Structural phase transition in substituted La2CuO4 and Pr2CuO4−y

The structural phase transition from orthorhombic (T) phase to tetragonal (T′) phase in substituted La_2−x R _x CuO₄ (R = Pr, Nd, Sm, Eu and Gd) and T′ to T-phase in Pr_2−x M _x CuO_4−y (M = Sr, Ca) has been studied by X-ray diffraction technique. The T-phase of La₂CuO₄ is transferred to T′ phase abruptly atx=0.8, 0.4, 0.4, 0.3 and 0.4 respectively for substitution of Pr, Nd, Sm, Eu and Gd for La in La₂CuO₄ without evidence of the T* phase. The T′ structure of Pr₂CuO₄ (x = 0.0) gets transformed to the T* structure at 30% Ca doping (x=0.6) and then to the T structure at 50% Ca doping (x=1.0), while for Sr-contentx=0.0, 0.4 and 1.0 it shows T′, T* and T structure respectively.     

X-Ray Absorption Spectroscopy of Strontium(II) Coordination

Detailed analyses of crystalline, hydrated, and precipitated strontium compounds and an aqueous strontium solution by synchrotron extended X-ray absorption fine structure (EXAFS) were used to quantify local thermal and static disorder and to characterize strontium coordination in a variety of oxygen-ligated bonding environments. Analysis of anharmonic vibrational disorder (i.e., significant contribution from a third cumulant term (C(3)) in the EXAFS phase-shift function) in compounds with low and high static disorder around strontium showed that first-shell anharmonic contributions were generally not significant above experimental error in the EXAFS fits (R+/-0.02 ? with and without C(3)). The only case in which a significant apparent decrease in Sr-O distance was observed with increasing temperature, and for which a third cumulant term was significant, was for dilute strontium in aqueous solution. Empirical parameterization of Debye-Waller factor (sigma(2)) for strontium compounds as a function of backscatterer atomic number (Z), interatomic Sr-Z distance, and temperature of spectral data collection showed systematic increases in sigma(2) as a function of increasing temperature and Sr-Z bond length. At values of sigma(2) greater than approximately 0.025 ?(2) (for N<12 and R(Sr)-Z>3 ?), backscattering was generally not significant above noise levels in spectra of compounds of known crystal structure. Comparison of the EXAFS spectra of freshly precipitated SrCO(3) (spectra collected wet) to that of dry, powdered strontianite (SrCO(3)(s)) indicated no significant differences in the local atomic structure around strontium. Analysis of partially hydrated strontium in natural Ca-zeolite (heulandite) showed that strontium is substituted only in the calcium (Ca2) site. Backscattering from aluminum and silicon atoms in the zeolite framework were apparent in the EXAFS spectra at low and room temperature at distances from central strontium of <4.2 ?. Comparison of strontium structural coordination determined in this and previous studies suggests that previous EXAFS determinations of hydrated strontium may have underestimated first-shell interatomic distances and coordination numbers because minor contributions to the EXAFS phase-shift and amplitude functions were not accounted for, either theoretically or empirically. Copyright 2000 Academic Press.     

Formation and stability of a Janus-Wedge type of DNA triplex

A new type of DNA targeting with the formation of a Janus-Wedge (J-W) triple helix is described. The "wedge" residue (W) attached to a PNA backbone is designed to insert itself into double-stranded DNA and base pair with both Watson-Crick faces. To study the stability of such an assembly, we have examined the formation of the J-W triplex with dC8 - T8 target sequence. The use of this target sequence permits the study of this new helix form without competing Watson-Crick interactions between the two target residues. Studies indicate that the W strand binds to both target strands, with defined polarity and a stability (-15.2 kcal/mol) that is roughly the sum of the two independent duplex interactions.     

Biomembrane phospholipid-oxide surface interactions: crystal chemical and thermodynamic basis

