RESUMO
The Na-doped perovskite-structure ruthenates Sr(1-x)Na(x)RuO3 and Ca(1-y)Na(y)RuO3 were prepared by high-pressure synthesis, which enables us to go beyond the previously reported Na doping limits and substitute Na for over 50% of the Sr in SrRuO3 and the Ca in CaRuO3. Gradual and systematic changes in the crystal structures were observed, and the decreases in the Ru-O bond lengths confirmed the Na substitution at the A site caused hole doping to Ru(4+) at the B site. In contrast to what has been previously reported, Sr(1-x)Na(x)RuO3 showed metallic conductivity. Magnetic properties were influenced by the Na substitution, but no long-range antiferromagnetic behavior was observed in Ca(1-y)Na(y)RuO3.
RESUMO
A-site-ordered perovskite Ce1/2Cu3Ti4O12 has been found to crystallize in two different forms, one with random and the other with ordered Ce/vacancy distribution at the A site of the prototype AA'3B4O12 structure. The random phase is isostructural with CaCu3Ti4O12, and the ordered phase is a new ordered derivative of the AA'3B4O12-type perovskite with two crystallographically distinct Cu sites. Although both phases form a G-type antiferromagnetic arrangement of Cu(2+) spins below 24 K, their magnetisms are quite different. A typical antiferromagnetic transition is observed in the random phase, whereas a small ferromagnetic moment appears below 24 K in the ordered phase, which rapidly decreases upon further cooling. A mean-field approximation approach revealed that this unusual behavior in the ordered phase is an L-type ferrimagnetism driven by the nonequivalent magnetizations of the two ferromagnetic Cu(2+) spin sublattices in the G-type spin structure. This unusual ferrimagnetism is a direct consequence of the Ce/vacancy ordering.
RESUMO
It is essential to know the detailed structure of the thin filament to understand the regulation mechanism of striated muscle contraction. Fluorescence resonance energy transfer (FRET) was used to construct an atomic model of the actin-tropomyosin (Tm)-troponin (Tn) core domain complex. We generated single-cysteine mutants in the 167-195 region of Tm and in TnC, TnI, and the ß-TnT 25-kDa fragment, and each was attached with an energy donor probe. An energy acceptor probe was located at actin Gln41, actin Cys374, or the actin nucleotide-binding site. From these donor-acceptor pairs, FRET efficiencies were determined with and without Ca(2+). Using the atomic coordinates for F-actin, Tm, and the Tn core domain, we searched all possible arrangements for Tm or the Tn core domain on F-actin to calculate the FRET efficiency for each donor-acceptor pair in each arrangement. By minimizing the squared sum of deviations for the calculated FRET efficiencies from the observed FRET efficiencies, we determined the location of Tm segment 167-195 and the Tn core domain on F-actin with and without Ca(2+). The bulk of the Tn core domain is located near actin subdomains 3 and 4. The central helix of TnC is nearly perpendicular to the F-actin axis, directing the N-terminal domain of TnC toward the actin outer domain. The C-terminal region in the I-T arm forms a four-helix-bundle structure with the Tm 175-185 region. After Ca(2+) release, the Tn core domain moves toward the actin outer domain and closer to the center of the F-actin axis.
Assuntos
Citoesqueleto de Actina/química , Actinas/química , Transferência Ressonante de Energia de Fluorescência/métodos , Imageamento Tridimensional/métodos , Modelos Moleculares , Tropomiosina/química , Troponina/química , Cálcio/farmacologia , Humanos , Estrutura Quaternária de Proteína , Estrutura Terciária de ProteínaRESUMO
A-site-ordered perovskites LaCu(3)(Ge(3/4)Ga(1/4))(4)O(12) and DyCu(3)(Ge(3/4)Ga(1/4))(4)O(12) were synthesized, and their magnetism was investigated. Ferromagnetic ordering of the square-planar-coordinated Cu(2+) spins was observed at 12-13 K in both compounds, and the Dy(3+) moment in DyCu(3)(Ge(3/4)Ga(1/4))(4)O(12) stayed paramagnetic below T(C). The decoupling of the magnetic behavior of Cu(2+) and Dy(3+) sublattices revealed the weak magnetic interaction between Cu(2+) and Dy(3+).
