Protonation Of NH3 Research Articles

Constraining ion transport in the diamagnetic cavity of comet 67P

ABSTRACT The European Space Agency Rosetta mission escorted comet 67P for a 2-yr section of its six and a half-year orbit around the Sun. By perihelion in 2015 August, the neutral and plasma data obtained by the spacecraft instruments showed the comet had transitioned to a dynamic object with large-scale plasma structures and a rich ion environment. One such plasma structure is the diamagnetic cavity: a magnetic field-free region formed by interaction between the unmagnetized cometary plasma and the impinging solar wind. Within this region, unexpectedly high ion bulk velocities have been observed, thought to have been accelerated by an ambipolar electric field. We have developed a 1D numerical model of the cometary ionosphere to constrain the impact of various electric field profiles on the ionospheric density profile and ion composition. In the model, we include three ion species: H2O+, H3O+, and $\mathrm{NH_4^+}$. The latter, not previously considered in ionospheric models including acceleration, is produced through the protonation of NH3 and only lost through ion–electron dissociative recombination, and thus particularly sensitive to the time-scale of plasma loss through transport. We also assess the importance of including momentum transfer when assessing ion composition and densities in the presence of an electric field. By comparing simulated electron densities to Rosetta Plasma Consortium data sets, we find that to recreate the plasma densities measured inside the diamagnetic cavity near perihelion, the model requires an electric field proportional to r−1 of around 0.5–2 mV m−1 surface strength, leading to bulk ion speeds at Rosetta of 1.2–3.0 km s−1.

Analytical descriptions of (multiple-contact) cross-polarization dynamics and spin-lattice relaxation in solid alanine

In this work, several exact and approximate analytical solutions to the quantum master equation are derived using both classical and non-classical coupling models to describe the kinetics of Hartmann-Hahn cross-polarization (HHCP) and multiple-contact CP (MCCP). Moreover, the analytical solution originally obtained by Naito and McDowell [J. Chem. Phys. 84 (1986) 4181.] is shown to be incorrect and the different regimes of spin diffusion and T1ρ relaxation are characterized by the amplitude of the second stage of the HHCP dynamics and the HHCP/MCCP crossing time. The analysis of the 1H–13C HHCP and MCCP dynamics together with (Lee-Goldburg) 1H T1ρ relaxation experimental data provides a consistent picture of spin dynamics in solid alanine and explains the apparent discrepancies previously observed between T1ρ and T1 relaxation measurements. The CH and CH3 protons relax as expected via spin diffusion towards the NH3 protons but the assumption of common proton spin temperature, in which the bottleneck of relaxation is at the NH3 sites, generally valid for T1 relaxation breaks down for T1ρ relaxation. A diffusion-limited situation in which nuclear Zeeman energy is transferred to the lattice faster than can be supplied by spin diffusion is observed instead.

Origin and trends in NH4+ observed in the coma of 67P

ABSTRACT The European Space Agency/Rosetta mission escorted comet 67P/Churyumov–Gerasimenko and witnessed the evolution of its coma from low activity (∼2.5–3.8 au) to rich ion-neutral chemistry (∼1.2–2.0 au). We present an analysis of the ion composition in the coma, focusing on the presence of protonated high proton affinity (HPA) species, in particular $\mathrm{{ NH}_{4}}^{+}$. This ion is produced through the protonation of NH3 and is an indicator of the level of ion-neutral chemistry in the coma. We aim to assess the importance of this process compared with other $\mathrm{{ NH}_{4}}^{+}$ sources, such as the dissociation of ammonium salts embedded in dust grains. The analysis of $\mathrm{{ NH}_{4}}^{+}$ has been possible thanks to the high mass resolution of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double Focusing Mass Spectrometer (ROSINA/DFMS). In this work, we examine the $\mathrm{{ NH}_{4}}^{+}$ data set alongside data from the Rosetta Plasma Consortium instruments, and against outputs from our in-house ionospheric model. We show that increased comet outgassing around perihelion yields more detections of $\mathrm{{ NH}_{4}}^{+}$ and other protonated HPA species, which results from more complex ion-neutral chemistry occurring in the coma. We also reveal a link between the low magnetic field strength associated with the diamagnetic cavity and higher $\mathrm{{ NH}_{4}}^{+}$ counts. This suggests that transport inside and outside the diamagnetic cavity is very different, which is consistent with 3D hybrid simulations of the coma: non-radial plasma dynamics outside the diamagnetic cavity is an important factor affecting the ion composition.

