Substate Populations Research Articles

Resonance Raman and EPR investigations of the D251N oxycytochrome P450cam/putidaredoxin complex.

We have performed resonance Raman and electron paramagnetic resonance (EPR) studies on the dioxygen bound state of the D251N mutant of cytochrome P450cam (oxy-P450cam) and its complex with reduced putidaredoxin (Pd). The D251N oxy-P450cam/Pd complex has a perturbed proton delivery mechanism and shows a significantly red-shifted UV-visible spectrum as observed in Benson et al. [Benson, D. E., Suslick, K. S., and Sligar, S. G. (1997) Biochemistry 36, 5104-5107]. The red shift has been interpreted to indicate a major perturbation of the electronic structure of the oxy-heme complex. However, we find no evidence that electron transfer has occurred from Pd to the heme active site of D251N oxy-P450cam. This suggests that both electron and proton transfer are perturbed by the D251N mutation and that these processes may be coupled. Three oxygen isotope sensitive Raman features are identified in the Pd complex, and occur at 1137, 536, and 399 cm(-1). These values are not significantly different from those for WT or D251N oxy-P450cam. However, a careful examination of the oxygen stretching feature near 1137 cm(-1) reveals the presence of three peaks at 1131, 1138, and 1146 cm(-1), which we attribute to the presence of conformational substates in oxy-P450cam. A significant change in the conformational substate population is observed for the D251N oxy-P450cam when the Pd complex is formed. We suggest that the conformational population redistribution of oxy-P450cam, along with the red-shifted electronic spectra, reflects a structural equilibrium of the oxy-heme that is perturbed upon Pd binding. We propose that this structural perturbation is connected to the effector function of Pd and may involve changes in the electron donation properties of the thiolate ligand.

Estimation of the BII Conformer Substate Population in Nonoriented Hydrated B-DNA via Curve Resolution of Infrared Spectra

Hydrated films of the self-complementary B-type d(CGCGAATTCGCG)2 dodecamer, with 13 or 14 water molecules per nucleotide, were studied by Fourier transform infrared (FT-IR) spectroscopy either as equilibrated samples between 290 and 260 K or, after quenching into the glassy state, as nonequilibrated films isothermally at 200 K. IR spectral changes on isothermal relaxation at 200 K revealed that difference spectra are caused by interconversion of B-DNA's conformer substates, BI converting into BII. Curve resolution of IR spectra in the spectral region containing the symmetric stretching vibration of the ionic phosphate group, with a composite band centered at 1087 cm−1 for BI and at 1109 cm−1 for BII, gives the BI/BII conformer substate population ratio and its dependence on temperature between 290 and 260 K, and on time for isothermal relaxation at 200 K. Curve resolution of the overlapping composite band profile was optimized by comparison of second-derivative and of difference curves, aiming thereby for optimal correspondence between experimental and curve-fitted bands as a criterion for goodness-of-fit. The spectroscopic aspects relevant for reliable curve resolution of the overlapping composite band profile are reported and discussed in detail.

Transport of Kr35+ Inner-Shells Through Solid Carbon Foils

New experimental data on the transport of Kr35+ inner-shells initially populated either by capture or by excitation processes are presented. Absolute Lyman (np → 1s) intensities, directly related to the np state populations, as well as 3lj substate populations have been determined over a range of carbon target thickness allowing to study the transport from single collision conditions to equilibrium. Results are compared with predictions of different transport simulations which take into account multiple collisions, the strong target polarization induced by the incoming HCI (the wake field), and radiative decay. Very good agreement is found between theory and experiment for the np populations up to n = 5 where induced wake mixing becomes visible. The simulations also explain the behavior of the 3lj populations of Kr35+ which exhibit a strong sensitivity to the presence of radiative decay during transport and the effective value of the wake field.

