Showing papers on "Proton published in 2005"

Spectral signatures of hydrated proton vibrations in water clusters.

Abstract: The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H 3 O + ) and Zundel (H 5 O 2 + ) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

Thermodynamics and Proton Transport in Nafion: II. Proton Diffusion Mechanisms and Conductivity

Abstract: A comprehensive pore transport model is proposed to describe proton diffusion within Nafion at various hydration levels by incorporating effects of water uptake and various proton transport mechanisms, namely, proton hopping along surface, Grotthuss diffusion, and ordinary mass diffusion of hydronium ions. The diffusion coefficients are predicted within a general random walk framework. The proton conductivity in contact with water vapor is accurately predicted as a function of relative humidity without any fitted parameters, considering the sorption isotherm proposed in the companion paper (Part I). A maximum conductivity in contact with liquid water is also predicted by the model for equivalent weight between 900 and 1000, in good agreement with the experimental measurements. The modeling framework could be extended to other proton conducting electrolytes for fuel cell applications. © 2005 The Electrochemical Society. All rights reserved.

Proton elastic form-factor ratios to Q**2 = 3.5-GeV**2 by polarization transfer

Abstract: The ratio of the proton elastic electromagnetic form factors, GEp/GMp, was obtained by measuring Pt and P, the transverse and longitudinal recoil proton polarization components, respectively, for the elastic epepreaction in the four-momentum transfer squared range of 0.5 to 3.5 GeV2. In the single-photon exchange approximation, GEp/GMp is directly proportional to Pt/P. The simultaneous measurement of Pt and P in a polarimeter reduces systematic uncertainties. The results for GEp/GMp show a systematic decrease with increasing Q2, indicating for the first time a definite difference in the distribution of charge and magnetization in the proton. The data have been reanalyzed and their systematic uncertainties have become significantly smaller than those reported previously. (Less)

A mechanistic principle for proton pumping by cytochrome c oxidase

Abstract: In aerobic organisms, cellular respiration involves electron transfer to oxygen through a series of membrane-bound protein complexes. The process maintains a transmembrane electrochemical proton gradient that is used, for example, in the synthesis of ATP. In mitochondria and many bacteria, the last enzyme complex in the electron transfer chain is cytochrome c oxidase (CytcO), which catalyses the four-electron reduction of O2 to H2O using electrons delivered by a water-soluble donor, cytochrome c. The electron transfer through CytcO, accompanied by proton uptake to form H2O drives the physical movement (pumping) of four protons across the membrane per reduced O2. So far, the molecular mechanism of such proton pumping driven by electron transfer has not been determined in any biological system. Here we show that proton pumping in CytcO is mechanistically coupled to proton transfer to O2 at the catalytic site, rather than to internal electron transfer. This scenario suggests a principle by which redox-driven proton pumps might operate and puts considerable constraints on possible molecular mechanisms by which CytcO translocates protons.

The effect of cooperative hydrogen bonding on the OH stretching-band shift for water clusters studied by matrix-isolation infrared spectroscopy and density functional theory.

Abstract: Infrared spectra of the water clusters have been measured in the N2 + O2 matrix. The aggregation process of water in the matrix has been monitored by annealing the deposited samples up to 40 K and UV irradiation. The monomer, dimer, cyclic trimer and cyclic pentamer are found as water clusters in the matrix. For the hexamer, several structures such as chair, cage, prism, bag 1 and/or book 1 are likely to exist. By UV irradiation, the cyclic pentamer is predominantly formed from the monomer and dimer. On the other hand, by annealing the deposited sample, several hexamers are formed. The theoretical calculation for water clusters has revealed that the formation of one hydrogen bonding in a hydrogen-bonded chain cooperatively enhances or diminishes the strength of another hydrogen bond. Both proton donor (D) and acceptor (A) participating in a hydrogen-bonding pair DA are capable of forming hydrogen bonding with the other water molecules; D can additionally accept two protons and donate one proton, and A can additionally donate two protons and accept one proton. We have proposed the classification of hydrogen-bonding patterns considering the cooperativity, denoting as d′a′DAd″a″, where d and a are integers indicating the number of proton donors and acceptors to D (the single prime) and A (the double prime), respectively. Then, a magnitude given by MOH = −d′ + a′ + d″ − a″ has been introduced, which is very useful for connecting the hydrogen-bonding patterns to their OH wavenumbers. As a result, it is revealed that the OH stretching bands of water clusters are characterized by eight indicators (free and MOH = −2, −1, 0, 1, 2, 3 and 4). The classification proposed here is applicable to the OH band analysis for the hydrogen-bonded water and alcohols in a condensed phase.