PH and temperature dual-sensitive molecular imprint polymers for BSA based on Cu2+ coordination

A novel kind of pH and temperature dual-sensitive Molecular imprint polymers (MIPs) combined with Cu2+ coordination (SiO2@CS/NIPAM-Cu2+-MIP) has been synthesized using glycidyl methacrylate-iminodiacetic acid (GMA-IDA) as a metalchelating ligand, with bovine serum albumin (BSA) as template protein, combined with pNIPAM and chiston (CS) as temperature and pH sensitive monomers, respectively. The coordination effect of GMA-IDA-Cu2+ has been shown to be beneficial to improve the adsorption capacity and adsorption specificity of BSA. The influence of pH not only changed the charge force between the polymer and protein in the imprinting system, but also deformed the imprinting cavity through the protonation of NH3+ on CS. Further, the thermo-sensitivity of the imprinted polymer was also found to be satisfactory. Withthe joint efforts of coordination, electrostatic action and good matching of imprinted sites, higher adsorption capacity (173.48 mg∙g-1) and imprinting factor (2.72) have been obtained at pH 4.6 and 35ºC. Although it took about 4 h to reach saturation adsorption, the cyclability of the SiO2@CS/NIPAM-Cu2+-MIP was found to be acceptable and the adsorptioncapacity was maintained at original 81.16% after six cycles. It is for the first time that GMA-IDA-Cu2+ has been used to prepare the pH and temperature dual-sensitive imprinted polymer for BSA.

Structural characterization, thermal properties, and molecular motions near the phase transition in hybrid perovskite [(CH2)3(NH3)2]CuCl4 crystals: 1H, 13C, and 14N nuclear magnetic resonance

The structural characterization of the [(CH2]3(NH3)2]+ cation in the perovskite [(CH2)3(NH3)2]CuCl4 crystal was performed by solid-state 1H nuclear magnetic resonance (NMR) spectroscopy. The 1H NMR chemical shifts for NH3 changed more significantly with temperature than those for CH2. This change in cationic motion is enhanced at the N-end of the organic cation, which is fixed to the inorganic layer by N–H···Cl hydrogen bonds. The 13C chemical shifts for CH2-1 increase slowly without any anomalous change, while those for CH2-2 move abruptly compared to CH2-1 with increasing temperature. The four peaks of two groups in the 14N NMR spectra, indicating the presence of a ferroelastic multidomain, were reduced to two peaks of one group near TC2 (= 333 K); the 14N NMR data clearly indicated changes in atomic configuration at this temperature. In addition, 1H and 13C spin–lattice have shorter relaxation times (T1ρ), in the order of milliseconds because T1ρ is inversely proportional to the square of the magnetic moment of paramagnetic ions. The T1ρ values for CH2 and NH3 protons were almost independent of temperature, but the CH2 moiety located in the middle of the N–C–C–C–N bond undergoes tumbling motion according to the Bloembergen–Purcell–Pound theory. Ferroelasticity is the main cause for the phase transition near TC2.

Temperature dependence of 1H NMR chemical shifts and diffusivity of confined ethylammonium nitrate ionic liquid

Some ionic liquids (ILs) change their dynamic properties when placed in a confinement between polar surfaces (Filippov et al., Phys. Chem. Chem. Phys. 2018, 20, 6316). The diffusivities of ions and NMR relaxation times in these ILs also reversibly change under a strong static magnetic field. The mechanisms of these phenomena are not clear, but it has been suggested that they involve modified hydrogen-bonding networks formed in these ILs in the presence of polar surfaces. To obtain a better understanding of these effects, we performed temperature-dependent measurements of chemical shifts and diffusion coefficients for ethylammonium nitrate (EAN) IL in the bulk phase (IB) and confined in layers with a thickness of ~4 μm between quartz plates unexposed (I phase) and exposed (IMF phase) to a static magnetic field of 9.4 T. It was shown that the NMR chemical shift of NH3 protons of EAN in the I phase is strongly shifted upfield, ~0.0145 ppm/K, which is due to weakening of the hydrogen-bonding network of the confined EAN. Exposure to the magnetic field leads to restitution of the hydrogen-bonding (H-bonding network). The temperature dependences of diffusion coefficients follow the order D(I) > D(IB) > D(IMF) and can be described by a Vogel-Fulcher-Tammann approach with variation of the pre-exponential factor, which is determined by the strength of the H-bonding network. Confinement of EAN between plates (IB → I) is an endothermic process, while processes occurring in a magnetic field, I → IMF and IMF → I, are exothermic and endothermic, respectively.