B-DNA's BII Conformer Substate Population Increases with Decreasing Water Activity. 2. A Fourier Transform Infrared Spectroscopic Study of Nonoriented d(CGCGAATTCGCG)2

Nonoriented hydrated films of the sodium salt of the d(CGCGAATTCGCG)2 dodecamer, with Γ (water molecules per nucleotide) = 20, 14, 8, and 6, were investigated by Fourier transform infrared spectros...

B-DNA's BII Conformer Substate Population Increases with Decreasing Water Activity. 1. A Molecular Dynamics Study of d(CGCGAATTCGCG)2

The BII substate of the biologically active B-DNA family is characterized by a different phosphate backbone geometry. In contrast to the current belief, the BII substate of the d(CGCGAATTCGCG)2 dodecamer is shownboth experimentally and by molecular dynamics simulationsto be significantly populated at water activities in the range of those in the cell nucleus. Three systems with different water contents show an increase in BI ⇌ BII transitions as the water activity decreases. In addition, the transition mechanism is investigated and the sequence dependence of these interconversion processes in the form of pyrimidine−purine interconversion dominance is demonstrated.

Unexpected BII Conformer Substate Population in Unoriented Hydrated Films of the d(CGCGAATTCGCG)2 Dodecamer and of Native B-DNA from Salmon Testes

Conformational substates of B-DNA had been observed so far in synthetic oligonucleotides but not in naturally occurring highly polymeric B-DNA. Our low-temperature experiments show that native B-DNA from salmon testes and the d(CGCGAATTCGCG)2 dodecamer have the same BI and BII substates. Nonequilibrium distribution of conformer population was generated by quenching hydrated unoriented films to 200K, and isothermal structural relaxation toward equilibrium by interconversion of substates was followed by Fourier transform infrared spectroscopy. BI interconverts into BII on isothermal relaxation at 200K, whereas on slow cooling from ambient temperature, BII interconverts into BI. Our estimation of the dodecamer's BI-to-BII conformer substate population by curve resolution of the symmetrical stretching vibration of the ionic phosphate is 2.4±0.5 to 1 at 200K, and it is 1.3±0.5 to 1 between 270 and 290K. Pronounced spectral changes upon BI-to-BII interconversion are consistent with base destacking coupled with migration of water from ionic phosphate toward the phosphodiester and sugar moieties. Nonspecific interaction of proteins with the DNA backbone could become specific by induced-fit-type interactions with either BI or BII backbone conformations. This suggests that the BI-to-BII substate interconversion could be a major contributor to the protein recognition process.

Installation of an on-line Lamb shift spin-filter polarimeter in the Triangle Universities Nuclear Laboratory atomic beam polarized ion sourcea)

An atomic beam polarized ion source, used heavily since 1989 for producing polarized H± and D± beams for experiments between 25 keV and 20 MeV, has been modified to accept a Lamb shift, spin-filter polarimeter. In this source, polarized ground-state H or D atoms enter an electron cyclotron resonance ionizer where they are stripped to produce an outgoing positive polarized ion beam. When negative ions are desired, cesium vapor is introduced into a downstream charge-exchange canal. The polarimeter, based on an atomic physics concept first developed to produce nuclear-spin-polarized beams at Los Alamos, is designed to monitor the polarization of 2S1/2 metastable H or D atoms emerging from the cesium canal. Metastable 2S1/2 atoms created by electron pickup in a collision with cesium are ‘‘filtered’’ by the polarimeter according to magnetic substate, as the magnetic field imposed on the polarimeter cavity is tuned between 53 and 61 mT. Photons produced by subsequent quenching of these filtered atoms to their ground state are monitored downstream by a phototube to reveal the magnetic substate population of the incident positive beam. To install the polarimeter cavity and phototube assembly, the existing polarized ion source was lengthened by 30 cm. Installation is complete, and comparisons with calibrated nuclear polarimeters have shown agreement to better than 0.023. Principles of operation, a description of the hardware, measurements for cross calibration, and impressions gained from its use are all presented.

