Neutron diffraction experiments over the momentum transfer range 0

We report the first neutron diffraction experiments of the equiatomic liquid alloys NaSn and KPb over a temperature range from 873 to 1873 K and pressures up to 60 bars. In the crystalline state, these are semiconducting compounds containing tetrahedrally coordinated polyanions, Sniand Pb; -. The diffraction results indicate the survival of polyanions in the expanded liquid alloys even up to the highest temperatures measured, as indicated by the presence of the first peak in the radial distribution function and by the persistence of the peak at Q = 1 A-1, which is indicative of intermediate-range order. The position of this peak shifts towards lower Q-values, its height decreases, and its width drastically increases with increasing temperature and correspondingly decreasing density. One can conclude that in these alloys, which are representative of a wider class of compounds containing Zintl ions in their crystalline state, the polyanions survive at these high temperatures and at densities which are about 25% lower than those at the melting point. * * *We are happy to acknowledge the assistance of the staff of ISIS at the Rutherford Appleton Laboratory during the course of these experiments. Work at ANL was supported by the US

Experimental results for fluid metals near the liquid-vapour critical point show that profound changes in the electronic structure of fluid metals occur in that region. A gradual transition from metallic to non-metallic behaviour occurs with decreasing density, which manifests itself in a corresponding strong thermodynamic state dependence of the interparticle interaction. The existence of this transition noticeably influences the thermodynamic features of the vapour-liquid phase transition of metals. Another equally important aspect of the problem is the size-dependent gradual evolution of metallic properties in isolated, microscopic metal clusters which clearly must be part of any detailed consideration of the liquid-vapour critical point phase transition of metals

We report on computer modelling studies of the structure of the equiatomic liquid alloy KPb over a temperature range from 873 K up to 1873 K. In the crystalline state, this semiconducting alloy contains tetrahedrally coordinated polyanions Pb44-. Recent neutron diffraction results indicate the survival of polyanions in the expanded liquid alloy even up to the highest temperature measured, as indicated by the persistence of a prepeak in the static structure factor S(Q) at a momentum transfer of Q approximately=IAA-1 up to 1873 K. Applying the reverse Monte Carlo technique, we demonstrate that there is a structure consistent with the experimental data, which consists of small clusters with a partially tetrahedral arrangement or an open loose structural network of lead atoms, typically with three or fourfold coordination. The calculations show that the peak at approximately I AA-1 observed in the diffraction patterns occurs predominantly in the partial structure factor SPbPb(Q), which is related to correlations between Pb clusters, for all temperatures. The Pb-Pb bonds in the clusters are fluctuating. With increasing temperature the tendency of Pb atoms to form particular clusters becomes a more general tendency to microphase separation.

Phosphoserine aminotransferase (EC 2.6.1.52), an enzyme of the “phosphorylated pathway” leading to the formation of serine, was purified from Scenedesmus obliquus, mutant C-2 A′. Purification started from the soluble supernatant of a crude cell homogenate and included different affinity and DEAE chromatographic techniques, as well as gel filtration. The purified phosphoserine aminotransferase was enriched 1537-fold and identified to be a homodimer with subunit molecular masses of 40 kDa, each. The absorption spectrum is consistent with the presence of pyridoxal-5-phosphate as cofactor. From the purified enzyme 18 amino acids of the N-terminus could be determined, showing at least 67% homology with the serC gene encoding phosphoserine aminotransferases from bacterial organisms.