Ammonia for Proton Conducting Medium in Metal-Organic Frameowkrs

Proton transport is essential and critical in a biological system and electrochemical devices. In particular, the fundamental understanding of proton conduction mechanism in conducting medium is important to design the efficient proton conductor. The overwhelming majority of proton conducting medium is water (H2O) which was intensively investigated because water can form the efficient hydrogen bonding networks by symmetric two proton donor and two acceptor sites resulting in degenerated proton conduction system (H3O++H2O ⇔ H2O+H3O+). In this point of view, we believe that ammonia (NH3) can be a next promising conducting medium due to its similar molecular system to water. Specifically, it shows a small molecular size (2.60 Å), three proton donor sites/ one acceptor site, and degenerated proton conduction system (NH4 ++NH3 ⇔ NH3+NH4 +). Considering the fluid phase of NH3, a confined environment should be required to achieve NH3 proton conductor. The porous crystalline metal-organic frameworks (MOFs), tunable solid, are emerging as a new class of solid electrolyte because of a considerable porosity and versatile sorptivity. In particular, the pore of MOFs is a suitable platform to confine the guest molecules. In addition, a simultaneous proton donation from framework to NH3 can induce the conjugated acid-base system in the pores. Here, we show, for the first time, proton conductivity through the simultaneously confined NH3 in MOFs by NH3 adsorption. We choose highly stable Al-based MOFs consisting of one-dimensional channel and MO4(OH)2 secondary building units (SBUs) interconnected by dicarboxylate groups. Fuethermore, we can tune the pore environment by using the different fuctionla groups of ligand. We found cooperative ammonia adsorption with simultaneous ammonium formation and the highly enhanced proton conductivity as NH3 pressure increases. Tthe structure analysis of MOFs incorporating NH3 by Rietveld refinement and dynamic motion study of confined NH3 by 2H solide-state NMR help to understand proton conduction mechanism. These results clearly support the Grotthuss mechanism not seen in bulk NH3 system. We believe that this research may provide a design strategy for material using NH3 as a conducting media as well as an energy carrier.

Controlling bromate formation in the Co(II)/peroxymonosulfate process by ammonia, chlorine-ammonia and ammonia-chlorine pretreatment strategies

The Co(II)/peroxymonosulfate (Co(II)/PMS) process, producing sulfate radicals (SO4•-), effectively removes organic pollutants in water, while producing a significant amount of bromate (BrO3−) in the presence of bromide (Br−). This paper investigates the ammonia (NH3) addition, chlorine-ammonia (Cl2-NH3) and ammonia-chlorine (NH3-Cl2) pretreatment strategies in controlling BrO3− formation in 20 min in the Co(II)/PMS process at pH 4.0. The addition of NH3 retarded the BrO3− formation, but only at a reduction level of about 9.5% for NH3 concentration of 50 μM, and was mainly attributed to the protonation of NH3 at pH 4 (99.99% as NH4+, did not react with HOBr). Both the Cl2-NH3 and NH3-Cl2 pretreatment strategies at HOCl and NH3 dosages of 15 and 50 μM, respectively, reduced 95% or more of the overall BrO3− formation and retarded the BrO3− formation, with the NH3-Cl2 pretreatment strategy outperforming Cl2-NH3. The reduction of the BrO3− formation was mainly attributed to the formation of monochloramine (NH2Cl) in both pretreatment strategies. NH2Cl effectively outcompetes SO4•- to react with HOBr and forms NHBrCl, with the apparent reaction rate constant between NH2Cl and HOBr more than 100 times faster than that between SO4•- and HOBr. However, the oxidation/degradation of NHBrCl in the Co(II)/PMS process reforms HOBr, and, although less in quantity, is oxidized to BrO3− at higher Co(II) and Br− concentrations. Thus, the NH3-Cl2 and Cl2-NH3 pretreatment strategies inhibit the BrO3− formation more significantly at lower Co(II) and Br− concentrations. In all cases, the generation of SO4•- in 20 min was not affected by the implementation of the three BrO3− pretreatment strategies.