Calculation of the spin deorientation in (α,2nγ) reactions

Abstract A general method for the calculation of the nuclear-spin orientation at various stages of compound-nucleus reactions is presented. Results of calculations performed by this method are compared with experimental data on the spin orientation of discrete 110,112 Cd levels populated in the 108,110 Pd(α,2nγ) reactions at various projectile energies. The validity of the Gaussian hypothesis concerning the side-feeding part of the m -substate population is discussed.

Installation of an on-line Lamb–shift spin–filter polarimeter in the Triangle Universities Nuclear Laboratory atomic beam polarized ion source (abstract)a)

An atomic beam polarized ion source, used heavily since 1989 for producing polarized H± and D± beams for experiments between 25 keV and 20 MeV, have been modified to accept a Lamb–shift, spin–filter polarimeter. In this source, polarized ground-state H or D atoms enter an electron cyclotron resonance ionizer where they are stripped to produce an outgoing positive polarized ion beam. When negative ions are desired, cesium vapor is introduced into a downstream charge-exchange canal. The polarimeter, based on an atomic physics concept first developed to produce nuclear–spin-polarized beams at Los Alamos, is designed to monitor the polarization of 2S1/2 metastable H or D atoms emerging from this cesium canal. Metastable 2S1/2 atoms created by electron pickup in a collision with cesium are ‘‘filtered’’ by the polarimeter according to a magnetic substate, as the magnetic field imposed on the polarimeter cavity is tuned between 53 and 61 mT. Photons produced by subsequent quenching of these filtered atoms to their ground state are monitored downstream by a phototube to reveal the magnetic substate population of the incident positive beam. To install the polarimeter cavity and phototube assembly, the existing polarized ion source was lengthened by 30 cm. Installation is complete, its calibration has been checked, and initial impressions gained from its use are presented.

Lipid-peptide interface: valine conformation and dynamics in the gramicidin channel.

High-resolution dynamic and structural characterizations have been achieved for each of the valine side chains of the gramicidin channel while solubilized in hydrated lipid bilayers. The characterizations have been achieved by 2H NMR spectra of both oriented and unoriented samples obtained at 36 and 5 degrees C, respectively. Powder patterns displaying intermediate time frame averaging provide dynamic information, and quadrupole splittings from aligned samples provide orientational constraints for the side chain structure. Librational amplitudes for each site throughout the side chain have also been characterized. Val6 and Val8 are shown to be fixed in rotameric states, potentially constraining two of the indole rings and the functionally important indole dipole moment orientations. Val1 and Val7 undergo three-state jump motions. The jump frequencies increase from the microsecond to nanosecond time frame upon increasing the temperature through the lipid phase transition. For the same temperature range, there is no evidence for changes in conformational state populations. Despite small differences in the substate populations for the two residues, the motions may be loosely coupled as indicated by the high-resolution structure.

New approach to spin assignments of intermediate structures in 12C(16O,12C

Magnetic substate populations for the ${2}^{+}$ state at 4.44 MeV in $^{12}\mathrm{C}$ were measured in resonant $^{12}\mathrm{C}$${+}^{16}$O inelastic scattering by the \ensuremath{\gamma}-ray recoil method. The m-substate population has been determined in the coordinate frame where a quantization axis is taken along the center of mass scattering angle of a reaction product. This coordinate frame gives an advantage that a resonance spin J can be obtained directly from m-substate angular distributions compared with the beam-axis frame. In addition a decay L value is sensitive to the relative intensity between each m-substate cross section. Our data suggest a spin assignment of ${\mathit{J}}^{\mathrm{\ensuremath{\pi}}}$=${16}^{+}$ or ${17}^{\mathrm{\ensuremath{-}}}$ for the structures at ${\mathit{E}}_{\mathrm{c}.\mathrm{m}.}$=29.8 MeV and 31.8 MeV. Both structures are determined to decay dominantly through the aligned configuration of L=J-2.

