Site 2a Research Articles

Hydrogenation induced valence change and metal atom site exchange at room temperature in the C14-type sub-structure of CeMn1.8Al0.2H4.4

CeMn1.8Al0.2 absorbs more than 4.4 hydrogen atoms near ambient conditions, thereby undergoing a volume expansion (ΔV/V∼43%) that is the largest known among metal hydrides. While the Mn/Al distribution in the C14-type alloy [P63/mmc, a=5.37393(6), c=8.76321(12) Å] is partially ordered (preference of Al for site 2a), it tends to become disordered in the hydride (equal Mn/Al occupancies for sites 2a and 6h). Only very slow hydrogenation while cooling the sample to −70°C is capable of maintaining a partial Mn/Al order. Deuterium in CeMn1.8Al0.2D4.36(7) [a=5.9788(6), c=9.7600(13) Å] occupies exclusively tetrahedral Ce2(Mn,Al)2 type interstices. Magnetic susceptibility data suggest that the volume expansion during hydrogenation is enhanced by a valence change of cerium (CeIV→CeIII).

Influence of postmenopausal hormone replacement therapy on platelet serotonin uptake site and serotonin 2a receptor binding

OBJECTIVE: To examine whether binding of [ 3H]paroxetine to the platelet serotonin transporter or binding of [ 3H]lysergic acid diethylamide (LSD) to the platelet 5-HT 2A receptor are influenced by postmenopausal estrogen/progestogen treatment. METHODS: Twenty-three postmenopausal women with climacteric symptoms completed this double-blind, randomized, crossover study. The women received 2 mg of estradiol continuously during four 28-day cycles. In the last 14 days of each cycle, 10 mg of medroxyprogesterone acetate, 1 mg of norethindrone acetate, or placebo was given. Before treatment, as well as once during the last week of each treatment, blood samples were collected for analysis of [ 3H]LSD and [ 3H]paroxetine binding. The power of the study setup was 81%. The study had an effect size of 0.36, corresponding to the ability to detect a 15% difference in [ 3H]paroxetine and [ 3H]LSD binding between treatments with α = .05 and β = .20, based on a previously reported standard deviation within cells of 20% of the mean binding values. RESULTS: The number of platelet receptors (B max), or the affinity of the radioligand to the receptor (K d), for [ 3H]paroxetine binding did not change during estrogen or estrogen-progestogen treatment, nor did B max or K d for [ 3H]LSD binding change during the different treatments. However, in a subgroup of depressed patients, the decrease in B max for [ 3H]LSD binding during treatment was significantly more pronounced than in the nondepressed subgroup ( P < .05). CONCLUSION: Estrogen treatment with or without the addition of progestogen does not affect binding to the serotonin transporter or to the serotonergic 5-HT 2A receptor in healthy postmenopausal women.

The Effect of Interlayer Cations on the Magnetic Properties of the Mixed-Metal Pnictide Oxides: A2MnZn2As2O2 (A = Sr, Ba)

The new pnictide oxide Sr2MnZn2As2O2 is reported. The crystal structure of this compound consists of a 1:1 intergrowth of square-planar MnO22- layers and ThCr2Si2-type Zn2As22- layers interspersed by Sr cations. This compound has been prepared by sintering the stoichiometric ratio of SrO, Mn, Zn, and As in a fused-silica ampule under 0.2 atm of Ar at 1000 °C for 1 week. The sample quality has been examined by Rietveld refinement of the powder X-ray diffraction data, and the transition metal site (2a and 4d) selectivity has been analyzed by Rietveld refinement of powder neutron diffraction data. Furthermore, effects of this structural order on the magnetic susceptibility and interaction between the magnetic layers have been analyzed by temperature-dependent magnetization measurements. The magnetic structures of both phases, A2MnZn2As2O2 (A = Sr, Ba), have been investigated by temperature-dependent powder neutron diffraction. The Ba analogue exhibits two-dimensional short-range order below about 40 K, whereas for the Sr analogue, no evidence for either long- or short-range order was found over the 2θ range investigated down to 4 K. The results of these characterizations are compared with those of the previously studied phase, Sr2Mn3As2O2.

The cooperative Jahn-Teller effect in BaFe 2.5Mn 9.5O 19

The distorted magnetoplumbite-type structure of BaFe 2.5Mn 9.5O 19 has been studied by powder neutron diffraction. The compound is orthorhombic, space group Cmcm ( a=5.826 Å, b=10.367 Å, c=23.004 Å, Z=4). Manganese is substituted into all five iron sites of the barium ferrite with population decreasing when going from the centre of the S block (100% of Mn for the 2a site) to the centre of the R block (40% for the 4e( 1 2 ) site). The structure includes two types of local Jahn-Teller distortions — the shortened Mn 3+O 6 octahedral in sites 4f IV and two thirds of sites 12k and the elongated octahedra in the remaining third. Manganese in the tetrahedral site 4f IV is bivalent and is compensated by Mn 4+ ions located preferentially in the octahedral site 2a. Ferrimagnetic arrangement below T c=220 K involves a large deal of static disorder. The ordered moments are aligned along the c-axis.

