Proton-induced Transition Research Articles

Recommended data on proton-ion collision rate coefficients for Fe X–Fe XXIII ions

Aims. Proton-ion collisions are important for excitation or de-excitation of certain excited states in a high-temperature low density astrophysical plasma. Methods. In the present work an evaluation of data obtained for proton-induced transitions in Fe X–Fe XXIII ions is carried out with the help of different theoretical methods. Results. A simple analytical formula with seven parameters is suggested. This formula allows us to describe the dependence of the proton rate coefficient on the proton temperature in a wide temperature range. The values of the free parameters have been determined by fitting the approximation formula to numerical data and are presented for recommended data together with fitting accuracies. By comparing proton collision rates with electron rates it is shown that proton impact excitation processes may be important for Fe X, XI, XIII–XV, XVII–XXIII ions. The results obtained can be used for plasma kinetics calculations and for the development of spectroscopic methods of astrophysical plasma diagnostics.

Final-state interactions in hypernuclear decay

We present an update of the one-meson-exchange (OME) results for the weak decay of s- and p-shell hypernuclei [A. Parre\~no, A. Ramos, and C. Bennhold, Phys. Rev. C 56, 339 (1997)], paying special attention to the role played by final state interactions between the emitted nucleons. The present study also corrects for a mistake in the inclusion of the K and ${K}^{*}$ exchange mechanisms, which substantially increases the ratio of neutron-induced to proton-induced transitions ${\ensuremath{\Gamma}}_{n}/{\ensuremath{\Gamma}}_{p}.$ With the most up-to-date model ingredients, we find that the OME approach is able to describe very satisfactorily most of the measured observables, including the ratio ${\ensuremath{\Gamma}}_{n}/{\ensuremath{\Gamma}}_{p}.$

Evidence for Hoogsteen GC base pairs in the proton-induced transition from right-handed to left-handed poly(dG-dC).poly(dG-dC).

The structure of double-helical poly(dG-dC).poly(dG-dC) is investigated at various pH values with Raman spectroscopy, absorption spectroscopy, and circular dichroism. A comparison is made between the B-form with Watson-Crick base pairing at 1 mM [Na+] and pH 7.2, the Z-form with Watson-Crick base pairing at 4 M [Na+] and pH 7.2, and a different structure at 1 mM [Na+] and pH 4.5 as well as at 150 mM [Na+] and pH 3.1. The CD spectrum of poly(dG-dC). poly(dG-dC) under the latter conditions does not show a negative band at 290 nm. The structure is a double-helical structure different from the B-form and the Z-form according to circular dichroism, Raman, and absorption spectroscopic studies. The Raman spectra evidence that the structure contains Hoogsteen base pairing. This can be accommodated in the double helix when the cytosine group is protonated and the sugar-guanine conformer has adopted a C2'-endo/syn conformation. It is shown that this antiparallel-stranded Hoogsteen base paired structure can be maintained under varying conditions, balancing the decrease in pH with an increased salt concentration. It is further concluded that the proton-induced transition from a Watson-Crick to a Hoogsteen base pair is aided by a decrease of [Na+] at pH 4.5 and occurs prior to a conversion from a right-handed helix to a left-handed helix.

Colloid Chemistry of Kaolinitic Tropical Soils

AbstractHighly weathered soils of the humid tropics may exhibit loss of aggregate stability and dispersion of hydroxyl‐bearing soil constituents as a result of management‐induced changes in soil solution chemistry. The effects of pH and 1:1 “indifferent” electrolyte concentration on the surface charge and flocculation of colloids extracted from four representative kaolinitic tropical soils were examined by microelectrophoresis and light scattering methods. Electrophoretic mobilities (u) of clay‐sized particles (0.1‐1.0‐µm diameter) were predominantly negative between pH 2 and 6 for all soils, but decreased in magnitude with increasing solution proton concentration. Measured u values were between −3.0 × 10−8 and +0.5 × 10−8 m2s−1V−1, with isoelectric points (i.e.p.) ≤ 2.5. Photon correlation spectroscopy revealed an abrupt change from small to large particle size with increasing proton concentration in the pH range 2.5 to 3.5. Light scattering by whole‐soil suspensions indicated a proton‐induced transition from kinetically stable to kinetically unstable suspensions in the same pH range. Rapid flocculation occurred in the electrophoretic mobility range −1.2 to −1.8 × 10−8 m2 s−1 V−1. The pH and ionic strength dependence of colloidal stability observed for the soils is consistent with an electrostatic stabilization mechanism among their clay‐sized particles.

