Neutral Water Molecules Research Articles

On the mechanism of hydrogen-deuterium exchange in bacteriorhodopsin

Continuous-flow resonance Raman experiments carried out in bacteriorhodopsin show that the exchange of a deuteron on the Schiff base with a proton takes place in times shorter than 3 ms. Exchange mechanisms based on a base-catalyzed deprotonation followed by reprotonation of the Schiff base are excluded. A mechanism is suggested in which a water molecule interacts directly with the Schiff base deuteron in a concerted exchange mechanism. It appears that in the dark, the binding site is more accessible to neutral water molecules than to charged protons.

The hydrogen bonds of potassium tetraoxalate: An X-ray redetermination of the crystal structure of KHC 2O 4·H 2C 2O 4·2H 2O

Salts called tetraoxalates have been known for many years. The IR spectrum of potassium tetraoxalate, KHC 2O 4·H 2C 2O 4·2H 2O, has a continuum which extends from 3500 cm −1 down to 800 cm −1. This is now explained as arising from the network of hydrogen bonding within the crystal, which contains not only the expected short bonds between oxalate and oxalic acid but also between neutral water molecules and oxalic acid. Though short none of these hydrogen bonds is symmetrical. The hydrogen bonding continuum is being produced by bonds that are neither charged nor symmetrical, as is normally assumed must be the case in order to generate such an IR spectrum.

Ion-molecule reactions in H 2O after charge transfer ionization

Consecutive ion—molecule reactions in water, up to the formation of H 7O 3 +, were studied in the pressure range 5 × 10 −6 to 4 × 10 −2 torr. The water molecules were ionized by charge transfer in a tandem mass spectrometer of perpendicular type, and H 2O + ions were formed in different electronic states. Formation of H 3O + from H 2O + initially present in the H 2O + ( X̃ 2 B 1),( Ã 2 A 1) and ( B̃ 2 B 2) states occurred with similar rate constants, although somewhat lower values were found for the excited states. The charge transfer reactions with Ne + and He + are not resonant and, as a consequence, accurate rate constants for the reactions of OH + and H + with H 2O could not be determined. It is shown that the initial formation of H 5O 2 + from H 3O + requires three consecutive collisions with neutral water molecules, while for the formation of H 7O 3 + from H 5O 2 + two collisions are sufficient. The rate of formation of the two higher clusters did not significantly depend on the initial ionization.

Optical absorption of Co2+ doped NH4Cl: Phase transformation studies

The absorption spectrum of Co2+ doped NH4Cl has been studied from the room temperature to the liquid nitrogen temperature. A sudden change in the spectrum is observed between 243° K and 233° K which is attributed to the phase transition in the crystal. From the observed spectrum it is suggested that Co2+ goes in interstitially as well as substitutionally. Both the types of centers exist at room temperature, but with decrease in temperature substitutional ions migrate to interstitial sites, the process being stimulated at the phase transformation point so that the 77° K spectrum seems to be mostly due to the interstitial centers. The 77° K spectrum is analyzed in the approximation of octahedral symmetry for interstitial ions and the band positions are fitted fairly well with B = 870 cm.−1 Dq = 850 cm.−1 and C = 4·4 B. A blue shift of about 100 cm.−1 is observed for4T1 (P) band at the phase transition which is attributed to the increase in Dq value with the anomalous lattice contraction at the phase transition. The decrease in the lattice parameter calculated from this blue shift is around 0·4% which is in good agreement with the results of X-ray measurements. Two possible models for the interstitial complex are examined and the one with fourfold chlorine coordination associated with two neutral water molecules at the first neighbour (NH4)+ site lying along direction is suggested to be more probable.

Calculation of the effects of hydration and divalent metal ions on nucleotide base pairs: single G-C and poly(G-C).

SCF calculations have been carried out for the electronic energy levels and wavefunctions of single G–C and poly(G–C) in the presence of a neutral water molecule or Mg1+ ion. The Pariser- Parr–Pople (P–P–P) approximation was used for the single base pair, and its equivalent, the semiempirical SCF–LCAO–crystal-orbital (CO) approximation, for the polymer. Energy levels and energy bands are presented for four different cases. According to the results, the presence of the Mg2+ ion very strongly changes the band structure of the poly(G–C). The probable consequences of this on the conductive properties of DNA and the possibility of ab initio calculations on periodic DNA models are discussed.

