Catherine Renard

Preliminary crystal structure of mixed-valency Sr4Ni3O9, the actual formula of the so-called Sr5Ni4O11

A crystal structure investigation of the so-called Sr5Ni4O11 from single-crystal X-ray data has shown that the composition of this oxide is in fact close to Sr4Ni3O9. The structure has been solved in the trigonal P321 space group. The final refinement gave an R factor of 0.045 for 512 independent reflections. The structure contains NiO3 chains with two NiO6 octahedra and one NiO6 trigonal prism alternating and sharing faces. The chains run along the three-fold axis and are connected by Sr ions. The Ni–O distances seem to indicate a possible repartition of NiIV in the octahedral sites and NiII in the trigonal prisms.

Molten Salt Synthesis of a Mixed-Valent Lanthanide(III/IV) Oxychloride with an Unprecedented Sillen X24 Structure: Ce1.3Nd0.7O3Cl

A new cerium neodymium oxychloride, Ce1.3Nd0.7O3Cl, has been synthesized by precipitation in a LiCl–CaCl2 molten salt by humid argon sparging. Chemical and structure characterization have been undertaken by powder X-ray diffraction, scanning electron microscopy, high-temperature X-ray diffraction, thermogravimetric analysis, and X-ray photoelectron scattering. This oxychloride crystallizes in space group P4/nmm, a = 3.9848(3) Å and c = 12.467(2) Å, in a new Sillen-type phase represented by the symbol X(2)(4) where “quadruple” fluorite-type layers [M4O6], containing Ce(IV) in “inner” sublayers and both CeIII and NdIII in “outer” sublayers, alternate with double-halide ion sheets. The structure is also described as a stacking of LnOCl and fluorite-type blocks and constitutes the term n = 2 of a possible series (MO2)n(NdOCl)2.

Steady-state properties of a model ternary substrate cycle: theoretical predictions

Numerous ternary substrate cycles are metabolically operative in vivo. The relative concentrations of the interconverted substrates are generally correlated with different physiological states. These cycles often include reversible and/or substrate-inhibited enzymic steps. The switch between one steady state (metabolic state) and another may be the consequence of either the effect of an exogeneous metabolite or signal, or the alteration of a cycle internal parameter. The interpretation of results obtained with currently designed experiments on substrate cycles seldom take into account the very dynamic and regulatory properties inherent in the cyclic and often autocatalytic nature of the pathway. In the present report, the various dynamic properties of a model ternary substrate cycle, bounded by moiety conservation, are investigated. Three situations with increasing complexity are considered: (i) the three enzymes are michaelian and catalyse irreversible steps; (ii) one of the enzymic steps is reversible; and (iii) one step is subjected to a destabilizing factor, i.e. inhibition by excess of substrate. The behavior(s) of the whole cycle is mainly controlled by four parameters, that is, ST, the total concentration of the substrate pool, and the three enzyme maximal velocities, VMi (i = 1,2,3). As ST (= S1 + S2 + S3) is constant, the Si steady-state concentrations (stable or not) can be represented in barycentric coordinates in a triangle (simplex). This convenient representation allows us to predict the different states of the system when one enzyme maximal activity is varied. The steady-state concentration dependencies as a function of one or several parameters may be either monostable (possibility of zero-order ultrasensitivity) or bistable (with or without reversible transitions). The physiological and experimental relevances of these observations are emphasized.