Biosynthetic insights provided by unusual sesterterpenes from the medicinal herb Aletris farinosa

A series of novel sesterterpenes (2–6) have been isolated from the roots of Aletris farinosa and structurally characterized by MS, NMR, and X-ray crystallography in conjunction with computational modeling. Their structures provide new insights into the mechanisms of sesterterpene biosynthesis. Specifically, we propose with support from density functional theory computations that the configuration at a single stereocenter determines the fate of a key tetracyclic carbocationic intermediate, derived from an oxidogeranylfarnesol precursor. Whereas one epimer of the carbocation undergoes H⁺ elimination to give 6, the other undergoes a spectacular cascade of seven 1,2-methyl and hydride migrations leading to the previously unreported carbon skeleton of 5. Theoretical calculations suggest that the cascade is triggered by substrate preorganization in the enzyme active site.     

Radiofluorination of a Pre‐formed Gallium(III) Aza‐macrocyclic Complex: Towards Next‐Generation Positron Emission Tomography (PET) Imaging Agents

As part of a study to investigate the factors influencing the development of new, more effective metal‐complex‐based positron emission tomography (PET) imaging agents, the distorted octahedral complex, [GaCl(L)]?2 H₂O has been prepared by reaction of 1‐benzyl‐1,4,7‐triazacyclononane‐4,7‐dicarboxylic acid hydrochloride (H₂L?HCl) with Ga(NO₃)₃?9 H₂O, which is a convenient source of Ga^III for reactions in water. Spectroscopic and crystallographic data for [GaCl(L)]?2 H₂O are described, together with the crystal structure of [GaCl(L)]?MeCN. Fluorination of this complex by Cl^?/F^? exchange was achieved in high yield by treatment with KF in water at room temperature over 90 minutes, although the reaction was complete in approximately 30 minutes if heated to 80 °C, giving [GaF(L)]?2 H₂O in good yield. The same complex was obtained by hydrothermal synthesis from GaF₃?3 H₂O and Li₂L, and has been characterised by single‐crystal X‐ray analysis, IR, ¹H and ¹⁹F{¹H} NMR spectroscopy and ESI⁺ MS. Radiofluorination of the pre‐formed [GaCl(L)]?2 H₂O has been demonstrated on a 210 nanomolar scale in aqueous NaOAc at pH 4 by using carrier‐free ¹⁸F^?, leading to 60–70 % ¹⁸F‐incorporation after heating to 80 °C for 30 minutes. The resulting radioproduct was purified easily by using a solid‐phase extraction (SPE) cartridge, leading to 98–99 % radiochemical purity. The [Ga¹⁸F(L)] is stable for at least 90 minutes in 10 % EtOH/NaOAc solution at pH 6, but defluorinates over this time scale at pH of approximately 7.5 in phosphate buffered saline (PBS) or human serum albumin (HSA). The subtle role of the Group 13 metal ion and co‐ligand donor set in influencing the pH dependence of this system is discussed in the context of developing potential new imaging agents for PET.     

Human mitochondrial amidoxime reducing component (mARC): An electrochemical method for identifying new substrates and inhibitors

As recently as 2006 the mitochondrial amidoxime reducing component (mARC) was identified as the fourth and last Mo enzyme present in humans. Its physiological role remains unknown. mARC is capable of reducing a variety of N-hydroxylated compounds such as amidoximes to their corresponding amidine and there is considerable interest in this enzyme from a pharmaceutical perspective. mARC is a target for N-hydroxylated pro-drugs that may be reductively activated intracellularly to release potent drugs such as cationic amidinium ions, which exhibit a broad spectrum of activity as antithrombotics and against various bacteria and parasites. In the quest for a rapid screen of new mARC substrates and inhibitors we present an electrochemical method which utilizes the natural electron partner of mARC, cytochrome b₅, coupled to an electrochemical electrode. Mediated electron transfer from the electrode via cytochrome b₅ to mARC results in a catalytic current in the presence of substrate.     

Background oriented schlieren for flow visualisation in hypersonic impulse facilities

Experiments to demonstrate the use of the background-oriented schlieren (BOS) technique in hypersonic impulse facilities are reported. BOS uses a simple optical set-up consisting of a structured background pattern, an electronic camera with a high shutter speed and a high intensity light source. The visualization technique is demonstrated in a small reflected shock tunnel with a Mach 4 conical nozzle, nozzle supply pressure of 2.2 MPa and nozzle supply enthalpy of 1.8 MJ/kg. A 20° sharp circular cone and a model of the MUSES-C re-entry body were tested. Images captured were processed using PIV-style image analysis to visualize variations in the density field. The shock angle on the cone measured from the BOS images agreed with theoretical calculations to within 0.5°. Shock standoff distances could be measured from the BOS image for the re-entry body. Preliminary experiments are also reported in higher enthalpy facilities where flow luminosity can interfere with imaging of the background pattern. A version of this paper was presented at the 25th International Symposium on Shock Waves in Bangalore in July 2005.     

