Water Oxygen Atoms Research Articles

Catalytic Highly Regioselective C-H Oxygenation Using Water as the Oxygen Source: Preparation of 17O/18O-Isotope-Labeled Compounds.

We found that the oxygen atom of water is activated to iodosylbenzene derivatives via reversible hydrolysis of PhI(OOCR)2 and can be used to the oxygen source for ruthenium(bpga)-catalyzed site-selective C-H oxygenation. Ru(bpga)/PhI(OOCR)2/H2O system, sterically less bulky methinic and methylenic C-H bonds in various compounds can be converted to desired oxygen functional groups in a site-selective manner. Using this method, oxygen-isotope labeled compounds such as d-[3-17O/18O]-mannose can be prepared in a multigram scale.

Crystal structure of bis-[cis-di-aqua-bis-(phen-an-thro-line)cobalt(II)] bis-(citrato)germanate(IV) dinitrate.

The asymmetric unit of the title compound, [Co(C12H8N2)2(H2O)2]2[Ge(C6H5O7)2](NO3)2, features two complex [(C12H8N2)2(H2O)2Co]2+ cations, two NO3 - anions as well as one centrosymmetric [(C6H5O7)2Ge]2- anion. Two HCit ligands (Cit = citrate, C6H4O7) each coordinate via three different oxygen atoms (hy-droxy-late, α-carboxyl-ate, β-carboxyl-ate) to the Ge atom, forming a slightly distorted octa-hedron. The coordination polyhedron of the Co atom is also octa-hedral, formed by coordination of four nitro-gen atoms from two phenanthroline mol-ecules and two water oxygen atoms. In the crystal, the cations and anions are linked by hydrogen bonds and form layers parallel to the bc plane. The structure exhibits disorder of the NO3 - anion [disorder ratio 0.688 (9) to 0.312 (9)]. There are also highly disordered solvent mol-ecules (presumably water and/or ethanol) in the crystal structure; explicit refinement of these mol-ecules was not possible, and the content of the voids was instead taken into account using reverse Fourier transform methods [SQUEEZE procedure in PLATON; Spek (2015 ▸). Acta Cryst. C71, 9-18]. The given chemical formula and other crystal data do not take into account the unknown solvent mol-ecule(s).

Force Field Benchmark of Amino Acids. 3. Hydration with Scaled Lennard-Jones Interactions.

Classical protein force fields were reported with too weak protein-water interactions relative to protein-protein interactions, leading to more compact structures and artificial protein aggregation. Here we investigated the impacts of scaled Lennard-Jones (LJ) interactions on the hydration of amino acids and the simulation of folded and intrinsically disordered proteins (IDPs). The obtained optimal scaling parameters reproduce accurately hydration free energies of neutral amino acid side chain analogues and do not affect the compactness and structural stability of folded proteins significantly. The scaling leads to less compact IDPs and varies from case to case. Strengthening the interactions between protein and water oxygen or hydrogen atoms by increasing the interacting LJ well depth (ε) appears more effective than weakening protein-protein interactions by reducing the interacting dispersion coefficients (C6). We demonstrate that weakening water-water interactions is a solution as well to obtaining more favorable protein-water interactions in an indirect way, although modern force fields like Amber ff19SB and a99SB-disp tend to use water models with strong water-water interactions. This is likely a compromise between strong protein-protein interactions and strong water-water interactions. Independent optimization of protein force fields and water models is therefore needed to make both interactions more close to reality, leading to good accuracy without bias or scaling.

Calcium Chelidonate: Semi-Synthesis, Crystallography, and Osteoinductive Activity In Vitro and In Vivo.

