Showing papers on "Proton published in 1992"

Proton transfer in reaction centers from photosynthetic bacteria

Abstract: Proton transfer in the bacterial RC associated with the reduction of the bound QB to the dihydroquinone is an important step in the energetics of photosynthetic bacteria. The binding of two protons by the quinone is associated with the transfer of the second electron to QB at a rate of ca. 10(3) s-1 (pH 7). Mutation of three protonatable residues, GluL212, SerL223, and AspL213, located near QB to nonprotonatable residues (Gln, Ala, and Asn, respectively) resulted in large reductions (by 2 to 3 orders of magnitude) in the rate or proton transfer to QB. These mutations can be grouped into two classes: those that blocked both proton transfer and electron transfer (SerL223, and AspL213) and those that blocked only proton transfer (GluL212). These results were interpreted in terms of a pathway for proton transport in which uptake of the first proton, required for the transfer of the second electron, occurs through a pathway involving AspL213 and SerL223. Uptake of the second proton, which follows electron transfer, occurs through a pathway involving GluL212 and possibly AspL213. Acidic residues near QB affect electron transfer rates via electrostatic interactions. One residue, with a pKa of ca. 10 interacting strongly with the charge on QB (delta pKa greater than 2), was shown to be GluL212. A second residue with a pKa of ca. 6, which interacts more weakly with the charge on QB (delta pK approximately 1), could be either AspL210 or AspL213. Several possible mechanisms for proton transfer are consistent with the observed experimental results and proposed proton pathways. These involve proton transfers from individual amino acid residues or internal water molecules either as single steps or in a concerted fashion. The determination of the dominant mechanism will require evaluation of the energetics of the various steps.

Electron production in proton collisions with atoms and molecules: energy distributions

Abstract: All known data on the energy distribution of secondary electrons from collisions of protons with atoms and molecules have been reviewed and differential cross sections have been collected. The two experimental methods used to obtain the data are discussed and possible sources of error pointed out. Theoretical treatments are reviewed and several methods of checking the consistency of the data are discussed. Two semiempirical models have been chosen to represent the differential cross sections, and parameters for these models are given which fit the average of the experimental data, subject to known constraints. Recommended values of differential cross sections are given for ten target gases by means of these models.

The mirror and ion cyclotron anisotropy instabilities

Abstract: The linear dispersion equation for fully electromagnetic waves and instabilities at arbitrary directions of propagation relative to a background magnetic field B(0) in a homogeneous Vlasov plasma is solved numerically for bi-Maxwellian particle distributions. For isotropic plasmas the dispersion and damping of the three modes below the proton cyclotron frequency are studied as functions of Beta(i) and T(e)/T(i). The transport ratios of helicity, cross-helicity, Alfven ratio, compressibility, and parallel compressibility are defined. Under the condition that the proton temperature perpendicular to B(0) is greater than the parallel temperature, the growth rates and transport ratios of the mirror instability and the ion cyclotron anisotropy instability are examined and compared. Both the proton parallel compressibility and the proton Alfven ratio are significantly different for the two growing modes.

Radiation-Induced Acid Generation Reactions in Chemically Amplified Resists for Electron Beam and X-Ray Lithography

Abstract: The radiation-induced reactions of onium salts in some kinds of solutions and model compound solutions of chemically amplified electron beam (EB) and X-ray resists have been studied by means of picosecond and nanosecond pulse radiolysis. The following reaction mechanisms of the chemically amplified EB and X-ray resists have been elucidated. The radiation-induced reaction mechanisms are complicated due to the presence of several proton donors. The onium salts directly produce small amounts of Bronsted acids by EB and X-ray exposure and most of the Bronsted acids are formed from proton adducts of the base polymer. The onium salts are strong electron scavengers and promote the generation of the proton adducts in the chemically amplified resists.

Proton halo of B 8 disclosed by its giant quadrupole moment

Abstract: The quadrupole moment of the $^{8}\mathrm{B}$ (${\mathit{I}}^{\mathrm{\ensuremath{\pi}}}$=${2}^{+}$, ${\mathit{T}}_{1/2}$=769 msec) nucleus was measured as \ensuremath{\Vert}Q${(}^{8}$B)\ensuremath{\Vert}=68.3\ifmmode\pm\else\textpm\fi{}2.1 mb by use of modified \ensuremath{\beta}-NMR. This value is twice as large as the prediction of the Cohen-Kurath shell model. It is found by subtracting the contribution of deeply bound neutrons that the protons in $^{8}\mathrm{B}$ carry more than 90% of the observed moment. The anomalous value is accounted for fairly well by the proton halo due to the loosely bound valence configuration. This is the first experimental evidence for the existence of a proton halo covering a neutron core.

