Perturbed Angular Correlation Data Research Articles

Molecular Rotational Correlation Times and Nanoviscosity Determined by 111m Cd Perturbed Angular Correlation (PAC) of γ-rays Spectroscopy.

The nanoviscosity experienced by molecules in solution may be determined through measurement of the molecular rotational correlation time, τc , for example, by fluorescence and NMR spectroscopy. With this work, we apply PAC spectroscopy to determine the rate of rotational diffusion, λ=1/τc , of a de novo designed protein, TRIL12AL16C, in solutions with viscosities, ξ, from 1.7 to 88 mPa⋅s. TRIL12AL16C was selected as molecular probe because it exhibits minimal effects due to intramolecular dynamics and static line broadening, allowing for exclusive elucidation of molecular rotational diffusion. Diffusion rates determined by PAC data agree well with literature data from fluorescence and NMR spectroscopy, and scales linearly with 1/ξ in agreement with the Stokes-Einstein-Debye model. PAC experiments require only trace amounts (∼1011 ) of probe nuclei and can be conducted over a broad range of sample temperatures and pressures. Moreover, most materials are relatively transparent to γ-rays. Thus, PAC spectroscopy could find applications under circumstances where conventional techniques cannot be applied, spanning from the physics of liquids to in-vivo biochemistry.

Towards the role of metal ions in the structural variability of proteins: CdII speciation of a metal ion binding loop motif

A de novo designed dodecapeptide (HS), inspired by the metal binding loops of metal-responsive transcriptional activators, was synthesized. The aim was to create a model system for structurally promiscuous and intrinsically unstructured proteins, and explore the effect of metal ions on their structure and dynamics. The interaction with Cd(II) was investigated by UV, synchrotron radiation CD, (1)H NMR, and perturbed angular correlation (PAC) of γ-rays spectroscopy, pH-potentiometry, and molecular modelling. The peptide mainly displays characteristics of random coil in the CD spectra, and the molecular dynamics simulations demonstrate that it is unstructured with transient and varying helical content. The spectroscopic studies revealed the formation of loop structures with the coordination of the two Cys-thiolates close to each end of the HS peptide, in the presence of one equivalent of Cd(II) per ligand. The imidazole moiety from histidine is also bound to Cd(II) at neutral pH and above. In the presence of 0.5 equivalent of Cd(II) per HS metal bridged structures with e.g. CdS(2)N(2) and possibly CdS(4) coordination geometries are formed above pH ~6. In an equilibrium of several co-existing species the peptide is exchanging between a number of structures also in its metal ion bound state(s), as indicated by NMR and PAC data.

Fully digital time differential perturbed angular correlation (TDPAC) spectrometer

A new generation time differential perturbed angular correlation (PAC) spectrometer has been designed and built. The design strategy and details of the data collection and reduction methodology are reported. First results obtained by the new spectrometer are reported and compared with PAC data obtained by more conventional means.

The Application of199Hg NMR and199mHg Perturbed Angular Correlation (PAC) Spectroscopy to Define the Biological Chemistry of HgII: A Case Study with Designed Two- and Three-Stranded Coiled Coils

The use of de novo designed peptides is a powerful strategy to elucidate Hg(II)-protein interactions and to gain insight into the chemistry of Hg(II) in biological systems. Cysteine derivatives of the designed alpha-helical peptides of the TRI family [Ac-G-(L(a)K(b)A(c)L(d)E(e)E(f)K(g))(4)-G-NH(2)] bind Hg(II) at high pH values and at peptide/Hg(II) ratios of 3:1 with an unusual trigonal thiolate coordination mode. The resulting Hg(II) complexes are good water-soluble models for Hg(II) binding to the protein MerR. We have carried out a parallel study using (199)Hg NMR and (199m)Hg perturbed angular correlation (PAC) spectroscopy to characterize the distinct species that are generated under different pH conditions and peptide TRI L9C/Hg(II) ratios. These studies prove for the first time the formation of [Hg{(TRI L9C)(2)-(TRI L9C-H)}], a dithiolate-Hg(II) complex in the hydrophobic interior of the three-stranded coiled coil (TRI L9C)(3). (199)Hg NMR and (199m)Hg PAC data demonstrate that this dithiolate-Hg(II) complex is different from the dithiolate [Hg(TRI L9C)(2)], and that the presence of third alpha-helix, containing a protonated cysteine, breaks the symmetry of the coordination environment present in the complex [Hg(TRI L9C)(2)]. As the pH is raised, the deprotonation of this third cysteine generates the trigonal thiolate-Hg(II) complex Hg(TRI L9C)(3)(-) on a timescale that is slower than the NMR timescale (0.01-10 ms). The formation of the species [Hg{(TRI L9C)(2)(TRI L9C-H)}] is the result of a compromise between the high affinity of Hg(II) to form dithiolate complexes and the preference of the peptide to form a three-stranded coiled coil.