Calcium chelidonate [Ca(ChA)(H2O)3]n was obtained by semi-synthesis using natural chelidonic acid. The structure of the molecular complex was determined by X-ray diffraction analysis. The asymmetric unit of [Ca(ChA)(H2O)3]n includes chelidonic acid coordinated through three oxygen atoms, and three water ligands. The oxygen atoms of acid and oxygen atoms of water from each asymmetric unit are also coordinated to the calcium of another one, forming an infinite linear complex. Calcium geometry is close to the trigonal dodecahedron (D2d). The intra-complex hydrogen bonds additionally stabilize the linear species, which are parallel to the axis. In turn the linear species are packed into the 3D structure through mutual intercomplex hydrogen bonds. The osteogenic activity of the semi-synthetic CaChA was studied in vitro on 21-day hAMMSC culture and in vivo in mice using ectopic (subcutaneous) implantation of CaP-coated Ti plates saturated in vitro with syngeneic bone marrow. The enhanced extracellular matrix ECM mineralization in vitro and ectopic bone tissue formation in situ occurred while a water solution of calcium chelidonate at a dose of 10 mg/kg was used. The test substance promotes human adipose-derived multipotent mesenchymal stromal/stem cells (hAMMSCs), as well as mouse MSCs to differentiate into osteoblasts in vitro and in vivo, respectively. Calcium chelidonate is non-toxic and can stimulate osteoinductive processes.

Bio-inspired construction of ion conductive pathway in covalent organic framework membranes for efficient lithium extraction

Summary As the most sophisticated separation system, biological membranes have served as natural prototypes for the design of artificial membranes because they provide high permeability and solute selectivity, owing to the presence of specialized ion channels. However, developing stable and selective artificial ion channels remains a formidable challenge. Herein, we demonstrate the construction of lithium nanochannels in two-dimensional covalent organic frameworks (COFs) to create biomimetic membranes. Implanted lithiophilic oligoethers conferred specificity and facilitated Li+ diffusion along the pore pathway of the COF. The ion channel characteristics were indicated by reversal potential measurements, showing that the relative permeability decreased in the order Li+ > K+ > Na+ > Ca2+ > Mg2+. The Li+ transfer was enhanced, while other ions were obstructed, allowing for high selectivity and permeability. A Li+/Mg2+ separation factor of 64 was achieved, confirming high lithium affinity. This study may serve as a design principle to develop selective artificial membranes for effective ion separation.

Origin of n type properties in single wall carbon nanotube films with anionic surfactants investigated by experimental and theoretical analyses

We investigated the origin of n-type thermoelectric properties in single-wall carbon nanotube (SWCNT) films with anionic surfactants via experimental analyses and first-principles calculations. Several types of anionic surfactants were employed to fabricate SWCNT films via drop-casting, followed by heat treatment at various temperatures. In particular, SWCNT films with sodium dodecylbenzene sulfonate (SDBS) surfactant heated to 350 °C exhibited a longer retention period, wherein the n-type Seebeck coefficient lasted for a maximum of 35 days. In x-ray photoelectron spectroscopy, SWCNT films with SDBS surfactant exhibited a larger amount of sodium than oxygen on the SWCNT surface. The electronic band structure and density of states of SWCNTs with oxygen atoms, oxygen molecules, water molecules, sulfur atoms, and sodium atoms were analyzed using first-principles calculations. The calculations showed that sodium atoms and oxygen molecules moved the Fermi level closer to the conduction and valence bands, respectively. The water molecules, oxygen, and sulfur atoms did not affect the Fermi level. Therefore, SWCNT films exhibited n-type thermoelectric properties when the interaction between the sodium atoms and the SWCNTs was larger than that between the oxygen molecules and the SWCNTs.