Microprojectile velocities in a gas driven launcher

Microprojectile methods of genetic engineering sometimes employ a macroprojectile which launches the microprojectiles when it hits a stopping plate. A study has been made of the relationship between the velocities of the macroprojectile and the microprojectile it launches. Macroprojectile velocities were measured using a beam blanking method and the microprojectile velocities were measured with a laser Doppler anemometer. Particles of tungsten, titanium dioxide and silicon carbide were investigated as microprojectiles. The microprojectile velocity was found to depend not only on the macroprojectile velocity but also on the microprojectile size and material, the bond strength with the macroprojectile and the gas flow that develops ahead of the macroprojectile. On average the microprojectile launch velocity was similar to the macroprojectile velocity. However, some tungsten particles had velocities twice this value. Experimental evidence suggests that these higher velocities were caused by early break-way of particles from the macroprojectile followed by further acceleration by aerodynamic drag arising from the gas flow ahead of the macroprojectile.We are especially grateful for the advice of Dr. M. Gharib on the use of the HSI color system. John Lyons provided expert help in the laboratory and materials for and advice on photography. We thank the staff of the SIO Hydraulics Laboratory for making the wind-wave channel available for our use, and the staff of UCSD library for enabling us to use the Barneyscanner photometer-digitizer. We thank an anonymous reviewer who pointed out a numerical error and improved the clarity of the text.     

Three-dimensional effects on line-of-sight visualization measurements of supersonic and hypersonic flow over cylinders

Flow visualization experiments were performed for supersonic and hypersonic nitrogen test gas flows over a cylinder. The results were used to quantify the influence of three-dimensional effects on optical line-of-sight visualization measurements. Images of cylindrical models of varying aspect ratios (length to diameter) were taken. Shock stand-off distance measurements for the models were compared with a two-dimensional approximation and numerical simulations. For aspect ratios of two and above, the two-dimensional approximation was acceptable within experimental uncertainty. The measured shock stand-off decreased by less than 5% from an asymptotic value for an infinite length cylinder. For smaller aspect ratios, a correction factor for the shock stand-off needs to be applied if comparisons between the two-dimensional approximation and experimental measurements are to be drawn. An estimate of this correction factor has been derived from an empirical fit to the available data.      

Hydrolytic degradation of POSS-PEG-lactide hybrid hydrogels

A polyhedral oligomeric silsesquioxane (POSS), functionalized with eight arms of poly(ethylene glycol) (PEG; MW 400) and then acrylated, was incorporated into a hydrogel network based on triblock copolymers of poly(lactide-b-ethylene glycol-b-lactide) diacrylates using a redox-initiated polymerization. The organic-inorganic hybrid hydrogels so prepared contained the inorganic crosslinker POSS from 1 to 28 wt.%. The degradation properties of the hydrogels in a pH 7.4 phosphate-buffered saline solution at 37 °C were studied using measurements of mass loss, cryogenic SEM, and ATR-FTIR spectroscopy. It was found that copolymerization of acrylated 1kPEG-lactide with increasing amounts of POSS created a more porous network which was more resistant to hydrolysis. The ATR-FTIR technique was used to monitor the progress of degradation with exposure time through the changes in the carbonyl and C-H deformation bands of the lactide and the Si-C stretching band of the POSS. Increasing POSS incorporation resulted in decreased rate of degradation due to its hydrophobic nature and inertness to hydrolysis. Conversely, an increase in lactide content increased the degradation rate due to the increased number of hydrolytically-sensitive ester groups in the network.     

Physical and chemical analysis of elemental sulfur formation during galena surface oxidation

The surface oxidation of sulfide minerals, such as galena (PbS), in aqueous solutions is of critical importance in a number of applications. A comprehensive understanding of the formation of oxidation species at the galena surface is still lacking. Much controversy over the nature of these oxidation products exists. A number of oxidation pathways have been proposed, and experimental evidence for the formation of elemental sulfur, metal polysulfides, and metal-deficient lead sulfides in acidic conditions has been shown and argued. This paper provides further insight into the electrochemical behavior of galena at pH 4.5. Utilizing a novel experimental system that combines in situ electrochemical control and AC mode atomic force microscopy (AFM) surface imaging, the formation and growth of nanoscopic domains on the galena surface are detected and examined at anodic potentials. AFM phase images indicate that these domains have different material properties to the underlying galena. Continued oxidation results in nanoscopic pitting and the formation of microscopic surface domains, which are confirmed to be elemental sulfur by Raman spectroscopy. Further clarification of the presence of elemental sulfur is provided by Cryo-XPS. Polysulfide and metal-deficient sulfide could not be detected within this system.     

Crystal structure of nickel paratungstate B Ni<Subscript>5</Subscript>[W<Subscript>12</Subscript>O<Subscript>40</Subscript>(OH)<Subscript>2</Subscript>]·37H<Subscript>2</Subscript>O

From the solution of the system Na₂WO₄-HNO₃-Ni(NO₃)₂-H₂O acidified to Z = ν(H⁺)/ν(WO ₄ ^2? ) = 1.29, the green crystals of nickel paratungstate B Ni₅[W₁₂O₄₀(OH)₂]·37H₂O are isolated. By FTIR spectroscopy the isopoly anion is shown to belong to the structural type of paratungstate B. Using single crystal X-ray analysis, the structure of Ni₅[W₁₂O₄₀(OH)₂]·37H₂O is solved (M _r = 3840.36, monoclinic, P2₁/c space group, a = 21.9061(6) Å, b = 14.9297(4) Å, c = 22.1391(6) Å, β = 107.609(3)°, V = 6901.4(3)ų at T = 293 K, Z = 4, d_x = 3.696 g/cm³, F ₀₀₀ = 6944, μ = 21.368 mm^?1, ?33 ≤ h ≤ 33, ?22 ≤ k ≤ 22, ?33 ≤ l ≤33; the final uncertainty values for the observed reflections are R _F = 0.0532, wR ² = 0.0831 (R _F = 0.1088, wR ² = 0.0894 over all independent reflections), S = 0.978; CSD-421468).