbolactone (VIII) and 9,10-dihydroxy-4,4-dimethylanthracen-1(4H)-one (VII) (55.49 and 60.70 kcal/mol, respectively) when compared with the remaining molecules (all less than 50 kcal/mol), suggests that the IHBs in VIII and VII are strongly resonance assisted. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

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Review

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Isotope Effects on Chemical Shifts in the Study of Intramolecular Hydrogen Bonds

by Poul Erik Hansen

Molecules 2015, 20(2), 2405-2424; https://doi.org/10.3390/molecules20022405 - 30 Jan 2015

Cited by 25 | Viewed by 7096

Abstract

The paper deals with the use of isotope effects on chemical shifts in characterizing intramolecular hydrogen bonds. Both so-called resonance-assisted (RAHB) and non-RAHB systems are treated. The importance of RAHB will be discussed. Another very important issue is the borderline between “static” and [...] Read more.

The paper deals with the use of isotope effects on chemical shifts in characterizing intramolecular hydrogen bonds. Both so-called resonance-assisted (RAHB) and non-RAHB systems are treated. The importance of RAHB will be discussed. Another very important issue is the borderline between “static” and tautomeric systems. Isotope effects on chemical shifts are particularly useful in such studies. All kinds of intramolecular hydrogen bonded systems will be treated, typical hydrogen bond donors: OH, NH, SH and NH⁺, typical acceptors C=O, C=N, C=S C=N⁻. The paper will be deal with both secondary and primary isotope effects on chemical shifts. These two types of isotope effects monitor the same hydrogen bond, but from different angles. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

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¹H-NMR as a Structural and Analytical Tool of Intra- and Intermolecular Hydrogen Bonds of Phenol-Containing Natural Products and Model Compounds

by Pantelis Charisiadis, Vassiliki G. Kontogianni, Constantinos G. Tsiafoulis, Andreas G. Tzakos, Michael Siskos and Ioannis P. Gerothanassis

Molecules 2014, 19(9), 13643-13682; https://doi.org/10.3390/molecules190913643 - 02 Sep 2014

Cited by 133 | Viewed by 29724

Abstract

Experimental parameters that influence the resolution of ¹H-NMR phenol OH signals are critically evaluated with emphasis on the effects of pH, temperature and nature of the solvents. Extremely sharp peaks (Δν_1/2 ≤ 2 Hz) can be obtained under optimized experimental conditions [...] Read more.

Experimental parameters that influence the resolution of ¹H-NMR phenol OH signals are critically evaluated with emphasis on the effects of pH, temperature and nature of the solvents. Extremely sharp peaks (Δν_1/2 ≤ 2 Hz) can be obtained under optimized experimental conditions which allow the application of ¹H-¹³C HMBC-NMR experiments to reveal long range coupling constants of hydroxyl protons and, thus, to provide unequivocal assignment of the OH signals even in cases of complex polyphenol natural products. Intramolecular and intermolecular hydrogen bonds have a very significant effect on ¹H OH chemical shifts which cover a region from 4.5 up to 19 ppm. Solvent effects on –OH proton chemical shifts, temperature coefficients (Δδ/ΔT), OH diffusion coefficients, and ⁿJ(¹³C, O¹H) coupling constants are evaluated as indicators of hydrogen bonding and solvation state of phenol –OH groups. Accurate ¹H chemical shifts of the OH groups can be calculated using a combination of DFT and discrete solute-solvent hydrogen bond interaction at relatively inexpensive levels of theory, namely, DFT/B3LYP/6-311++G (2d,p). Excellent correlations between experimental ¹H chemical shifts and those calculated at the ab initio level can provide a method of primary interest in order to obtain structural and conformational description of solute-solvent interactions at a molecular level. The use of the high resolution phenol hydroxyl group ¹H-NMR spectral region provides a general method for the analysis of complex plant extracts without the need for the isolation of the individual components. Full article

(This article belongs to the Special Issue Intramolecular Hydrogen Bonding)

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