Magnetic field effects dynamics of ethylammonium nitrate ionic liquid confined between glass plates.

Self-diffusion and NMR relaxation of the ethylammonium (EA) cation were studied in the protic ionic liquid, ethylammonium nitrate (EAN), confined between polar glass plates separated by a few μm distance and exposed to an external magnetic field of 9.4 T. The diffusion coefficient of EA (D) and the transverse NMR relaxation rate (1/T2) of -NH3 protons were increased immediately after placing the sample in the magnetic field by factors of ∼2 and ∼22, respectively, in comparison with those of bulk EAN. Further exposure of the sample to the magnetic field led to gradual changes in D, T1 and T2 towards their bulk values with a time constant of ∼70 min. Complete "recovery" of the sample to the "accelerated" D and "shortened" T2 values occurred at longer than 24 hours after the removal of the EAN sample from the magnet. Because the observed characteristic times of the change far exceed the times of molecular processes in EAN, we suggested that this phenomenon is related to reversible phase transformations occurring in confined EAN.

Cation synergies affect ammonia adsorption over VOX and (V,W)OX dispersed on α-Al2O3 (0001) and α-Fe2O3 (0001)

The catalytic behavior of oxide-supported metal oxide species depends on the nature of the support and the presence of co-catalysts. We use density functional theory (DFT) to explore the relationship between the structure and chemical behavior of vanadium oxide in light of its industrial use for the selective catalytic reduction of nitric oxide with ammonia (NO-SCR). The relative stabilities of dispersed VOX monomers, dimers, and long-chain oligomers on two model oxide support surfaces with similar structure but drastically different chemical behavior, α-Al2O3 (0001) and α-Fe2O3 (0001), are determined. The effect of added tungsten, known to promote NO-SCR, is also investigated on the relatively inert α-Al2O3 (0001) support. We find that the adsorption behavior of NH3, representing the first step of the NO-SCR reaction, depends strongly on the VOX local structure. Protonation of NH3 to NH4+ over surface hydroxyls is energetically favorable over VOX-WOX dimers and VOX oligomers, which are stabilized by the reducible α-Fe2O3 (0001) support.

Competitive formation of DNA linkage isomers by a trinuclear platinum complex and the influence of pre-association.

2D [(1)H, (15)N] HSQC NMR spectroscopy has been used to monitor the reaction of fully (15)N-labelled [{trans-PtCl(NH3)2}2(μ-trans-Pt(NH3)2{NH2(CH2)6NH2}2)](4+) (BBR3464 ((15)N-1)) with the 14-mer duplex (5'-{d(ATACATG(7)G(8)TACATA)}-3'·5'-{d(TATG(18)TACCATG(25)TAT)}-3' or I) at pH 5.4 and 298 K, to examine the possible formation of 1,4 and 1,5-GG adducts in both 5'-5' and 3'-3' directions. In a previous study, the binding of the dinuclear 1,1/t,t to I showed specific formation of the 5'-5' 1,4 G(8)G(18) cross-link, whereas in this case a mixture of adducts were formed. Initial (1)H NMR spectra suggested the presence of two pre-associated states aligned in both directions along the DNA. The pre-association was studied in the absence of covalent binding, by use of the "non-covalent" analog [{trans-Pt(NH3)3}2(μ-trans-Pt(NH3)2{NH2(CH2)6NH2}2)](6+) (AH44, 0). Chemical shift changes of DNA protons combined with NOE connectivities between CH2 and NH3 protons of 0 and the adenine H2 protons on I show that two different molecules of 0 are bound in the minor groove. Molecular dynamic simulations were performed to study the interaction of 0 at the two pre-association sites using charges derived from density functional theory (DFT) calculations. Structures where the central platinum is located in the minor groove and the aliphatic linkers extend into the major groove, in opposite directions, often represent the lowest energy structures of the snapshots selected. In the reaction of (15)N-1 and I, following the pre-association step, aquation occurs to give the mono aqua monochloro species 2, with a rate constant of 3.43 ± 0.03 × 10(-5) s(-1). There was evidence for two monofunctional adducts (3, 4) bound to the 3' (G8) and 5' (G7) residues and the asymmetry of the (1)H,(15)N peak for 3 suggested two conformers of the 3' adduct, aligned in different directions along the DNA. The rate constant for combined monofunctional adduct formation (0.6 ± 0.1 M(-1)) is ca. 2-fold lower for 1 compared to 1,1/t,t, whereas the rate constant for conversion of the combined monofunctional species to combined bifunctional adducts (5) (8.0 ± 0.2 × 10(-5) s(-1)) is two-fold higher.