Comment on “Nature and entropy content of the ordering transitions inRCo2”

In their analysis of our perturbed angular correlation (PAC) study of the magnetic phase transitions of $R{\mathrm{Co}}_{2}$ [Phys. Rev. B 68, 014409 (2003)], Herrero-Albillos et al. [Phys. Rev. B 73, 134410 (2006)] come to the conclusion that it is very difficult for PAC spectroscopy to distinguish a first-order from a second-order phase transition. The statement is incorrect and does not resolve the conflict between the conclusion drawn from the PAC data and the differential scanning calorimetry data of Herrero-Albillos et al. on the order of the magnetic phase transitions of $\mathrm{Pr}{\mathrm{Co}}_{2}$ and $\mathrm{Nd}{\mathrm{Co}}_{2}$. In this Comment we show that measurements of hyperfine interactions by PAC and other microscopic techniques are a very powerful tool for the investigation of phase transitions which may provide details on the transition not accessible to macroscopic methods. We explain why the PAC data leave no alternative to the conclusion that the spontaneous magnetization of $\mathrm{Pr}{\mathrm{Co}}_{2}$ and $\mathrm{Nd}{\mathrm{Co}}_{2}$ undergoes a discontinuous, first-order phase transition at ${T}_{\mathrm{C}}$.

PH-dependent structural change of reduced spinach plastocyanin studied by perturbed angular correlation of γ-rays and dynamic light scattering

In this study the pH-dependent structural changes of reduced spinach plastocyanin were investigated using perturbed angular correlation (PAC) of gamma-rays and dynamic light scattering (DLS). PAC data of Ag-substituted plastocyanin indicated that the coordinating ligands are two histidine residues (His37, His87) and a cysteine residue (Cys84) in a planar configuration, whereas the methionine (Met92) found perpendicular to this plane is not a coordinating ligand at neutral pH. Two slightly different conformations with differences in the Cys-metal ion-His angles could be observed with PAC spectroscopy. At pH 5.3 a third coordination geometry appears which can be explained as the absence of the His87 residue and the coordination of Met92 as a ligand. With DLS the aggregation of reduced plastocyanin could be observed below pH 5.3, indicating that not only the metal binding site but also the aggregation properties of the protein change upon pH reduction. Both the structural changes at the metal binding site and the aggregation are shown to be reversible. These results support the hypothesis that the pH of the thylakoid lumen has to remain moderate during steady-state photosynthesis and indicate that low pH induced aggregation of plastocyanin might serve as a regulatory switch for photosynthesis.

Hyperfine Interactions in the Cubic Phase of BaTi x Hf 1− x O 3

The hyperfine quadrupole interaction at probes in sites with cubic point group symmetry was measured in many perovskite-type compounds. This interaction is commonly associated to the presence of oxygen vacancies close to probes. The effect of this point defect on 181 Ta probe in BaTi x Hf 1 m x O 3 for x = 0.70, 0.50, 0.30, 0.10, 0.05, and 0.01, is studied. The lattice constant of these oxides at room temperature were measured using XRD technique. The quadrupole parameters corresponding to probe-defect interaction were obtained by means of Perturbed Angular Correlation (PAC) spectroscopy as functions of both, composition and temperature. A static, asymmetric and distributed quadrupole interaction was fitted to all PAC data. This interaction was observed to be strongly composition dependent. These results and those corresponding to compositions 0.75 h x < 1 were compared to the ones obtained using the point charge method for the calculation of the electric field gradient. The proposed model took into account polarized oxygen vacancies, different covalence of Ti--O and Hf--O bonds and using computational simulation for cations and oxygen vacancies lattice positions.

