Intra- vs intermolecular hydrogen bonding. The crystal structure of 5-methyl-2-hydroxyacetophenone thiosemicarbazone monohydrate and salicylaldehyde-2-methylthiosemicarbazone monohydrate

The crystal structure of 5-methyl-acetophenonethiosemicarbazone monohydrate,A, and salicylaldehyde-2-methylthiosemicarbazone monohydrate,B, were determined using single crystal X-ray diffraction.A crystallizes in the monoclinic space groupC2/c, with lattice parametersa=14.161(2),b=15.753(1) Å,c=11.084(1) Å, β=112.59(1)° andZ=4, yielding a calculated density ofD _calc=1.352 mg/m³.B crystallizes in the triclinic space groupP1, witha=7.233(2) Å,b=7.371(2) Å,c=11.841(2) Å, α=82.77(2)°, β=78.33(2)°, γ=63.06(2)° andD _calc=1.371 mg/m³ forZ=2,. In bothA andB the immine nitrogen and the sulfur atom areanti with respect to N2-C8. WhileA presents the usual intramolecular six membered hydrogen bond ring,B has instead an intermolecular hydrogen bond between the hydroxy moiety of the salicyladehyde and a water molecule. AM1 calculations agree with the experimental conformations observed in both compounds.     

Structures of 8α-(2′-methylbutyryloxy)-9α-hydroxymontahibisciolide,a new skeletal type of sesquiterpene lactone,and of its precursor 8α-isobutyryloxy-9-oxo-germacra-4E, 1(10)Z-dien-6β,12-olide

The structures of a new skeletal type of sesquiterpene lactone, containing a seven, five, fivering system, and of its putative precursor, a 9-oxo-melampolide, have been confirmed by single crystal X-ray structure determination. The central 5-membered ring of the new skeleton istrans-fused to the cycloheptene ring andcis-fused to the lactone ring. Transannular interactions and the geometry of thetrans carbon-carbon double bond of the oxo-melampolide and the keto-group at C9, suggest that the new seven, five, five tricyclic skeleton-type of montahibisciolide can easily be reached by a transannular cyclization process. The isobutyrloxy group of the oxo-melampolide is disordered into two partially populated conformations. 8-(2-Methylbutyryloxy)-9-hydroxymontahibisciolide,3, C₂₀H₂₈O₅, crystallizes in orthorhombic space groupP2₁2₁2₁ witha=8.294(2),b=9.740(2),c=22.675(1) Å,V=1832(1)ų,Z=4. The structure was refined toR=0.043 andR _w=0.048 for 2898 observed reflections with CuK radiation. 8-isobutyryloxy-9-oxo-germacra-4E, 1(10)Z-dien-6, 12-olide,4, C₁₉H₂₄O₅, crystallizes in monoclinic space groupC2 witha=32.879(7),b=5.793(2),c=9.884(4)Å, =99.01(3)°,V=1859.5(9)å³,Z=4. The structure was refined toR=0.054 andR _w=0.064 for 1010 observed reflections with MoK radiation.     

Structure of fruticuline B: A new natural anthracene quinodimethide of abietanoid origin

The title compound is C₁₉H₁₈O₄·1/2 CH₃OH, triclinic,P¯1,a=9.891(2),b=13.273(4),c=13.860(4) Å,a=66.47(2),=86.91(2), and =85.59(2)°. The structure was solved by direct methods and refined by least-squares techniques to anR factor of 0.082 for 2282 observed reflections. The X-ray structure uniquely resolves the one remaining ambiguity, namely the assignment of the correct structure4. There are two crystallographically independent molecules (A and B) and one disordered methanol molecule. Both molecular skeletons show small distortions from planarity with inclinations of 1.7 and 4.0° between the outermost rings A/C in molecules A and B, respectively. Intramolecular hydrogen bonds of the form O(3A)-HO(2A) and O(3B)-HO(2B) are observed in molecules A and B, respectively. The molecules are linked by pairs of O(3A)-HO(2B) and O(3B)-HO(2A) hydrogen bonds. The dimerlike structures are stabilized by intermolecular C-HO interactions and van der Waals forces.     

Stereochemistry and hydrogen bonding of cytokinins: Crystal and molecular structure of 6-(4-hydroxy-3-methyl-cis-2-butenyl) aminopurine (cis-zeatin)

The title compound is C₁₀H₁₃N₅O, monoclinic,P2₁/a,a=13.829 (7),b=7.834 (7),c=20.138 (9) Å, and =100.75 (6)°. The structure was solved by direct methods and refined by least-squares techniques to anR factor of 0.079 for 1581 observed reflections. There are two crystallographically independent molecules (I andI). The N⁶-substituent is distal to the imidazole ring of the adenine moiety. It is interesting to note that the conformation of the N⁶-substituent relative to the adenine ring base in these two crystallographically independent molecules shows considerable resemblance with the preferred one usually adopted in other cytokinins. The angles between the mean planes of the adenine base and the N⁶-substituent are 65.6 and 63.3° for theI andI molecules, respectively. The molecules are linked by pairs of N(9)-H·N(3) hydrogen bonds. The dimer-like structures are stabilized by a three-dimensional network of O-H·O, O-H·N, N-H·O, and C-H·N hydrogen bonds.     