Electron-Transfer-Induced Acidity/Basicity and Reactivity Changes of Purine and Pyrimidine Bases. Consequences of Redox Processes for Dna Base Pairs

Abstract: Changes in the oxidation state of the DNA bases, induced by oxidation (ionization) or by reduction (electron capture), have drastic effects on the acidity or basicity, respectively, of the molecules. Since in DNA every base is connected to its complementary base in the other strand, any change of the electric charge status of a base in one DNA strand that accompanies its oxidation or reduction may affect also the other strand via proton transfer across the hydrogen bonds in the base pairs. The free energies for electron transfer to or from a base can be drastically altered by the proton transfer processes that accompany the electron transfer reactions. Electron-transfer (ET) induced proton transfer sensitizes the base opposite to the ET-damaged base to redox damage, i.e., damage produced by separation of charge (ionization) has an increased change of being trapped in a base pair. Of the two types of base pair in DNA, A-T and C-G, the latter is more sensitive to both oxidative and reductive processes than the former. Proton transfer induced by ET does not only occur between the heteroatoms (O and N) of the base pairs (intra-pair proton transfer), but also to and from adjacent water molecules in the hydration shell of DNA (extra-pair proton transfer). These proton transfers can involve carbon and as such are likely to be irreversible. It is the A-T pair which appears to be particularly prone to such irreversible reactions.

Electron and Proton Transfer in Chemistry and Biology

Abstract: Mechanisms of Electron Transfer (R.D. Cannon). Electron Transfer Effect in Chemical Compounds (B. Jezowska-Trzebiatowska and W. Wojciechowski). Light Induced Electron Transfer of Metal Complexes (A. Vogler and H. Kunkely). Coordinative Aspects of Single Electron Exchange between Main Group or Transition Metal Compounds and Unsaturated Organic Substrates (W. Kaim). Electron Hopping and Delocalization in Mixed-Valence Metal-Oxygen Clusters (H. So and M.T. Pope). Electron Transfer in Semiconducting Colloids and Membranes, Applications in Artificial Photosynthesis (M. Gratzel). Electron Transfer in Photosynthetic Reaction Centers (Ch.C. Moser et al.). Exchange Interaction in Electron Transfer Proteins and their Model Compounds (W. Haase and S. Gehring). Multi-Electron Transfer Processes in Nitrogen Fixation and other Natural Systems (D.J. Lowe). Electron Transfer in Anaerobic Microorganisms (A. Kroger et al.). The Importance of Inhibitors as an Analytic Tool for the Study of the Quinol Oxidation Centre and the Quinol Oxidase Reaction (G. von Jagow and U. Brandt). Electron and Proton Transfer through the Mitochondrial Respiratory Chain (T.A. Link). Coupled Proton and Electron Transfer Pathways in the Acceptor Quinone Complex of Reaction Centers from Rhodobacter Sphaeroides (E. Takahashi et al.). The Water Oxidizing Enzyme - An Alternative Model (E.K. Pistorius). Proton Pumps, Proton Flow and Proton ATP Synthases in Photosynthesis of Green Plants (W. Junge et al.). Diffusion of Proton in Microscopic Space: Effect of Geometric Constraints and Dielectric Discontinuities (M. Gutman et al.). Protonation of the Schiff Base Chromophore in Rhodopsins (C. Sandorfy). Proton Transfer along the Hydrogen Bridge in Some Hydrogen-Bonded Molecular Complexes (H. Ratajczak). Hydrogen-Bonded Systems with Large Proton Polarizability due to Collective Proton Motion as Pathways of Protons in Biological Systems (G. Zundel). NMR Studies of Multiple Proton and Deuteron Transfers in Liquids, Crystals and Organic Glasses (H.-H. Limbach). Proton Transfer Reactions in Solutions: A Molecular Approach (D. Borgis). Recent Developments in Solitonic Model of Proton Transfer in Quasi-One-Dimensional Infinite Hydrogen-Bonded Systems (E. Kryachko). Index of Contributing Authors. Subject Index.

