Proton Reservoir Research Articles

Proton Spin–Lattice Relaxation in Wood—T1Related to Local Specific Gravity Using a Fast-Exchange Model

The spin–lattice relaxation and cross relaxation of western red-cedar sapwood (moisture contents 216 to 15%) and western hemlock sapwood (moisture contents 144 to 15%) were investigated using proton magnetic resonance. Below the fiber saturation point (FSP), solid wood and cell-wall water were found to have the sameT1indicating a fast exchange of proton magnetization between these two environments. Above the FSP, multiexponentialT1decay was observed. A one-dimensional profile across the growth rings showed that the multiple components of spin–lattice relaxation in wood were due to variations in local specific gravity. Within a small volume, a singleT1was observed for protons in lumen water, cell-wall water, and solid wood. The mixing of proton reservoirs was attributed to water diffusion between the lumen and cell wall, and cross relaxation between the protons in the cell-wall water and the solid wood. This cross-relaxation time was found to be a few milliseconds, which is fast on theT1time scale of tens of milliseconds. A fast-magnetization-exchange model which relatedT1values to local specific gravity was verified for protons in all environments in wood. Using this model, the distribution of local specific gravity was derived for the western red-cedar and hemlock samples.

Magic angle carbon‐13 NMR study of solid poly(ethylene naphthalene‐2,6‐dicarboxylate)

AbstractAmorphous (1) and semicrystalline (2) samples of poly(ethylene naphthalene‐2,6‐dicarboxylate) (PEN) have been investigated by cross‐polarization/magic angle spinning (CP/MAS) 13C NMR at 26°C (1 and 2), 100°C (1) and 120°C (2) in order to study the phase structure and the local motion of polymer chain segments at temperatures below and close to Tg (120°C). The lineshape of the ethylene unit 13C signal in sample 2 is consistent with the presence of two components which were assigned to trans and gauche conformations. The first component arises mainly from the crystalline regions and the second one from the amorphous part. Cross‐polarization curves were traced by changing the contact time between carbon and proton reservoirs. TCH (cross relaxation time) and proton T1p (spin‐lattice relaxation time in the rotating frame) values were obtained as best fit parameters by fitting calculated curves to the experimental data. All 13C NMR data are consistent with the presence of highly rigid ethylene units in both semicrystalline and amorphous samples within the temperature range (T) investigated. This result is in disagreement with the 1H NMR wide line spectra which showed a noticeable narrowing of the linewidth with increasing temperature in the same range, hence indicating a great mobility of the chain segments. To account for this discrepancy a qualitative model based on the existence of two distinct dynamic regions, one where motion is highly restricted and the other one where large reorientations of ethylene group torsional angles take place, is suggested. The NMR results led to the conclusion that three structural phases are present in PEN: crystalline, very rigid amorphous, and very mobile amorphous. © 1995 John Wiley & Sons, Inc.

Thermodynamics of benzylic C–H activation in 18- and 19-electron iron sandwich complexes: determination of pKavalues and bond dissociation energies

The pKa values and bond dissociation energies of 19-electron complexes have been determined for the first time, supporting the concept of proton and hydrogen atom reservoirs for late transition-metal sandwich complexes.

Distribution of acid sites and differential heat of NH3 chemisorption on some aluminas and zeolites

The distribution and the strength of acid sites in some aluminas and zeolites were obtained from microcalorimetry measurements of ammonia adsorption. They were compared to the results of FTIR of adsorbed CO and of1H→27Al cross-polarization (CP) MAS NMR using chemisorbed ammonia as proton reservoir. FTIR of CO adsorption yields the numbers of Lewis sites and CP MAS NMR gives the coordination of surface aluminum. In spite of the variety of catalysts and of acidity encountered, interesting correlations were found with both the energy of adsorption and singularities in the microcalorimetric titration. In particular, in zeolites a clear difference exists in the distribution of the Lewis and Bronsted acid site strengths with respect to the amount of irreversibly adsorbed NH3.

Photoelectric response of oriented purple membrane electrodeposited onto poly(vinyl alcohol) film

Purple membrane (PM) fragments isolated from Halobacterium halobium were electrodeposited onto a wet poly(vinyl alcohol) (PVA) film. Then the oriented PM fragments were covered with another layer of PVA (PVA cast). Two gold electrodes Au(1) and Au(2) were vacuum deposited onto the PVA film and the PVA cast respectively and a sandwich cell Au(1)∣PVA cast∣PM∣PVA film∣Au(2) was fabricated. On irradiation, the cell showed a photovoltaic response such that the Au(1) electrode was always positive, indicating unidirectional proton movement from the PVA film to the PVA cast. The photocurrent of the cell was affected markedly by the relative humidity (RH). The photocurrent exhibited a maximal value at 66% RH. The cell showed a stationary photocurrent even after prolonged irradiation (4h), suggesting that humid PVA serves as a proton source and reservoir. The cell may be used as a humidity sensor. The blue-light effect was observed for the photocurrent and photovoltage: a negative photosignal was induced by a second blue-light excitation superimposed on strong white or green light. The cell might also be used as a novel type of colour sensor.