Complex Weblike Hydrogen Bonding in Large “Drain Pipe” Channels of Wightmanite Revealed by New X-Ray and Spectroscopic Measurements

ABSTRACT The wallpaper-type crystal structure of wightmanite, Mg5(BO3)O(OH)5·1–2H2O, has been reanalyzed in order to better understand the position and bonding of hydrogen atoms. Single-crystal structure refinement yielded the monoclinic I2/m unit cell a = 13.5165(18), b = 3.0981(3), c = 18.170(3)Å, ß = 91.441(6)°, and V = 760.65(17)Å3, Z = 4. Hydrogen atoms of OH groups pointing to the inside of the elliptical channels oriented parallel to [010] are arranged in the form of two overlying, a–c parallel planar pentagons. The two pentagons point in opposite directions. Hydrogen-bond analysis shows that the hydroxyl groups are linked by complex polyfurcated, intra-molecular hydrogen bonds forming a web-like network coating the walls of the channels. The longest distance between hydrogens (7.226 Å) is observed in the pentagonal planes of the channel. The anisotropically refined oxygen atoms of the zeolitic water show their strongest vibration parallel to the b axis and in the direction of the largest diameter of the elliptical channel and similarly form a complex inter-molecular hydrogen-bond system to the hydroxyl groups coating the channel walls. This complex bonding is expressed in the Raman spectrum by a broad band between 3100 and 3300 cm–1 that is assigned to the OH / H2O stretching mode and one strong band at 3661 cm–1 attributable to an OH-stretching mode. Infrared spectra also show a pronounced broad band between 3200 and 3700 cm–1 attributed to H2O and OH-stretching modes. The weak bands around 1600 cm–1 observed in the Raman and IR spectra are probably due to relatively weakly bound water in the channels.

Computational Study of Helical and Helix-Hinge-Helix Conformations of an Anti-Microbial Peptide in Solution by Molecular Dynamics and Vibrational Analysis.

Many classical antimicrobial peptides adopt an amphipathic helical structure at a water-membrane interface. Prior studies led to the hypothesis that a hinge near the middle of a helical peptide plays an important role in facilitating peptide-membrane interactions. Here, dynamics and vibrations of a designed hybrid antimicrobial peptide LM7-2 in solution were simulated to investigate its hinge formation. Molecular dynamics simulation results on the basis of the CHARMM36 force field showed that the α-helix LM7-2 bent around two or three residues near the middle of the peptide, stayed in a helix-hinge-helix conformation for a short period of time, and then returned to a helical conformation. High-resolution computational vibrational techniques were applied on the LM7-2 system when it has α-helical and helix-hinge-helix conformations to understand how this structural change affects its inherent vibrations. These studies concentrated on the calculation of frequencies that correspond to backbone amide bands I, II, and III: vibrational modes that are sensitive to changes in the secondary structure of peptides and proteins. To that end, Fourier transforms were applied to thermal fluctuations in C-N-H angles, C-N bond lengths, and C═O bond lengths of each amide group. In addition, instantaneous all-atom normal mode analysis was applied to monitor and detect the characteristic amide bands of each amide group within LM7-2 during the MD simulation. Computational vibrational results indicate that shapes and frequencies of amide bands II and especially III were altered only for amide groups near the hinge. These methods provide high-resolution vibrational information that can complement spectroscopic vibrational studies. They assist in interpreting spectra of similar systems and suggest a marker for the presence of the helix-hinge-helix motif. Moreover, radial distribution functions indicated an increase in the probability of hydrogen bonding between water and a hydrogen atom connected to nitrogen (HN) in such a hinge. The probability of intramolecular hydrogen bond formation between HN and an amide group oxygen atom within LM7-2 was lower around the hinge. No correlation has been found between the presence of a hinge and hydrogen bonds between amide group oxygen atoms and the hydrogen atoms of water molecules. This result suggests a mechanism for hinge formation wherein hydrogen bonds to oxygen atoms of water replace intramolecular hydrogen bonds as the peptide backbone folds.

Microsolvation of myrtenal studied by microwave spectroscopy highlights the role of quasi-hydrogen bonds in the stabilization of its hydrates.