Electronic Absorption Spectrum of Ni2+ Doped in NH4Cl Single Crystal

The absorption spectrum of Ni2+ doped ammonium chloride is studied in the temperature range of 293–77°K. From the observed spectrum it is concluded that Ni2+ goes interstitially. Different possible coordinations at the metal ion site are discussed. It is suggested that the divalent nickel ion has very likely a fourfold chlorine coordination associated with two neutral water molecules at the first-neighbor NH4+ sites along 〈100〉 direction. An abrupt blue shift of about 100–150 cm−1 is observed at 235°K in the positions of the band maxima and is attributed to the anomalous lattice contraction of the host lattice at this temperature.

Intermolecular proton transport along the hydrogen bond and the dielectric properties of ice crystals

We have attempted here to explain some of the dielectric properties of ice crystals by a concept involving the activated transport of a proton along the hydrogen bond. The choice of the almost symmetrical curve with two minima for a proton in the hydrogen bridge and the rejection of tunnel transport of a proton makes it possible to suggest a theory which explains adequately the dependence of the low frequency dielectric constant on temperature and the phenomenon of the dielectric relaxation of ice. The electrical polarization of an ice crystal according to our model is produced by the defect structure of the ice, the ion pairs, which arises when the proton moves between two neutral water molecules.

Electrification associated with the evaporation of water and ionic solutions

Measurements indicate that any electrification accompanying the evaporation of pure water must proceed at a rate less than 9 × 10 −5 e.s.u. g −1. This result is in good agreement with that of Israël and Knopp (1962). However, a definite charging effect has been measured during the evaporation of NaCl and KCl solutions over a restricted range of normalities; for example, when 0.1 N KCl solution is evaporated at a temperature of 63°C it loses mass at a rate dm dt = −1.67 × 10 −3 g sec −1 and becomes positively charged at a rate i =5 × 10 −14 A with a corresponding negative charge being given to the air. It is shown that energetic considerations preclude the possibility that the charge is removed on unassociated ions but that agreement between theory and experiment exists if the charge leaves the evaporating liquid surface as an agglomerate of about ten neutral water molecules containing one negatively charged ion. Tentative calculations indicate that the contribution of evaporation of sea-water to the electrical budget of the atmosphere may be around 40°A in a direction reinforcing the fineweather current.

Strength and Static Fatigue of Abraded Glass Under Controlled Ambient Conditions: IV, Effect of Surrounding Medium

The strength and static fatigue behavior of abraded glass specimens tested in various surrounding media have been studied. The test media included distilled water, nitrogen atmospheres of varying humidity, methyl and isopropyl alcohol and mixtures of these with distilled water, and various acidic and basic solutions. For intermediate durations of load the strength values in atmospheres of 0.5 and 43% relative humidity were 45 and 20% greater, respectively, than in distilled water. The value of t0.5, the characteristic duration for static fatigue, under these three test conditions was 3500, 200, and 8 seconds, respectively, for the particular grit-blast abrasion studied, indicating that liquid water is the most effective agent for promoting a high rate of static fatigue and very dry air with a small concentration of water vapor is least effective. In reagent-grade methyl or isopropyl alcohol (0.01 to 0.05% H2O) the strength at an intermediate load duration of 10 seconds was about 40% greater than in distilled water and the slope of the static fatigue curve differed somewhat from that obtained for tests in water. With the addition of water to the alcohol solutions the fatigue curve became parallel to that for water, and as the water content was increased, the curves shifted toward the water curve. For tests in acidic and basic solutions the strength at an intermediate load duration was independent of pH over the range pH 1 to 13. For more acidic solutions the strength was slightly less and for more basic solutions slightly greater than in this range. The results are discussed in terms of possible mechanisms for static fatigue. It is concluded that the static fatigue of abraded glass results from changes in the shape or size of surface abrasion cracks under the combined action of stress and the surrounding medium. In general, water or water vapor is the primary agent in promoting fatigue, and, unlike the usual chemical attack, the interaction appears to involve primarily the neutral water molecule and the SiO2 network of the glass. The presence of other ions or molecules is relatively unimportant except as it may serve to reduce the concentration of water and its availability to the tip of the abrasion crack.

On the Dissociation Constants of Acids in Light and Heavy Water

The ratio of dissociation constants of acids in light and heavy water is shown to become the larger the weaker the acid. This is a consequence of the differences in zero-point energy of the proton (deuteron) when attached to the anion or the neutral water molecule.