Robert S. Armstrong

Molecular Mechanism of AHSP-Mediated Stabilization of α-Hemoglobin

Hemoglobin A (HbA), the oxygen delivery system in humans, comprises two alpha and two beta subunits. Free alpha-hemoglobin (alphaHb) is unstable, and its precipitation contributes to the pathophysiology of beta thalassemia. In erythrocytes, the alpha-hemoglobin stabilizing protein (AHSP) binds alphaHb and inhibits its precipitation. The crystal structure of AHSP bound to Fe(II)-alphaHb reveals that AHSP specifically recognizes the G and H helices of alphaHb through a hydrophobic interface that largely recapitulates the alpha1-beta1 interface of hemoglobin. The AHSP-alphaHb interactions are extensive but suboptimal, explaining why beta-hemoglobin can competitively displace AHSP to form HbA. Remarkably, the Fe(II)-heme group in AHSP bound alphaHb is coordinated by the distal but not the proximal histidine. Importantly, binding to AHSP facilitates the conversion of oxy-alphaHb to a deoxygenated, oxidized [Fe(III)], nonreactive form in which all six coordinate positions are occupied. These observations reveal the molecular mechanisms by which AHSP stabilizes free alphaHb.

Fourier Transform Raman Microscopic Mapping of the Molecular Components in a Human Tooth

Fourier transform Raman microscopic mapping was used to examine the distribution of the mineral and organic components in a cross-section of a human tooth. The relative distributions of carbonate and phosphate ions as well as the organic matrix components were plotted using functional group maps. The distribution of organic components was examined using the C–H stretch band between 2880 and 2700 cm-1. The distributions of phosphate and carbonate were determined using their respective symmetric stretching vibrations, νs(PO) at 961 cm-1 and νs(CO) at 1070 cm-1. Maps of the enamel–dentine junction at a high resolution of 10 μm showed that the amount of phosphate is lowest in this region. A series of spectra at 2 cm-1, ca. 20000 scans, used for curve fitting of the νs(CO) band was used to show that the concentration of carbonate ions in B-hydroxyapatite positions increases on going from the outside of the enamel towards the enamel–dentine junction. The results reveal that the deconvolution of the band envelope of νs(CO) and νas(PO), the antisymmetric stretching vibration of phosphate is more complicated than previously reported. The advantage of functional group Raman mapping is that, in addition to identifying the chemical constituents, valuable microstructural information is obtained including species distribution across the surface, and in particular at the enamel–dentine interface. Minimal, non-chemical sampling preparation means that spectral information obtained can be directly related to the physical properties and biological function of such tissues.

Vibrational spectroscopic studies of trivalent hexa-aqua-cations: single-crystal Raman spectra between 275 and 1 200 cm–1 of the caesium alums of titanium, vanadium, chromium, iron, gallium, and indium

Oriented single-crystal Raman spectra have been recorded at 80 or 90 K for CsM(SO4)2·12H2O (M = Ti, V, Cr, Fe, Ga, or In), CsM(SeO4)2·12H2O (M = Cr or Fe), and CsFe(SO4)2·12D2O. A complete assignment of the spectra between 275 and 1 200 cm–1 has been made. The total symmetric metal–water stretching modes of the trivalent hexa-aqua-cations occur at 517 (Ti), 525 (V), 540 (Cr), 523 (Fe), 537 (Ga), and 505 cm–1(In), significantly higher than values previously reported. The wavenumbers of these modes vary with the reciprocal of the metal(III)–water bond lengths.

Vibrational spectroscopic mapping and imaging of tissues and cells

Vibrational spectroscopic mapping (point-by-point measurement) and imaging of biological samples (cells and tissues) covering Fourier-transform infrared (FTIR) and Raman spectroscopies has opened up many exciting new avenues to explore biochemical architecture and processes within healthy and diseased cells and tissues, including medical diagnostics and drug design.

