Cooperative hydrogen bonds of macromolecules. 2. Two-dimensional cooperativity in the binding of poly(4-vinylpyridine) to poly(4-vinylphenol)

The hydrogen bond interaction of poly(4-vinylphenol) (PVF), ligated by a 20 mol/mol excess of pyridine-d(5) (PD) in tetrahydrofuran-d(8), with poly(4-vinylpyridine) (PVP) was studied using liquid and solid-state NMR and quantum mechanical calculations. Because of its cooperative interaction, PVP substitutes PD in its hydrogen bond with PVF, thus forming a PVF-PVP complex, which gradually precipitates from solution. On the basis of the 1H/13C NMR spin-diffusion experiments and density functional theory quantum calculations, the complex is shown to have the fairly regular structure of a polymer sheet with intermittent H-bond links between PVF and PVP chains. The cooperativity of PVP interaction with PVF was studied by measuring the dependence of the binding degree alpha of PVP on its polymerization degree (P(n), being 10, 17, 30, 36, 48, 65, and 84) at various PVP/PVF molar ratios. The value of alpha was established indirectly by measuring the fraction of liberated PD using its 2H quadrupolar relaxation and pulsed field-gradient spin-echo measurement of self-diffusion. The cooperativity is shown to be of a higher order and two-dimensional, that is, dependent on both the polymerization degree of PVP and its ratio to PVF. A mathematical model of such two-dimensional cooperativity based chiefly on a proximity effect is suggested.     

Water's hydrogen bonds in the hydrophobic effect: a simple model

We propose a simple analytical model to account for water's hydrogen bonds in the hydrophobic effect. It is based on computing a mean-field partition function for a water molecule in the first solvation shell around a solute molecule. The model treats the orientational restrictions from hydrogen bonding, and utilizes quantities that can be obtained from bulk water simulations. We illustrate the principles in a 2-dimensional Mercedes-Benz-like model. Our model gives good predictions for the heat capacity of hydrophobic solvation, reproduces the solvation energies and entropies at different temperatures with only one fitting parameter, and accounts for the solute size dependence of the hydrophobic effect. Our model supports the view that water's hydrogen bonding propensity determines the temperature dependence of the hydrophobic effect. It explains the puzzling experimental observation that dissolving a nonpolar solute in hot water has positive entropy.     

Cooperative effect of hydrogen bonds in the complexes of aliphatic alcohols with proton acceptors in chloroform

Effect of solvation on the frequency of stretching vibrations of O-H groups of aliphatic alcohols in complexes with acetonitrile, dimethyl sulfoxide, diethyl ether, pyridine, tetrahydrofuran, and triethylamine in various media was studied by IR spectroscopy. We found that the strength of hydrogen bonds in the studied complexes increased considerably at the interaction with chloroform as a solvent. The contribution of cooperative effect was estimated. The enthalpies of cooperative effect in the complexes consisting of three different molecules are estimated for the first time.     

Interstitial water and the formation of low barrier hydrogen bonds: A computational model study

The B3LYP/D95+(d,p) analysis of the uncharged low barrier hydrogen bond (LBHB) between 4‐methyl‐1H‐imidazole (Mim) and acetic acid (HAc) shows that uncharged LBHBs can be formed either by adding three water molecules around the cluster or by placing the Mim–HAc pair in a dielectric environment created by a polarizable continuum model with a permittivity larger than 20.7. The permittivity of environment around uncharged LBHB can be lowered significantly by including water molecules into the system. A Mim–HAc LBHB stabilized with one water molecule observed in diethyl ether (ε = 4.34), with two water molecules in toluene (ε = 2.38), and with three water molecules in vacuo (ε = 1). Solvation models with different numbers of water molecules predict average differences in the proton affinities of the hydrogen bonded bases (ΔPA) for stable uncharged LBHB systems in vacuo to be 91.5 kcal/mol being different from the ΔPA values close to zero in charge‐assisted LBHB systems. The results clearly indicate that small amounts of interstitial water molecules at the active site of enzymes do not preclude the existence of LBHBs in biological catalysis. Our results also show that interstitial water molecules provide a useful clue in the search for uncharged LBHBs in an enzymatic environment and the number of water molecules can be used as a relative measure for the polarity around the direct environment of LBHBs. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2012     