Strange-quark contributions to parity-violating asymmetries in the forward G0 electron-proton scattering experiment

Abstract: We have measured parity-violating asymmetries in elastic electron-proton scattering over the range of momentum transfers 0.12 ≤ Q^2 ≤ =1.0 GeV^2. These asymmetries, arising from interference of the electromagnetic and neutral weak interactions, are sensitive to strange-quark contributions to the currents of the proton. The measurements were made at Jefferson Laboratory using a toroidal spectrometer to detect the recoiling protons from a liquid hydrogen target. The results indicate nonzero, Q^2 dependent, strange-quark contributions and provide new information beyond that obtained in previous experiments.

The rosetta experiment: atmospheric soft error rate testing in differing technology FPGAs

Abstract: Results are presented from real-time experiments that evaluated large field programmable gate arrays (FPGAs) fabricated in different CMOS technologies (0.15 /spl mu/m, 0.13 /spl mu/m, and 90 nm) for their sensitivity to radiation-induced single-event upsets (SEUs). These results are compared to circuit simulation (Qcrit) studies as well as to Los Alamos Neutron Science Center (LANSCE) neutron beam results and Crocker Nuclear Laboratory (University of California, Davis) cyclotron proton beam results.

Proton, helium, and electron spectra during the large solar particle events of October-November 2003

Abstract: The extraordinary period from late October through early November 2003 was marked by more than 40 coronal mass ejections (CME), eight X-class flares, and five large solar energetic particle (SEP) events. Using data from instruments on the ACE, SAMPEX, and GOES-11 spacecraft, the fluences of H, He, O, and electrons have been measured in these five events over the energy interval from ∼0.1 to >100 MeV/nucleon for the ions and ∼0.04 to 8 MeV for electrons. The H, He, and O spectra are found to resemble double power laws, with a break in the spectral index between ∼5 and ∼50 MeV/nucleon which appears to depend on the charge-to-mass ratio of the species. Possible interpretations of the relative location of the H and He breaks are discussed. The electron spectra can also be characterized by double power laws, but incomplete energy coverage prevents an exact determination of where and how the spectra steepen. The proton and electron fluences in the 28 October 2003 SEP event are comparable to the largest observed during the previous solar maximum, and within a factor of 2 or 3 of the largest SEP events observed during the last 50 years. The 2-week period covered by these observations accounted for ∼20% of the high-energy solar-particle fluence over the years from 1997 to 2003. By integrating over the energy spectra, the total energy content of energetic protons, He, and electrons in the interplanetary medium can be estimated. After correcting for the location of the events, it is found that the kinetic energy in energetic particles amounts to a significant fraction of the estimated CME kinetic energy, implying that shock acceleration must be relatively efficient in these events.