Dihydrogen Bonding under High Pressure: A Raman Study of BH3NH3 Molecular Crystal

The effects of high pressures (up to 40 kbar) on the dihydrogen-bonded BH3NH3 molecular crystal were investigated using Raman spectroscopy in diamond and moissanite anvil cells. The stretching mode frequencies of the NH3 proton donor groups exhibited moderate red shifts with increasing pressures, as found in many conventional hydrogen-bonded systems of weak to medium strength. The stretching modes corresponding to the BH3 proton acceptor group, on the other hand, showed large blue shifts with increasing pressure, which, however, are not related to the changes in the N−H···H−B interactions. The BH3NH3 crystals undergo a pressure-induced disorder−order phase transition around 8 kbar, which is facilitated by the presence of dihydrogen bonds.

A comparative study of anharmonicity and matrix effects on the complexes XH:NH3, X=F, Cl, and Br

Ab initio calculations have been performed to investigate the structures and infrared spectra of the complexes FH:NH3, ClH:NH3, and BrH:NH3, and the effects of the presence of inert gas atoms on structures and spectra. Two-dimensional MP2/6-31+G(d,p) potential energy surfaces were constructed for the complexes XH:NH3, and model two-dimensional Schrödinger equations were solved for the proton stretching and dimer (heavy-atom) stretching modes. Although all complexes have equilibrium structures characterized by traditional hydrogen bonds, their infrared spectra differ significantly. In FH:NH3 both the ground (v=0) and first excited state for the proton stretching mode (v=1) are confined to the potential well describing the equilibrium structure. In this case the harmonic approximation is appropriate, and matrix effects are unimportant. In ClH:NH3 the v=1 proton stretching vibration accesses the more polar, proton-shared region of the potential surface. Here the harmonic treatment leads to a significant overestimation of the experimental proton-stretching frequency. Significant improvement results from an anharmonic treatment, which shows some coupling between proton and dimer modes. The presence of rare gas atoms preferentially stabilizes the proton-shared region of the surface, lowering the energy of the v=1 state. Further improvement results if the MP2/aug’-cc-pVDZ potential energy surface is used. In BrH:NH3 the proton-shared region of the potential surface is accessible in both v=0 and v=1 vibrational states, and an anharmonic treatment is required to obtain reasonable agreement with experiment. In BrH:NH3 proton stretching and dimer stretching modes are highly coupled, and rare gas atoms have structural and spectral effects.

Acquisition and assimilation of gaseous ammonia as revealed by intracellular pH changes in leaves of higher plants

Atmospheric ammonia (NH3) from various anthropogenic sources has become a serious problem for natural vegetation. Ammonia not only causes changes in plant nitrogen metabolism, but also affects the acid-base balance of plants. Using the pH-sensitive fluorescent dyes pyranine and esculin, cytosolic and vacuolar pH changes were measured in leaves of C3 and C4 plants exposed for brief periods to concentrations of NH3 in air ranging from 1.33 to 8.29 μmol NH3 · mol-1 gas (0.94–5.86 mg · m-3). After a lag phase, uptake of NH3 from air at a rate of 200 nmol NH3 · m - 2 leaf area · s- 1 into leaves of Zea mays L. increased pyranine fluorescence indicating cytosolic alkalinisation. The increase was much larger in the dark than in the light. In illuminated leaves of the C3 plant Pelargonium zonale L. and the C4 plants Z. mays and Amaranthus caudatus L., NH3-dependent cytosolic alkalinisation was particularly pronounced when CO2 was supplied at very low levels (16 or 20 μmol CO2 · mol- 1 gas, containing 210 mmol O2 · mol- 1 gas). An increase in esculin fluorescence, which was smaller than that of pyranine, was indicative of trapping of some of the NH3 in the vacuoles of leaves of Spinacia oleracea L. and Z. mays. Photosynthesis and transpiration remained unchanged during exposure of illuminated leaves to NH3, yielding an influx of 200 nmol NH3 · m-2 leaf area · s-1 for up to 30 min, the longest exposure time used. Both CO2 and O2 influenced the extent of cytosolic alkalinisation. At 500 μmol CO2 · mol-1 gas the cytosolic alkalinisation was suppressed more than at 16 or 20 μmol CO2 · mol-1 gas. The suppressing effect of CO2 on the NH3induced alkalinisation was larger in illuminated leaves of the C4 plants Z. mays and A. caudatus than in leaves of the C3 plant P. zonale. A reduction of the O2 concentration from 210 to 10 mmol O2 · mol -1 gas, which inhibits photorespiration, increased the NH3induced cytosolic alkalinisation in C3 plants. Suppression by CO2 or O2 of the alkaline pH shift caused by the dissolution and protonation of NH3 in queous leaf compartments, and possibly by the production of organic compounds synthesised from atmospheric NH3, indicates that NH3 which enters leaves is rapidly assimilated if photosynthesis or photorespiration provide nitrogen acceptor molecules.