A propensity rule indicated by the observed dominance of substate populations for double-atom excitation in He-He collisions

A new interpretation is given to experiments on double-atom excitation in He-He collisions that were focused on the relative population and coherence of atomic substate combinations. It is shown that a proposed two-state propensity rule may describe basic characteristics of the measurements. More generally by choosing the quantization axis perpendicular to the beam axis it can be understood why the two-state propensity model is successful. It also turns out that the diagonal elements of the new density matrix are independent of the kinetic energy. From experimental excitation parameters the angular part of the atomic charge density for double-atom excitation is viewed by fixing the direction of one of the excited electrons and showing the angular charge distribution of the other. The shape of the atoms changes dramatically with the collision energy, especially if the direction of the first electron is 'frozen' in the direction of the initial relative velocity.

Analysis of hemoglobin oxygen equilibrium curves are unique solutions possible?

As a hemoglobin tetramer is oxygenated, it converts from a form with low ligand affinity to a high-affinity structure. This allosteric transition occurs in partially liganded molecules, typically after two or three ligands are bound. As a result of the co-operative nature of the process, the populations of the partially liganded forms are low. The relative proportions and precise properties of these intermediate substrates are therefore difficult to measure and are subject to controversy. The problem is compounded by compensating effects; for example, over-estimation of the oxygen affinity of triply liganded forms will result in under-estimation of the proportion of these species. Specifically, published values for the oxygen affinity of the triply liganded species vary for identical conditions by a factor of more than 6. In analyses based on the highest affinity values, the triply liganded species virtually disappears. However, this affinity is usually calculated from the last few per cent of the oxygenation curve, and this part of the curve is extremely sensitive to the normalization of the data. We conclude that unique solutions for the ligand affinity and substate populations may be impossible to achieve, and that unusual mechanisms based on particular combinations of parameters, therefore, should be viewed with caution.

Information on magnetic substate population obtained by particle-particle angular correlations

A simple closed-form expression of the in-plane b-c angular correlation for a sequential process of the type A(a, b)B(c)C is given. In particular, it is outlined how the amplitudes for a reaction A(a, b)B can be obtained. As an example, the application to the in-plane α′-t angular correlation for the sequential process7Li(α, α′)7Li(t)4He at 27.2 MeV is quoted.

The radio-frequency electric resonance technique?a tool for investigating ion-atom collisions

The rf electric resonance technique is discussed with special regard to its applicability to the analysis of excitation processes in ion-atom collisions. In particular, it is useful for investigating substate populations of excited Helium atoms and other He-like systems. A convenient set-up for the rf resonance circuit and interaction region is described. Within the frequency range 100 MHz≲ν≲1000 MHz, rf electric fieldsE1≲100 V/cm were applied to He atoms excited by ion impact.E1 can accurately be measured using level-crossing techniques, and thus a detailed analysis of the shapes and amplitudes of resonance signals becomes possible.

Nuclear spin polarized alkali beams (Li and Na): Production and acceleration

Recent improvements of the Heidelberg source for polarized heavy ions (PSI) are described. By means of optical pumping in combination with the existing multipole separation magnet the beam figure of merit (polarization 2 × intensity) was doubled. 7Li and 23Na atomic beams can now be produced in pure hyperfine magnetic substrates. Fast switching of the polarization is achieved by an adiabatic medium field transition. The hyperfine magnetic substate population is determined by laser-induced fluorescence spectroscopy. In routine operation atomic beams with nuclear polarization p α ≧ 0.85 ( α = z, zz) are obtained. The acceleration of polarized 23na − ions by a 12 MV tandem accelerator introduces a new problem: the energy at the terminal stripper foil is not sufficient to produce a usable yield of naked ions. For partially stripped ions hyperfine interaction of the remaining electrons with the nuclear spin reduces the polarization. Using in addition the Heidelberg postaccelerator 23Na 9+ beams of energies between 49 and 184 MeV were obtained with an alignment on target of P zz ≊ 0.45 . 7Li beams have also been accelerated up to 45 MeV with an alignment of P zz = 0.69.