Proton binding within a membrane protein by a protonated water cluster

Abstract: Proton transfer is crucial for many enzyme reactions. Here, we show that in addition to protonatable amino acid side chains, water networks could constitute proton-binding sites in proteins. A broad IR continuum absorbance change during the proton pumping photocycle of bacteriorhodopsin (bR) indicates most likely deprotonation of a protonated water cluster at the proton release site close to the surface. We investigate the influence of several mutations on the proton release network and the continuum change, to gain information about the location and extent of the protonated water network and to reveal the participating residues necessary for its stabilization. We identify a protonated water cluster consisting in total of one proton and about five water molecules surrounded by six side chains and three backbone groups (Tyr-57, Arg-82, Tyr-83, Glu-204, Glu-194, Ser-193, Pro-77, Tyr-79, and Thr-205). The observed perturbation of proton release by many single-residue mutations is now explained by the influence of numerous side chains on the protonated H bonded network. In situ hydrogen/deuterium exchange Fourier transform IR measurements of the bR ground state, show that the proton of the release group becomes localized on Glu-204 and Asp-204 in the ground state of the mutants E194D and E204D, respectively, even though it is delocalized in the ground state of wild-type bR. Thus, the release mechanism switches between the wild-type and mutated proteins from a delocalized to a localized proton-binding site.

Proton acceleration mechanisms in high-intensity laser interaction with thin foils

Abstract: The interaction of short and intense laser pulses with plasmas or solids is a very efficient source of high-energy ions. This paper reports the detailed study, with particle-in-cell simulations, of the interaction of such a laser pulse with thin, dense targets, and the resulting proton acceleration. Depending on the laser intensity and pulse duration, the most energetic protons are found to come from the front, the core, or the back of the target. The main accelerating mechanisms discussed in this paper are plasma expansion acceleration, where proton acceleration is driven by the hot electron population, and shock acceleration, originating from the laser ponderomotive potential imposed at the front target surface. Three main regimes of proton acceleration are defined and the parameters for which each regime is dominant are obtained. For irradiances close to 10^20 W/cm^2, the highest proton energies are obtained from thin foils efficiently heated by relativistic transparency. At larger intensities, a complex interplay between collisionless shock acceleration and plasma expansion acceleration is evidenced.

Density resolution of proton computed tomography.

Abstract: Conformal proton radiation therapy requires accurate prediction of the Bragg peak position. Protons may be more suitable than conventional x rays for this task since the relative electron density distribution can be measured directly with proton computed tomography (CT). However, proton CT has its own limitations, which need to be carefully studied before this technique can be introduced into routine clinical practice. In this work, we have used analytical relationships as well as the Monte Carlo simulation tool GEANT4 to study the principal resolution limits of proton CT. The noise level observed in proton CT images of a cylindrical water phantom with embedded tissue-equivalent density inhomogeneities, which were generated based on GEANT4 simulations, compared well with predictions based on Tschalar's theory of energy loss straggling. The relationship between phantom thickness, initial energy, and the relative electron density resolution was systematically investigated to estimate the proton dose needed to obtain a given density resolution. We show that a reasonable density resolution can be achieved with a relatively small dose, which is comparable to or even lower than that of x-ray CT.

Fundamental excitations of the shared proton in the H3O2- and H5O2+ complexes.

Abstract: We exploit recent advances in argon predissociation spectroscopy to record the spectroscopic signature of the shared proton oscillations in the H3O2- system and compare the resulting spectrum with that of the H5O2+ ion taken under similar conditions Very intense 1 ← 0 transitions are observed below 1100 cm-1 in both cases and are surprisingly sharp, with the 697 cm-1 transition in H3O2- being among the lowest in energy of any shared proton system measured to date The assignments of the three fundamental transitions associated with the three-dimensional confinement of the shared proton in H3O2- are carried out with full-dimensional (DMC) calculations to treat this strongly anharmonic vibrational problem

UHE nuclei propagation and the interpretation of the ankle in the cosmic-ray spectrum

