Hydrated proton hydrogen bonding

Like other ions in aqueous solution, both hydronium and hydroxide ions are hydrated. Moreover, hydrogen bonds are involved in attracting water molecules to hydronium and hydroxide ions. In both cases, three water molecules appear to be rather rigidly held, yielding formulas H30(H20)3 (or H90 ) and OH (H20)3 (or H7C>4). However, for convenience, the proton is usually discussed as though it occurred in the form of H+. Hydroxide ions, OH, also occur as hydrated ions, but like H+, they are written as though they were not hydrated. The ionization of water is thus written as... 

N-Protonation and -Alkylation, Hydration via Hydrogen Bonds from Water... 

The quest for additional conformational information has led to the investigation of hydroxyl protons in aqueous solution. Samples are dissolved in mixed methanol-water or acetone-water solvents, and analysed in capillary NMR tubes at low (-5 to -15°C) temperatures. Chemical exchange of hydroxyl protons is reduced to the point that it is possible to use them as probes of hydration and hydrogen bonding. Distance information can also be extracted from NOESY or ROESY spectra under these conditions. 

The hydration reaction has been extensively studied because it is the mechanistic prototype for many reactions at carbonyl centers that involve more complex molecules. For acetaldehyde, the half-life of the exchange reaction is on the order of one minute under neutral conditions but is considerably faster in acidic or basic media. The second-order rate constant for acid-catalyzed hydration of acetaldehyde is on the order of 500 M s . Acid catalysis involves either protonation or hydrogen bonding at the carbonyl oxygen. 

Indeed, because most hydrogen atoms in liquid water are hydrogen-bonded to a neighboring water molecule, this protonic hydration is an instantaneous process and the ion products of water are and OH ... 

Since hydration of biomolecules is of particular importance in molecular biology, uracil - water (U-W) complexes have been studied by many groups [98 JCS(F) 1277, 98JST307, 99JPC(A)1611, 00PCCP1281]. In the cyclic U-W complex the most stable hydrogen bond is formed at the site characterized by the lowest proton... 

Water is a special liquid that forms unique bonds involving protons between the oxygen atoms of neighboring molecules, the so-called hydrogen bond. The solvation forces are then due not simply to molecular size effects, but also and most importantly to the directional nature of the bond. They can be attractive or hydrophobic (hydration forces between two hydrophobic surfaces) and repulsive or hydrophilic (between two hydrophilic surfaces). These forces arise from the disruption or modification of the hydrogen-bonding network of water by the surfaces. These forces are also found to decay exponentially with distance [6]. 

McMahon and Kebarle (1986) studied (MeF)2H as a model for (HF)2H . They thought this to be reasonable because the hydrogen bond of a proton bound to two methanols or dimethyl ethers, e.g. [MejO - H OMe2], gives cations with very similar energies to that of the hydrated oxonium ion [H2O H OHj] (Grimsrud and Kebarle, 1973 Meot-Ner, 1984). 

Variations of R with A suggest a two-step hydration process solvation and formation of disconnected water clusters centered on polar head groups, followed by the formation of a continuous hydrogen-bond network. At low A, Ri depends logarithmically on co, suggesting bidimensional diffusion of protons in the interfacial region between polymer and water. 

Structural diffusion is favored by conditions that enhance the stiffness of the hydrogen-bonded network between water molecules low temperatures and low acid concentration. The decrease in water content leads to an effective increase in the concentration of acid protons, which in turn suppresses the contribution of structural diffusion, as found in aqueous acidic solutions. This agrees with the finding of an enhanced contribution of vehicular transport in PEMs at low hydration. Such an observation is also supported by recent studies of molecular mechanisms of proton transport in PEMs at minimal hydration. ... 

The crystal structure of the cobalt-substituted enzyme was obtained with bicarbonate bound to the metal (Iverson et al. 2000). The structure shows Asn 202 and Gln75 hydrogen bonded to the metal-bound bicarbonate, suggestzing potential roles for these residues in either transition-state stabilization or orientation and polarization of CO2 for attack from the zinc-hydroxyl (Fig. 11.5). The crystal structure also shows three discrete conformations for Glu 84, suggesting a role for this residue in the transfer of protons out of the active site indeed, kinetic analyses of Glu 84 variants combined with chemical rescue experiments establish this residue as critical for proton transfer (Tripp and Ferry 2000). The location of Glu 62 adjacent to Glu 84 suggests a potential role in proton transfer as well. Although kinetic analyses of site-specific variants establish an essential role for Glu 62 in the CO2 hydration steps (Eqs. 11.3 and 11.4), the results were inconclusive regarding an additional role in proton transfer (Eqs. 11.5 and 11.6). 

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