Extra Water Molecules Research Articles

Synthesis, crystal structure and photophysical properties of a dinuclear MnII complex with 6-(di-ethyl-amino)-4-phenyl-2-(pyridin-2-yl)quinoline.

A new quinoline derivative, namely, 6-(di-ethyl-amino)-4-phenyl-2-(pyridin-2-yl)quinoline, C24H23N3 (QP), and its MnII complex aqua-1κO-di-μ-chlorido-1:2κ4 Cl:Cl-di-chlorido-1κCl,2κCl-bis-[6-(di-ethyl-amino)-4-phenyl-2-(pyridin-2-yl)quinoline]-1κ2 N 1,N 2;2κ2 N 1,N 2-dimanganese(II), [Mn2Cl4(C24H23N3)2(H2O)] (MnQP), were synthesized. Their compositions have been determined with ESI-MS, IR, and 1H NMR spectroscopy. The crystal-structure determination of MnQP revealed a dinuclear complex with a central four-membered Mn2Cl2 ring. Both MnII atoms bind to an additional Cl atom and to two N atoms of the QP ligand. One MnII atom expands its coordination sphere with an extra water mol-ecule, resulting in a distorted octa-hedral shape. The second MnII atom shows a distorted trigonal-bipyramidal shape. The UV-vis absorption and emission spectra of the examined compounds were studied. Furthermore, when investigating the aggregation-induced emission (AIE) properties, it was found that the fluorescent color changes from blue to green and eventually becomes yellow as the fraction of water in the THF/water mixture increases from 0% to 99%. In particular, these color and intensity changes are most pronounced at a water fraction of 60%. The crystal structure contains disordered solvent mol-ecules, which could not be modeled. The SQUEEZE procedure [Spek (2015 ▸). Acta Cryst. C71, 9-18] was used to obtain information on the type and qu-antity of solvent mol-ecules, which resulted in 44 electrons in a void volume of 274 Å3, corresponding to approximately 1.7 mol-ecules of ethanol in the unit cell. These ethanol mol-ecules are not considered in the given chemical formula and other crystal data.

Heterotropic roles of divalent cations in the establishment of allostery and affinity maturation of integrin αXβ2.

SUMMARYAllosteric activation and silencing of leukocyte β2-integrins transpire through cation-dependent structural changes, which mediate integrin biosynthesis and recycling, and are essential to designing leukocyte-specific drugs. Stepwise addition of Mg2+ reveals two mutually coupled events for the αXβ2 ligand-binding domain—the αX I-domain—corresponding to allostery establishment and affinity maturation. Electrostatic alterations in the Mg2+-binding site establish long-range couplings, leading to both pH− and Mg2+-occupancy-dependent biphasic stability change in the αX I-domain fold. The ligand-binding sensorgrams show composite affinity events for the αX I-domain accounting for the multiplicity of the αX I-domain conformational states existing in the solution. On cell surfaces, increasing Mg2+ concentration enhanced adhesiveness of αXβ2. This work highlights how intrinsically flexible pH− and cation-sensitive architecture endows a unique dynamic continuum to the αI-domain structure on the intact integrin, thereby revealing the importance of allostery establishment and affinity maturation in both extracellular and intracellular integrin events.

Unraveling Flow Separation at the Water–Carbon Nanotube Interface: An Atomic-Scale Overview by Molecular Dynamics Simulation

Flow separation near the fluid-solid surface has attracted attention for decades. It is critical to understand the behavior of separated flow adjacent to the solid walls to broaden its range of potential applications. Therefore, we conducted molecular dynamics investigations to consider water flow separation at the water-carbon nanotube (CNT) interface for different diameters of CNTs between 13 and 50 Å and different pressures of 0.1-1.254 GPa. Density heat maps indicated that water flow separation is observed for all CNTs under high pressures, and an empty space of water molecules or evacuation is formed behind the CNTs. It is shown that in CNTs with small diameters, (10, 10) and (20, 20), the structure of the first layer (FL) of water molecules or hydrated layer adjacent to the CNT wall is completely preserved, indicating that evacuation occurs from behind the CNTs. In (30, 30) and (40, 40) CNTs, flow separation occurred from the FL of water molecules near the solid surface, and the layered structure of water around CNTs is completely destroyed. Our findings of fluid-solid and fluid-fluid interaction energies suggested that the flow separation can be due to an attraction between the FL of water molecules and CNT and a repulsion between the water molecules in the hydrated layer and the outer layers. Moreover, analyzing the relationship between the CNT size and flow separation revealed that in the case of small CNTs, there are extra water molecules that contribute to the structural stability of the hydrated layer by strengthening the repulsive interaction in the liquid-liquid surface.