Abstract: We consider the stochastic propagation of high-energy protons and nuclei in the cosmological microwave and infrared backgrounds, using revised photonuclear cross-sections and following primary and secondary nuclei in the full 2D nuclear chart. We confirm earlier results showing that the high-energy data can be fit with a pure proton extragalactic cosmic ray (EGCR) component if the source spectrum is ∝E −2.6 . In this case the ankle in the CR spectrum may be interpreted as a pair-production dip associated with the propagation. We show that when heavier nuclei are included in the source with a composition similar to that of Galactic cosmic-rays (GCRs), the pair-production dip is not present unless the proton fraction is higher than 85%. In the mixed composition case, the ankle recovers the past interpretation as the transition from GCRs to EGCRs and the highest energy data can be explained by a harder source spectrum ∝E −2.2 –E −2.3 , reminiscent of relativistic shock acceleration predictions, and in good agreement with the GCR data at low-energy and holistic scenarios.

A system for the delivery of proton therapy

Abstract: A system for an intensity-modulated proton therapy of a predetermined volume (T) within an object (M) is disclosed, comprising: a) a proton source in order to generate a proton beam (B) being adjustable with respect to the beam intensity; b) a degrader being optionally disposable in the proton beam (B) in order to attenuate the energy of the protons in the proton beam (B) to a desired proton energy in the proton beam (B); c) a number of proton beam bending and/or focusing units; d) a beam nozzle (N) having an outlet for the proton beam (B) to penetrate the predetermined volume (T) of, the object (M); e) a beam bending magnet (A3) being disposed upstream of the nozzle (N); and f) a couple of sweeper magnets (WT, WU) being disposed upstream of said beam bending magnet (A3) in order to sweep the proton beam (B) in both lateral directions (T, U) before the proton beam (B) enters into the beam bending magnet (A3), g) said beam nozzle (N) is defining a cross-sectional scanning area (SF) substantially perpendicular to the proton beam (B) in the range of 10 to 30 cm2, and h) said sweeper magnets (WT, WU) and said beam bending magnet (A3) are controlled in order to guarantee a parallel beam orientation over the complete cross-sectional scanning area (SF)

Multi chiral-doublets in one single nucleus

Abstract: Adiabatic and configuration-fixed constraint triaxial relativistic mean field (RMF) approaches are developed for the first time and a new phenomenon, the existence of multi chiral-doublets (M$\chi$D), i.e., more than one pairs of chiral doublets bands in one single nucleus, is suggested for nuclei in A~100 region, typically for $^{106}$Rh, based on the triaxial deformations together with their corresponding proton and neutron configurations.

Electron transfer versus proton transfer in gas-phase ion/ion reactions of polyprotonated peptides.

Abstract: The ion/ion reactions of several dozen reagent anions with triply protonated cations of the model peptide KGAILKGAILR have been examined to evaluate predictions of a Landau-Zener-based model for the likelihood for electron transfer. Evidence for electron transfer was provided by the appearance of fragment ions unique to electron transfer or electron capture dissociation. Proton transfer and electron transfer are competitive processes for any combination of anionic and cationic reactants. For reagent anions in reactions with protonated peptides, proton transfer is usually significantly more exothermic than electron transfer. If charge transfer occurs at relatively long distances, electron transfer should, therefore, be favored on kinetic grounds because the reactant and product channels cross at greater distances, provided conditions are favorable for electron transfer at the crossing point. The results are consistent with a model based on Landau-Zener theory that indicates both thermodynamic and geometric criteria apply for electron transfer involving polyatomic anions. Both the model and the data suggest that electron affinities associated with the anionic reagents greater than about 60-70 kcal/mol minimize the likelihood that electron transfer will be observed. Provided the electron affinity is not too high, the Franck-Condon factors associated with the anion and its corresponding neutral must not be too low. When one or the other of these criteria is not met, proton transfer tends to occur essentially exclusively. Experiments involving ion/ion attachment products also suggest that a significant barrier exists to the isomerization between chemical complexes that, if formed, lead to either proton transfer or electron transfer.