Computational studies of acid sites in ZSM 5: dependence on cluster size

Abstract: An ab initio molecular orbital study of the proton affinities, OH stretching frequencies, and ammonia interactions of Si{sub x}TO{sub y}H{sub z} (T = Si, Al) clusters representing the ZSM-5 zeolite framework has been carried out. The dependence of these properties on cluster size was examined. The proton affinity of the central bridging Si-O-Al site was found to be slowly convergent with cluster size. In calculations on clusters containing up to two shells of silicons and two shells of oxygens beyond the bridge, the proton affinity varies by as much as 30 kcal/mol with each additional shell. Addition of a silicon shell reduces the proton affinity varies by as much as 30 kcal/mol with each additional shell. Addition of a silicon shell reduces the proton affinity and addition of an oxygen shell increases the proton affinity. This is due to the long range electrostatic interaction of the added shells on the bridging hydroxyl site. The clusters containing a central Si-O-Si bridge exhibit a similar slow convergence of the proton affinity. The OH stretching frequencies are much less dependent on cluster size than the proton affinities. The theoretical proton affinities and OH stretching frequencies are much less dependent on cluster size thanmore » the proton affinities. The theoretical proton affinities and OH stretching frequencies from the T = Al clusters are consistent with experiment. The interaction energy of the `ionic` structure that results from interaction of the ammonia with the acid site, Z{sup {minus}}...HNH{sub 3}{sup +}, has a slow convergence with cluster size similar to the proton affinity and the `ionic` structure increases instability relative to the `covalent` structure, ZH...NH{sub 3}, with increasing cluster size. The NH{sub 3} desorption energy is inversely proportional to the proton affinities of the clusters of the same size representing the ZSM-5 zeolite. 54 refs., 8 figs., 7 tabs.« less

Lipid-suppressed single- and multisection proton spectroscopic imaging of the human brain.

Abstract: Spectroscopic images of the brain have great potential in disease diagnosis and treatment monitoring. Unfortunately, interfering lipid signals from subcutaneous fat and poor water suppression due to magnetic field inhomogeneities can make such images difficult to obtain. A pulse sequence that uses inversion recovery for lipid suppression and a spectral-spatial refocusing pulse for water suppression is introduced. In contrast to methods that eliminate fat signal by restricting the excited volume to lie completely within the brain, inversion-recovery techniques allow imaging of an entire section without such restrictions. In addition, the spectral-spatial pulse was designed to provide water suppression insensitive to a reasonable range of B0 and B1 inhomogeneities. Several data processing algorithms have also been developed and used in conjunction with the new pulse sequence to produce metabolite maps covering large volumes of the human brain. Images from single- and multisection studies demonstrate the performance of these techniques.

Molecular‐dynamics study of adiabatic proton‐transfer reactions in solution

Abstract: A molecular‐dynamics study of adiabatic proton transfer between two ions in a polar solvent is presented. The proton is treated as a quantum particle in three dimensions and the polar solvent is composed of classical rigid, dipolar molecules. The coupled Schrodinger and Newton’s equations are solved to determine the proton charge density and solvent configuration. The rate coefficient for the proton transfer is computed from correlation function expressions and corrections to transition‐state theory due to recrossing of a free‐energy barrier are determined. The simulation results are compared with a simple two‐state model.

Direct comparison between protons and alpha-particles of the same LET: I. Irradiation methods and inactivation of asynchronous V79, HeLa and C3H 10T1/2 cells.

Abstract: A direct comparison was carried out of the biological effectiveness of protons and alpha-particles of the same linear energy transfer (LET) under identical conditions with a variety of in vitro biological systems. Monolayers of mammalian cells were irradiated with accelerated beams of protons (1.2 and 1.4 MeV) and alpha-particles (30 and 35 MeV) corresponding to LETs of 23 and 20 keV microns-1 for each particle type. For V79-4 cells it was observed that the linear term of the dose-response for cell inactivation by protons was significantly greater than that for alpha-particles of the same LET. For HeLa and HeLa S3 cells, also, the linear term appeared to be greater for protons, but this was not observed with more limited data for C3H 10T1/2 cells. The result for V79 cells is in agreement with the report of Belli et al. (1989) who observed that the biological effectiveness of protons rose sharply between 17 and 30 keV microns-1 in strong contrast to alpha-particles which reached a peak effectiveness at greater than 100 keV microns-1. These results place new constraints on the biologically relevant features of the microscopic structure of radiation tracks, and have implications for the mechanistic and practical comparison between radiations.

Excited-state proton transfer in 1-naphthol/ammonia clusters

Abstract: Excited‐state proton transfer dynamics are reported for the 1‐naphthol(NH3)n cluster system for n=3 and 4. Picosecond time‐ and mass‐resolved pump (S1←S0)–probe (I←S1) experiments demonstrate the following results: (1) excited‐state proton transfer occurs for n=3 and 4 clusters only; (2) for n=5 clusters the proton is transferred in the ground state and for n=2 clusters no proton transfer can be observed; (3) the proton transfer time in the n=3 cluster at the 000 transition is ca. 60 ps; (4) this time is reduced to ca. 40 ps and ca. 10 ps for 800 and 1400 cm−1 of vibrational energy in S1, respectively; (5) for the n=4 clusters these times are approximately 70, 70, and 30 ps, for 0, 800, and 1400 cm−1 of vibrational energy in S1, respectively; (6) both n=3 and 4 clusters exhibit a second low‐amplitude decay component, which is about an order of magnitude slower than the initial decay; and (7) 1‐naphthol‐d1(ND3)n clusters have a greatly reduced rate constant for the excited‐state proton transfer dynamics. T...

