An investment model with switching costs and the option to abandon

We develop a complete analysis of a general entry–exit–scrapping model. In particular, we consider an investment project that operates within a random environment and yields a payoff rate that is a function of a stochastic economic indicator such as the price of or the demand for the project’s output commodity. We assume that the investment project can operate in two modes, an “open” one and a “closed” one. The transitions from one operating mode to the other one are costly and immediate, and form a sequence of decisions made by the project’s management. We also assume that the project can be permanently abandoned at a discretionary time and at a constant sunk cost. The objective of the project’s management is to maximise the expected discounted payoff resulting from the project’s management over all switching and abandonment strategies. We derive the explicit solution to this stochastic control problem that involves impulse control as well as discretionary stopping. It turns out that this has a rather rich structure and the optimal strategy can take eight qualitatively different forms, depending on the problems data.     

Stereocontrolled Synthesis of (2R,3S)‐2‐Methylisocitrate,a Central Intermediate in the Methylcitrate Cycle

2‐Methylisocitrate (=3‐hydroxybutane‐1,2,3‐tricarboxylic acid) is an intermediate in the oxidation of propanoate to pyruvate (=2‐oxopropanoate) via the methylcitrate cycle in both bacteria and fungi (Scheme 1). Stereocontrolled syntheses of (2R,3S)‐ and (2S,3R)‐2‐methylisocitrate (98% e.e.) were achieved starting from (R)‐ and (S)‐lactic acid (=(2R)‐ and (2S)‐2‐hydroxypropanoic acid), respectively. The dispiroketal (6S,7S,15R)‐15‐methyl‐1,8,13,16‐tetraoxadispiro[5.0.5.4]hexadecan‐14‐one ( 2a ) derived from (R)‐lactic acid was deprotonated with lithium diisopropylamide to give a carbanion that was condensed with diethyl fumarate (Scheme 3). The configuration of the adduct diethyl (2S)‐2‐[(6S,7S,14R)‐14‐methyl‐15‐oxo‐1,8,13,16‐tetraoxadispiro[5.0.5.4]hexadec‐14‐yl]butanedioate ( 3a ) was assigned by consideration of possible transition states for the fumarate condensation (cf. Scheme 2), and this was confirmed by a crystal‐structure analysis. The adduct was subjected to acid hydrolysis to afford the lactone 4a of (2R,3S)‐2‐methylisocitrate and hence (2R,3S)‐2‐methylisocitrate. Similarly, (S)‐lactic acid led to (2S,3R)‐2‐methylisocitrate. Comparison of 2‐methylisocitrate produced enzymatically with the synthetic enantiomers established that the biologically active isomer is (2R,3S)‐2‐methylisocitrate.     

Encapsulation of Microbial Cells into Silica Gel

This work deals with changes in microbial phenol degradation and cell proliferation caused by immobilization into silica gel. Mixed microbial culture and the yeast Candida tropicalis were immobilized in silica layers and pieces prepared by mixing of prepolymerized tetraethoxysilane with cell suspension. The phenol degradation rate of cells entrapped in silica gel was compared with those immobilized into an organic polymer-polyurethane. The phenol degradation efficiency decreased in the following order: free cell suspension > cells entrapped into polyurethane foam > cells entrapped into prepolymerized TEOS. Inside the silica there was no growth observed by optical microscope. The immobilization of bacterium Pseudomonas species 2 into silica gel, cells which co-metabolize PCBs with biphenyl, did not result in substantial change of intermediate concentration.     

A Mechanistic Puzzle: Variations on Decarboxylative Elimination

This experiment expands on a previous study of decarboxylative elimination. A number of cinnamic acids were subjected to alkene bromination. The resulting dibromides were given to pairs of students, who treated the acids with a weak base in either aqueous or organic solvent. The resulting products of decarboxylative elimination displayed different stereochemistries, depending not only on the conditions employed but also on the substituents present. The differences in stereochemistry of the alkenes formed can be understood in terms of carbocation stabilities as well as the electron-donating and electron-withdrawing effects of substituents on aromatic rings. In this application, the experiment was presented to the students as an open-ended investigation; that is, students did not know what the product of the reaction would be. A variety of instrumental techniques (¹H NMR, ¹³C NMR/DEPT, IR, and GC–MS) was used to arrive at product structures and subsequently propose a reaction mechanism.     

Voltammetric in situ measurements of trace metals in coastal waters

Developments in instrument miniaturisation and automation have resulted in the manufacture of portable electrochemical instrumentation for continuous trace-metal measurements from the banks of estuaries and on board ships. The most recent developments in flow cells with gel-coated iridium (Ir) micro-electrode arrays have resulted in submersible in situ voltammetric probes that allow long-term trace-metal monitoring at sub-nanomolar concentrations in coastal waters. This article overviews the design and the application of field-deployable voltammetric instrumentation for trace-metal monitoring.     

Asymmetric lithium(I) and copper(II) alkoxy-N-heterocyclic carbene complexes; crystallographic characterisation and Lewis acid catalysis

A one-pot synthesis of a wide range of bidentate, alkoxide-N-heterocyclic carbene ligands provides new lithium alkoxy-carbenes and a range of covalently bound organometallic Cu(II) carbene complexes, which are catalytically active, in some cases enantioselectively, for conjugate addition reactions.     