Multiple roles of carbonic anhydrase in cellular transport and metabolism.

Carbonic anhydrase (CA) is a central enzyme to both transport and metabolic processes at the cellular level. In metabolically active tissue such as muscle, CA in the cytoplasm and on the sarcolemma appears to be important in facilitating CO2 transport out of the cell. Membrane-associated CA, with an extracellular orientation, also appears to be important in acidifying the outer boundary layer through the catalyzed hydration of excreted CO2. This facilitates cellular ammonia transport by providing H+ ions for the protonation of NH3, thus maintaining the trans-membrane NH3 gradient. Mitochondrial CA is known to supply HCO3- for the initial reactions of gluconeogenesis and ureagenesis in mammalian tissues, but systematic comparative studies of CA as a metabolic enzyme are lacking. CA probably evolved as an enzyme of trans-membrane facilitated CO2 transport and took on a secondary metabolic role later in metazoan evolution.

Quasidiabatic states for intramolecular charge transfer. Application to the protonation of NH3

We have constructed ab initio adiabatic and quasidiabatic surfaces for the description of the protonation of NH3. For the diabatic states, we applied a recently developed method which is based on the propagation along the reaction coordinate, starting from the dissociation region, of the maximized nonorthonormal overlap between diabatic states calculated at successive geometries. In agreement with earlier calculations [Kaldor et al., J. Chem. Phys. 90, 6395 (1989)], it was found that the adiabatic surfaces cannot explain this charge-transfer process. On the contrary, a single diabatic potential curve correlates smoothly the ground state of NH+4 with the NH3+H+ limit.

Interaction of ammonia with a zeolitic proton: ab initio quantum-chemical cluster calculations

The interaction of NH3 and a zeolitic cluster as well as the protonation of NH3 by zeolitic protons are studied by quantum-chemical calculations on small clusters at different levels of approximation. The focus of the paper is on a comparison of results obtained by the different methods. The clusters are studied at the SCF level as well as at the correlated level. Electron correlation is included through second-order Moller-Plesset perturbation theory. The basis-set superposition error (BSSE) was avoided by using the counterpoise scheme. Monodentate singly bonded NH3, that is NH3 being attached to one oxygen atom, forms a strong hydrogen bond with the zeolitic OH group. This bond has a strength of 60 or 67 kJ/mol, depending on the geometry of the zeolitic cluster. This value is approximately half the experimentally found heat of desorption. For this case, the O⋯N distance is found to be very short (2.74 or 2.73 A) and the intermolecular O-H-N stretching frequency is calculated to be 185 or 193 cm-1. The latter values agree reasonably with experimental data. Upon complexation with NH3, the OH stretching frequency shows a red shift of 551 cm-1. Proton transfer from the zeolitic cluster to NH3 is calculated to be unfavorable by 52 kJ/mol, as long as NH4+ is considered to be monodentate coordinated. The description of the hydrogen-bonded form is only slightly dependent on the basis set used. However, the proton-transfer energy does strongly depend on the basis set used. Electron correlation makes the proton transfer more favorable. The BSSE has a large influence on the description of the structures, especially if electron correlation is included. Although electron correlation has a nonnegligible effect on the proton-transfer energy, some conclusions can be drawn from SCF calculations on doubly and triply coordinated NH4+. The computed energy of adsorption now is approximately twice that computed for the hydrogen-bonded and singly coordinated NH3 and close to experimentally observed values of ammonia adsorption. From these results, it follows that these adsorption modes are prefered over the singly bonded form. These forms are preferred because of the favorable electrostatic stabilization of NH4+ when bonded to the cluster by two or three hydrogen bonds.

