N,N-coordinated pi radical anions of S-methyl-1-phenyl-isothiosemicarbazide in two five-coordinate ferric complexes [Fe III(L Me*)(2)X] (X = CH3S-, Cl-)

The reaction between [Fe(III)(dmf)(6)](ClO(4))(3) and the ligand S-methyl-1-phenyl-isothiosemicarbazide, H(2)[L(Me)], and triethylamine (1:3:6) in methanol under an argon blanketing atmosphere at elevated temperatures (reflux) yields a purple solution from which upon cooling to 20 degrees C dark green crystals of [Fe(III)(L(Me)(*))(2)(SCH(3))] (1) were obtained in 15% yield. From a similar reaction mixture using FeCl(3) as starting material in the solvent acetone under anaerobic conditions at -80 degrees C, dark green crystals of [Fe(III)(L(Me)(*))(2)Cl] (2) were obtained in 21% yield. The structures of complexes 1 and 2 have been determined by single-crystal X-ray crystallography at 100 K. Both complexes are five-coordinate square base pyramidal ferric species containing two N,N-coordinated, monoanionic pi radicals, (L(Me)(*))(1)(-), of the parent S-methyl-1-phenyl-isothiosemicarbazide(2-) dianion in the basal positions whereas the axial position is occupied by methylthiolate in 1 and chloride in 2, respectively. The electronic structure of both species has been elucidated by their electronic spectra, magnetic properties, and X-band EPR and M?ssbauer spectra. Both possess an S(t) = (1)/(2) ground state which is attained via an antiferromagnetic coupling between the spins of an intermediate spin ferric ion (S(Fe) = (3)/(2)) and two ligand pi radical anions (S(rad) = (1)/(2)).     

A Quantitative Spectrometric Assay for the Determination of Solution Concentration of para-sulphonato-calix[n]arenes and Their Derivatives

A spectrometric assay for the determination of concentration of para-sulphonato-calix[n]arenes and their derivatives has been developed using dimethylmethylene blue (DMMB) as a probe. Interaction with para-sulphonato-calix[n]arenes leads to a metachromatic shift in the spectrum of DMMB with appearance of a peak at 536 nm and diminution of the spectral intensity of the peaks at 594 and 649 nm. The method shows good linearity in the concentration range 0–6 μg/ml for para-sulphonato-calix[n]arenes. in final form: 27 December 2004     

Mispair-aligned N3T-alkyl-N3T interstrand cross-linked DNA: synthesis and characterization of duplexes with interstrand cross-links of variable lengths

Therapeutic bifunctional alkylating agents generate interstrand cross-links in duplex DNA. As part of our continuing studies on DNA duplexes that contain alkyl interstrand cross-links, we have synthesized a cross-link that bridges the N(3) positions of a mismatched thymidine base pair. This cross-link, which is similar to the N(3)C-alkyl-N(3)C cross-link that has been observed between mismatched cytosine base pairs, was introduced by first incorporating a cross-linked phosphoramidite unit at the 5'-end of an oligonucleotide chain. Fully cross-linked duplexes were then synthesized using an orthogonal approach to selectively remove protecting groups, thus allowing construction of the cross-linked duplex via conventional solid-phase oligonucleotide synthesis. Short DNA duplexes with alkyl cross-links of various lengths (two, four, and seven methylene units) were prepared, and their physical properties were studied via UV thermal denaturation and circular dichroism spectroscopy. These linkers were found to stabilize the duplexes by 37, 31, and 16 degrees C for the two-, four-, and seven-carbon linkers, respectively, relative to a non-cross-linked duplex. Circular dichroism spectra suggested that these lesions induce very little deviation in the global structure relative to the non-cross-linked duplex DNA control. Molecular models show that the two-carbon cross-link spans the distance between the N(3) atoms of the T-T mismatch without perturbing the helix structure, whereas the longer linkers, particularly the seven-carbon linker, tend to push the thymines apart, creating a local distortion. This perturbation may account for the lower thermal stability of the seven-carbon versus two-carbon cross-linked duplex.     

Pyridine analogs of (−)-cytisine and varenicline: cholinergic receptor probes

A series of bicyclic pyridine containing alkaloids related to (−)-cytisine and varenicline are described. Synthetic access via regioisomeric metalation of alkoxy- and halo-pyridines gains entry to all four isomeric [3.3.1]-bicyclic targets. Regioselective and sequential oxidative cleavage of dicyclopentadiene generates a related [3.2.1]-bicyclic analog.     

Design and synthesis of hierarchical materials from ordered zeolitic building units

The crystallization of colloidal silicalite-1 from clear solution is one of the best understood zeolite formation processes. Colloidal silicalite-1 formation involves a self-assembly process in which nanoslabs and nanotablets with a silicalite-1 type connectivity are formed at intermediate stages. During the assembly process, with strongly anisometric particles present, regions appear with orientational correlations, as evidenced with measurements of dynamic light scattering, viscosity, and rotation of polarized light. The presence of such regions rationalizes the unexpected differences between the crystallization kinetics under microgravity and on earth. The discovery of the locally oriented regions sheds new light on currently poorly understood hydrodynamic effects on the zeolite formation processes, such as the influence of stirring on the phases obtained and the subsequent kinetics. Addition of surfactants or polymers modifies the ordering of the zeolitic building units in the correlated regions, and new types of hierarchical materials named zeogrids and zeotiles can be obtained.     

Synthesis of structurally diverse and defined bivalent mannosides on saccharide scaffolding

[Structure: see text] The synthesis of bivalent mannosides by the grafting of alpha-D-mannopyranoside onto monosaccharide acceptors and conjugation to terephthalic acid or phenylenediamine is described. Computational methods were used to predict accessible orientations and distances between the mannose units.     

Atomistic simulation of charged iron oxyhydroxide surfaces in contact with aqueous solution

Molecular dynamics simulations of aqueous solution/goethite interfaces show that the classical models of the electrical double layer do not accurately describe the distribution of ions near the surface (such a distribution is present even when the surface is neutral) and that the explicit treatment of solvent molecules is essential to capture the effects of the surface on the liquid phase.     

Constraints on changes in fundamental constants from a cosmologically distant OH absorber or emitter

We have detected the four 18 cm OH lines from the z approximaetely 0.765 gravitational lens toward PMN J0134-0931. The 1612 and 1720 MHz lines are in conjugate absorption and emission, providing a laboratory to test the evolution of fundamental constants over a large lookback time. We compare the HI and OH main line absorption redshifts of the different components in the z approximately 0.765 absorber and the z approximately 0.685 lens toward B0218 + 357 to place stringent constraints on changes in F triple-bond g(p)[alpha(2)/mu](1.57). We obtain [DeltaF/F] = (0.44 +/- 0.36(stat) +/- 1.0(sys)t) x 10(-5), consistent with no evolution over the redshift range 0 < z < or = 0.7. The measurements have a 2sigma sensitivity of [Deltaalpha/alpha] < 6.7 x 10(-6) or [Deltamu/mu] < 1.4 x 10(-5) to fractional changes in alpha and mu over a period of approximately 6.5 G yr, half the age of the Universe. These are among the most sensitive constraints on changes in mu.     

Convergent expansions for tunneling

A new method to compute effects of tunneling in one-dimensional multiple well is developed. A tunneling parameter built with physical quantities is introduced to measure the splitting between eigenvalues due to tunneling. These splittings are given by convergent series in term of this tunneling parameter for a wide class of double well.