Hydrates of myrtenal (C10H14O) · (H2O)n (n = 1, 2, 3) were experimentally investigated in a molecular jet using a cavity-based Fourier transform microwave spectrometer in the 2.6 GHz-15 GHz frequency range. The assignment of the spectra was made possible, thanks to computationally optimized structures at the B3LYP-D3BJ/def2-TZVP and MP2/6-311++G(d,p) levels using the Gaussian 16 software. The spectra of two mono- and two dihydrates and those of the lowest energy conformer among the two expected trihydrates could be assigned. A similar study replacing normal water by 18O labeled water allowed the identification of the spectra of all possible isotopomers, leading to the calculation of the substitution coordinates of water oxygen atoms and that of the effective structure of the water molecule arrangements around myrtenal, except for the trihydrate. The structure of the latter species was nevertheless confirmed by the analysis of the spectrum of the isotopomer with three H2 18O molecules. The computational rotational constants and structural parameters were found quite close to the experimental ones at the density functional theory B3LYP-GD3BJ/def2-TZVP and ab initio MP2/6-311++G(d,p) levels. Symmetry adapted perturbation theory calculations reveal that the aldehyde hydrogen atom strongly interacts with water oxygen atoms in the case of di- and trihydrates.

Di-aqua-bis-{μ-1,5-bis-[(pyridin-2-yl)methyl-idene]carbonohydrazide(1-)}di-μ-chlorido-tetra-chlorido-tetra-zinc(II).

A tetra-nuclear ZnII complex, [Zn4(C13H11N6O)2Cl6(H2O)2] or {[Zn2(HL)(H2O)(Cl2)](μCl)2[Zn2(HL)(H2O)(Cl)]}2, was synthesized by mixing an equimolar amount of a methanol solution containing ZnCl2 and a methanol solution containing the ligand H2 L [1,5-bis-(pyridin-2-yl-methyl-ene)carbono-hydrazide]. In the tetra-nuclear complex, each of the two ligand mol-ecules forms a dinuclear unit that is connected to another dinuclear unit by two bridging chloride anions. In each dinuclear unit, one ZnII cation is penta-coordinated in a N2OCl2 in a distorted square-pyramidal geometry, while the other ZnII cation is hexa-coordinated in a N3OCl2 environment with a distorted octa-hedral geometry. The basal plane around the penta-coordinated ZnII cation is formed by one chloride anion, one oxygen atom, one imino nitro-gen atom and one pyridine nitro-gen atom with the apical position occupied by a chloride anion. The basal plane of the hexa-coordinated ZnII cation is formed by one chloride anion, one hydrazinyl nitro-gen atom, one imino nitro-gen atom and one pyridine nitro-gen atom with the apical positions occupied by a water oxygen atom and a bridged chloro anion from another dinuclear unit, leading to a tetra-nuclear complex. A series of intra-molecular C-H⋯Cl hydrogen bonds is observed in each tetra-nuclear unit. In the crystal, the tetra-nuclear units are connected by inter-molecular C-H⋯Cl, C-H⋯O and N-H⋯O hydrogen bonds, forming a planar two-dimensional structure in the ac plane.

Gas-Phase Hydration of Perillaldehyde Investigated by Microwave Spectroscopy Assisted by Computational Chemistry.

The microsolvated complexes of two equatorial conformers of perillaldehyde were experimentally investigated in a supersonic molecular jet coupled to a cavity-based Fourier transform microwave spectrometer, in the 2.3-8 GHz frequency range. The structures of hydrates C10H14O·(H2O)n (n = 1,2,3) were first optimized at the MP2/6-311++G(d,p) and B3LYP-D3BJ/def2-TZVP levels of theory. The spectral signatures of four monohydrates and of two dihydrates could then be obtained. Additional rotational constants from the analysis of the spectra of their 18O isotopologues allowed the calculation of the substitution coordinates of the water oxygen atoms of each hydrate. They were found to be in good agreement with those of the optimized structures. SAPT2 calculations and noncovalent interaction analysis highlight the role of dispersion and quasi-hydrogen bonds in the stabilization of the structures.