Cooperative organic hydrogen bonds: the librational modes of cyclic methanol clusters

Intermolecular hydrogen bond libration modes of isolated cyclic methanol trimers (approximately 613 cm(-1)) and tetramers (695 and 760 cm(-1)) are observed in pulsed jet Fourier transform infrared spectra and found to exhibit sizeable anharmonicity and mode coupling effects, opening the way for a microscopic interpretation of the broad librational bands of alcohols. The correlation of experimental OH stretching and OH libration band intensities provides important constraints for theoretical band strengths, cluster densities, and size assignments.     

Theoretical study of some nitriles: Intramolecular hydrogen bonds and anomeric effect

An ab initio study of 3-chloro-, 3-hydroxy-, 3-mercapto-, and 3-amino-propanenitrile and 4-chloro-bu- tanenitrile was carried out at several levels of theory. The calculated stabilities and geometrical trends are interpreted in terms of the effects of intramolecular hydrogen bonds and anomeric interactions, and compared with available experimental data.     

Cooperative interaction of hydrogen bonds in proton exchange between the bases of nucleic acids

The tautomeric equilibrium in the H-bonded pair guanine-cytosine was studied by the semiempirical MNDO/HB quantum-chemical method. It was shown that hydration of the bases leads to an appreciable reduction of the activation barrier in the synchronous exchange of protons between the bases. Desolvation of the active center of the polymerase reduces the probability of error in the synthesis of the complementary chain of the nucleic acid by 10⁶ times. Deprotonation of the phosphate groups of the nucleotides destabilizes the transition state in proton exchange. The genetic information is conserved as a result of the formation of nucleoprotein complexes with an ionic bond between the phosphate residue of the saccharophosphate skeleton of the nucleic acid and an amino (imino) group of the protein.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1828–1832, August, 1991.     

The effect of entropy on protein hydrogen bonds

Hydrogen bonds are essential elements in protein structure and dynamics. We show that such bonds are strongly dominated by entropic contributions of the environment that can be at least as important as energetic considerations. These contributions are global in that they are not due to the internal degrees of freedom. The molecular dynamics (MD) is carried out as a computer experiment and agrees very well with the known energies of hydrogen bonds, both in water and in the isolated molecule. The entropy in water as a solvent is found to modulate the hydrogen bond breaking and reformation rate by two orders of magnitude, in a negative direction.     

Polymer complexes stabilized through hydrogen bonds: A semiquantitative theoretical model

Many studies have been devoted to the formation and behavior of polymer complexes. Unfortunately, the complexity of such systems do not allow a rigorous theoretical approach presently. In this paper we develop a semiquantitative theoretical model for cooperative association between macromolecules. The model is especially adapted for reversible polymer complexes stabilized through weak interactions, e.g., through hydrogen bonds. Theoretical predictions are compared with potentiometric and viscometric experimental results of poly(acrylic acid)/polybase system and a good agreement is established. The influence of parameters such as degree of neutralization of the polyacid, concentration, and molecular weight of polymers is also conveniently predicted.     

Cooperative interactions of hydrogen bonds in proton-transfer processes involving water molecules. Simulation of biochemical systems

Quantum-chemical calculations of molecular complexes simulating the proton channel of influenza A virus and the proton-transfer system of the active site of carboanhydrase enzyme were performed. These complexes comprise a proton-donor and a proton-acceptor groups bridged by a chain of water molecules. Calculations of the methylimidazole (H⁺)-H₂O-CH₃COO^? complex as a model of influenza M₂ virus revealed free translation motion of the water molecule between the donor and acceptor, as well as concerted proton transfer in both H bonds. The barrier for proton transfer is independent of the position of the bridging water molecule and varies linearly with the difference in the electrostatic potentials between the donor and acceptor. With elongation of the H-bond bridge between the donor and acceptor groups, the H-bond lengths and proton shifts in the chain links vary periodically. This process can be defined as an H-bond deformation wave (proton wave). It was shown that motion of one proton along the H bond is associated with vibrational motion of protons in other links, which results in wave propagation along the chain. The calculation results allowed the rate of the proton wave and the time of proton transfer from the donor to acceptor to be estimated.     

