Development of structural order during supercooling of a fragile oxide melt.

The authors have studied the structural evolution of the fragile glass-forming liquid CaAl2O4 during supercooling from the stable liquid phase to the cold glass below Tg. The evolution is characterized by a sharpening of the first diffraction peak and a shortening of the average nearest-neighbor bond length around 1.25Tg, indicating an increase in the degree of both intermediate-range and short-range orders occurring close to the dynamical crossover temperature. The cooling curve developed a kink at this temperature, indicating a simultaneous change in thermodynamic properties.

Longitudinal excitations in Mg-Al-O refractory oxide melts studied by inelastic x-ray scattering.

The dynamic structure factor S(Q,omega) of the refractory oxide melts MgAl2O4 and MgAl4O7 is studied by inelastic x-ray scattering with aerodynamic levitation and laser heating. This technique allows the authors to measure simultaneously the elastic response and transport properties of melts under extreme temperatures. Over the wave vector Q range of 1-8 nm-1 the data can be fitted with a generalized hydrodynamic model that incorporates a slow component described by a single relaxation time and an effectively instantaneous fast component. Their study provides estimates of high-frequency sound velocities and viscosities of the Mg-Al-O melts. In contrast to liquid metals, the dispersion of the high-frequency sound mode is found to be linear, and the generalized viscosity to be Q independent. Both experiment and simulation show a weak viscosity maximum around the MgAl4O7 composition.

Structure of molten yttrium aluminates: a neutron diffraction study.

We used the aerodynamic levitation technique combined with CO2 laser heating to study the structure of liquid yttrium aluminates above their melting point with neutron diffraction. For various yttria contents, we determined the structure factors and corresponding pair correlation functions describing the short-range order in the liquids. In particular, we derived Al-O and Y-O bond distances and coordination numbers. Experimental data are compared with ab initio molecular dynamics, carried out using the VASP code where the interatomic forces are obtained from density functional theory. In particular, partial pair correlation functions have been calculated and are in relatively good agreement with the experimental observations.

Magnetic critical scattering in solid Co80Pd20.

Using small-angle neutron scattering combined with a containerless aerodynamic levitation technique for high temperatures, we have measured the temperature dependence of the correlation length ξ of near-critical magnetic fluctuations in the solid phase of the completely miscible fcc alloy Co80Pd20. A fit to our data yields a critical exponent ν = 0.76 ± 0.05 for the divergence of ξ(T) above the ferromagnetic transition temperature Tc. This value of ν is consistent with the prediction of the three-dimensional Heisenberg model for magnetic critical scattering.

Structural properties of molten dilute aluminium-transition metal alloys.

The short-range order in liquid binary Al-rich alloys (Al-Fe, Al-Ti) was studied by x-ray diffraction. The measurements were performed using a novel containerless technique which combines aerodynamic levitation with inductive heating. The average structure factors, S(Q), have been determined for various temperatures and compositions in the stable liquid state. From S(Q), the pair correlation functions, g(r), have been calculated. The first interatomic distance is nearly temperature-independent, whereas the first-shell coordination number decreases with increasing temperature for all the alloys investigated. For the Al-Fe alloys, room-temperature scanning electron microscropy (SEM) studies show the formation of a microstructure, namely the existence of Al(13)Fe(4) inclusions in the Al matrix.

Copper binding before polypeptide folding speeds up formation of active (holo) Pseudomonas aeruginosa azurin.

Cofactors often stabilize the native state of the proteins; however, their effects on folding dynamics remain poorly understood. To uncover the role of one cofactor, we have examined the folding kinetics of Pseudomonas aeruginosa azurin, a small blue-copper protein with a copper cofactor uniquely coordinated to five protein residues. Copper removal produces apo-azurin which adopts a folded structure identical to that of the holo-form. The folding and unfolding kinetics for apo-azurin follow two-state behavior. The extrapolated folding time in water, tau approximately 7 ms, is in good agreement with the topology-based prediction. Copper uptake by folded apo-azurin, to govern active (holo) protein, is slow (tau approximately 14 min, 50:1 copper-to-protein ratio). In contrast, the formation of active (holo) azurin is much faster when copper is allowed to interact with the unfolded polypeptide. Refolding in the presence of 10:1, 50:1, and 100:1 copper:protein ratios yields identical time-trajectories: active azurin forms in two kinetic phases with folding times, extrapolated to water, of tau = 10 +/- 2 ms (major phase) and tau = 190 +/- 30 ms (minor phase), respectively. Correlating copper-binding studies, with a small peptide derived from the metal-binding region of azurin, support that initial cofactor binding is fast (tau approximately 3.7 ms) and thus not rate-limiting. Taken together, introducing copper prior to protein folding does not speed up the polypeptide-folding rate; nevertheless, it results in much faster (> 4000-fold) formation of active (i.e., holo) azurin. Living systems depend on efficient formation of functional biomolecules; attachment of cofactors prior to polypeptide folding appears to be one method to achieve this.

Probing copper ligands in denatured Pseudomonas aeruginosa azurin: unfolding His117Gly and His46Gly mutants.

Azurin is a single-domain beta-barrel protein with a redox-active copper cofactor. Upon Pseudomonas aeruginosa azurin unfolding, the cofactor remains bound to the polypeptide, coordinating three ligands: cysteine-112, one histidine imidazole, and a third, unknown ligand. In order to identify which histidine (histidine-117 and histidine-46 both coordinate copper in native azurin) is involved in copper coordination in denatured azurin, two single-site (histidine to glycine) mutants, His117Gly and His46Gly azurin, are investigated here. Equilibrium denaturation experiments of His46Gly azurin loaded with copper demonstrate that copper remains bound to this mutant in high urea concentrations where the protein's secondary structure is lost. In contrast, for copper-loaded His117Gly azurin, copper does not stay coordinated upon polypeptide unfolding. The copper absorption at 370 nm in denatured His46Gly azurin agrees with that for copper in complex with a peptide corresponding to residues 111-123 in azurin, suggesting similar metal coordination. We conclude that histidine-117 (and not histidine-46) is the histidine copper ligand in denatured azurin. This is also in accord with the proximity of histidine-117 to cysteine-112 in the primary sequence.

Lack of correlation between complications after smallpox vaccination and the ABO blood-group system.

Blood group [ABO system] distribution was analysed in 245 children with complications after smallpox vaccination. One hundred and thirty-two children had neurological and 113 and skin or mucosal complications. 41.6% of the children were blood-group A or AB. 58.4% were blood group O or B, which was not a significant difference from the normal population [44.7%---48.7% and 51.0%---55.9%, respectively]. The analysis did not show in persons possessing blood groups A or AB an enhanced susceptibility to complications following antismallpox immunization or to particularly severe clinical forms of these complications. Immunological advantages in persons with blood group O or B were not found either.