Chemical reactions in clusters

Abstract: : Four different classes of cluster chemical reactions are reviewed and specific examples from our laboratory are given for each class. Solvent induced electron transfer reactions are illustrated by our studies of 4- dimethylaminobenzonitrile/acetonitrile clusters. The electron transfer reaction depends on solvent polarity and on cluster structure. The reaction is induced by only one properly oriented CH 3 CN solvent molecule. Proton transfer reactions in neutral clusters are exemplified by the 1-naphthol/ammonia cluster system. Proton transfer occurs in the first excited singlet state of the 1-naphthol (NH 3)3 cluster of the proper geometry. Two time decays are measured for this event: one dealing with proton transfer and the other with solvent reorganization or relaxation following the transfer event. Isotope, energy, cluster size dependence, and model calculations demonstrate a proton tunneling mechanism is appropriate for this reaction. Cluster ion chemistry for toluene, toluene-d3, benzyl alcohol, benzyl-alpha, alpha-d2 alcohol, benzyl-alpha, a-Me2 alcohol/ammonia and water clusters is also discussed. These ionic reactions are characterized by benzyl-like radical formation, solvation of protons, and extensive cluster fragmentation following both ion formation and proton transfer. Finally, a preliminary study of radical reactions in clusters is presented for the benzyl radical clustered with ethylene, propylene, and actylene.

The Fate of 7Be in the Sun

Abstract: The electron- and proton-capture rates of 7 Be important to the solar neutrino «problem» are re-examinated. Although the assumptions implied by the traditional Debye approximation for plasma screening are not valid, a careful numerical study changes the electron capture rate by less than 2%. We extrapolate experimental data on the proton capture reaction to astrophysically relevant energies using an energy dependence that includes d-wave scattering and is shown to be relatively independent of the model space and interaction used. It is found that the solar proton capture rate is lowered by approximately 7% from the currently accepted value

Energy loss of 70 MeV protons in elements

Abstract: Measurements of the average stopping power relative to Al of 70 MeV protons losing energies of about 30 MeV in many elements and in water have been made with a simple apparatus, using Bragg ionization curves. Based on an l -value of 166 eV for aluminum, the value l = 79.7 eV was obtained for water. The standard deviation of the ratio of average stopping powers is ±0.08% for aluminum and water, and is estimated to be ±0.25% for other substances. Fluctuations of up to 1.2% in the mean range of the proton beam from the cyclotron were measured.

Proton transfer during the reaction between fully reduced cytochrome c oxidase and dioxygen: pH and deuterium isotope effects.

Abstract: The pH dependence of proton uptake and electron transfers during the reaction between fully reduced cytochrome c oxidase and oxygen has been studied using the flow-flash method. Proton uptake was monitored using different pH indicators. We have also investigated the effect of D2O on the electron-transfer reactions. Proton uptake was biphasic throughout the pH range studied (6.3-9.3), and the decrease of the observed rate constants at increasing pH could be described by titration curves with pKa values of 8-8.5. Of the four phases resolved in the redox reaction, the rate constants for the first two were independent of pH, whereas that of the third decreased at increasing pH with a pKa of 7.9. All phases except the first were slower in D2O than in H2O. The values obtained for kH/kD were 1.0 for the first phase, 1.4 for the second and third phases, and 2.5 for the fourth phase. We suggest from these results that the fast phase of proton uptake is initiated by the second phase of the redox reaction and that this step includes a partially rate-limiting internal proton transfer. The third and fourth phases of the redox reaction are suggested to be rate limited by proton uptake from the medium. The pH dependencies of the proton uptake reactions are consistent with the participation of a titrable group in the protein in proton transfer from the medium to the oxygen-binding site.

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

Abstract: 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.

Measurement of particle production in proton-induced reactions at 14.6 GeV/c.

Abstract: Particle production in proton-induced reactions at 14.6 GeV/{ital c} on Be, Al, Cu, and Au targets has been systematically studied using the E-802 spectrometer at the BNL-Alternating Gradient Synchrotron. Particles are measured in the angular range from 5{degree} to 58{degree} and identified up to momenta of 5, 3.5, and 8 GeV/{ital c} for pions, kaons, and protons, respectively. Mechanisms for particle production are discussed in comparison with heavy-ion-induced reactions at the same incident energy per nucleon.