Conformation of cyclo(Gly-L-Pro-L-Pro-Gly-L-Pro-L-Pro)(2)Mg complex crystallized from CH(3)CN solution

The cyclic hexapeptides (Gly-L-Pro-L-Pro-Gly-L-Pro-L-Pro) in the (peptide—Mg—peptide)²⁺ complex have nearly identical asymmetric conformations. Each has two cis Pro-Pro linkages and lacks any intraring hydrogen bonds. The Mg²⁺ ion forms six ligands in a regular octahedral array with the carbonyl oxygen atoms of the two Gly residues and one Pro residue of each peptide. The “sandwich” complex has an approximate 2-fold rotation axis through the Mg²⁺ relating the two peptide moieties. Cyclo(Gly-Pro-Pro-Gly-Pro-Pro)₂Mg(ClO₄)₂· 4C²H₃CN crystallizes in space group P3₁ with a = b = 15.744(4) Å, c = 24.002(6) Å, γ = 120°, and Z = 3. A highlight of the structure determination is the ready location of the Mg self-vector in a Harker section and the development of the entire structure by use of the tangent formula starting with the known position of the Mg atom.     

On the ion selectivity in Ca-binding proteins: the cyclo(-L-Pro-Gly-)3 peptide as a model.

Calcium plays a crucial role in many cellular processes. Its functions are directly dependent on the high specificity for Ca2+ exhibited by the proteins and ion carriers that bind divalent ions. To elucidate the basis for this specificity we have calculated the relative energies of solvation of calcium and magnesium ions in complexes with cyclo(-L-Pro-Gly-)3, a small synthetic peptide that binds Ca2+ with an affinity comparable to those of the naturally occurring proteins. The results show that the ion selectivity of the peptide resides in the difference in the solvation energies of the competing ions in water. Although the peptide is able to complex Mg2+ better than Ca2+ in the stoichiometries in which cyclo(-L-Pro-Gly-)3 binds divalent ions, it is not always able to provide as much stabilization for Mg2+ as water does. These results also explain why cyclo(-L-Pro-Gly-)3 binds Ca2+ and Mg2+ with different stoichiometries and indicate the source for expected differences in the structures of complexes of the two ions.     

Properties and Microstructure of Na2CO3-Activated Binders Modified with Ca(OH)2 and Mg(OH)2

Delayed strength development and long setting times are the main disadvantageous properties of Na₂CO₃-activated slag cements. In this work, combined auxiliary activators of Ca(OH)₂ and Mg(OH)₂ were incorporated in one-part Na₂CO₃-activated slag binders to accelerate the kinetics of alkali activation. The properties and microstructure evolution were investigated to clarify the reaction mechanism. The results showed that the additions of auxiliary activators promoted the hardening of the pastes within 2 h. The 28 days compressive strengths were in the range of 39.5–45.5 MPa, rendering the binders practical cementitious materials in general construction applications. Ca(OH)₂ was more effective than Mg(OH)₂ in accelerating the kinetics of alkali activation. The dissolution of Ca(OH)₂ released more OH⁻ and Ca²⁺ ions in the aqueous phase to increase alkalinity in the aqueous phase and promote the formation of the main binding gel phase of calcium-aluminosilicate hydrate (C-A-S-H). An increase in the Ca(OH)₂/Mg(OH)₂ ratios increased autogenous shrinkage and decreased drying shrinkage of the binders. The formation of a compact pore structure restricted the water evaporation from the binders during the drying procedure.     

High T(c) electron doped Ca10(Pt3As8)(Fe2As2)5 and Ca10(Pt4As8)(Fe2As2)5 superconductors with skutterudite intermediary layers

It has been argued that the very high transition temperatures of the highest T(c) cuprate superconductors are facilitated by enhanced CuO(2) plane coupling through heavy metal oxide intermediary layers. Whether enhanced coupling through intermediary layers can also influence T(c) in the new high T(c) iron arsenide superconductors has never been tested due the lack of appropriate systems for study. Here we report the crystal structures and properties of two iron arsenide superconductors, Ca(10)(Pt(3)As(8))(Fe(2)As(2))(5) (the "10-3-8 phase") and Ca(10)(Pt(4)As(8))(Fe(2)As(2))(5) (the "10-4-8 phase"). Based on -Ca-(Pt(n)As(8))-Ca-Fe(2)As(2)- layer stacking, these are very similar compounds for which the most important differences lie in the structural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to T(c) of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The often-cited empirical rule in the arsenide superconductor literature relating T(c) to As-Fe-As bond angles does not explain the observed differences in T(c) of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase arising from the two-channel interlayer interactions and the metallic nature of its intermediary Pt(4)As(8) layer. The interlayer coupling is thus revealed as important in enhancing T(c) in the iron pnictide superconductors.     