Synthesis,characterization, DNA interaction studies and anticancer activity of platinum–clotrimazole complexes

New trans-platinum complexes of clotrimazole (CTZ) have been prepared and characterized. Their interaction with DNA and activity against tumour cell lines were evaluated. [Pt (CTZ)₂I₂] (1) was synthesized by the reaction of K₂PtCl₄, KI and CTZ, and [Pt(CTZ)₂Cl₂] (2) by direct reaction of K₂PtCl₄ with CTZ. These complexes were characterized by a combination of elemental analysis, molar conductivity, UV–Vis, IR, and NMR spectroscopy in one and two dimensions. DNA–platinum complex interactions were studied by spectroscopic, thermal denaturation and viscosity titration measurements. Covalent interaction studies were also performed. From these results we suggest that complexes 1 and 2 interact with the minor groove of DNA. Both complexes showed growth inhibitory effects on four out of the six tumour cells lines with GI₅₀ (50% growth inhibition) values in the 5–25 μM range, but there was no indication of cytotoxicity over the range of concentrations tested.     

Role of synaptic filtering on the firing response of simple model neurons

During active states of the brain neurons process their afferent currents with an effective membrane time constant much shorter than its value at rest. This fact, together with the existence of several synaptic time scales, determines to which aspects of the input the neuron responds best. Here we present a solution to the response of a leaky integrate-and-fire neuron with synaptic filters when long synaptic times are present, and predict the firing rate for all values of the synaptic time constant. We also discuss under which conditions this neuron becomes a coincidence detector.     

Membrane potential and response properties of populations of cortical neurons in the high conductance state

Because of intense synaptic activity, cortical neurons are in a high conductance state. We show that this state has important consequences on the properties of a population of independent model neurons with conductance-based synapses. Using an adiabaticlike approximation we study both the membrane potential and the firing probability distributions across the population. We find that the latter is bimodal in such a way that at any particular moment some neurons are inactive while others are active. The population rate and the response variability are also characterized.     

Intrinsic character of the (3 x 3) to (square root 3 x square root 3) phase transition in Pb/Si(111)

We have investigated the (3 x 3) to (square root 3 x square root 3) reversible phase transition in Pb/Si(111) by means of variable temperature scanning tunneling microscopy and density functional first-principles calculations. By tracking exactly the same regions of the surface with atomic resolution in a temperature range between 40 and 200 K, we have observed the phase transition in real time. The ability to prepare and track exceptionally large domains without defects has allowed us to detect the intrinsic character of the phase transition at temperatures around 86 K. This intrinsic character is in full agreement with our first-principles calculations. Moreover, our results show that the hypothesis that point defects play a fundamental role as the driving force, reported for similar systems, can be discarded for Pb/Si(111).     

Numerical Schottky Uniformizations

A real algebraic curve of algebraic genus g ≥ 2 is a pair (S, τ), where S is a closed Riemann surface of genus g and τ is a reflection on S (anticonformal involution with fixed points). In this note, we discuss a numerical (Burnside) program which permits to obtain a Riemann period matrix of the surface S in terms of an uniformizing real Schottky group. If we denote by Aut₊(S, τ) the group of conformal automorphisms of S commuting with the real structure τ, then it is a well known fact that |Aut₊(S,τ)| ≥ 12(g−1). We say that (S,τ) is maximally symmetric if |Aut₊(S,τ)|=12(g−1). We work explicitly such a numerical program in the case of maximally symmetric real curves of genus two. We construct a real Schottky uniformization for each such real curve and we use the numerical program to obtain a real algebraic curve, a Riemann period matrix and the accessory parameters in terms of the corresponding Schottky uniformization. In particular, we are able to give for Bolza’s curve a Schottky uniformization (at least numerically), providing an example for which the inverse uniformization theorem is numerically solved.Partially supported by Projects Fondecyt 1030252 1030373 and UTFSM 12.03.21     

A 1:1:2 complex between 1,4,8,11-tetraazacyclotetradecane (cyclam), anthraflavic acid, and water

A 1:1:2 complex of 1,4,8,11-tetraazacyclotetradecane (cyclam), anthraflavic acid, and water has been obtained and characterized by X-ray diffraction. C₂₄H₃₆N₄O₆, monoclinic, P2₁/c, a = 11.966(1) Å, b = 11.570(1) Å, c = 9.604(1) Å, = 107.16(1)°,V = 1270.5(2) ų, Z = 2. Two of the four N atoms of the macrocyclic ring are protonated, and the corresponding anthraflavic acid exists as a dianion. The following types of H-bonding have been found in this complex: (1) between protonated NH groups of the macrocycle and O atoms of the ionized anthraflavic hydroxyls; (2) between protonated and nonprotonated NH groups of the macrocycle; (3) between the water molecules and O atoms of the ionized anthraflavic hydroxyls. The above interactions give rise to the formation of a 2D-supramolecular network in the cyclam complex; however, the major difference from the azamacrocyclic anthraflavates studied previously is that there is no direct interaction between the anthraflavic anions.Deceased