A tetragonal form of dysprosium orthomolybdate at room temperature

In the present tetragonal modification of dysprosium orthomolybdate, Dy₂(MoO₄)₃, the Dy, one Mo and one O atom are located on a mirror plane with Wyckoff symbol 4e, while another Mo atom is located on a fourfold inverse axis, Wyckoff symbol 2a. A single crystal was selected from a polycrystalline mixture of the Dy₂O₃–ZrO₂–MoO₃ system and was stable at room temperature for at least three months. The structure refinement does not indicate the presence of Zr on the Dy sites (to within 1% accuracy). Thus, the stabilization of the tetragonal form is due to disordered positions for a second O atom and split positions for a third O atom that also maintain the DyO₇ coordination, which is not expected for short Dy—O distances [2.243 (6)–2.393 (5) Å].     

Do Liposome‐binding Constants of Porphyrins Correlate with Their Measured and Predicted Partitioning Between Octanol and Water?¶

We tested correlations between lipophilicity parameters and the partitioning of sensitizers into membranes. For this purpose we investigated 17 porphyrins and two chlorins having various chemical structures. Some of these compounds possess an amphiphilic structure (including hematoporphyrin, deuteroporphyrin, mesoporphyrin, chlorin e6 and more). The others are very symmetrical sensitizers [meso-tetra(N-methyl-4-pyridyl)porphyrin, tetra-benzoporphyrin, coproporphyrin I dihydrochloride (CP), meso-tetra(4-carboxyphenyl)porphyrin (TCP) and meso-tetra(m-hydroxyphenyl)chlorin]. Our investigation also included two series of hematoporphyrins and protoporphyrins with varying lengths of alkylcarboxylate side groups. The partitioning of these compounds between the bulk aqueous phase and liposomes was studied by fluorescence methods, and a liposome-binding constant, Kb, was obtained. It was found that CP and TCP do not incorporate into the lipid phase at pH 7.3. An n-octanol-water partition coefficient (log P) and a distribution coefficient (log D) were predicted with a modeling software. The values of log D were also obtained experimentally. We found that for the studied molecules, the predicted log D correlated well with the measured values. The values of log D as well as log P, in turn, did not correlate nicely, for the whole group of studied compounds, with the binding constants to liposomes. However, in the case of porphyrins that share a similar structure, the Kb showed good linear correlation with both log P and log D. For the series of hematoporphyrins and protoporphyrins with different lengths of alkylcarboxyl groups, it was shown that prolongation of this group caused an increase in the lipophilicity and the liposome-binding constant. This effect is more pronounced for the proto- than for the hematoporphyrin series. The results highlight the possible use, as well as limitations, of lipophilicity parameters for the prediction of membrane binding.     

Echo counting and echo integration with a sector scanning sonar

An investigation is described of the possible application of sector scanning sonar for measuring the spatial density of marine organisms. The two assessment techniques investigated are called echo counting and echo integration. These techniques have been used extensively with single transducer echo sounding equipment for fisheries assessment. One of the main limitations of single transducer assessment systems is that they cannot provide both high resolution and good volume coverage. However, an acoustic assessment system in which a sector scanning sonar is used can provide both. The problems of implementing the two assessment techniques when using a scanning sonar are discussed. The performance of the techniques is evaluated by calculating the normalized rms error in the estimate of spatial density they provide. It is shown that, under certain conditions, the sector scanning assessment will system provide an estimate with a significaantly smaller error than that obtained with a conventional single transducer system.     