Bifunctional N-Doped Tungsten Trioxide Microspheres as Electrode Materials for Lithium-Ion Batteries and Direct Methanol Fuel Cells

Oxygen vacancies (OVs) have emerged as an efficient strategy to modulate the electronic structures, conductivity, and electrochemical properties of WO3. However, OVs are easily absorbed by water molecules or oxygen atoms, resulting in the poor stability of its electrocatalytic reaction. In the present work, we have demonstrated that nitrogen doping provides an alternative to improve the electrochemical properties of WO3, which plays a series of roles in reducing the band gap, improving the electron mobility, accelerating the Li+ ion diffusion, and providing more active sites for catalytic reactions. Consequently, N-doped WO3 microspheres exhibit low discharge–charge voltage gaps and high capacity retention as well as exceptional rate performance when used as anode materials for lithium-ion batteries (LIBs). Furthermore, the as-prepared Pt/N-WO3 catalyst also exhibits excellent electrocatalytic properties toward methanol electro-oxidation (MOR) including high activity, strong poison tolerance, and outstanding long-term stability owing to the synergetic effects between Pt and N-WO3.

Coordination Polymer and Monomer with the Cd(NO3)2 Fragment Containing 2-Amino-5-Bromopyridine: Synthesis, Structures, NMR Study, and Luminescence Properties

The reactions of Cd(NO3)2 ⋅ 4H2O with 2-amino-5-bromopyridine (Аbp) afford compounds [Cd(NO3)2(Аbp)(H2O)]n (I) and [Cd(NO3)2(Abp)2(H2O)2] (II). The structures of both complexes are determined by single-crystal X-ray structure analysis (CIF files CCDC nos. 1938624 (I) and 1959680 (II)). Compound I is a coordination 1D polymer in which two chelate-bonded $${\text{NO}}_{3}^{ - }$$ groups act as bridges. The coordination polyhedron of the octacoordinated central cadmium atom in compound I consists of seven oxygen atoms (one oxygen atom of water and six atoms of the $${\text{NO}}_{3}^{ - }$$ groups) and one nitrogen atom of Abp, being a triangular dodecahedron. Complex II is a mononuclear molecule in which the octahedral coordination polyhedron of Cd is formed by four oxygen atoms of two water molecules and two $${\text{NO}}_{3}^{ - }$$ groups and two oxygen atoms of two molecules of the Abp cycle. In both complexes, the amino group of Abp is not involved in coordination with the metal. Compound I is studied by 1H, 31C, and 15N NMR spectroscopy of a solution of the polymer complex in CD3CN, and the most substantial difference in the chemical shifts of the bound and free Abp ligands is observed in the 15N NMR spectra (37 ppm). According to the data of luminescence spectroscopy, compounds I and II exhibit an emission in a range of 430–690 nm.

Synthesis, Spectroscopic Studies and Crystal Structure Determination of a Novel Mn(II) Complex with N,N-1,5-bis(2-acetylpyridinyl)carbonohydrazone Ligand

A new Mn(II) complex prepared by the reaction of 1,5-bis(1-(pyridin-2-yl)ethylidene)carbonohydrazide (H2L) with Mn(II) ion is reported in this paper. The H2L ligand is structurally characterized by elemental analysis, NMR and infrared spectroscopies. The mononuclear complex [Mn(H2L)(Cl2)(H2O)] (1), is characterized by infrared spectroscopy, elemental analysis, conductance, magnetic room temperature measurement and single X-ray diffraction. The compound crystallizes in the triclinic system in the space group P-1 with the unit cell parameters a = 8.5453 (13) A, b = 9.4231 (10) A, c = 12.2273 (17) A, α = 78.168 (10)°, β = 81.361 (13)° and γ = 83.187 (11)°, Z = 2, R1 = 0.084 and wR2 = 0.257 (I > 2σ(I)). H2L acts as a tridentate ligand which coordinated metal atom through one pyridine nitrogen atom, one hydrazone nitrogen atom and one hydrazone oxygen atom. The Mn2+ ion has a distorted octahedral environment with three atoms from the ligand, one water oxygen atom and two chloride atoms. Numerous hydrogen bonds form layers propagating in bc plane and ensure the cohesion of crystalline structure in three-dimensional network.