Spectral properties and reactivity towards azide of Dicrocoelium dendriticum met-hemoglobin

The spectroscopic properties of Dicrocoelium dendriticum met-hemoglobin, investigated between pH 3.8 and 10.5, display two proton-induced transitions with apparent pK values of 8.1 and 4.7. The spectral changes, over the pH region 6.5 to 10.5, correspond to the high to low spin transition usually observed in ferric hemoproteins. The relaxation time (tau = 3.2 ms at the pK) associated with this transition is closely similar to that observed in Aplysia limacina met-myoglobin, but approximately 1000-fold slower than that of sperm whale met-myoglobin. The spectral changes associated with the more acid transition are in the opposite direction and have not been resolved by the temperature jump method. The rate constants for the reaction of azide with Dicrocoelium dendriticum met-hemoglobin were measured between pH 3.8 and 6.7 by the temperature jump method. Two kinetic schemes, both consistent with the pH dependence of the apparent rate constant for binding of azide, were identified. No objective way of discriminating between the two models is available. However, one of them is amenable to a physical interpretation based on a comparison with the structural, kinetic and spectral properties of other monomeric hemoproteins.

The intensities of the resonance lines of highly ionized hydrogen-like ions

The collisional-radiative model for the distribution of population densities among the energy states of a hydrogen-like ion plasma has been extended to lower electron densities by including the additional atomic processes of two-photon decay of the 2S level and electron- and proton-induced transitions between the component angular momentum sublevels in principal levels 2 and 3. The intensity ratio of the Lyman alpha (1S-2P) to Lyman beta (1S-3P)lines in the hydrogen-like ions (Lyman ratio) is calculated for a wide range of electron temperature and density. New experimental measurements give Lyman ratios about 60-80% of theoretical predictions, compared with about 35% for previously reported measurements. It is believed that these discrepancies are due to self-absorption.

A synthetic DNA with unusual base-pairing.

The alternating polydeoxynucleotide poly[d(A‐s4T) · d(A‐s4T)] with 4‐thiothymidine substituting for thymidine shows anomalous spectral and hydrodynamic properties which are inconsistent with a Watson‐Crick structure but agree with a model of a left‐handed double helix built up from Haschemeyer‐Sobell (reversed Hoogsteen) base‐pairs instead.Optical‐rotatory‐dispersion and circular‐dichroism spectra exhibit a strong negative Cotton effect centered near 400 nm that is assigned to a n→π* transition of the thioamide chromophor and that is at least unexpected if the sulfur atom participates in the hydrogen bridge. From the inversion of the Cotton effects between 370 and 400 nm with respect to the monomer or randomcoil state it is inferred that 4‐thiothymidine attains the unusual syn conformation in the ordered polymer. At pH below 4.5 the polymer is protonated at N‐1 of the deoxyadenosine residues and changes into a different helical conformation. As the inflection point of the proton‐induced transition falls close to the basic pK of deoxyadenosine monophosphate, also N‐1 (A) is probably not involved in hydrogen bonding. Therefore, only N‐7 (A) and 0‐2 (s4T) remain as proton acceptors for base‐pairing.The polymer undergoes cooperative temperature‐dependent transitions in concentrated solutions of 1:1 electrolytes. Two more helical conformations are characterized by ultraviolet, optical‐rotatory dispersion, and circular dichroism. In addition, changes in hydrodynamic parameters, e.g. sedimentation coefficient, buoyant density and partial specific‐volume are reported. A structural hysteresis is observed in a helix‐helix transition and is discussed in terms of metastable states indicated from a phase diagram.