A twist boat: Structure and conformation oftrans-2,4,6-tris(trichloromethyl)-1,3,5-trithian

Trans-2,4,6-tris(trichloromethyl)-1,3,5-trithian, (CCl₃·CHS)₃, has been prepared free from persistent impurification bycis-trans-2,4,6-tris(trichloromethyl)-1,3,5-oxadithian and its conformation determined by single crystal x-ray crystallography. The title compound is orthorhombic,Pbca, a=10.479(10),b=27.957(14),c=11.093(3) Å,V=3250 ų,Z=8. The structure was solved by direct methods, from data collected at room temperature on an Enraf-Nonius CAD4 diffractometer, and refined by full-matrix least squares to a finalR value at 0.074 using 1186 reflections. The molecule exhibits a twist boat conformation with the –CCl₃ groups occupying pseudoequatorial positions. The torsion angles in the trithian ring are successively 29(1), –79(1), 41(1), 41(1), –84(1), 39(1)°. Average values are: S-C 1.82(2), C(ring)-C(Cl₃) 1.56(4), C-Cl 1.76(2) Å, S-C-S 112.7(2)°. The C-S-C angles are 97.4(7), 101.4(7), 97.3(7)°.     

High-quality Gaussian basis sets for fourth-row atoms

Summary Energy-optimized Gaussian basis sets of triple-zeta quality for the atoms Rb-Xe have been derived. Two series of basis sets are developed; (24s 16p 10d) and (26s 16p 10d) sets which we expand to 13d and 19p functions as the 4d and 5p shells become occupied. For the atoms lighter than Cd, the (24s 16p 10d) sets with triple-zeta valence distributions are higher in energy than the corresponding double-zeta distribution. To ensure a triple-zeta distribution and a global energy minimum the (26s 16p 10d) sets were derived. Total atomic energies from the largest basis sets are between 198 and 284E _H above the numerical Hartree-Fock energies.     

Weak ergodicity and products of random matrices

A representation for a weakly ergodic sequence of (nonstochastic) matrices allows products of nonnegative matrices which eventually become strictly positive to be expressed via products of some associated stochastic matrices and ratios of values of a certain function. This formula used in a random setup leads to a representation for the logarithm of a random matrix product. If the sequence of random matrices is in addition stationary then automatically almost all sequences are weakly ergodic, and the representation is expressed in terms of an one-dimensional stationary process. This permits properties of products of random matrices to be deduced from the latter. Second moment assumptions guarantee that central limit theorems and laws of the iterated logarithm hold for the random matrix products if and only if they hold for the corresponding stationary process. Finally, a central limit theorem for some classes of weakly dependent stationary random matrices is derived doing away with the restriction of boundedness of the ratios of colum entries assumed by previous studies. Extensions beyond stationarity are discussed.     

Applications of the zero–one law to problems connected with Cauchy's functional equation

Summary Marek Kuczma's book, entitled An Introduction To The Theory Of Functional Equations And Inequalities, mentions a certain setV ₀ in several places and presents references as to where this set is discussed in the literature. The main result of this paper is a proof of the fact that the setA _M (V ₀)={xV ₀ f(x)>M} is saturated non-measurable for each additive discontinuous functionf and each real numberM. Other results aboutV ₀ are also presented. Connections between measure and category are stressed. The main tool in our proofs is a certain so-called zero–one law and its topological analogue. In addition it is shown that the zero–one law is equivalent to Smital's lemma.     

Interpenetrating polymer networks of poly(carbonate-urethane) and polymethyl methacrylate

We report the synthesis and characterization of interpentrating polymer networks (IPN's), pseudo INPN's, and blends of urethane-containing aliphatic polycarbonates (PCU) and polymethyl methacrylate (PMMA). The simultaneous full IPN's of PCU's and PMMA over the whole composition range have only one T_g, as determined by DSC and DMA which, together with transmission electron microscopy observations, suggest a single phase morphology even though the linear chain blends are completely immiscible. The full IPN's exhibit a maximum tensile strength at a certain composition, and superior solvent and heat resistance as compared to the pseudo IPN's, linear blends, and the pure crosslinked PCU's and PMMA.     

