Showing papers on "Proton published in 1991"
TL;DR: In this paper, the QCD corrections to the fusion cross section in the Higgs mass range below the top-antitop threshold are presented, and the corrections are large, being of the order of 60%.
765 citations
492 citations
TL;DR: In this paper, a dynamical theory for proton transfer rates in solution can be implemented in a molecular-dynamics simulation for a model reaction system, in which the transfer occurs via quantum tunneling of the proton.
Abstract: It is shown how a dynamical theory for proton transfer rates in solution can be implemented in a molecular‐dynamics simulation for a model reaction system. The reaction is in the nonadiabatic limit, in which the transfer occurs via quantum tunneling of the proton. The importance of the coupling of the proton to the solvent and to an intramolecular vibration is illustrated, and the simulation results are successfully compared with analytic rate‐constant expressions in several limiting regimes.
305 citations
TL;DR: In this article, the authors proposed an out-and-back magnetization transfer approach for 3D triple-resonance measurements of proteins of a substantially larger size, which can be applied to proteins of substantially larger sizes.
281 citations
TL;DR: In this paper, a CaZrO 3 -based ceramics, in which trivalent cations such as In, Ga and Sc were substituted for Zr, were synthesized.
218 citations
TL;DR: The data suggest that nuclear magnetic resonance imaging is sensitive in detecting changes in proton-associated parameters during early cerebral ischemia and confirm significant changes in the temporal evolution of transverse relaxation times, proton densities, and diffusion coefficients following middle cerebral artery occlusion.
Abstract: We studied the effect of focal cerebral ischemia on the "state" of brain water using proton nuclear magnetic resonance imaging. Focal cerebral ischemia was induced in five halothane-anesthetized rats via tandem occlusion of the left common carotid artery and the left middle cerebral artery. The proton transverse relaxation time, the proton density, and the water diffusion coefficient were measured at various times from the same region of brain tissue from 1.5 to 168 hours after occlusion. Early measurements indicated significant changes in the transverse relaxation time (p = 0.004) and water diffusion coefficient (p = 0.002) of ischemic brain tissue compared with a homologous region from the contralateral hemisphere. However, the transverse relaxation time, proton density, and water diffusion coefficient in ischemic brain tissue showed different temporal evolutions over the study period. Diffusion coefficient weighting was superior to relaxation time and proton density weighting for the visualization of early cerebral ischemia. Our data suggest that nuclear magnetic resonance imaging is sensitive in detecting changes in proton-associated parameters during early cerebral ischemia and confirm significant changes (p less than or equal to 0.01) in the temporal evolution of transverse relaxation times, proton densities, and diffusion coefficients following middle cerebral artery occlusion.
207 citations
TL;DR: In this article, a four-dimensional 13C/13C-edited NOESY experiment is described which dramatically improves the resolution of protein NMR spectra and enables the straightforward assignment of nuclear Overhauser effects involving aliphatic and/or aromatic protons in larger proteins.
Abstract: A four-dimensional 13C/13C-edited NOESY experiment is described which dramatically improves the resolution of protein NMR spectra and enables the straightforward assignment of nuclear Overhauser effects involving aliphatic and/or aromatic protons in larger proteins. The experiment is demonstrated for uniformly (greater than 95%) 13C-labeled interleukin 1 beta, a protein of 153 residues and 17.4 kDa, which plays a key role in the immune response. NOEs between aliphatic and/or aromatic protons are first spread out into a third dimension by the 13C chemical shift of the carbon atom attached to the originating proton and subsequently into a fourth dimension by the 13C chemical shift of the carbon atom attached to the destination proton. Thus, each NOE cross peak is labeled by four chemical shifts. By this means, ambiguities in the assignment of NOEs that arise from chemical shift overlap and degeneracy are completely removed. Further, NOEs between protons with the same chemical shifts can readily be detected providing their attached carbon atoms have different 13C chemical shifts. The design of the pulse sequence requires special care to minimize the level of artifacts arising from undesired coherence transfer pathways, and in particular those associated with "diagonal" peaks which correspond to magnetization that has not been transferred from one proton to another.(ABSTRACT TRUNCATED AT 250 WORDS)
153 citations
TL;DR: It is found that the gramicidin channel saturates at approximately 530 pS of protonic current in 4 M HCl, more than sufficient for typical ATP synthesis rates, and it follows that proton diffusion to a putative channel in Fo, rather than the channel itself, may limit ATP synthesis rate.
135 citations
TL;DR: In this article, a balloon-borne superconducting magnetic spectrometer was used to measure the cosmic-ray proton and helium spectra during the 1987 solar minimum using a balloon.