The crystal structures of CdAl2S4, HgAl2S4, and HgGa2S4

The crystal structures of CdAl2S4, HgAl2S4, and HgGa2S4 were solved and refined to R = 0.032, 0.042, and 0.046, respectively. CdAl2S4 and HgGa2S4 crystallize with the thiogallate type structure, space group I[unk]. The positional parameters of S are x = 0.2660(1), y = 0.2770(1), z = 0.1352(1) for CdAl2S4 and x = 0.2718(3), y = 0.2675(3), z = 0.1374(1) for HgGa2S4. HgAl2S4 crystallizes with space group I[unk]2m. Hg is situated at the special site (2a) 0,0,0, Al1 at (2b) 0,0,1/2, whereas Al2 and the vacancy are randomly distributed at (4d) 0,1/2,1/4. The positional parameters for S at (8i) x,x,z are x = 0.2272(11) and z = 0.3633(8).

Electron-spin resonance and fluorescence investigation of LaMgAl11O19:Ti3+, a potential tunable laser material

Single crystals of LaMgAl11O19:Ti (LMA:Ti) have been grown from the melt by the Verneuil (flame fusion) process. As-grown crystals contain titanium in both 3+ and 4+ oxidation states. Ti4+ ions can be reduced into the 3+ state by annealing at 1200 °C under hydrogen atmosphere. The crystals exhibit a broadband fluorescence extending from ∼600 to 1200 nm and peaking around 775 nm. The fluorescence lifetime is found to be ∼6 μs at 10 K against ∼4 μs at 300 K; it does not depend significantly on the emission wavelength, this indicating that fluorescence originates mainly from one type of Ti3+ active ions. Electron-spin-resonance (ESR) investigations reveal that Ti3+ ions are distributed among the three octahedral sites of the magnetoplumbite structure of the host, with a marked predominance for the regular octahedral site (2a), which contains about 90% of the titanium detected by ESR. Crystal field splitting of the t2g level has been estimated for the three sites and it is proposed that Ti3+ in the (12k) site is adjacent to a cationic defect. This new material is of potential interest for near infrared tunable laser application.

Antigenic Structure of Polioviruses of Serotypes 1, 2 and 3

The antigenic sites recognized by monoclonal antibodies with neutralizing activity for the Sabin vaccine strains of poliovirus of serotypes 1, 2 and 3 have been studied by the isolation and characterization of mutants resistant to neutralization by antibody. Three distinct sites have been identified which are designated site 1, site 2 and site 3. Site 1 includes a region of 12 amino acids of VP1, from residues 89 to 100, and a corresponding region of VP1 has been identified as an antigenic site for poliovirus 2. This site was strongly immunodominant in type 2 and type 3 but was not detected for poliovirus 1. Site 2 is a complex site including residues 220 to 222 from VP1 (site 2a) with residues including 169 and 170 and others of VP2 (site 2b). Both site 2a and site 2b have been detected in type 1 poliovirus, while as yet only site 2b has been detected in type 3 poliovirus. Site 3 is a complex site including residues 286 to 290 from VP1 (site 3a) with residues including 58 and 59 and others of VP3 (site 3b). Both sites 3a and 3b have been detected in type 3 poliovirus, while as yet only site 3b has been detected in type 1 poliovirus.

On the location of the H+-extruding steps in site 2 of the mitochondrial electron transport chain.

The location of the H+-translocating reactions within energy-conserving Site 2 of the mitochondrial electron transport chain was evaluated from two sets of data. In the first, the H+/2e- ejection ratios and Ca2+/2e- uptake ratios were compared for electron flow from succinate dehydrogenase, whose active site is on the matrix side of the inner membrane and from glycerol phosphate dehydrogenase, whose active site is on the cytosolic side. In intact rat liver mitochondria both substrates yielded H+/2e- ejection ratios close to 4.0 and Ca2+/2e- uptake ratios close to 1.0 during antimycin-sensitive reduction of ferricyanide. With rat liver mitoplasts and ferricytochrome c as electron acceptor, both substrates again gave the same stoichiometric ratios. The second approach involved determination of the sidedness of H+ formation during electron flow from succinate to ferricyanide via bypass of the antimycin block of the cytochrome b.c1 complex provided by N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), under conditions in which the TMPD-TMPD+ couple does not act as a membrane-penetrating protonophore. Electron flow in this system was inhibited by 2-then-oyltrifluoroacetone, indicating that TMPD probably accepts electrons from ubiquinol. The 2 H+ formed in this system were not delivered into the matrix but appeared directly in the medium in the absence of a protonophore. To accommodate the available evidence on Site 2 substrates, it is concluded that the substrate hydrogens are first transferred to ubiquinone, 2 H+ per 2e then appear in the medium by protolytic dehydrogenation of a species of ubiquinol or ubiquinol-protein having the appropriate sidedness (designated Site 2A), and the other 2 H+ are translocated from the matrix to the medium on passage of 2e- through the cytochrome b x c1 complex (designated Site 2B).