Masses, deformations and charge radii: Nuclear ground-state properties in the relativistic mean field model

Abstract: We perform a systematic study of the ground-state properties of all the nuclei from the proton drip line to the neutron drip line throughout the periodic table employing the relativistic mean field model. The TMA parameter set is used for the mean-field Lagrangian density, and a state-dependent BCS method is adopted to describe the pairing correlation. The ground-state properties of a total of 6969 nuclei with Z, N ≥ 8a ndZ ≤ 100 from the proton drip line to the neutron drip line, including the binding energies, the separation energies, the deformations, and the rms charge radii, are calculated and compared with existing experimental data and those of the FRDM and HFB-2 mass formulae. This study provides the first complete picture of the current status of the descriptions of nuclear ground-state properties in the relativistic mean field model. The deviations from existing experimental data indicate either that new degrees of freedom are needed, such as triaxial deformations, or that serious effort is needed to improve the current formulation of the relativistic mean field model.

UHE nuclei propagation and the interpretation of the ankle in the cosmic-ray spectrum

Abstract: We consider the stochastic propagation of high-energy protons and nuclei in the cosmological microwave and infrared backgrounds, using revised photonuclear cross-sections and following primary and secondary nuclei in the full 2D nuclear chart. We confirm earlier results showing that the high-energy data can be fit with a pure proton extragalactic cosmic ray (EGCR) component if the source spectrum is \propto E^{-2.6}. In this case the ankle in the CR spectrum may be interpreted as a pair-production dip associated with the propagation. We show that when heavier nuclei are included in the source with a composition similar to that of Galactic cosmic-rays (GCRs), the pair-production dip is not present unless the proton fraction is higher than 85%. In the mixed composition case, the ankle recovers the past interpretation as the transition from GCRs to EGCRs and the highest energy data can be explained by a harder source spectrum \propto E^{-2.2} - E^{-2.3}, reminiscent of relativistic shock acceleration predictions, and in good agreement with the GCR data at low-energy and holistic scenarios.

Quantum and dynamical effects of proton donor-acceptor vibrational motion in nonadiabatic proton-coupled electron transfer reactions

Abstract: This paper presents a general theoretical formulation for proton-coupled electron transfer (PCET) reactions. The solute is represented by a multistate valence bond model, and the active electrons and transferring proton(s) are treated quantum mechanically. This formulation enables the classical or quantum mechanical treatment of the proton donor-acceptor vibrational mode, as well as the dynamical treatment of the proton donor-acceptor mode and the solvent. Nonadiabatic rate expressions are presented for PCET reactions in a number of well-defined limits for both dielectric continuum and molecular representations of the environment. The dynamical rate expressions account for correlations between the fluctuations of the proton donor-acceptor distance and the nonadiabatic PCET coupling. The quantities in the rate expressions can be calculated with a dielectric continuum model or a molecular dynamics simulation of the full system. The significance of the quantum and dynamical effects of the proton donor-acceptor mode is illustrated with applications to model PCET systems.

Influence of protons on Fe-Mg interdiffusion in olivine

Abstract: [1] We report the experimental measurement of Fe-Mg interdiffusivity in olivine along the [001] crystallographic direction in a water-saturated environment at pressures of 0.1 to 6 GPa and temperatures between 1373 and 1450 K. The concentration of water-derived protons in olivine was controlled by varying the water fugacity. The oxygen fugacity was set by the Ni-NiO solid state reaction, while the activity of silica was controlled by the presence of orthopyroxene. In this work, we report diffusivity as a function of temperature, pressure, and water fugacity following the relation Fe–Mg = exp [−(Q + PV*)/RT] m2s−1, where log(Do) = (−14.8 ± 2.7), r = 0.9 ± 0.3, Q = 220 ± 60 kJ/mol, and V* = (16 ± 6) × 10−6m3/mol. The approximately linear increase in diffusivity with increasing water fugacity is consistent with incorporation of protons associated with octahedral cation vacancies to form defect complexes. Our results indicate that cation diffusion in water-saturated olivine is ∼50 times faster than under water-absent conditions at a pressure of 5 GPa and a temperature of 1373 K.

