W. Tröger

The Leipzig high-energy ion nanoprobe: A report on first results

Abstract The high-energy ion nanoprobe LIPSION at the University of Leipzig has been operational since October 1998. Its magnetic quadrupole lens system, arranged as a separated Russian quadruplet, has been developed by the Microanalytical Research Centre (MARC), Melbourne. The ultrastable single-ended 3.5 MV SINGLETRON™ accelerator (High Voltage Engineering Europa) supplies H+ and He+ ion beams with a beam brightness in the range of 10–20 A rad −2 m −2 eV −1 [D.J.W. Mous, R.G. Haitsma, T. Butz, R.-H. Flagmeyer, D. Lehmann, J. Vogt, Nucl. Instr. and Meth. B 130 (1997) 31]. Due to this high brightness, the excellent optical properties of the focusing system of the nanoprobe and the suppression of mechanical vibrations, lateral resolutions of 100 nm for the low current mode (STIM) and 340 nm at a current of 10 pA (PIXE, RBS, SEI modes) were achieved. Further improvements are expected.

Ion exchanger in the brain: Quantitative analysis of perineuronally fixed anionic binding sites suggests diffusion barriers with ion sorting properties.

Perineuronal nets (PNs) are a specialized form of brain extracellular matrix, consisting of negatively charged glycosaminoglycans, glycoproteins and proteoglycans in the direct microenvironment of neurons. Still, locally immobilized charges in the tissue have not been accessible so far to direct observations and quantifications. Here, we present a new approach to visualize and quantify fixed charge-densities on brain slices using a focused proton-beam microprobe in combination with ionic metallic probes. For the first time, we can provide quantitative data on the distribution and net amount of pericellularly fixed charge-densities, which, determined at 0.4–0.5 M, is much higher than previously assumed. PNs, thus, represent an immobilized ion exchanger with ion sorting properties high enough to partition mobile ions in accord with Donnan-equilibrium. We propose that fixed charge-densities in the brain are involved in regulating ion mobility, the volume fraction of extracellular space and the viscosity of matrix components.

Optical and TDPAC spectroscopy of Hg(II)-rubredoxin: model for a mononuclear tetrahedral [Hg(CysS)4]2− center

Rubredoxins possess a well-defined mononuclear tetrahedral tetrathiolate metal binding site, a feature exploited by several investigations to study the spectroscopic characteristics and the coordination chemistry of different metal ions at this binding site. In the present work, Hg(II)-substituted rubredoxin (Rd) from Desulfovibrio gigas has been studied by electronic absorption, circular dichroism (CD), magnetic circular dichroism (MCD), and time differential perturbed angular correlation of γ-rays (TDPAC) spectroscopies. The TDPAC spectrum of 199mHg-Rd at pH 8 exhibits a prevailing nuclear quadrupole interaction (NQI) with a precession frequency of ω1=0.09 Grad/s and an asymmetry parameter η=0, features characteristic of a slightly distorted tetrahedral tetrathiolate metal coordination, i.e, a HgCysS4 center. In addition, three minor populated NQIs have also been detected. They may represent a trigonal HgS3 (ω1=1.13 Grad/s, η=0.21), a digonal HgS2 (ω1=1.34 Grad/s, η=0.20), and a digonal Hg(II) coordination (ω1=1.58 Grad/s, η= 0.18) with unidentified ligands. Since similar studies at pH 2.5 revealed a time-dependent increase of the HgCysS4 population, the low populated sites may represent intermediate Hg(II) complexes formed prior to the generation of the thermodynamically stable structure. The metal-induced absorption envelope of Hg-Rd reveals three distinct transitions with Gaussian-resolved maxima located at 230, 257, and 284 nm, which are paralleled by dichroic features in the corresponding difference CD spectrum of Hg(II)-Rd versus apo-Rd. Based on the optical electronegativity theory of Jørgensen, the lowest energy transition has been attributed to a CysS-Hg(II) charge-transfer excitation. The Td type of metal coordination in Hg-Rd is supported by the presence of an unresolved A-term with a negative lobe at 295 nm in the difference MCD spectrum. These results point to the usefulness of optical and TDPAC spectroscopies for studying Hg(II) sites in other proteins.

Nuclear probes in life sciences

The nuclear quadrupole interaction at metal sites in macromolecules monitored by time differential perturbed angular correlation provides information on the local environment of metal ions residing at these sites. This information is obtained in the ultra-trace regime and is, therefore, extremely valuable for research on reaction pathways of enzymatic heavy metal detoxification, for the development of radiopharmaceuticals, for investigations into protein architectures on surfaces, and for environmental research.