Tailoring the Growth of Nanosized α-Zirconium Phosphate.

Nanosized α-zirconium phosphate (α-ZrP), unlike microcrystalline α-ZrP, takes less time to prepare and is synthesized directly in organic solvents. During the synthesis of nanosized α-ZrP, the heating step is critical because during which the particle growth/aggregation is ongoing. In order to explore more details of this step, extra water molecules were intentionally introduced to the ethanol gel of nanosized α-ZrP so that the evaporation of the solvent was slowed. To heat the gels at 65 °C for different periods (1-6 days), one could control with ease the synthesized α-ZrP in size, from 63 to 155 nm, and in morphology, from amorphous to highly crystalline. Furthermore, the heating step also affects the intercalated solvent and the dangling propionate groups on the edge of the nanosized α-ZrP, which could be deduced from the intercalation/exfoliation behavior of the nanocrystals. This modified synthesis method of nanosized α-ZrP offers an alternate means to tune the size and morphology of the nanosized α-ZrP for broad applications.

Structural properties and stability of the Betaine-Urea natural deep eutectic solvent

This work focuses on the stability and supramolecular structure of the betaine-urea-water (B:U:W) natural deep eutectic solvent. Solutions spanning a range of molar ratios of betaine, urea, and water were prepared, varying the temperature and preparation times, and were analyzed by attenuated total reflection Fourier-transform infrared spectroscopy and Nuclear Magnetic Resonance. Density Functional Theory and the Natural Bond Orbital analysis were employed to obtain the most stable conformations for each mixture. The experimental results show that, in non-anhydrous conditions, betaine:urea (1:1), a minimum of two moles of water are needed to form a metastable transparent liquid, and a minimum of three moles of water is required to have a stable NADES. Comparison of the 13C NMR spectra of B:U:W 1:1:2 and 1:1:3 shows for the latter that the carbonyl groups of betaine and urea form stronger hydrogen bonds with water, and that the CH3 group of betaine becomes more deprotected by the addition of the extra water molecule, making 1:1:3 a more stable solution. Our experimental and computational results show that water is of crucial importance to the NADES supramolecular structure and stability. A better understanding of the structural characteristics of NADES can lead to better envisage applications for these green solvents.

The origins of binding specificity of a lanthanide ion binding peptide

Lanthanide ions (Ln3+) show similar physicochemical properties in aqueous solutions, wherein they exist as + 3 cations and exhibit ionic radii differences of less than 0.26 Å. A flexible linear peptide lanthanide binding tag (LBT), which recognizes a series of 15 Ln3+, shows an interesting characteristic in binding specificity, i.e., binding affinity biphasically changes with an increase in the atomic number, and shows a greater than 60-fold affinity difference between the highest and lowest values. Herein, by combining experimental and computational investigations, we gain deep insight into the reaction mechanism underlying the specificity of LBT3, an LBT mutant, toward Ln3+. Our results clearly show that LBT3-Ln3+ binding can be divided into three, and the large affinity difference is based on the ability of Ln3+ in a complex to be directly coordinated with a water molecule. When the LBT3 recognizes a Ln3+ with a larger ionic radius (La3+ to Sm3+), a water molecule can interact with Ln3+ directly. This extra water molecule infiltrates the complex and induces dissociation of the Asn5 sidechain (one of the coordinates) from Ln3+, resulting in a destabilizing complex and low affinity. Conversely, with recognition of smaller Ln3+ (Sm3+ to Yb3+), the LBT3 completely surrounds the ions and constructs a stable high affinity complex. Moreover, when the LBT3 recognizes the smallest Ln3+, namely Lu3+, although it completely surrounds Lu3+, an entropically unfavorable phenomenon specifically occurs, resulting in lower affinity than that of Yb3+. Our findings will be useful for the design of molecules that enable the distinction of sub-angstrom size differences.

Formation of Formamide from HCN + H2O: A Computational Study on the Roles of a Second H2O as a Catalyst, as a Spectator, and as a Reactant.