Abstract: The differential cosmic-ray proton and helium spectra have been measured during the 1987 solar minimum using a balloon-borne superconducting magnetic spectrometer launched from Prince Albert, Canada. The changing geomagnetic cutoff along the balloon trajectory was observed in the low-energy proton data to be about 25 percent below the nominal calculated values. The absolute particle fluxes were approximately equal to the highest fluxes observed at the previous solar minimum in 1977. Above 10 GV the observed spectra are represented by a power law in rigidity with spectral indices of 2.74 + or - 0.02 for protons and 2.68 + or - 0.03 for helium. The measurements above 200 MeV per nucleon are consistent with rigidity power-law interstellar spectra modulated with the solar modulation parameter phi = 500 MV. The energy dependence of the proton-to-helium ratio is consistent with rigidity power-law injection spectra and rigidity-dependent propagation without reacceleration.
126 citations
TL;DR: In this paper, the fluorescence decay of I-hydroxypyrene-1,3,6-trisulfonate due to proton transfer to solvent in water-methanol mixtures was measured.
Abstract: We have measured the fluorescence decay of I-hydroxypyrene-1,3,6-trisulfonate due to proton transfer to solvent in water-methanol mixtures. The data agree quantitatively with the reversible, diffusion-influenced proton-transfer mechanism in all solutions. No significant deviations from a short-time exponential decay or a long-time power law decay were found that could support a recently suggested water molecule diffusion mechanism. Solvent-induced variations in the deprotonation rate coefficient are very close to those in the corresponding equilibrium coefficient. Analysis of measured and literature pK data indicates that the composition dependence is mainly due to localized counterion stability in water-rich solutions and to proton stability in methanol-rich solutions. Quantitative agreement with literature compilations of proton-transfer free energy is obtained. These observations challenge the prevailing idea that a minimal cluster of four water molecules is the required proton acceptor in these solutions.
121 citations
TL;DR: In this paper, the authors studied the temperature dependence of the OH vibrational band in LiNbO 3 crystals and showed that the absorption bands consist of at least four components in contrast to more or less perfect stoichiometric crystals which reveal only one to three components.
TL;DR: In this paper, the electronic properties of the acidic site upon aluminum substitution at each of the twelve distinct tetrahedral sites are presented. And the T12 site is found to be the most preferred site for aluminum substitution.
Abstract: Semiempirical molecular orbital calculations have been performed to study aluminum siting in H-ZSM-5 zeolites. Local structural rearrangements upon substituting aluminum (with a charge compensating proton) for silicon are found to be important. The T12 site is found to be the most preferred site for aluminum substitution. However, the calculated energetics for substitution show that several tetrahedral sites are energetically comparable with regard to aluminum siting. Results pertaining to the electronic properties of the acidic site upon aluminum substitution at each of the twelve distinct tetrahedral sites are presented. The acidic center is found to be a rather soft species, with the HOMO-LUMO energy gap being roughly 8 eV.
TL;DR: In this paper, the proton dissociation reaction of excited 1-aminopyrene follows a simple structure-reactivity correlation in water-methanol, water-DMSO and water-dioxane binary mixtures.
Abstract: We demonstrate that the proton dissociation reaction of excited 1-aminopyrene follows a simple structure-reactivity correlation in water-methanol, water-DMSO, water-dioxane, and water-acetonitrile binary mixtures.
TL;DR: The first observation of laser-induced recombination, performed with merged beams of protons, electrons, and laser photons in an ion storage ring, is reported and could be well described by a simple extension of the field-free quantum-mechanical theory.
Abstract: The first observation of laser-induced recombination, performed with merged beams of protons, electrons, and laser photons in an ion storage ring, is reported. The process was used to study the photorecombination spectrum with high resolution. Deviations from earlier theoretical predictions turned out to be due to a weak external electric field and could be well described by a simple extension of the field-free quantum-mechanical theory, based on properties of the classical electron trajectories in the combined external and Coulomb fields.
TL;DR: In this paper, it was shown that a small positive drift velocity between alpha particles and protons can lead to alpha particle velocities well in excess of the proton bulk velocity.
Abstract: Preferential acceleration of alpha particles interacting with left-hand polarized ion cyclotron waves is studied. It is shown that a small positive drift velocity between alpha particles and protons can lead to alpha particle velocities well in excess of the proton bulk velocity. During the acceleration process, which is assumed to take place at heliocentric distances less than 0.3 AU, the alpha particle drift velocity should exceed the proton bulk velocity, and then the gap which exists around the alpha particle gyrofrequency should disappear. It is also shown that for proton thermal anisotropies of the order of those observed in fast solar wind, the waves either grow or are not damped excessively, so that the waves can exist and might thus lead to the observed differential speeds. However, the way in which the alpha particles exceed the proton velocity remains unexplained.
TL;DR: The first of a long series of experiments on highly inelastic electron scattering was started at the two-mile accelerator at the Stanford Linear Accelerator Center (SLAC) using liquid hydrogen and, later, liquid deuterium targets as mentioned in this paper.