Nuclear quadrupole interaction of111Cd on type-1 Cu-sites in blue copper proteins

The nuclear quadrupole interaction (NQI) of111Cd substituted for Cu(II) on type-1 sites in blue copper proteins is characterized by high values of ω0 in the region of 300 Mrad/s, close to that for the catalytic zinc site in alcohol dehydrogenase. Type-1 Cu has usually two sulfur ligands and two nitrogen ligands and in some cases an oxygen ligand in either a distorted tetrahedral geometry or in a trigonal bipyramidal geometry. The near tetrahedral arrangement together with the ligand sphere containing the same number of sulfur ligands explains the value of ω0 in the blue copper proteins. The present work determined the partial NQI for methionine using the known structure of azurin. This value was then used in the angular overlap model to calculate the NQI for ascorbate oxidase the structure of which is also known and gave good agreement with experiment. NQI data for laccase and stellacyanin the structures of which are unknown, are also given.

RBS studies of the intercalation compound HgxTiS2: Morphology and staging

Abstract In order to study the intercalation process of Hg into the layered crystal TiS 2 we performed Rutherford Backscattering (RBS) and channeling experiments on TiS 2 and on the intercalation compound Hg x TiS 2 with variable Hg uptake. After the intercalation of Hg in TiS 2 we observed a clear Hg-signal in the RBS-spectrum, increasing with reaction time and temperature. The variation of the Hg RBS-signal depending on the lateral position of the analysing beam cannot be explained by vertical reaction fronts or by a spatially homogenous intercalation process. We conclude from these measurements that the intercalation process proceeds parallel to the TiS 2 layers in concave reaction fronts. The empty TiS 2 crystal exhibits a good channeling effect ( χ min ≈ 20%). Owing to Hg uptake, however, the ion channeling effect in the “misfit compound” Hg x TiS 2 vanishes. We attribute this to the incommensurability of the Hg and TiS 2 sublattices as well as to a bending of the crystal induced by the intercalation process.

Small Scale Intramolecular Flexibility in 111mCd-Plastocyanin

Abstract The effect of mutations in the vicinity of the putative electron transfer path on the metal center of the electron transfer protein plastocyanin (spinacea) is investigated by monitoring the nuclear quadrupole interaction of 111mCd in Cd-derivatives of the protein via time differential perturbed angular correlation. The spectra for the wild type protein and the mutants were rather similar. All spectra exhibit a peculiar line profile which points towards a small scale intramolecular flexibility of the metal center.

Hg(II) Coordination Studies in Penicillamine Enantiomers by 199mHg-TDPAC

In order to study the binding of the toxic heavy metal ion Hg2+ to penicillamine, complexes with the D- and L-enantiomers of penicillamine were investigated by the nuclear quadrupole interaction of 199Hg monitored by time differential perturbed angular correlation spectroscopy. It was found that bound Hg(II) occurs in two-fold, three-fold and four-fold coordinations.

Investigation of the Negative Thermal Expansion of ZrW2O8

The negative thermal expansion in ZrW2O8 was investigated on a microscopic scale by temperature dependent measurements of the electric field gradients at the nuclear probe 187W(β-) 187Re using time differential perturbed angular correlation spectroscopy. Two distinct nuclear quadrupole interactions I VzzRe1 l= 18.92(4) • 10 21 V/m2 , ηRe1 = 0.0 and I VzzRe1 l = 4.55(2) • 1021 V/m2 , ηRe1 = 0.053(3) were observed at 295 K, which are assigned to the two crystallographically distinct W0 4 tetrahedra of the room temperature structure. Ab initio calculations of electron densities and electric field gradients with 1:7 Re-impurities using the full potential linearized augmented plane wave package WIEN97 yield the electric field gradients VzzRe1 = 12.63 • 10 21 V/m2 , ηRe1 = 0.0 and VzzRe2 =4.90 • 10 21 V/m2 , ηRe2 =0.0. The observed temperature dependence of the nuclear quadrupole interactions agrees well with the structural phase transition at 428 K observed by neutron and x-ray diffraction. Our experiments corroborate the suggested mechanism of coupled librations of rigid ZrO6 octahedra and WO4 tetrahedra, which is an alternative description of transverse vibrations of oxygen atoms in Zr-O-W bonds, for the negative thermal expansion in ZrW2 O8

Hg-coordination studies of oligopeptides containing cysteine, histidine and tyrosine by 199mHg-TDPAC

In order to study the interaction of histidine- and tyrosine-containing peptide chains with Hg(II), the nuclear quadrupole interaction (NQI) of 199mHg in the Hg complexes of the oligopeptides Alanyl–Alanyl–Histidyl–Alanyl–Alanine-amid (AAHAA–NH2) and Alanyl–Alanyl–Tyrosyl–Alanyl–Alanine-amid (AAYAA–NH2) was determined by time differential perturbed angular correlation and is compared with previous data on Alanyl–Alanyl–Cysteyl–Alanyl–Alanyl (AACAA–OH). The 199mHg–NQIs depend on the oligopeptide to Hg(II) stoichiometry and indicate that two-fold and four-fold coordinations occur for the bound Hg(II).