Oxygen reduction on Ru-oxide pyrochlores bonded to a proton-exchange membrane

Oxygen reduction at a gas-fed, porous, ruthenium-pyrochlore electrode attached to a Dow Developmental Fuel Cell Membrane was measured in solutions of various pH. Electrode assemblies containing high surface area Pb2Ru2−xPbxO7−y or Bi2Ru2−xBixO7−y with different amounts of Teflon content with/without the incorporation of Dow gel in the active part of the electrode with/without a CO2-treated Vulcan XC-72 carbon substrate were tested. The oxide pyrochlores were found to be chemically stable and to show their lowest overpotential if separated from a 2.5 M H2SO4 proton reservoir by the membrane. Interesting oxygen reduction activity at room temperature was obtained with the Pb2Ru1.74Pb0.26O7−y electrode bonded with 22% by weight Teflon and incorporating 5% by weight Dow gel. The performance of the oxides against B-site Pb concentration and a measurement of the surface charge on the particles indicate that, in this configuration, the active sites for the oxygen reduction reaction are OH− species at the O-site positions of the A2B2O6′O1−y pyrochlores, especially the bridging oxygen with one Ru and one Pb near neighbour, i.e. Pb−Ob−Ru. Evidence that oxide particles precipitated on CO2-treated carbon transfer electrons to the substrate is also presented.

ATP formation onset lag and post-illumination phosphorylation initiated with single-turnover flashes. III. Characterization of the ATP formation onset lag and post-illumination phosphorylation for thylakoids exhibiting localized or bulk-phase delocalized energy coupling.

When 100 mM KCl replaced sucrose in a chloroplast thylakoid stock suspension buffer, the membranes were converted from a localized proton gradient to a delocalized proton gradient energy coupling mode. The KCl-suspended but not the sucrose-suspended thylakoids showed pyridine-dependent extensions of the ATP onset lag and pyridine effects on post-illumination phosphorylation. The ATP formation assays were performed in a medium of identical composition, using about a 200-fold dilution of the stock thylakoid suspension; hence the different responses were due to the pretreatment, and not the conditions present in the phosphorylation assay. Such permeable buffer effects on ATP formation provide a clear indicator of delocalized proton gradients as the driving force for phosphorylation. The pyridine-dependent increases in the onset lags (and effects on post-illumination phosphorylation) were not due to different ionic conductivities of the membranes (measured by the 515 nm electrochromic absorption change), H+/e- ratios, or electron transport capacities for the two thylakoid preparations. Thylakoid volumes and [14C]pyridine equilibration were similar with both preparations. The KCl-induced shift toward a bulk-phase delocalized energy coupling mode was reversed when the thylakoids were placed back in a low-salt medium. Proton uptake, at the ATP-formation energization threshold flash number, was much larger in the KCl-treated thylakoids and they also had a longer ATP formation onset lag, when no pyridine was present. These results are consistent with the salt treatment exposing additional endogenous buffering groups for interaction with the proton gradient. The concomitant appearance of the pyridine buffer effects implies that the additional endogenous buffering groups must be located on proteins directly exposed in the aqueous lumen phase. Kinetic analysis of the decay of the post-illumination phosphorylation in the two thylakoid preparations showed different apparent first-order rate constants, consistent with there being two different compartments contributing to the proton reservoirs that energize ATP formation. We suggest that the two compartments are a membrane-phase localized compartment operative in the sucrose-treated thylakoids and the bulk lumen phase into which protons readily equilibrate in the KCl-treated thylakoids.

Effect of weak acids on amino acid transport by Penicillium chrysogenum: evidence for a proton or charge gradient as the driving force.

A variety of weak acids at and below their pK(a) are potent inhibitors of transport in Penicillium chrysogenum. The effective compounds include sorbate, benzoate, and propionate (common antifungal agents), indoleacetate (a plant hormone), acetylsalicylate (aspirin), hexachlorophene, and a yellow pigment produced by the mycelia under nutrient-deficient conditions, as well as the classical uncouplers 2,4-dinitrophenol, p-nitrophenol, and azide. The results suggest that a proton gradient or charge gradient is involved in energizing membrane transport in P. chrysogenum. The unionized form of the weak acids could discharge the gradient by diffusing through the membrane and ionizing when they reach an interior compartment of higher pH. Experiments with 2,4-dinitrophenol and p-nitrophenol established that the ionized species are not absorbed by the mycelium to any great extent. The transport inhibitors also caused a decrease in cellular adenosine 5'-triphosphate (ATP) levels, but there was no constant correlation between inhibition of transport and suppression of cellular ATP. A decrease in aeration of the mycelial suspension had the same effect on transport and ATP levels as the addition of a weak organic acid. The effects on transport rates and ATP levels were reversible. The instantaneous inhibition of [(14)C]l-leucine transport by NH(4) (-) (and vice-versa) in nitrogen-starved mycelia at pH values of 7 or below can be explained by competition for a common energy-coupling system. The inhibition is not observed in carbon-starved mycelia in which the NH(4) (+) transport system is absent or inactive (but the general amino acid transport is fully active), or in iodoacetate-treated mycelia in which the NH(4) (+) transport system has been differentially inactivated. At pH values greater than 7.0, NH(3) and HPO(4) (2-) inhibit transport, presumably by discharging the membrane proton or charge gradient. Aniline counteracts the inhibitory effect of NH(3) and HPO(4) (2-) possibly by acting as a proton reservoir or buffer within the membrane.