Optimization of Processing Steps for Superior Functional Properties of (Ba,Ca)(Zr,Ti)O3 Ceramics

Lead-free piezoelectric ceramics with nominal composition at morphotropic phase boundary Ba_0.85Ca_0.15Ti_0.9Zr_0.1O₃ (BCTZ) prepared by different processing routes and sintered either by conventional solid-state reaction or by spark plasma sintering (SPS) techniques were comparatively investigated to observe the role of structural modifications and of microstructures on the dielectric, ferroelectric, piezoelectric and electrocaloric responses. The ceramics presented relative densities from 75% to 97% and showed variations in their phase composition as a result of variable mixing and different synthesis and sintering parameters providing local compositional heterogeneity. As result, all of the ceramics showed diffuse phase transition and ferroelectric switching responses, with parameters affected mostly by density (P_r between 3.6 to 10.1 μC/cm²). High values for the electrocaloric response in the Curie range were found for the ceramics with predominantly orthorhombic character. Field-induced structural modifications were probed by tunability anomalies and by XRD experiments in remanence conditions. Piezoelectric effects with notably high figure of merit values were assigned to the better densification and poling efficiency of BCTZ ceramics.     

Blue copper proteins: Synthesis, spectra, and structures of CuN(3)(SR) and CuN(3)(SR) active site analogues

The reaction of Cu(SR) or [Cu(SR)][ClO₄] derivatives (SR = p-nitrobenzenethiolate or O-ethylcysteinate) with potassium hydrotris(3,5-dimethyl-1-pyrazolyl)borate produces redox pairs of the stoichiometry Cu^IN₃(SR) and Cu^IIN₃(SR). These complexes are well-defined synthetic approximations to the proposed N₃S binding sites of blue (type 1) copper electron transfer proteins. The compounds were investigated by a variety of chemical and spectral (optical, resonance Raman, and electron paramagnetic resonance) techniques; the complex K[Cu(hydrotris(3,5-dimethyl-1-pyrazolyl)borate)(p- NO₂C₆H₄S]-2 acetone was also studied by single-crystal x-ray diffraction methods. The spectrochemical characteristics of the Cu^IIN₃(SR) species are in large part similar to the native system and thus provide some perspective regarding the origin of the unique type 1 spectral parameters and electron transfer properties.     

Enhanced Piezoelectric Properties in a Single-Phase Region of Sm-Modified Lead-Free (Ba,Ca)(Zr,Ti)O3 Ceramics

In ferroelectric materials, phase boundaries such as the morphotropic phase boundary (MPB) and polymorphic phase boundary (PPB) have been widely utilized to enhance the piezoelectric properties. However, for a single-ferroelectric-phase system, there are few effective paradigms to achieve the enhancement of piezoelectric properties. Herein, we report an unexpected finding that largely enhanced piezoelectric properties occur in a single-tetragonal-ferroelectric-phase region in the Sm-modified (Ba_0.85Ca_0.15)(Zr_0.1Ti_0.9)O₃ (BCZT-xSm) system. An electrostrain maximum (0.13%) appears in the single-phase region of the BZCT-0.5Sm composition with the maximum polarization (P_m = 18.37 µC/cm²) and piezoelectric coefficient (d₃₃ = 396 pC/N) and the minimum coercive field (E_C = 3.30 kV/cm) at room temperature. Such an enhanced piezoelectric effect is due to the synergistic effect of large lattice distortion and domain miniaturization on the basis of the transmission electron microscope (TEM) observation and X-ray diffraction (XRD) Rietveld refinement. Our work may provide new insights into the design of high-performance ferroelectrics in the single-phase region.     

Conformation of uncomplexed natural antamanide crystallized from CH(3)CN/H(2)O

Natural antamanide, a cyclic decapeptide from Amanita phalloides, has been shown to have the same conformation for the backbone and comparable side groups as the biologically active synthetic [Phe⁴, Val⁶]antamanide. Packing in the crystal is quite different for the two compounds. Large channels with a polar lining are formed in the crystals of [Phe⁴, Val⁶]antamanide, whereas in natural antamanide large channels in the crystal are completely surrounded by the lipophilic side groups of the Pro, Phe, and Ala residues. Antamanide, C₆₄H₇₈N₁₀O₁₀·8H₂O·X, crystallizes in space group P2₁2₁2₁ with a = 15.88 Å, b = 34.88 Å, and c = 13.61 Å.     

