Neutron diffraction and simulation studies of CsNO3 and Cs2CO3 solutions

Neutron diffraction with isotopic substitution (NDIS) experiments and molecular dynamics (MD) simulations have been used to study the structuring in aqueous solution of two cesium salts, cesium carbonate, and cesium nitrate. As was previously found for guanidinium salts of carbonate, mesoscopic-scale clusters were seen to form in the Cs2CO3 solution both in the MD simulations and in the diffraction experiments. No such large scale ion clusters were found in the CsNO3 solutions in either the modeling or experiments. The results are dominated by the strength and geometry of the direct first-neighbor interactions, which explain the differences in the clustering behavior between the two solutions without need to refer to longer-range water-water structuring.     

Synthesis and biological activity of medium molecular weight polymers of camptothecin

A new monomer, 3,6-endo-methylene-1,2,3,6-tetrahydrophthalimidohexanoylcamptothecin (ETHCPT) was synthesized from 3,6-endo-methylene-1,2,3,6-tetrahydrophthalimidohexanoic acid. Its homopolymer and copolymer with acrylic acid (AA) were synthesized and spectroscopically characterized. The ETHCPT content in poly(ETHCPT-co-AA) obtained by elemental analysis was 37 wt.%. The number-average molecular weights of the polymers determined by gel permeation chromatography were as follows: M_n=9700 for poly(ETHCPT), for poly(ETHCPT-co-AA). The IC₅₀ value of ETHCPT and its polymers against cancer cells was much larger than that of CPT. The in vivo antitumor activity of all polymers in Balb/C mice bearing the sarcoma 180 tumor cell line was greater than that of CPT at a dose of 100 mg/kg.     

K(π 1)'s for Artin Groups of Finite Type

We construct K( 1)'s for Artin groups of type C _n and D _n , using the lattice of elements preceding a Coxeter element in the partial order defined by reflection length.     

Cytoskyrins A and B, new BIA active bisanthraquinones isolated from an endophytic fungus

[structure] The biochemical induction assay (BIA) is a rapid (colorimetric) bacterial assay used to identify compounds that damage DNA or inhibit DNA synthesis and thereby identify potential natural product anticancer agents. Bisanthraquinones based on a 1,3,6, 8-tetrahydroxyanthraquinone-type carbon skeleton were isolated from an endophytic fungus and characterized by NMR and X-ray crystallography. Cytoskyrin A (1) is highly active in the biochemical induction assay, while the closely related cytoskyrin B (2) has no detectable activity in this assay.     

The cytosporones, new octaketide antibiotics isolated from an endophytic fungus

[structure] Organic extracts from cultures of endophytic fungi collected in the Guanacaste Conservation Area of Costa Rica were screened for antibiotic activity. Two endophytes CR200 (Cytospora sp.) and CR146 (Diaporthe sp.) were found to have potent antibiotic activity. Bioassay-guided fractionation of the extracts from these fungi led to the identification of cytosporones D and E, antibacterial active trihydroxybenzene lactones, and three related but inactive metabolites. The five new octaketides were characterized using X-ray crystallography and NMR.     

Carbon-13 and proton NMR parameters of neopentyl-and trimethylsilylmethyl-thallium(III) derivatives and the x-ray crystal structure of bis(trimethylsilylmethyl)chlorothallium(III)

Carbon-13 and proton NMR spectra have been determined for organothallium (III) derivatives of the types RTlX₂ and R₂ TlX (R  (CH₃)₃CCH₂ or (CH₃)₃SiCH₂; X  Cl, Br or O₂CCH(CH₃)₂). The dependence of coupling of ¹³C and ¹H to thallium on the number and nature of R groups is discussed in terms of the Fermi contact mechanism for spinspin coupling.The crystal structure of [(CH₃)₃SiCH₂]₂ TlCl has been determined. The compound crystallises in the monoclinic space group P2₁/n, with a 10.618, b 24.492, c 6.017 Å, β 99.76°. The molecule is dimeric with each four-coordinate thallium atom bonded unequally to two bridging chlorine atoms. The CTlC angle is 168°.     

Spin-state alteration from sterically enforced ligand rotation in bis(indenyl)chromium(II) complexes

The rotational orientation of cyclopentadienyl rings usually has no effect on d-orbital energy levels and splitting in transition metal complexes. With related but less symmetrical carbocyclic ligands, however, the magnetic properties of the associated complexes can be altered by the alignment of the ligands. Examples of this effect are found in substituted organochromium(II) bis(indenyl) complexes. The monosubstituted compounds (1-RC(9)H(6))(2)Cr (R = t-Bu, SiMe(3)) are prepared from the substituted lithium indenides and CrCl(2) in THF; they are high-spin species with four unpaired electrons. Their spin state likely reflects that in the unknown monomeric (C(9)H(7))(2)Cr, which is calculated to have a high-spin (S = 2) ground state in the staggered configuration (180 degrees rotation angle). However, the analogous bis(indenyl) complexes containing t-Bu or SiMe(3) groups in both the 1 and 3 positions on the indenyl ligands ((1,3-R(2)C(9)H(5))(2)Cr) are low-spin compounds with two unpaired electrons. X-ray diffraction results indicate that [1-(t-Bu)C(9)H(6)](2)Cr exists in a staggered conformation, with Cr-C (av) = 2.32(4) A. In contrast, the average Cr-C distances in [1,3-(t-Bu or SiMe(3))(2)C(9)H(5)](2)Cr are 2.22(2) and 2.20(2) A, respectively, and the rings are in a gauche configuration, with rotation angles of 87 degrees. The indenyl conformations are sterically imposed by the bulk of the t-Bu and SiMe(3) substituents. The change from a staggered to a gauche indenyl orientation lowers the symmetry of a (C(9)H(7))(2)M complex and allows greater mixing of metal and ligand orbitals. Calculations indicate that previously nonbonding pi orbitals of the indenyl anion are able to interact with the chromium d orbitals, producing bonding and antibonding combinations. The latter remain unpopulated, and the resulting increase in the HOMO-LUMO gap forces the complexes to adopt a low-spin configuration. The possibility of using sterically imposed ligand rotation as a means of spin-state manipulation makes indenyl compounds a potentially rich source of magnetically adjustable molecules.