Simulated Quadrupolar Broadening in Condensed Matter PAC Spectroscopy

Perturbed angular correlation (PAC) spectroscopy of materials containing static defects must often deal with quadrupolar broadening due to randomly distributed defects. The electric field gradient (EFG) due to defects adds to the crystal lattice EFG causing quadrupolar frequencies to differ from site to site in a crystal. We report a simulation study of the static defect EFG using a uniform random distribution of point charges. In this simulation, the distribution of random EFG's is found to depend on the ratio γ of the most probable values of the lattice and random EFG magnitudes, γ=V zz lat/V zz ran. We present fits from the results of the simulations to Hf-doped cubic niobium and Nb-doped tetragonal ZrO2 PAC data. The single scaling parameter fitting for γ=0 provides an unprecedented excellent description for the cubic PAC data.

Stability studies of Hg implanted YBa 2Cu 3O 6+x

High quality YBa 2Cu 3O 6+ x (YBCO) superconducting thin films were implanted with the radioactive 197mHg ( T 1/2=24 h) isotope to low fluences of 10 13 atoms/cm 2 and 60 keV energy. The lattice location and stability of the implanted Hg were studied combining the Perturbed Angular Correlation (PAC) and Emission Channeling (EC) techniques. We show that Hg can be introduced into the YBCO lattice by ion implantation into a unique regular site. The EC data show that Hg is located on a highly symmetric site on the YBCO lattice, while the PAC data suggests that Hg occupies the Cu(1) site. Annealing studies were performed under vacuum and O 2 atmosphere and show that Hg starts to diffuse only above 653 K.

High temperature study of yttrium--barium--copper oxides near the YBa2Cu3Ox composition by 111In/Cd time differential \gamma--\gamma perturbed angular correlation spectroscopy

The local atomic order around very dilute indium impurities in c-axis-oriented YBa2Cu3O7 and YBa2Cu3O6.25 films at room temperature and in YBa2Cu3Ox bulk powders held in air or oxygen over a wide temperature range were investigated using 111In/Cd \gamma--\gamma perturbed angular correlation (PAC) spectroscopy. The probe atoms occupy a single substitutional lattice position in YBa2Cu3Ox, and evidence reported here strongly supports previous claims that this is the yttrium position. In YBa2Cu3Ox powders, the local atomic order changes continuously with temperature. At room temperature the electric field gradient measured by PAC in bulk YBa2Cu3Ox powder is indistinguishable from that in the fully oxygenated YBa2Cu3O7 film. Near the decomposition temperature, the bulk YBa2Cu3Ox powder data are semiquantitatively similar to data in the room temperature YBa2Cu3O6.25 film, with quantitative differences that we attribute to temperature averaging in the former. Other sites usually found in PAC spectra of YBa2Cu3Ox bulk samples arise from 111In dissolved in Y2BaCuO5 and Y2Cu2O5 impurity phases. The latter phase apparently arises because of significant barium loss during processing; the barium deficiency is clearly demonstrated by comparison of PAC data with the alloy phase diagram above the decomposition temperature. PAC data on pure Y2Cu2O5 are reported here also.

MAGNETIC PHASE TRANSITIONS IN TRANSITIONAL METAL OXIDES STUDIED WITH PERTURBED ANGULAR CORRELATIONS

The Perturbed Angular Correlation (PAC) method with radioactive tracer nuclei is well-suited to study the critical behaviour of magnetic hyperfine fields. Here we present the results of PAC measurements with 111 Cd probes, concerning the antiferromagnetic phase transitions in CuO, Co 3 O 4 and MnO. The measured Larmor frequencies were fitted with a Curie-Weiss type temperature dependence ωL (T) /ωL(0) = (1- T/T N)β. In CuO the PAC probe is subject to a high electric field gradient (efg) with the coupling constant νQ = 434(2) MHz and the asymmetry parameter η = 0.42(1). Therefore, combined electric/magnetic hyperfine interaction is observed below T N. Similar to neutron diffraction experiments, a second magnetic transition was observed at T 1 = 213 K . The analysis of the PAC data gave T N = 224(2) K and β = 0.33(1) for T 1 ≤ T ≤ T N. In the spinel Co 3 O 3, the measured critical parameters for 111 Cd at cation sites with tetrahedral oxygen coordination are T N = 39(1) K and β = 0.34(2). This site has a nearly cubic oxygen coordination and therefore a vanishing efg. In Mn 1−x O (defective NaCl structure, x ≤ 0.15), the critical parameters were found to sensitively depend on the annealing conditions. They will be discussed in comparison with those in Fe 1−x O .