Formamide (NH2CHO), being the smallest and fundamental building block of life (with a peptide linkage), has recently been able to attract much interests, in the field of astrochemistry, astrophysics, and astrobiology. In this work, using quantum mechanical computations, reactions between HCN and H2O, leading to the formation of formamide, have been analyzed. For the first time, an alternative and competing reaction channel, which proceeds via a geminal diol intermediate, for the formation of formamide, has been proposed. In this alternative channel, an extra water molecule (second H2O) was found to be acting as a reactant, in the second step of the reaction path. Effects of second H2O molecule in the reaction paths, providing catalytic assistance to the reaction or behaving like a spectator (concept is introduced for the first time for this reaction), have also been analyzed. Usefulness of spectator behavior is highlighted for the reactions happening on the rigid water-ice surfaces, where the water-ice may not be getting involved for any catalytic assistance. In light of catalytic assistances provided by the second H2O, prominent effects in reducing the barrier heights drastically (even for the second step of the reaction, the barrier height was found to be below the reactants), through a hydrogen relay transport mechanism, were observed. In addition to the mechanism studies, interstellar feasibilities of all the reaction channels and their significances are discussed in detail.

Defect-driven water migration in MgCl2 tetra- and hexahydrates

This molecular dynamics simulations study elucidates how water diffusion in MgCl2·nH2O (n=4 and 6) is facilitated by defects and dopants. Both the impact of a single vacancy (one water molecule was removed) and an interstitial (one water molecule was added) on long range water motion has been investigated. Spontaneous vacancy-interstitial pair defect formation was not observed, which is in line with the predicted high energy costs for defect formation: 150 and 200 kJ/mol for the tetrahydrate and hexahydrate, respectively. The vacancy defects did not show long range mobility, which we relate to the strong bonding of water in the coordination shell of the Mg-ion that prevents water molecules from shifting to neighbouring magnesium ions. On the contrary, the addition of an extra water molecule, facilitates long range motion, which was found as a sequence of hopping events. The interstitial motion is anisotropic in both tetrahydrate and hexahydrate crystals, as interstitials preferentially reside at locations of unfavourable Cl-Cl interactions.Strikingly, Mg ↔ Ca substitutional defects neither increase the mobility of vacancies nor facilitates interstitial motion in the MgCl2 lattice. While Ca dopants slightly facilitate vacancy formation, it also stabilizes interstitial water molecules by incorporating these molecules in its coordination shell. As a consequence, the interstitial becomes trapped and loses its mobility. Therefore, Ca dopants will not increase the hydration/dehydration kinetics of MgCl2 hydrates and cannot be used to boost the power output of MgCl2-based heat storage devices.

Nucleophilic Attack by OH2 or OH–: A Detailed Investigation on pH-Dependent Performance of a Ru Catalyst

The considerable rate enhancements along with the increase in pH values may be due to the direct involvement of hydroxide anion in attacking electrophilic [RuV(tda)(py)2O]+ (1; tda = [2,2′:6′,2″-terpyridine]-6,6″-dicarboxylate, py = pyridine). The enhanced reaction rate is well in agreement with the descending activation barriers in our calculation. The addition of four extra water molecules in the geometry optimization plays a key role in stabilizing hydroxide anion as well as building a reasonable hydrogen-bonding network, and three of these molecules are required to stabilize the OH as an anion instead of a radical.

Mechanical properties of C-S-H globules and interfaces by molecular dynamics simulation

At meso-scale, Calcium Silicate Hydrate (C-S-H) can be considered as randomly packed globules (about 4.2 nm), which forms the basic unit cell, with water molecules and voids. In this paper, the nanostructures for the globules are developed based on some plausible atomic structures of C-S-H. The mechanical properties for the C-S-H globules are determined through molecular dynamics simulation. Interfaces between the C-S-H globules are also simulated with different amount of water molecules. Key material parameters, e.g., Young’s modulus, strength and fracture energy, are obtained. It has been found that longer mean chain length of silicate tends to increase the strength of C-S-H and change the fracture behavior from brittle to ductile failure, in the chain length direction. In the other direction, however, silicate chains do not play an important role while interlayer structure matters. Moreover, pores in the C-S-H nanostructures can considerably reduce the strength of the globule structures in the normal direction to silicate chain but the weakening effect becomes substantially less in silicate chain direction. Further, it has been found that for all types of the interfaces between C-S-H globules, the interface with no extra water molecules has the greatest tensile/shear strength. The mechanical properties obtained in this paper for C-S-H nanostructures and interfaces could be necessary inputs to the meso-scale modelling of C-S-H via either granular mechanics, i.e., DEM, or continuum mechanics, i.e., FEM.