Magnetic substate population in [formula omitted] inelastic scattering

Using the λ-ray recoil method we have measured the spin alignment for the single 2 + excitation in 12C+ 12C inelastic scattering over the energy range 19⩽ E cm ⩽55 MeV. The angular distribution of the magnetic substate population has been obtained. As quantization axis for the description of the magnetic substate has been chosen the direction of the center-of-mas scattering angle of a decaying nucleus. The all-over behavior of the angular distribution is satisfactorily reproduced by a simple DWBA calculation.

Spin alignment and phase angles inC12+C12inelastic scattering

The spin alignment $A$ and the relative phase angle $\ensuremath{\beta}$ between the $m=+2 \mathrm{and} \ensuremath{-}2$ substates populated in $^{12}\mathrm{C}$+$^{12}\mathrm{C}$(${2}_{1}^{+}$) inelastic scattering were measured over a wide range of particle scattering angles ($53\ifmmode^\circ\else\textdegree\fi{}<{\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}<101\ifmmode^\circ\else\textdegree\fi{}$) and interaction energies ($15<{E}_{\mathrm{c}.\mathrm{m}.}<33$ MeV). Although the substate population parameters depend rather strongly on reaction angle, the angle-averaged alignment is markedly enhanced at energies where either broad or narrow maxima occur in $\ensuremath{\sigma}(E)$, at least below ${E}_{\mathrm{c}.\mathrm{m}.}=26$ MeV. Above that energy variations in the alignment become damped with respect to both bombarding energy and reaction angle. The relative phase $\ensuremath{\beta}(\ensuremath{\theta})$ is approximately linear in $\ensuremath{\theta}$ at most energies. For energies below ${E}_{\mathrm{c}.\mathrm{m}.}=26$ MeV corresponding to gross structure maxima in the inelastic cross sections, the measured dependence of the phase on reaction angle suggests that the reaction is dominated at these energies by individual entrance and exit channel partial waves $L$ and ${L}^{\ensuremath{'}}$, respectively, and that these are related by ${L}^{\ensuremath{'}}=L\ensuremath{-}2$. Thus a strongly aligned configuration can be associated with the maxima. At neighboring energies no evidence for such alignment was found. Calculations with the band-crossing, diffraction, distorted-wave Born approximation, and barrier-top models were compared to the measurements. None provide an adequate representation of the data.NUCLEAR REACTIONS $^{12}\mathrm{C}(^{12}\mathrm{C}, ^{12}\mathrm{C}\ensuremath{\gamma})$, ${E}_{\mathrm{lab}}=30\ensuremath{-}65$ MeV, ${\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}(^{12}\mathrm{C})=53\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}101\ifmmode^\circ\else\textdegree\fi{}$. Measured particle-$\ensuremath{\gamma}$ angular correlations; deduced magnetic substate alignments, phase angles; compared with model predictions.

Energy and projectile dependence of nuclear alignment in fusion-evaporation reactions

Nuclear alignment attenuation coefficients of excited levels in 67Ga have been measured via the reactions 64 Zn + α and 57 Fe + 12 C for incident energies between 9 and 22 MeV and 32 and 48 MeV, respectively. The data were deduced from angular distributions of four pure electric quadrupole transitions in 67Ga. For the heavy-ion induced reaction a slight decrease of alignment with incident energy has been observed while there is a strong energy dependence of nuclear alignment for the α-induced reaction due to the feeding mechanism. The attenuation coefficients are consistent with the Gaussian magnetic substate population hypothesis except where direct and cascade feeding compete with comparable intersities.

Improved analysis of γ-ray angular distributions in (particle, xn) experiments

Carefully measured angular distributions in the 165Ho(α, 2n) 167Tm reaction are reported. It is shown that the Gaussian substate population hypothesis is inadequate. An improved population model is proposed which takes into account the direct feeding through discrete γ-ray transitions in the yrast region. The experimental angular distribution coefficients can then be properly reproduced by the calculation. Unambiguous and nuclear model independent spin determinations are obtained.