Liposome binding constants and singlet oxygen quantum yields of hypericin, tetrahydroxy helianthrone and their derivatives: studies in organic solutions and in liposomes

The spectroscopy and photophysics of several hypericin and helianthrone derivatives were studied in methanol and when bound to liposomes. The singlet oxygen quantum yields (phi(delta)) were measured indirectly relative to Rose Bengal and hematoporphyrin IX, employing 9,10-dimethylanthracene as a singlet oxygen trap. Hypericin was found to have a phi(delta) of 0.39+/-0.01 in methanol, and 0.35+/-0.05 in lecithin vesicles, in agreement with literature values. A heavy atom effect was evident upon bromination, resulting in phi(delta) for tetrabromohypericin of 0.72+/-0.02, presumably due to enhanced intersystem crossing. Elimination of the anionic hydroxyls by methylation also enhanced phi(delta) to 0.81+/-0.01. Conversely, addition of anionic sulfate groups drastically reduced phi(delta) resulting in phi(delta)'s of 0.12+/-0.01, 0.052+/-0.003 and 0.40+/-0.01 for hypericin disulfonate, hypericin tetrasulfonate and hexamethyl hypericin tetrasulfonate, respectively. The non-sulfonated helianthrones exhibited low phi(delta)'s in solution. The liposome binding constants, Kb, were measured using a spectroscopic assay. Except for hexamethyl hypericin, all non-sulfonated compounds bound well with Kb's ranging from 15.5+/-0.1 to 48.7+/-3.9 (mg/ml)(-1). None of the tetrasulfonated compounds bound, however the hypericin disulfonate had a Kb of 4.1+/-0.2 (mg/ml)(-1). The phi(delta)'s of the compounds capable of binding were measured and, in the case of the hypericin derivatives, were found not to vary dramatically from those in the free state. Liposome-bound helianthrone and dimethyl tetrahydroxy helianthrone both exhibited high phi(delta)'s, i.e. >0.5. The variations in binding constant and sensitization efficiencies are explained in conjunction with the molecular structure. The relevance of the above data to photodynamic therapy is briefly discussed.     

Random signal fluctuations can reduce random fluctuations in regulated components of chemical regulatory networks

Many intracellular components are present in low copy numbers per cell and subject to feedback control. We use chemical master equations to analyze a negative feedback system where species X and S regulate each other's synthesis with standard intracellular kinetics. For a given number of X-molecules, S-variation can be significant. We show that this signal noise does not necessarily increase X-variation as previously thought but, surprisingly, can be necessary to reduce it below a Poissonian limit. The principle resembles Stochastic Resonance in that signal noise improves signal detection.     

A new tetragonal structure type for Li2B2C

The ternary dilithium diboron carbide, Li₂B₂C (tetragonal, space group Pm2, tP10), crystallizes as a new structure type and consists of structural fragments which are typical for structures of elemental lithium and boron or binary borocarbide B₁₃C₂. The symmetries of the occupied sites are .m. and 2mm. for the B and C atoms, and m2 and 2mm. for the Li atoms. The coordination polyhedra around the Li atoms are cuboctahedra and 15‐vertex distorted pseudo‐Frank–Kasper polyhedra. The environment of the B atom is a ten‐vertex polyhedron. The nearest neighbours of the C atom are two B atoms, and this group is surrounded by a deformed cuboctahedron with one centred lateral facet. Electronic structure calculations using the TB–LMTO–ASA method reveal strong B...C and B...B interactions.     

X‐ray powder diffraction structural characterization of iron(III) chloro‐ and bromo‐complexes of triphenylphosphine oxide

Iron (III) complexes of the composition [FeX₂(OPPh₃)₄][FeX₄], where X is Cl or Br were synthesised by several methods. It was found that each particular synthesis has yielded solids with slightly different powder patterns and also their comparison to those of calculated from single crystal data showed remarkable differences. A series of constrained Rietveld refinements has been performed on the title complexes to explain these differences. In a course of Rietveld refinement it was found that preferred orientation is not the only factor affecting the shape of the pattern and atoms parameters were included into refinement too. The results of Rietveld refinement allowed to conclude that the structures of compounds prepared by different methods are identical. In addition, it was shown that also in the case of rather complicated structure like the title complexes the powder diffraction method can lead to reasonable results.