Photoproduction of eta mesons off protons for 0.75 GeV

Abstract: Total and differential cross sections for the reaction p(gamma,eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S-11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into peta. At medium energies we find evidence for a new resonance N(2070)D-15 with (M,Gamma)=(2068+/-22, 295+/-40) MeV. At gamma energies above 1.5 GeV, a strong peak in the forward direction develops, signaling the exchange of vector mesons in the t channel.

Monte Carlo dose calculations for spot scanned proton therapy.

Abstract: Density heterogeneities can have a profound effect on dose distributions for proton therapy. Although analytical calculations in homogeneous media are relatively straightforward, the modelling of the propagation of the beam through density heterogeneities can be more problematical. At the Paul Scherrer Institute, an in-house dedicated Monte Carlo (MC) code has been used for over a decade to assess the possible deficiencies of the analytical calculations in patient geometries. The MC code has been optimized for speed, and as such traces primary protons only through the treatment nozzle and patient's CT. Contributions from nuclear interactions are modelled analytically with no tracing of secondary particles. The MC code has been verified against measured data in water and experimental proton radiographs through a heterogeneous anthropomorphic phantom. In comparison to the analytical calculation, the MC code has been applied to both spot scanned and intensity modulated proton therapy plans, and to a number of cases containing titanium metal implants. In summary, MC-based dose calculations could provide an invaluable tool for independently verifying the calculated dose distribution within a patient geometry as part of a comprehensive quality assurance protocol for proton treatment plans.

In vivo 17O NMR approaches for brain study at high field.

Abstract: O is the only stable oxygen isotope that can be detected by NMR. The quadrupolar moment of 17 Os pin (I ¼ 5/2) can interact with local electric field gradients, resulting in extremely short T1 and T2 relaxation times which are in the range of several milliseconds. One unique NMR property of 17 O spin is the independence of 17 O relaxation times on the magnetic field strength, and this makes it possible to achieve a large sensitivity gain for in vivo 17 O NMR applications at high fields. In vivo 17 O NMR has two major applications for studying brain function and cerebral bioenergetics. The first application is to measure the cerebral blood flow (CBF) through monitoring the washout of inert H2 17 O tracer in the brain tissue following an intravascular bolus injection of the 17 O-labeled water. The second application, perhaps the most important one, is to determine the cerebral metabolic rate of oxygen utilization (CMRO2) through monitoring the dynamic changes of metabolically generated H2 17 O from inhaled 17 O-labeled oxygen gas in the brain tissue. One great merit of in vivo 17 O NMR for the determination of CMRO2 is that only the metabolic H2 17 O is detectable. This merit dramatically simplifies both CMRO2 measurement and quantification compared to other established methods. There are two major NMR approaches for monitoring H2 17 O in vivo, namely direct approach by using 17 O NMR detection (referred as direct in vivo 17 O NMR approach) and indirect approach by using 1 H NMR detection for measuring the changes in T2 -o rT1� -weighted proton NMR signals caused by the 17 O- 1 H scalar coupling and proton chemical exchange (referred as indirect in vivo 17 O NMR approach). Both approaches are suitable for CBF measurements. However, recent studies indicated that the direct in vivo 17 O NMR approach at high/ultrahigh fields appears to offer significant advantages for quantifying and imaging CMRO2 .N ew developments have further demonstrated the feasibility for establishing a completely noninvasive in vivo 17 O NMR approach for imaging CMRO2 in a rat brain during a brief 17 O2 inhalation. This approach should be promising for studying the central role of oxidative metabolism in brain function and neurological diseases. Finally, the similar approach could potentially be applied to image CMRO2 noninvasively in human brain. Copyright # 2005 John Wiley & Sons, Ltd.