Abstract: In late 1967 the first of a long series of experiments on highly inelastic electron scattering was started at the two-mile accelerator at the Stanford Linear Accelerator Center (SLAC) using liquid hydrogen and, later, liquid deuterium targets. Carried out by a collaboration from the Massachusetts Institute of Technology (MIT) and SLAC, the object was to look at large-energy-loss scattering of electrons from the nucleon (the generic name for the proton and neutron), a process soon to be dubbed deep inelastic scattering. Beam energies up to 21 GeV, the highest electron energies then available, and large electron fluxes made it possible to study the nucleon to very much smaller distances than had previously been possible. Because quantum electrodynamics provides an explicit and well-understood description of the interaction of electrons with charges and magnetic moments, electron scattering had, by 1968, already been shown to be a very powerful probe of the structures of complex nuclei and individual nucleons.
TL;DR: In this paper, the authors studied the postcollision interaction distortion of the Ne K-L2.32(1D2) Auger electron line excited by proton bombardment in the energy range 0.7-2.0 MeV.
Abstract: The authors have studied the post-collision interaction distortion of the Ne K-L2.32(1D2) Auger electron line excited by proton bombardment in the energy range 0.7-2.0 MeV. A systematic study of both the bombarding energy and the observation angle dependence has been performed for the first time. They have found the effect to be most pronounced at 0' observation angle and at the bombarding energy where the projectile and the outgoing Auger electron have about the same velocity. The results are in a reasonable agreement with theories which assume PCI between the Auger electron and the outgoing projectile.
TL;DR: A recent theoretical model due to Brereton is shown to account for the complex shape of the transverse relaxation function observed for high molecular weight polyethylenes with low polydispersity as discussed by the authors.
Abstract: A recent theoretical model due to Brereton is shown to account for the complex shape of the transverse relaxation function observed for high molecular weight polyethylenes with low polydispersity showing that this has its origin in the restrictions on molecular mobility imposed by the entangled nature of the polymer melt
TL;DR: In this article, an improved heteronuclear chemical-shit-and-on-elation experiment is introduced which can be applied to the analysis of H13C and other HN correlations in a wide variety of crystalline and amorphous materials.
TL;DR: The measurements carried out by the plasma spectrometer, Aspera, on board the Phobos-2 spacecraft show that protons originating in the extended hydrogen corona of Mars, were observed at altitudes not greater than 7500 km as mentioned in this paper.
Abstract: The measurements carried out by the plasma spectrometer, Aspera, on board the Phobos-2 spacecraft show that protons, originating in the extended hydrogen corona of Mars, were observed at altitudes not greater than 7500 km The cyclotron instability of these pickup ions appears to generate Alfven waves observed by the Magma magnetometer Analysis of the plasma data shows that weak pitch-angle diffusion of a ring-distribution of pickup protons occurs The altitude profiles of pickup proton fluxes and number densities of the parent hydrogen atoms are derived
TL;DR: The spectroscopic results from 24 muscle studies on 17 young male volunteers show remarkable intra‐ and interindividual differences in the absolute signal intensities of mobile lipids.
Abstract: Localized proton NMR spectroscopy using stimulated echoes (STEAM) has been used to study metabolites in different proximal skeletal muscles of normal volunteers at rest. Single scan water-suppressed proton NMR spectra obtained at 1.5 and 2.0 T (Siemens Magnetom) from a 64-ml volume-of-interest (VOI) yield resonances due to triglycerides, phosphocreatine plus a minor contribution from creatine, and betaines comprising carnitine and choline-containing compounds. The observation of the pH-dependent resonances of carnosine required multiple acquisitions and echo times as short as 20 ms. T1 and T2 relaxation times of muscle metabolites were obtained by varying the repetition time and echo time of the STEAM sequence, respectively. Although rather long T2 values such as 180 ms for (phospho-) creatine correspond to natural resonance linewidths of only 2 Hz, the observed linewidths of typically 10-12 Hz are entirely determined by the short T2 relaxation times (25-30 ms) of the water protons used for shimming. The spectroscopic results from 24 muscle studies on 17 young male volunteers show remarkable intra- and interindividual differences in the absolute signal intensities of mobile lipids. Further metabolic variations were observed for the relative concentrations of betaines (by a factor of 2) and carnosine (by a factor of 3) when total creatine is assumed to be constant.
TL;DR: In this paper, the authors investigated the mechanism of back-bending in a fully quantum mechanical way using the angular-momentum projection method and pointed out three hindrance mechanisms for second and higher order back bending.
Journal Article•
TL;DR: In this article, the excited singlet state intermolecular proton transfer reaction in jet-cooled clusters of 1-naphthol/ammonia and water is investigated employing mass resolved excitation, threshold photoionization, and emission spectroscopy.