Quartz has the least favored surface among many oxides for bacterial attachment and for lipid bilayer or micelle interactions. Tetrahedrally coordinated crystalline silica polymorphs are membranolytic toward liposomes, lysosomes, erythrocytes, and macrophages. Amorphous silica, the octahedral silica polymorph, (stishovite), and oxides such as Al2O3, Fe2 O3, and TiO2 are less cytotoxic. Existing theories for membrane rupture that invoke interactions between oxide surfaces and cell membrane phospholipids (PLs) do not adequately explain these differences in membranolytic potential of the oxides. The author presents a crystal chemical, thermodynamic model for the initial interaction of oxide surfaces with the quaternary ammonium component of the PL's polar head group. The model includes solvation energy changes and electrostatic forces during adsorption, represented by the dielectric constant of the solid and the charge-to-radius ratio of the adsorbing solute. The nature of oxide-solute interactions compared with oxide-water, solute-water, and water-water interactions determines the membranolytic activity of the oxide, where the solute is TMA+, the quaternary ammonium moeity. Significant membrane rupture, as on quartz, requires unfavorable adsorption entropy (DeltaS(ads,TMA+)<0) to maximize disruption of normal membrane structure and requires favorable Gibbs free energy of exchange between TMA+ and the ambient Na+ ions (DeltaG(exc,TMA+/Na+) = DeltaG(ads,TMA+)-DeltaG(ads,Na+)<0) to maximize the extent of membrane affected. For amorphous silica, DeltaS(ads,TMA+) >0, so disruption of structure is limited, even though G(exc,TMA+/Na+) is <0. Stishovite and other oxides have DeltaS(ads,TMA+) <0, but now DeltaG(exc,TMA+/Na+) is>0 at the acidic to circumneutral pHs of cellular and subcellular organelle fluids. The model predicts the correct sequence of membranolytic ability: quartz > or = amorphous SiO2 >Al2O3 >Fe2O3 >TiO2. The model thus explains the relatively poor adhesion of bacterial cells to quartz and the lack of quartz as a biomineral. It is proposed that one function of extracellular polymeric substances exuded by bacteria is to render mineral surfaces more hydrophilic.     

Synthesis, Characterization and Antibacterial Activity of Some Transition Metal cis-3,7-dimethyl-2,6-octadiensemicarbazone Complexes

The synthesis and characterization of some transition metal cis-3,7-dimethyl-2,6-octadiensemicarbazone (CDOSC) complexes are reported. The ligand CDOSC yields: [ML₂ Cl₂] and [ML₂ Cl₂] Cl type complexes, where M = Cr^III, Mn^II, Fe^III, Co^II, Ni^II, Cu^II, Zn^II, Cd^II and Hg^II, L = CDOSC. Structures of the complexes were determined using elemental analysis, molar conductivity, magnetic measurements, i.r. and electronic, as well as n.m.r spectra. CDOSC acts as a bidentate ligand in all the complexes. All the newly synthesized metal complexes, as well as the ligand, were screened for their antibacterial activity. All the complexes exhibit strong inhibitory action against Gram (+) bacteria Staphylococcus aureus and Gram (−) bacteria Escherichia coli. The antibacterial activities of the complexes are stronger than those of the ligand CDOSC itself.     

Effect of bovine serum albumin on the surface properties of ionic liquid-type Gemini surfactant

The effect of bovine serum albumin on the surface properties of IL-type gemini surfactant ([C₁₀-4-C₁₀im]Br₂), have been investigated by surface tension method. The critical micelle concentration (CMC) as a function of BSA concentrations at various temperatures was investigated. The CMC of [C₁₀-4-C₁₀im]Br₂ increases with increasing the concentration of BSA as well as the temperature of the system. The interfacial parameters viz; maximum surface excess concentration (Γ_max), the minimum area per molecule (A_min), and surface pressure at CMC (Π_cmc) were calculated. In addition, thermodynamic parameters of adsorption and micellization were evaluated by using surface tension data. The results indicated that the binding of [C₁₀-4-C₁₀im]Br₂ to BSA is spontaneous and exothermic in nature. The process is entropy driven and hydrophobic interactions are the major driving forces.     

Quantitative Relationships Between Molecular Descriptors,Chromatographic Retention Behavior,and In Vitro Antituberculosis Activity of Phytol Derivatives

JPC – Journal of Planar Chromatography – Modern TLC - The lipophilic character of phytol derivatives has been studied using reverse-phase planar chromatographic procedures....     

Synthesis,Structural, and Antibacterial Studies of Some Mixed Ligand Complexes of Zn(II), Cd(II), and Hg(II) Derived From Citral Thiosemicarbazone and N-Phthaloyl Amino Acids

A series of new mixed ligand complexes of Zn(II), Cd(II), and Hg(II) with cis-3,7-dimethyl-2,6-octadienthiosemicarbazone (CDOTSC; LH) and N-phthaloyl amino acids (AH) have been synthesized by the reaction of metal dichloride with ligands CDOTSC and N-phthaloyl derivative of DL-glycine (A₁H), L-alanine (A₂H), or L-valine (A₃H) in a 1:1:1 molar ratio in dry refluxing ethanol. All the isolated complexes have the general composition [M(L)(A)]. The plausible structure of these newly synthesized complexes has been proposed on the basis of elemental analyses, molar conductances, molecular weight measurement, and various spectral (IR, ¹H NMR, and ¹³C NMR) studies, and four coordinated geometries have been assigned to these complexes. All the complexes and ligands have been screened for their antibacterial activity.