An electrostatic model for hydrogen bonds in alcohols

We have measured the Raman spectra of liquid methanol at temperatures between 50° and –77°C. The weak O–H stretching bands appear, under amplification, more and more asymmetric as the temperature is lowered. They can be decomposed into three Gaussian components centered at about 3220, 3310, and 3400 cm^–1. The former, predominant at low temperature, corresponds to single, linear hydrogen bonds (LHB) between two molecules. The other two are assigned to branched hydrogen bonds, respectively bifurcated (BHB), between three molecules, and trifurcated (THB), between four molecules. We conclude that the molecular structure of liquid alcohols is not chain-like, as presumed so far, but a three-dimensional network featuring a mixture of single (LBH), and multiple hydrogen bonds (BHB, and THB). They are mainly electrostatic in nature, their relative proportions and geometry governed by the packing conditions for minimum energy. They form distinct trimers and tetramers in dilute solutions of alcohols in inert solvents and frozen matrices, and the latter even in the vapor.Deceased December 25, 1987.     

Vibrational study of intermolecular hydrogen bonds in dichloroanilines

The stretching infrared bands of the amine groups associated by intermolecular hydrogen bonding in solid polyethylene solution have been identified at 140 cm⁻¹ in 3,4 dichloaniline and at 176 and 215 cm⁻¹ in 2,6 dichloroaniline. That assignment has been carried out using medium- and far-infrared spectroscopy and some theoretical considerations.     

Cation-anion hydrogen bonds: a new class of hydrogen bonds that extends their strength beyond the covalent limit. A theoretical characterization

The existence of O-H···O hydrogen bonds having a strength within the -80 to -210 kcal/mol range, that is, in the range of strength of covalent bonds and well beyond the so-called covalent limit (-50 kcal/mol), is reported on complexes where the O-H proton donor and O acceptor groups are located in ions of opposite sign. A complete analysis of short distance O-H···O hydrogen bonds between charged fragments was performed for cases where the OH and O groups are both located on charged molecules. It shows that these interactions (a) are nonsymmetrical for the O-H and H···O distances, (b) have a noncovalent H···O bond critical point, and (c) have a strong and energetically stable electrostatic component when the OH and O groups are located in oppositely charged molecules. These cation-anion O-H···O interactions are energetically stable, satisfy the usual topology for hydrogen bonds, HBs, and also have the same directionality found in other HBs. Therefore, they should be considered as a new class of HBs, the cation-anion hydrogen bonds.     

Semirubin. A novel dipyrrinone strapped by intramolecular hydrogen bonds

(4Z)-9-(5-Carboxypentyl)-2,3,7,8-tetramethyl-(10H)-dipyrrin- 1-one (1, semirubin), a new dipyrrinone model for one-half of bilirubin, the yellow-orange neurotoxic pigment of jaundice, was synthesized following Friedel-Crafts acylation of 2,3,7, 8-tetramethyl-(10H)-dipyrrin-1-one (5) with the half-ester acid chloride of adipic acid. Unlike other dipyrrinone models for bilirubin, such as the xanthobilirubic acids, which engage only in intermolecular hydrogen bonding, 1 is unique in having been designed and found to engage in intramolecular hydrogen bonding, between the carboxylic acid and the dipyrrinone lactam and pyrrole. This important conformation-determining structural characteristic, shared by 1 and bilirubin, renders them less polar than their methyl esters and leaves them monomeric in nonpolar solvents, where their esters are dimeric. The corresponding 10-oxo analogue (3) of 1 serves as a model for 10-oxo-bilirubin, a presumed bilirubin metabolite in alternate pathways for bilirubin excretion. Like 1, 3 is found to engage in intramolecular hydrogen bonding. Unlike the methyl ester of 1, the ethyl ester of 3 is not intermolecularly hydrogen bonded in nonpolar solvents.     