PAC analysis of defect motion by Blume's stochastic model forI=5/2 electric quadrupole interactions

Using Blume's stochastic model and the approach of Winkler and Gerdau, we have computed time-dependent effects on perturbed angular correlation (PAC) spectra due to defect motion in solids in the case ofI=5/2 electric quadrupole interactions. We report detailed analysis for a family of simple models: “XYZ+Z” models, in which the symmetry axis of an axial EFG is allowed to fluctuate among orientations alongx, y, andz axes, and a static axial EFG oriented along thez axis is added to the fluctuating EFGs. When the static EFG is zero, this model is termed the “XYZ” model. Approximate forms are given forG2(t) in the slow and rapid fluctuation regimes, i.e. suitable for the low and high temperature regions, respectively. Where they adequately reflect the underlying physical processes, these expressions allow one to fit PAC data for a wide range of temperatures and dopant concentrations to a single model, thus increasing the uniqueness of the interpretation of the defect properties. Application of the models is illustrated with data from a PAC study of tetragonal zirconia.

Hyperfine interactions of 111In-implanted tin oxide thin films

Thin Sn-O films were prepared by thermal evaporation of high-purity tin in a low-pressure oxygen atmosphere. The films were monitored by conversion electron Mossbauer spectroscopy (CEMS). Radioactive 111In+-ions were implanted into the films and perturbed angular correlation (PAC) measurements were carried out in air after different annealings up to 1023 K. Two phases were sufficient to describe the PAC data, which are attributed to SnO and SnO2. The temperature dependence of these phases was found to agree with CEMS measurements on Sn-O films after similar annealing programmes.

The Zn-Site in Bovine Copper, Zinc Superoxide Dismutase Studied by111Cd Pac

The active site in bovine copper, zinc superoxide dismutase (Cu2, Zn2SOD) has been studied by 111Cd time differential Perturbed Angular Correlation (PAC) on enzyme with Zn2+ replaced by excited 111Cd2+. The PAC spectra obtained for both the oxidized and the reduced form of Cu2Cd2SOD show no asymmetry between the two Zn-sites in the dimeric enzyme. The spectra further reveal that a significant change has taken place at the Zn-site in the reduced form compared to the oxidized form. Semi-empirical calculations based on the Angular Overlap Model (AOM) and coordinates from the crystal structure of the native enzyme are in agreement with the experimental PAC data of the oxidized enzyme. The results indicate that Cd2+ coordinates in the same manner as Zn2+ and that the crystal structure of SOD is valid for the enzyme in solution. The PAC spectrum of the reduced enzyme can be explained by extending the AOM calculations to the enzyme in the reduced form and assuming that the imidazol ring of His61 is no longer bridging the copper and cadmium ions in the reduced state.

Structural characterization of a high-temperature, ionic conducting ceramic using perturbed angular correlation spectroscopy.

Perturbed angular correlation (PAC) spectroscopy has been used to characterize several structural aspects of a high-temperature, ionic conducting ceramic, ${\mathrm{CaZr}}_{3.95}$${\mathrm{Hf}}_{0.05}$${\mathrm{P}}_{6}$${\mathrm{O}}_{24}$. Hafnium was introduced into the material to provide the PAC probe nuclei, $^{181}\mathrm{Hf}$${/}^{181}$Ta, which were located primarily at Zr sites. PAC measurements were made over a range of temperatures from 77 to 1180 K, and they have been analyzed and interpreted using several simple models. The distorted octahedral crystal field at the Zr site produced a (low-frequency) static electric quadrupole interaction which can be accurately described by the point-charge model. But, the temperature dependence of the associated electric field gradient (EFG) cannot be described accurately by purely static considerations via the point-charge model and high-temperature x-ray diffraction data. Although a high-frequency static interaction was also observed, the measurements were not sufficiently accurate to identify its origin unambiguously. Some of the high-temperature measurements show evidence of a time-varying interaction, which may result from ${\mathrm{Ca}}^{2+}$-ion jumping. But, jump frequencies derived classically from high-temperature electrical dc conductivity measurements are too low to agree with those indicated by the PAC data. However, the dc conductivity measurements support a simple model of thermally activated ${\mathrm{Ca}}^{2+}$-ion transport. The temperature dependence of the EFG (corresponding to the low-frequency interaction) was used to determine an effective Debye-Waller factor. As a result of using this approach to analyze this type of PAC data, this factor was shown also to agree qualitatively with the predictions of the Debye crystal model, although significant theoretical limitations were encountered. These particular results suggest that the PAC technique may provide new insights into understanding advanced ceramic materials.