Proton NMR structural characterization of a recombinant kringle 2 domain from human tissue-type plasminogen activator

The kringle 2 domain of human tissue-type plasminogen activator (t-PA) has been characterized via 1H NMR spectroscopy at 300 and 620 MHz. The experiments were performed on the isolated domain obtained by expression of the 174-263 portion of t-PA in Escherichia coli [Cleary et al. (1989) Biochemistry 28, 1884-1891]. The spectrum of t-PA kringle 2 is characteristic of a globular structure and shows overall similarity to that of the plasminogen (PGN) kringle 4. Spectral comparison with human and bovine PGN kringle 4 identifies side-chain resonances from Leu46, which afford a fingerprint of kringle folding, and from most of the aromatic ring spin systems. Assignment of signals arising from the His13, His48a, and His64 side chains, which are unique to t-PA kringle 2, was assisted by the availability of a His64----Tyr mutant. Ligand-binding studies confirm that t-PA kringle 2 binds L-lysine with an association constant Ka approximately 11.9 mM-1. The data indicate that homologous or conserved residues relative to those that compose the lysine-binding sites of PGN kringles 1 and 4 are involved in the binding of L-lysine to t-PA kringle 2. These include Tyr36 and, within the kringle inner loop, Trp62, His64, Trp72, and Tyr74. Acid/base titration of aromatic singlets in the presence of L-lysine yields pKa* approximately 6.25 and approximately 4.41 for His13 and His64, respectively, and shows that the His48a imidazole group does not protonate down to pH* approximately 4.3. Thus, the His48a and His64 side chains are in solvent-shielded locations. As observed for the PGN kringles, the Trp62 indole group titrates with pKa* approximately 4.60, which indicates proximity of the side chain to a titratable carboxyl group, most likely that of Asp57 at the binding site. Several labile NH protons of t-PA kringle 2 exhibit retarded H-exchange kinetics, requiring more than a week in 2H2O for full deuteration in the presence of L-lysine at 37 degrees C. This reveals that kringle 2 is endowed with a compact, dynamically stable conformation. Proton Overhauser experiments in 1H2O, centered on well-resolved NH resonances between 9.8 and 12 ppm, identify signals arising from the His48a imidazole NH3 proton and the three Trp indole NH1 protons. A strong dipolar interaction was observed among the Trp25 indole NH1, the Tyr50 amide NH, and the His48a imidazole CH2 protons, which affords evidence for an aromatic cluster in t-PA kringle 2 similar to that found at the hydrophobic kernel of PGN kringles.(ABSTRACT TRUNCATED AT 400 WORDS)

NMR Studies on Intercalation Compounds of Layered Chalcogenides with Methylamines and Ammonia

Abstract NMR wideline and pulse techniques have been used to study orientation and molecular motions of the intercalation compounds of niobium disulfide with monolayers of mono-, di-, and trimethylamine, tetramethylammonium and ammonia. Specific information has been obtained from the 1H and 2D spectra and from spin-lattice relaxation measurements of the selectively deuterated monomethylamines. Models for the arrangement of the various molecules within the layers and for the motions have been derived. One-dimensional rotation of CH3 and NH3 groups, rapid intermolecular exchange of NH, NH2 and NH3 protons, as well as translational and reorientational motions of whole molecules are of importance. Interpretation of spin-lattice relaxation rates at various temperatures and frequencies yields details of some of these motion and numerical values for the activation energies.

Effect of hydration on the FH... NH3 proton transfer reaction: Comparison of some quantum chemical approximations

The proton transfer energy curve for the hydrogen fluoride-ammonia system was studied by semi-empirical methodsin vacuo and in a hydrated form. It was found that the simple point-charge approximation cannot be used to estimate the Coulombic interaction term rather the molecular electrostatic potential has to be considered. Furthermore, even inclusion of the mutual polarization term is insufficient if quantitative agreement with CNDO/2 supermolecule results is desired since charge transfer plays an important role.