Abstract: The excited singlet state intermolecular proton transfer reaction in jet‐cooled clusters of 1‐naphthol/ammonia and water is investigated employing mass resolved excitation, threshold photoionization, and emission spectroscopy. The complete data set indicates that no proton transfer occurs for 1‐naphthol(NH3)1,2 and (H2O)n, n=1,...,20 clusters. Proton transfer occurs for (at least) one configuration of the 1‐naphthol(NH3)3 cluster, as well as all 1‐naphthol(NH3)n, n≥4, clusters. The (at least) two configurations of 1‐naphthol(NH3)3 clusters are distinguished by threshold photoionization studies. The 1‐naphthol(NH3)3 cluster for which proton transfer is indicated has a threshold photoionization energy ∼2000 cm−1 below the other 1‐naphthol(NH3)3 cluster configurations. These results are employed to explain the previous discrepancy between static spectroscopic experiments and picosecond time resolved dynamic experiments concerning proton transfer in the 1‐naphthol(NH3)3 cluster. Calculations of cluster geomet...
TL;DR: In this article, the authors estimate the cross section for Higgs-boson production in proton-proton collisions, and they find that most of the cross-section comes from a two-gluon annihilation process, in which the gluons couple to Higgs bosons via heavy-quark loops.
Abstract: We estimate the cross section for Higgs-boson production in proton-proton collisions. We find that most of the cross section comes from a two-gluon annihilation process, in which the gluons couple to Higgs bosons via heavy-quark loops.
TL;DR: In this paper, X-ray photoelectron spectroscopy (XPS) data of Pd clusters in NaY, MgY and HY zeolites show that the chemical shift of the Pd 3d5/2 line is dependent on the concentration of the protons and their position in the zeolite.
Abstract: X-Ray photoelectron spectroscopy (XPS) data of Pd clusters in NaY, MgY and HY zeolites show that the chemical shift of the Pd 3d5/2 line is dependent on the concentration of the protons and their position in the zeolite. The positive charge or ‘electron deficiency’ of the Pd particles is significant even for Pd/NaY which contains only the protons created by H2 reduction of Pd2+ ions. In these samples the Pd 3d5/2 binding energy exceeds that of neutral Pd by ca. 0.4 eV. The positive charge of the Pd clusters is much higher in HY and MgY, reflecting the higher proton concentration in the supercages; the binding energy is shifted by 0.8 eV. Additional relevant information on the metal cluster environment is obtained from analysis of the Auger parameter of Pd. It decreases with increasing number of protons interacting with the Pd particles, and it increases upon substituting protons by polarizable ions, e.g. Na+ or NH+4. These data are consistent with the model which suggests that metal–proton adducts, [Pdn–Hm]m+ with m 1, are formed in proton-containing zeolites.
01 Jan 1991
TL;DR: In this article, it was shown that the fluorescence signal from an excited, negatively charged hydroxypyrene derivative in solution has a non-exponential tail, attributed to reversible geminate recombination in the excited state.
Abstract: Proton dissociation in solution is traditionally described via a two step kinetic mechanism of complex formation and proton escape. This mechanism leads to an ultimate exponential decay of the acid concentration. In contrast, we find that the fluorescence signal from an excited, negatively charged hydroxypyrene derivative in solution has a non-exponential tail, attributed to reversible geminate recombination in the excited state. The theory of reversible diffusion influenced reactions predicts a t -3/2 asymptotic decay, in quantitative agreement with experiment.
TL;DR: Calculations for mammalian cell cultures show that at high dose, at which intertrack effects become important, protons deliver damage similar to that produced by gamma rays, and with fragmentation the relative biological effectiveness of protons increases moderately from unity, while at low dose, where sublethal damage is unimportant, the contribution from target fragments dominates, causing the proton effectiveness to be very different from that of gamma rays with a strongly fluence-dependent RBE.
Abstract: High-energy protons traversing tissue produce local sources of high-linear-energy-transfer ions through nuclear fragmentation. The contribution of these target fragments to the biological effectiveness of high-energy protons using the cellular track model is examined. The effects of secondary ions are treated in terms of the production collision density using energy-dependent parameters from a high-energy fragmentation model. Calculations for mammalian cell cultures show that at high dose, at which intertrack effects become important, protons deliver damage similar to that produced by gamma rays, and with fragmentation the relative biological effectiveness (RBE) of protons increases moderately from unity. At low dose, where sublethal damage is unimportant, the contribution from target fragments dominates, causing the proton effectiveness to be very different from that of gamma rays with a strongly fluence-dependent RBE. At high energies, the nuclear fragmentation cross sections become independent of energy. This leads to a plateau in the proton single-particle-action cross section, below 1 keV/micron, since the target fragments dominate.