N-H...S hydrogen bonds in a ferredoxin model

The Fe4S4 complex {(CH3)3NCH2CONH2}2[Fe4S4((tBuS)4] (1) was synthesized to replicate the ferredoxin active site with a subset of its N-H...S hydrogen bonds. The two cationic counterions mimic the polypeptide backbone of ferredoxin (Fd) as amide hydrogen-bond donors to sulfur atoms of the iron-sulfur cluster. X-ray crystallographic data show that the organic sulfur (Sgamma) of one tert-butylthiolate ligand and one inorganic sulfur of the cluster core serve as N-H...S hydrogen-bond acceptors. The cluster core of complex 1 is tetragonally elongated in contrast to that of Fd, which is tetragonally compressed. This is the first observation of an elongated [Fe4S4]2+ cluster core. Additionally, this is the first synthetic Fd model in which N-H...S hydrogen bonding to a cluster has been achieved.     

Calorimetric study of the proximity effect in aliphatic diesters

Excess molar enthalpies,H ^E , at atmospheric pressure and 303.15 K are reported for binary mixtures of 1-chlorohexane with diethyl oxalate, diethyl malonate, diethyl succinate, diethyl glutarate, diethyl adipate, diethyl pimelate and diethyl sebacate. These experimental results, together with those obtained previously on the excess enthalpies of aliphatic diester+n-hexane mixtures, are interpreted in terms of molecular surface interactions between aliphatic, chlorinated and carboxylate groups. Comparison of the enthalpy interchange parameters reveals a decrease of the intermolecular interactions with decreasing intramolecular COO-COO distance in the diester molecule.

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Zusammenfassung Bei Atmosphärendruck und 303,15 K gemessene molare Überschußenthalpien (H ^E werden für binäre Gemische von 1-Chlorhexan mit Diäthyloxalat, Diäthylmalonat, Diäthylsuccinat, Diäthylglutarat, Diäthyladipat, Diäthylpimelat bzw. Diäthylsebacat angegeben. Diese experimentellen Ergebnisse werden zusammen mit den früher für Gemische von aliphatischen Diestern und n-Hexan erhaltenen Resultaten als molekulare Oberflächenwechselwirkungen zwischen aliphatischen, chlorierten und Carboxylatgruppen interpretiert. Ein Vergleich der Enthalpieaustauschparameter läßt eine Verminderung der intermolekularen Wechselwirkungen mit abnehmendem intramolekularem Abstand zwischen den COO-Gruppen erkennen.

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(H ^E ) 303,15 K 1- , , , , , . , + -, , . COO-COO .

     

Multiple hydrogen bonds. Mass spectra of hydrogen bonded heterodimers. A comparison of ESI- and REMPI-ReTOF-MS

Resonance enhanced multi-photon ionization-reflectron time of flight mass spectrometry is the analytical method of choice to observe hydrogen bonded supramolecules in the gas phase when protonation of basic centers competes with cluster formation.     

Tuning the interaction energy of hydrogen bonds: the effect of the substituent

The effect of the substituent R in the hydrogen bonding properties of FH···FR (R = H, Al, Li, Cl and CCH) complexes has been studied by theoretical calculations. The dependency of the interaction energy with the hydrogen bond distance and R is explained in terms of the topologies of the electron density and the electrostatic potential. A simple model of the hydrogen bond interaction energy, which can be assimilated to an interaction potential, is defined in terms of a stabilizing mutual polarization of the monomers and an overall destabilizing contribution associated with the electron density reorganization when the overlap of the closed shells is large enough. This model shows an excellent agreement with the ab initio interaction energies and is common for all the analyzed complexes. The substituent effect is represented in the model by a single parameter that can be calculated from the electron distribution in the acceptor atom region. The perturbation in the hydrogen bonding interaction induced by the change of R presents a close similarity with that produced by an external electric field of the same order of magnitude than those found in crystalline solids, indicating that both perturbations should play a significant and similar role on the properties of hydrogen bonds in condensed matter.