Molecular rotation-caused autocorrelation behaviors of thermal noise in water

The finite autocorrelation time of thermal noise is crucial to unidirectional transportation on the molecular scale. Therefore, it is important to understand the cause of the intrinsic picosecond autocorrelation time of thermal noise in water. In this work, we use molecular dynamics simulations to compare the autocorrelation behaviors of the thermal noise, hydrogen bonds, and molecular rotations found in water. We found that the intrinsic picosecond autocorrelation time for thermal noise is caused by finite molecular rotation relaxation, in which hydrogen bonds play the role of a bridge. Furthermore, the simulation results show that our method of calculating the autocorrelation of thermal noise, by observing the fluctuating force on an oxygen atom of water, provides additional information about molecular rotations. Our findings may advance the understanding of the anomalous dynamic nanoscale behavior of particles, and the applications of terahertz technology in measuring the structural and dynamical information of molecules in solutions.

Bifunctional Hydrogen Bonding of Imidazole with Water Explored by Rotational Spectroscopy and DFT Calculations

Laser vaporization of imidazole in the presence of an argon buffer gas has allowed the generation and isolation of two isomers of an imidazole monohydrate complex, denoted herein as imid···H2O and H2O···imid, within a gas sample undergoing supersonic expansion. Imidazole and water are respectively proton-accepting and proton-donating in imid···H2O, but these roles are reversed in the H2O···imid complex. Both isomers have been characterized by chirped-pulse Fourier transform microwave spectroscopy between 7.0 and 18.5 GHz. The ground-state rotational spectra of four isotopologues of imid···H2O and three isotopologues of H2O···imid have been measured. All spectra have been assigned and fitted to determine rotational (A0, B0, C0), centrifugal distortion (DJ, DJK), and nuclear quadrupole coupling constants (χaa(N1), [χbb(N1) - χcc(N1)], χaa(N3), and [χbb(N3) - χcc(N3)]). Structural parameters (r0 and rs) have been accurately determined from measured rotational constants for each isomer. The imid···H2O complex contains a nonlinear hydrogen bond (∠(O-Hb···N3) = 172.1(26)° in the experimentally determined, r0 geometry) between the pyridinic nitrogen of imidazole and a hydrogen atom of H2O. The DFT calculations find that the H2O···imid complex also contains a nonlinear hydrogen bond between the oxygen atom of water and the hydrogen attached to the pyrrolic nitrogen of imidazole (∠(O···H1-N1) = 174.7°). Two states observed in the spectrum of H2O···imid, assigned as 0- and 0+ states, confirm that large amplitude motions occur on the time scale of the molecular rotation. Density functional theory has been performed to characterize these large amplitude motions.

Kinetic energy of oxygen atoms in water and in silica hydrogel

The effect of temperature on the mean kinetic energy of Oxygen, Ke(O), in silica hydrogel (constituting of water confined in a silica Xerogel matrix) was computed between 5 and 325 K. The results are higher by up to 40% than those deduced from a recent deep inelastic neutron scattering (DINS) measurement. Note that when the mean kinetic energy of Hydrogen Ke(H) was computed using the same method a good agreement was obtained with the DINS measurement. This seems to indicate that the measured Ke(O) values are strongly underestimated due to the DINS method of analysis of the n-scattering intensities as discussed in the text.