Theoretical insight into the role of urea in the hydrolysis reaction of NO2 as a source of HONO and aerosols

Environmental contextUrea is an important component of dissolved organic nitrogen in rainfall and aerosols, but the sources and the mechanisms of its production are not well understood. This computational study explores the effects of urea and water on the hydrolysis of NO2 and urea nitrate production. The results will aid our interpretation of the role of urea in the formation of atmospheric secondary nitrogen contaminants and aerosols. AbstractThe effects of urea on the hydrolysis reaction 2NO2 + mH2O (m = 1–3) have been investigated by theoretical calculations. The energy barrier (−2.67 kcal mol−1) of the urea-promoted reaction is lower than the naked reaction by 14.37 kcal mol−1. Urea also has a better catalytic effect on the reaction than methylamine and ammonia. Urea acts as a catalyst and proton transfer medium in this process, and the produced HONO may serve as a source of atmospheric nitrous acid. In addition, the subsequent reactions include clusters of nitrite, urea, and nitric acid. Then urea nitrate (UN), which is a typical HNO3 aerosol, can be formed in the subsequent reactions. The production of the acid-base complex (UN-2) is more favourable with an energy barrier of 0.10 kcal mol−1, which is 3.88 kcal mol−1 lower than that of the zwitterions NH2CONH3+NO3− (UN-1). The formation of zwitterions and the hydrolysis reaction are affected by humidity. The multi water-promoted hydrolysis reactions exhibit better thermodynamic stability when the humidity is increased. The extra water molecules act as solvent molecules to reduce the energy barrier. The natural bond orbital (NBO) analysis is employed to describe the donor-acceptor interactions of the complexes. The hydrogen bond interaction between the urea carbonyl and nitric acid of UN-2 is the strongest. The potential distribution maps of the urea nitrate and hydrate are examined, and the result shows that they tend to form zwitterions.

Quantum Refinement Does Not Support Dinuclear Copper Sites in Crystal Structures of Particulate Methane Monooxygenase.

Particulate methane monooxygenase (pMMO) is one of the few enzymes that can activate methane. The metal content of this enzyme has been highly controversial, with suggestions of a dinuclear Fe site or mono-, di-, or trinuclear Cu sites. Crystal structures have shown a mono- or dinuclear Cu site, but the resolution was low and the geometry of the dinuclear site unusual. We have employed quantum refinement (crystallographic refinement enhanced with quantum-mechanical calculations) to improve the structure of the active site. We compared a number of different mono- and dinuclear geometries, in some cases enhanced with more protein ligands or one or two water molecules, to determine which structure fits two sets of crystallographic raw data best. In all cases, the best results were obtained with mononuclear Cu sites, occasionally with an extra water molecule. Thus, we conclude that there is no crystallographic support for a dinuclear Cu site in pMMO.

Quantum Refinement Does Not Support Dinuclear Copper Sites in Crystal Structures of Particulate Methane Monooxygenase

AbstractParticulate methane monooxygenase (pMMO) is one of the few enzymes that can activate methane. The metal content of this enzyme has been highly controversial, with suggestions of a dinuclear Fe site or mono‐, di‐, or trinuclear Cu sites. Crystal structures have shown a mono‐ or dinuclear Cu site, but the resolution was low and the geometry of the dinuclear site unusual. We have employed quantum refinement (crystallographic refinement enhanced with quantum‐mechanical calculations) to improve the structure of the active site. We compared a number of different mono‐ and dinuclear geometries, in some cases enhanced with more protein ligands or one or two water molecules, to determine which structure fits two sets of crystallographic raw data best. In all cases, the best results were obtained with mononuclear Cu sites, occasionally with an extra water molecule. Thus, we conclude that there is no crystallographic support for a dinuclear Cu site in pMMO.

Possible interstellar formation of glycine from the reaction of CH2=NH, CO and H2O: catalysis by extra water molecules through the hydrogen relay transport.

