Kurt Nielsen

On the akaganeite crystal structure, phase transformations and possible role in post-excavational corrosion of iron artifacts

Abstract The crystal structure of akaganeite and the akaganeite to hematite transition has been studied by means of conventional and synchrotron X-ray and neutron powder diffraction. The chemical formula of akaganeite can be written as FeO 0.833 (OH) 1.167 Cl 0.167 . The crystal structure does not contain free water. Heating below 200 °C will not alter the akaganeite structure. Initial water loss can be attributed to a large amount of adsorbed water due to a very small particle size; 0.15 μm by 0.03 μm. Chloride is released from the structure only in connection with the transformation to hematite. Due to its stability, the presence of akaganeite does not in itself posses a threat to iron artifacts, but it is rather a symptom of the presence of high concentrations of chloride in an acidic environment.

Gas-sensitive properties and structure of nanostructured ( - materials prepared by mechanical alloying

The gas-sensitive properties of nanostructured 94, 85, 71 and 7 mol% materials prepared by high-energy ball milling have been investigated in atmospheres containing alcohol, CO and gases. It has been found that the materials show a high sensitivity with a short response time to the alcohol gas and almost no sensitivity to the other two gases, CO and in the concentration ranges studied. Furthermore, the gas-sensitive behaviour of these materials in response to the alcohol gas has been discussed on the basis of the microstructure of the materials. It is found that the tin ion content in and tin ions on the surface of particles may play an important role in the enhancement of the gas sensitivity.

Molecular structures of viruses from Raman optical activity

A vibrational Raman optical activity (ROA) study of a range of different structural types of virus exemplified by filamentous bacteriophage fd, tobacco mosaic virus, satellite tobacco mosaic virus, bacteriophage MS2 and cowpea mosaic virus has revealed that, on account of its sensitivity to chirality, ROA is an incisive probe of their aqueous solution structures at the molecular level. Protein ROA bands are especially prominent from which, as we have shown by comparison with the ROA spectra of proteins with known structures and by using a pattern recognition program, the folds of the major coat protein subunits may be deduced. Information about amino acid side-chain conformations, exemplified here by the determination of the sign and magnitude of the torsion angle chi(2,1) for tryptophan in fd, may also sometimes be obtained. By subtracting the ROA spectrum of the empty protein capsid (top component) of cowpea mosaic virus from those of the intact middle and bottom-upper components separated by means of a caesium chloride density gradient, the ROA spectrum of the viral RNA was obtained, which revealed that the RNA takes up an A-type single-stranded helical conformation and that the RNA conformations in the middle and bottom-upper components are very similar. This information is not available from the X-ray crystal structure of cowpea mosaic virus since no nucleic acid is visible.

Structure and Behaviour of Proteins, Nucleic Acids and Viruses from Vibrational Raman Optical Activity

On account of its sensitivity to chirality Raman optical activity (ROA), which may be measured as a small difference in vibrational Raman scattering from chiral molecules in right- and left-circularly polarized incident light, is a powerful probe of structure and behaviour of biomolecules in aqueous solution. Protein ROA spectra provide information on the secondary and tertiary structure of the polypeptide backbone, hydration, side chain conformation and structural elements present in denatured states. Nucleic acid ROA spectra provide information on the sugar ring conformation, the base stacking arrangement and the mutual orientation of the sugar and base rings around the C-N glycosidic link. The ROA spectra of intact viruses provide information on the folds of the coat proteins and the nucleic acid structure. The large number of structure-sensitive bands in protein ROA spectra is especially favourable for fold determination using pattern recognition techniques. This article gives a brief account of the ROA technique and presents the ROA spectra of a selection of proteins, nucleic acids and viruses that illustrate the applications of ROA spectroscopy in biomolecular research.

Solution structures of potato virus X and narcissus mosaic virus from Raman optical activity

Potato virus X (PVX) and narcissus mosaic virus (NMV) were studied using vibrational Raman optical activity (ROA) in order to obtain new information on the structures of their coat protein subunits. The ROA spectra of the two intact virions are very similar to each other and similar to that of tobacco mosaic virus (TMV) studied previously, being dominated by signals characteristic of proteins with helix bundle folds. In particular, PVX and NMV show strong positive ROA bands at approximately 1340 cm(-1) assigned to hydrated alpha-helix and perhaps originating in surface exposed helical residues, together with less strong positive ROA intensity in the range approximately 1297-1312 cm(-1) assigned to alpha-helix in a more hydrophobic environment and perhaps originating in residues at helix-helix interfaces. The positive approximately 1340 cm(-1) ROA band of TMV is less intense than those of PVX and NMV, suggesting that TMV contains less hydrated alpha-helix. Small differences in other spectral regions reflect differences in some loop, turn and side-chain compositions and conformations among the three viruses. A pattern recognition program based on principal component analysis of ROA spectra indicates that the coat protein subunit folds of PVX and NMV may be very similar to each other and similar to that of TMV. These results suggest that PVX and NMV may have coat protein subunit structures based on folds similar to the TMV helix bundle and hence that the helical architecture of the PVX and NMV particles may be similar to that of TMV but with different structural parameters.

Neutron diffraction investigation of the atomic defect structure of Y-doped SrCeO3, a high-temperature protonic conductor

The structures of SrCeO3 and its Y-doped equivalent Sr[Ce0.85Y0.15]O2.925 have been examined by neutron powder diffraction at room temperature. Both compounds crystallize in a distorted perovskite-like structure and were refined in space group Pnma(no. 62) by full-profile Rietveld methods. The structure refinements show that substitution of yttrium into SrCeO3 creates an increased lattice distortion relative to the cubic perovskite structure. Changes in neutron scattering densities, Δρ, due to this substitution are illustrated by difference scattering density maps around the atoms constructed from Fobs, phased by the Fcalc, obtained from the Rietveld refinement. Δρ is computed as ρ[Sr(Ce0.85Y0.15)O2.925]–ρ(SrCeO3) with the coordinates of the atoms in question translated to the origin (0,0,0). These maps provide a direct picture of the average in space and time of thermal vibrations and occupancies on atomic sites less biased by least-squares methods than parameters obtained from refinement. Oxygen vacancies induced in order to maintain electroneutrality upon substitution of Ce4+ by Y3+ are confined to one of two non-equivalent oxygen sites. This structural feature is consistent with experiments which show a very low contribution to the total conductivity from oxygen ion conductivity relative to that of protonic conductivity.

Using X-ray powder diffraction and principal component analysis to determine structural properties for bulk samples of multiwall carbon nanotubes

It has been attempted to derive the structural properties of bulk multiwall carbon nanotubes (MWCNTs) from a combination of powder diffraction and principal component analysis (PCA). By a transformation of the direct PCA basis functions to a structural parameter set it was possible to obtain average values of inner radius, number of turns and d-spacing. The true tube lengths cannot be correctly estimated due to correlations to other properties, tube bending and defects in the tubes. Improvements can be expected by including distributions of the structural properties, further developing the functional relationships between the PCA and parameter functions and including the chiral angle (rolling direction) as a separate parameter.