Hirshfeld surface analysis, crystal structure and physicochemical studies of a new Zn(II) complex with the 2,6-dimethylpyrimidin-4-(1H)-one ligand

A new Zn(II) complex with the monodentate ligand 2,6-dimethylpyrimidin-4-(1H)-one, [ZnCl(C6H8N2O)2H2O]NO3·H2O, has been prepared and characterized by single crystal X-ray diffraction, elemental analysis, CP–MAS NMR and IR spectroscopy. The Zn(II) cation is tetra-coordinated, in a distorted tetrahedral fashion, by two nitrogen atoms of two organic ligands, one water oxygen atom and one chlorine atom. In the atomic arrangement, the [ZnCl(C6H8N2O)2H2O]+ neighbor complexes are connected to each other via O–H ….O, O–H⋯Cl and N–H⋯O hydrogen bonds to form a ribbon running along the b-axis direction. The ribbons are interconnected by the nitrate anions via O–H⋯O and N4–H⋯O hydrogen bonds to form layers parallel to (102‾) plane. Intermolecular interactions were investigated by Hirshfeld surfaces. Globally, the structure is maintained primarily by the metal atom coordination as well as strong and weak (O,N,C)–H⋯O hydrogen bonds and C–H … π interactions. Besides a few strong electrostatic interactions (coordination and H-bonds), the nitrate and chloride anions are surrounded by many electropositive CH3 groups. The 13C and 15N CP–MAS NMR spectra are discussed and the vibrational absorption bands were identified by infrared spectroscopy. Electronic properties such as HOMO and LUMO energies were derived.

Gas hydrate formation by allyl alcohol and CH4: Spectroscopic and thermodynamic analysis

We discovered a new structure II (sII) hydrate forming agent, allyl alcohol (AA), in the presence of methane (CH4) for the first time, and characterized the crystal structure, guest distribution, and phase equilibria of the (AA+CH4) hydrate. Using solid-state 13C NMR and Raman spectroscopy, the crystal structure of the (AA+CH4) hydrate was confirmed to be a sII hydrate, and the CH4 molecule was found to be encapsulated in both the large and small cages of the sII hydrate. In addition, AA was found to be included in the large cages of the sII hydrate in the Gauche-Gauche form based on the measured- and calculated-NMR spectra. Notably, we investigated the free OH signal of AA in the Raman spectra to determine whether hydrogen bonding occurred between host and guest molecules; however, we could not determine whether the existence of the free OH signal was consistent with this host-guest interaction. To clearly identify the crystal structure and possible host-guest interactions, a high-resolution powder X-ray diffraction (HRPD) pattern of our (AA+CH4) hydrate sample was analyzed using Rietveld analysis with the direct space method. The crystal structure of the (AA+CH4) hydrate was assigned as the cubic Fd3m structure with a lattice constant of 17.1455 A. In particular, the shortest distance between the AA molecule in the hydrate cages and an oxygen atom in the host water was estimated to be 2.55 A; thus, we concluded that the hydroxyl group of the AA molecule was hydrogen-bonded to the host water framework. Finally, we measured the phase equilibrium conditions of the binary (AA+CH4) hydrate and found that the equilibrium pressure conditions of the binary (AA+CH4) hydrate were slightly higher than those of the pure CH4 hydrate.

Inverted Hydration Layers on Bio-Magnesium Surfaces in the Initial Degradation Stage and their Influence on Adsorption of Amino Acid Analogues: The Metadynamics Simulations.

Deeply exploring the interaction of biomolecules with magnesium in solution is essential to understand the formation of complex bio-magnesium interfaces accompanied with corrosion products. Using the accelerated metadynamics simulations, we have investigated the interactions of amino acid analogues on clean and hydroxylated Mg(0001) surfaces by identifying their free energy barriers and adsorption sites. We find that there are two hydration layers stacked on the clean Mg(0001) surfaces and the hydroxylated Mg(0001) surfaces, which mainly determine the free energy barriers and adsorbed configurations. Further studies reveal that the water molecules in double hydration layers present two opposite orientations, depending on the charge distribution of the substrate. Specifically, oxygen atoms of water concentrate in the center of double hydration layers for a clean Mg surface but transfer to the outside surface once the Mg substrate is degraded. The reversed hydration layers greatly reduce the binding affinities of positively charged and electroneutral analogues. Overall, our simulation findings provide new insights into the interaction mechanism of biomolecules on a bio-magnesium device in the implantation initial stage, which is noteworthy for revealing the magnesium degradation mechanism in vivo.