"How the fundamental life elements are created in the interstellar medium (ISM)?" is one of the intriguing questions related to the genesis of life. Using computational calculations, we have discussed the reaction of CH2=NH, CO and H2O for the formation of glycine, the simplest life element. This reaction proceeds through a concerted mechanism with reasonably large barriers for the cases with one and two water molecules as reactants. For the two water case we found that the extra water molecule exhibits some catalytic role through the hydrogen transport relay effect and the barrier height is reduced substantially compared to the case with one water molecule. These two cases can be treated as ideal cases for the hot-core formation of the interstellar glycine. With an increasing number of water molecules as the reactants, we found that when the numbers of water molecules are three or more than three, the barrier height reduced so drastically that the transition states were more stable than the reactants. Such a situation gives a clear indication that with excess water molecules as the reactants, this reaction will be feasible even under the low temperature conditions existing in the cold interstellar clouds and the exothermic nature of the reaction will be the driving force.

Interaction of OH- with xylan and its hydrated complexes: structures and molecular dynamics study using elongation method.

The interaction of OH(-) group with (xylan)12 and its hydrated complexes were theoretically studied using elongation optimization (ELG-OPT) method and elongation ab initio molecular dynamics simulation (ELG-MD) method. OH(-) group could abstract a H-atom from the terminal xylan ring to form a complex (xylan)12(-)-H2O without any energy barrier. One and two extra water molecules were also added to the same terminal xylan ring. All the geometry optimization results obtained using elongation method were compared with conventional calculation results, and it suggested that ELG-OPT method worked well for (xylan)12, (xylan)12-OH(-), and its hydrated complexes. Moreover 10 ps ab initio molecular dynamics simulations were performed for complexes (xylan)12(-)-H2O, (xylan)12(-)-2H2O, and (xylan)12(-)-3H2O at 300 K, 500 K, and 700 K. (xylan)12(-)-H2O complex was stable at room temperature. However H2O molecule which was formed by OH(-) group could move at 500 K. At 700 K the H-abstract reaction reversed. Adding an extra water molecule only accelerated the water transfer reaction, but no more chemical reactions occurred, and the transfer time decreased when the temperature increased. The complex (xylan)12(-)-H2O became very stable when adding two extra water molecules even at high temperature, and it indicated that two extra water molecules stabilized the complex (xylan)12(-)-H2O.

Encapsulated discrete octameric water cluster, 1D water tape, and 3D water aggregate network in diverse MOFs based on bisimidazolium ligands

Four new metal–organic frameworks, [Zn(2-mBIM)2(SO3CF3)2·(H2O)4] (1), [Zn(BMIE)(1,4-BDC)]·(H2O)3 (2), [Cd(BIM)2(OH)(H2O)2(PF6)]·(H2O)4 (3), and [Cd(PA-BIM)2 (ClO4)2]·11.33H2O (4) (2-mBIM=bis(2-methylimidazol-1-yl)methane, BMIE=1,2-bis[1-(2-methylimidazole)-diethoxy]ethane, BIM=bis(imidazol-1-yl)methane, and PA-BIM=1,1-bis [(2-phenylazo)imidazol-1-yl]methane) have been prepared and structurally characterized. Complex 1 exhibits an infinite 1D cationic beaded-chain structure, which encapsulated discrete octameric water clusters that are comprised of a chair-like hexameric water cluster with two extra water molecules dangling on two diagonal vertices of the chair. Complex 2 forms a 1D infinite zigzag metal–organic chain structure with a 1D T4(0)A(4) water tape. Complexes 3 show a 2D grid-like sheet structure with the 1D water tape T4(0)A(0)2(0) motif. Complex 4 is a porous 3D MOF with tetrahedron-coordinated Cd(II) centers and trans-conformation PA-BIM ligands. These holes are occupied by a fascinating three-dimensional water clathrate network, which consists of cage-shaped structural tetradecameric water cluster (H2O)14 units and six independent bridged water molecules. The results suggest that the bisimidazolium ligands and anions play crucial roles in the formation of the different host structures and different guest water aggregations. Additionally, the thermal stabilities and photoluminescence spectra of the complexes have been discussed.