On the homology of congruence subgroups and k(3)(z)

Let Γ(n;p) be the congruence subgroup of SL(n;Z) of level p. We study the homology and cohomology of Γ(n;p) as modules over SL(n;F_p) and apply our results to obtain an upper bound for the order of K₃(Z).     

Formation of hybrid complexes between Ca and the ionophores bromolasalocid (Br-X537A) and A23187

The ionophores A23187 and bromolasalocid (Br-X537A) acted synergistically in translocating Ca(2+) from an aqueous into an organic immiscible phase or in mediating Ca(2+) transport across the organic phase, the effects obtained in the simultaneous presence of both ionophores being greater than those expected from a summation of the individual effects of each ionophore. The nuclear magnetic resonance spectrum obtained when the complexation of Ca(2+) occurred in the presence of both ionophores differed strikingly from the individual spectra obtained with the A23187-Ca and Br-X537A-Ca complexes. These findings indicate that the two ionophores are able to form hybrid complexes with Ca(2+).     

Acute lymphoblastic leukemia (L3) with t(2;3)(p12;q27), t(14;18)(q32;q21), and t(8;22)(q24;q11)

A 50-year-old woman developed a subcutaneous tumor in the left lower leg. A biopsy led to the diagnosis of lymphoid malignancy. The malignant cells showed a B-cell immunophenotype. Karyotyping of the cells revealed t(14;18) and t(2;3). The patient was treated with chemotherapy, resulting in a transient response. Subsequently, tumor regrowth and bone marrow recurrence developed. Karyotyping of the bone marrow at relapse revealed a t(8;22) in addition to t(14;18) and t(2;3), which led to a diagnosis of acute lymphoblastic leukemia (ALL)-L3 (FAB). Although the patient was treated with several chemotherapy regimens, the disease was refractory to all the treatments. Fluorescence in situ hybridization (FISH) and the nested polymerase chain reaction (PCR) technique demonstrated rearrangements of the c-myc, bcl-2, and bcl-6 genes. ALL-L3 associated with t(14;18) is known to be complicated frequently with cerebrospinal infiltration and extramedullary lesions, and has a poor prognosis. In our case, the presence of the additional t(2;3) may have enhanced this patient's refractoriness to the treatment.     

Configurationally homogeneous diastereomers of a linear hexa(tertiary phosphine): enantioselective self-assembly of a double-stranded parallel helicate of the type (P)-[Cu(3)(hexaphos)(2)](PF(6))(3)

Three configurationally homogeneous diastereomers of the linear hexa(tertiary phosphine) Ph(2)PCH(2)CH(2)P(Ph)CH(2)CH(2)P(Ph)CH(2)CH(2)P(Ph)CH(2)CH(2)P(Ph)CH(2)CH(2)PPh(2) (hexaphos) have been isolated in enantiomerically pure form, namely (R,S,S,R)-, (R,S,S,S)-, and (S,S,S,S)-hexaphos. The strongly helicating (R,S,S,R)-(-) form of the ligand combines with copper(I) ions to generate by stereoselective self-assembly the P enantiomer of a parallel helicate of the type [Cu(3)(hexaphos)(2)](PF(6))(3), which has been characterized by x-ray crystallography. Theoretical modeling of the cation indicates that it is the relationship between the helicities of the two 10-membered rings containing the three copper ions, each of which has the twist-boat-chair-boat conformation, and the configurations of the three chiral, tetrahedral copper stereocenters of P configuration that determines the stereochemistry of the parallel and double alpha-helix conformers of the double-stranded trinuclear metal helicate.     

Bis[cyclo(histidylhistidine)]copper(II) complex that mimicks the active center of superoxide dismutase has its catalytic activity.

The formation of copper complexes with bis[cyclo( histidylhistidine )] copper(II) was determined potentiometrically; the maximum coordination number was four deprotonated histidine residues per Cu(II) ion. This complex mimicks the active center of Cu/Zn superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1). Its absorption spectrum showed a broad band above 600 nm as compared with the 670- to 680-nm band of the enzyme. Its CD spectrum had a negative 600- to 605-nm band and a more intense positive band near 780 nm. The band positions were identical with, and the intensities were higher than, those of the enzyme, but the corresponding bands had opposite signs. This can be attributed to the difference in the distorted planar structure between the complex and enzyme. The complex catalyzed the dismutation of superoxide anion; the activity was 1/10th that of the enzyme on a molar basis, but about three times that of the enzyme on a weight basis.