A combined ESR/X-ray diffraction study of the Y2O3–BaO–CuO phase diagram

Measurements at both X- and Q-band frequencies of various phases in the Y–Ba–Cu–O system are reported. Signals close to the free-spin region are attributed to localised Cu2+ species with g-value anisotropies noticeably dependent upon the local crystal structure. These materials include BaCuO2, Y2BaCuO5, Y2Cu2O5; characteristic g values are discussed in terms of crystal-field models for electronic structure.Importantly, we find that high purity YBa2Cu3O7–x(x≲ 0.1) superconducting samples (Tc= 92 K), prepared at lower temperature by solution or sol–gel techniques, have no intrinsic ESR signals in the free-spin region. We propose that this ‘ESR silence’ is, in fact, not due to the absence of intrinsic paramagnetism in these superconducting oxides, but rather due to the extremely efficient spin–lattice relaxation of the d-band conduction electrons. Here, electron spin relaxation in these metallic systems for T > Tc proceeds via an Elliott-type mechanism involving the modulation of spin–orbit interactions via itinerant conduction electrons. This efficient relaxation mechanism would lead to extremely large ESR linewidths, typically in the range 50–100 kG.

Polarities and anisotropic electron polarisabilities of neutral ligands

Dipole moments and molar Kerr constants are reported for the molecules Me3MX (M = P or As, and X = lone pair, O, or S) as solutes in dioxan and in benzene. The results are analysed to provide quantitative estimates of the anisotropic electron polarisabilities for each of these neutral ligands. The parameters so obtained are potentially of value in studies of metal–ligand interactions. Evidence is presented also of stereospecific Me3MX,C6H6 complex formation in the aromatic solvent.

Infrared and Raman spectra of (η6-C6H6−nXn)Cr(CO)3 complexes where X = Me (n = 0–6) or OMe (n = 0–2). A study of metal—ligand interactions

Abstract A systematic study is reported of the infrared and Raman spectra of methyl and methoxy substituted benzene chromium tricarbonyl complexes (η6-C6H6−nXn)Cr(CO)3 where X = Me (n = 0–6) or OMe (n = 0–2). Bands have been assigned and intensities calculated for a number of vibrational modes. Trends in the wavenumbers and band intensities of the arenemetal, metalcarbonyl and carbonyl bonds are examined to gain an insight into metal—ligand interactions and intramolecular electron rearrangements with progressive substitution on the arene. The results are discussed with regard σ-, π- and δ-contributions to arenemetal bonding and of σ- and π-components in metal-carbonyl and carbonyl bonding. The intensities are related to the polarities and polarizabilities of individual bonds within the molecules, and variations in these bond electronic properties with arene substitution are examined.

Analysis of the near-infrared spectra of C60−

Abstract Band analyses of the NIR spectra of two C 60 − salts in nine solvents reveal a previously unnoticed low-energy transition arising from a thermally populated excited electronic state. The viability of vibronic origin for three intermediate-energy transitions is confirmed. A multiplicity of weak high-energy transitions presently defies unique deconvolution and assignment.

ESR studies of high critical-temperature superconductors :absorption at low magnetic fields

One of the most intriguing aspects of ESR studies of oxide superconductors is the presence of a broad, and intense, low-field microwave absorption which grows in intensity upon cooling the sample through the superconducting transition temperature (Tc). In the present investigations we exploit the low-field response as a screening technique for superconductivity in a range of oxide materials. The materials investigated, derivatives of the 90 K superconductor YBa2Cu3O7, were specifically chosen so that Tc could be drastically modified either by oxygen loss or cation substitution. In all of these materials a clear correlation exists between the onset temperature of the low-field absorption and the superconducting transition temperature, as gauged by both a.c. inductive and electrical resistivity measurements. The temperature range over which the superconductivity onset was detected by these complementary methods extended from ca. 90 K down to ca. 30 K in the case of a Zn-substituted YBa2Cu3O7. Measurements on the corresponding Bi–Sr–Ca–Cu–O system, which becomes superconducting for temperatures below 110 K, also illustrate that this absorption phenomenon is not unique to YBa2Cu3O7 and hence will have widespread applicability in detecting superconductivity.

Crystal structures of the α alums CsM[SO4]2·12H2O (M = Rh or Ir)

Structure determinations of the alums CsM[SO4]2·12H2O have been carried out for M = rhodium(III) and iridium(III), providing the first structurally characterized examples of the hexa-aqua-rhodium(III) and -iridium(III) species. Structures were determined at 295(1) K and refined to R= 0.034 and 0.028 for 546 and 696 independent ‘observed’ reflections for Rh and Ir respectively. Crystals are cubic, space group Pa3 with a= 12.357(5) and 12.395(3)A respectively; Rh–O is 2.016(3) and Ir–O 2.041(3)A. Both alums are of the α type. Single-crystal Raman spectra of the S–O stretching vibrations are reported which distinguish between the α and β caesium alums.