A Water-Promoted Mechanism of Alcohol Oxidation on a Au(111) Surface: Understanding the Catalytic Behavior of Bulk Gold

To understand the catalytic mechanism of alcohol oxidation with molecular oxygen on bulk metallic gold catalysts, we have systematically studied the oxidative dehydrogenation of methanol on Au(111) using density functional theory. It is found that molecular oxygen can be activated via a hydroperoxyl (OOH) intermediate produced by abstracting a hydrogen atom from co-adsorbed methanol or water. Interestingly, extra water molecules significantly promote the hydrogen-transfer reactions between CH3OH center dot center dot center dot O-2 and H2O center dot center dot center dot O-2 co-adsorbates, lowering the activation barrier of OOH formation from similar to 0.90 to similar to 0.45 eV. The formed OOH intermediate either directly reacts with methanol to produce formaldehyde or dissociates into adsorbed atomic oxygen and hydroxyl. Further calculations demonstrate that the oxidative dehydrogenation of methanol by OOH, atomic oxygen, and hydroxyl is extremely facile with low barriers between 0.06 and 0.30 eV. These results provide an explanation for the activation mechanism of molecular oxygen on bulk gold and reveal a possible pathway for alcohol oxidation with dioxygen.

The Stability of Bisulfite and Sulfonate Ions in Aqueous Solution Characterized by Hydration Structure and Dynamics

The aqueous solutions of bisulfite (SO(3)H(-)) and sulfonate (HSO(3)(-)) were simulated by the ab initio quantum mechanical charge field molecular dynamics (QMCF MD) formalism. All superimposed trajectories for the atomic coordinates of solutes with three-dimensional alignment here illustrated the reactivities of the ions. Power spectra were evaluated on the basis of the velocity autocorrelation functions (VACFs) with the normal-mode analysis, presenting a higher frequency of the symmetric SO(3) deformation (δ(s)(SO(3))) than the asymmetric SO(3) deformation (δ(as)(SO(3))) modes for the sulfonate ion. The different influence of solvent on the frequency of the O-H and S-H stretching suggests a higher stability of hydrated sulfonate ion. The bisulfite shows a slightly stronger molecular hydration shell than the sulfonate ion with the average number of ion-solvent hydrogen bonds (H-bonds) of 5.3 and 5.0, respectively. Extra water molecules within the molecular hydration shell are found for bisulfite (1.2) and for sulfonate (1.6). The mean residence times for the water ligands classify each ion as a structure maker, while the S-H bond within the sulfonate ion displays a hydrophobic behavior. No tautomerization was observed within the simulation period.

Protein Kinase A (PKA) Phosphorylation of Na+/K+-ATPase Opens Intracellular C-terminal Water Pathway Leading to Third Na+-binding site in Molecular Dynamics Simulations

Phosphorylation is one of the major mechanisms for posttranscriptional modification of proteins. The addition of a compact, negatively charged moiety to a protein can significantly change its function and localization by affecting its structure and interaction network. We have used all-atom Molecular Dynamics simulations to investigate the structural consequences of phosphorylating the Na(+)/K(+)-ATPase (NKA) residue Ser(936), which is the best characterized phosphorylation site in NKA, targeted in vivo by protein kinase A (PKA). The Molecular Dynamics simulations suggest that Ser(936) phosphorylation opens a C-terminal hydrated pathway leading to Asp(926), a transmembrane residue proposed to form part of the third sodium ion-binding site. Simulations of a S936E mutant form, for which only subtle effects are observed when expressed in Xenopus oocytes and studied with electrophysiology, does not mimic the effects of Ser(936) phosphorylation. The results establish a structural association of Ser(936) with the C terminus of NKA and indicate that phosphorylation of Ser(936) can modulate pumping activity by changing the accessibility to the ion-binding site.

Modifications on the hydrogen bond network by mutations of Escherichia coli copper efflux oxidase affect the process of proton transfer to dioxygen leading to alterations of enzymatic activities

CueO has a branched hydrogen bond network leading from the exterior of the protein molecule to the trinuclear copper center. This network transports protons in the four-electron reduction of dioxygen. We replaced the acidic Glu506 and Asp507 residues with the charged and uncharged amino acid residues. Peculiar changes in the enzyme activity of the mutants relative to the native enzyme indicate that an acidic amino acid residue at position 506 is essential for effective proton transport. The Ala mutation resulted in the formation of a compensatory hydrogen bond network with one or two extra water molecules. On the other hand, the Ile mutation resulted in the complete shutdown of the hydrogen bond network leading to loss of enzymatic activities of CueO. In contrast, the hydrogen bond network without the proton transport function was constructed by the Gln mutation. These results exerted on the hydrogen bond network in CueO are discussed in comparison with proton transfers in cytochrome oxidase.