Redox couples of inducible nitric oxide synthase

We report direct electrochemistry of the iNOS heme domain in a DDAB film on the surface of a basal plane graphite electrode. Cyclic voltammetry reveals FeIII/II and FeII/I couples at -191 and -1049 mV (vs Ag/AgCl). Imidazole and carbon monoxide in solution shift the FeIII/II potential by +20 and +62 mV, while the addition of dioxygen results in large catalytic waves at the onset of FeIII reduction. Voltammetry at higher scan rates (with pH variations) reveals that the FeIII/II cathodic peak can be resolved into two components, which are attributable to FeIII/II couples of five- and six-coordinate hemes. Digital simulation of our experimental data implicates water dissociation from the heme as a gating mechanism for ET in iNOS.     

The effect of polymer chain length and surface density on the adhesiveness of functionalized polymersomes

Giant cell-like polymer vesicles, polymersomes, made from the diblock copolymer poly(ethylene oxide)-polybutadiene (PEO-PBD), have bilayer structures similar to the cell membrane but have superior and tunable properties for storage and stability. We have modified the terminal hydroxyl of the hydrophilic block with biotin-lysine (biocytin), a biologically derived group that imparts specific adhesiveness to a polymer colloid coated with avidin. The functionalized polymer will form vesicles, either on its own or when mixed with unmodified block copolymers that also form vesicles. The incorporation and mixing of the functionalized polymer into vesicle bilayers is measured using a fluorescent version ofbiocytin with confocal microscopy. The fluorescence signal associated with the vesicle is in proportion with the concentration of functional polymer added during vesicle construction. The adhesiveness of polymer vesicles containing functionalized biotinylated polymer to avidin coated microspheres is measured with micropipet aspiration. Two types of polymer vesicles were constructed: one where the functionalized polymer (molecular weight (MW), 10400 Da) was longer than the surrounding unfunctionalized polymer (MW, 3600 Da) and one where the functionalized polymer (MW, 10400 Da) was the same length as the unfunctionalized polymer. In all cases, the avidin-biotin bonds form kinetically trapped crossbridges that impart little tension as they form but require significantly more tension to break. The relative length of the functionalized polymer on the surface of the vesicle is an important determinant for the adhesion of a polymer vesicle but not for the adsorption of soluble avidin. Greater adhesion strengths are seen where the functionalized polymer is longer than the surrounding polymer. The concentration of functionalized polymer at which adhesion is maximal depends on the relative lengths of the polymers. When the functionalized polymer is the same length as the surface brush of the polymersome membrane, the critical tension is maximal at 10 mol % functionalized polymer concentration. However, when the biocytin groups are attached to a polymer which is larger than the surface brush, the critical tension is maximal at 55 mol % functionalized polymer. These results indicate that polymer mixing and length can control the interfacial adhesion of polymer brushes and must be understood to tune polymersome adhesiveness.     

Mapping the cytochrome C folding landscape

The solution to the riddle of how a protein folds is encoded in the conformational energy landscape for the constituent polypeptide. Employing fluorescence energy transfer kinetics, we have mapped the S.cerevisiae iso-1 cytochrome c landscape by monitoring the distance between a C-terminal fluorophore and the heme during folding. Within 1 ms after denaturant dilution to native conditions, unfolded protein molecules have evolved into two distinct and rapidly equilibrating populations: a collection of collapsed structures with an average fluorophore-heme distance (r) of 27 A and a roughly equal population of extended polypeptides with r > 50 A. Molecules with the native fold appear on a time scale regulated by heme ligation events ( approximately 300 ms, pH 7). The experimentally derived landscape for folding has a narrow central funnel with a flat upper rim on which collapsed and extended polypeptides interchange rapidly in a search for the native structure.     

From green algae to calcium inside buckyballs

Coorongite and carbonaceous residues from coorongite pyrolysis at 450 and 500°C were studied by laser ablation Fourier transform mass spectrometry. Raw coorongite gave positive-ion spectra having mainly protonated species of m/z 80–300 when laser ablated with a high laser power density. Endohedral fullerene positive ions of calcium were observed during the laser ablation of coorongite pyrolysis residues. Pyrolysis of the raw coorongite at 450 and 500°C produced residues which on laser ablation using the fundamental frequency of an Nd: YAG laser (1064 nm) gave a series of calcium fullerides. These ions were observed using low laser power densities (100–600 kW cm^–2) Mixing the coorongite pyrolysis residue with barium sulphate gave M@C_n⁺ ions

1 The symbol ‘@’, as in Ca@₆₀, is used to represent an endohedral complex, i.e. the adduct is located inside the carbon cage.

where M = calcium or barium. Mixing the coorongite pyrolysis residue with strontium oxalate also gave M@C_n⁺ ions where M = calcium or strontium. No ions containing two or more metals were detected.