MCAT is not required for in vitro polyketide synthesis in a minimal actinorhodin polyketide synthase from Streptomyces coelicolor

Background: It has been proposed that Streptomyces malonyl CoA:holo acyl carrier protein transacylases (MCATs) provide a link between fatty acid and polyketide biosynthesis. Two recent studies have provided evidence that the presence of MCAT is essential for polyketide synthesis to proceed in reconstituted minimal polyketide synthases (PKSs). In contrast to this, we previously showed that the holo acyl carrier proteins (ACPs) from type II PKSs are capable of catalytic self -malonylation in the presence of malonyl CoA, which suggests that MCAT might not be necessary for polyketide biosynthesis.Results: We reconstituted a homologous actinorhodin (act) type II minimal PKS in vitro, When act holo-ACP is present in limiting concentrations, MCAT is required by the synthase complex in order for polyketide biosynthesis to proceed. When holo-ACP is present in excess, however, efficient polyketide synthesis proceeds without MCAT, The rate of polyketide production increases with holo-ACP concentration, but at low ACP concentration or equimolar ACP:KS:CLF (KS, ketosynthase; CLF, chain length determining factor) concentrations this rate is significantly lower than expected, indicating that free holo-ACP is sequestered by the KS/CLF complex.Conclusions: The rate of polyketide biosynthesis is dictated by the ratio of holo-ACP to KS and CLF, as well as by the total protein concentration, There is no absolute requirement for MCAT in polyketide biosynthesis in vitro, although the role of MCAT during polyketide synthesis in vivo remains an open question. MCAT might be responsible for the rate enhancement of malonyl transfer at very low free holo-ACP concentrations or it could be required to catalyse the transfer of malonyl groups from malonyl CoA to sequestered holo-ACP.     

Structural events in the photocycle of green fluorescent protein

Picosecond time-resolved mid-infrared absorption changes of the wild type green fluorescent protein from Aequorea victoria are reported on structural events during the photocycle. Concomitant with rapid H/D transfer following excitation of the neutral A state at 400 nm, a transient signal at 1721/1711 cm(-1) (H/D) developed belonging to protonated glutamate 222, which was definitively assigned using the E222D mutant from the altered proton-transfer kinetics to aspartate in addition to the altered band position and intensity in the spectra. A transient at 1697 cm(-1), assigned to a structural perturbation of glutamine 69, had a H/D kinetic isotope effect of >32, showing the conformational dynamics to be sensitive to the active site H/D vibrations. The kinetic data up to 2 ns after excitation in the 1250-1800 cm(-1) region in D2O provided 10 and 75 ps time constants for the excited-state deuteron transfer and the associated A1* - A1 and A2* - A2 difference spectra and showed the radiative intermediate I state vibrations and the transient accumulation of the long-lived ground-state intermediate I2. Assignments of chromophore modes for the A1, A2, and I2 ground states are proposed on the basis of published model compound studies (Esposito, A. P.; Schellenberg, P.; Parson, W. W.; Reid, P. J. J. Mol. Struct. 2001, 569, 25 and He, X.; Bell, A. F.; Tonge, P. J. J. Phys. Chem. B 2002, 106, 6056). Tentative assignments for the singlet-state intermediates A1*, A2*, and I* are discussed. An unexpected and unassigned band that may be a C=C chromophore vibration was observed in the ground state (1665 cm(-1)) as well as in all photocycle intermediates. Optical dumping of the transient I population was achieved using an additional 532 nm pulse and the directly obtained I2 - I* difference spectrum was highly similar to the I2 - I* photocycle spectrum. The pump-dump-probe spectrum differed from the pump-probe photocycle difference spectrum with respect to the intensity of the phenol 1 mode at 1578 cm(-1), suggesting stronger delocalization of the negative charge onto the phenolic ring of the anionic chromophore in the dumped I2 state. Indication for structural heterogeneity of the chromophore, Glu 222, and the chromophore environment was found in the two parallel proton-transfer reactions and their distinct associated ground- and intermediate-state vibrations. Vibrational spectral markers at 1695 cm(-1) assigned to Gln 69, at 1631 cm(-1) belonging to a C=C mode, and at 1354 cm(-1) belonging to a phenolate vibration further indicated the I2 and I* states to be unrelaxed.     

Simple flow-injection system for the simultaneous determination of nitrite and nitrate in water samples

A novel spectrophotometric reaction system was developed for the determination of nitrite as well as nitrate in water samples, and was applied to a flow-injection analysis (FIA). The spectrophotometric flow-injection system coupled with a copperised cadmium reductor column was proposed. The detection was based on the nitrosation reaction between nitrite ion and phloroglucinol (1,3,5-trihydroxybenzene), a commercially available phenolic compound. Sample injected into a carrier stream was split into two streams at the Y-shaped connector. One of the streams merged directly and reacted with the reagent stream: nitrite ion in the samples was detected. The other stream was passed through the copperised cadmium reductor column, where the reduction of nitrate to nitrite occurred, and the sample zone was then mixed with the reagent stream and passed through the detector: the sum of nitrate and nitrite was detected. The optimised conditions allow a linear calibration range of 0.03–0.30 μg NO₂⁻-N ml⁻¹ and 0.10–1.00 μg NO₃⁻-N ml⁻¹. The detection limits for nitrite and nitrate, defined as three times the standard deviation of measured blanks are 2.9 ng NO₂⁻-N ml⁻¹ and 2.3 ng NO₃⁻-N ml⁻¹, respectively. Up to 20 samples can be analyzed per hour with a relative standard deviation of less than 1.5%. The proposed method could be applied successfully to the simultaneous determination of nitrite and nitrate in water samples.     

Hydrodynamic sequential injection system for a rapid dichlorophenol indophenol precipitation test for hemoglobin E

Dichlorophenol indophenol precipitation (DCIP) test is a screening method for unstable hemoglobins (Hb). An automatic system for a rapid DCIP test was developed based on stopped-flow hydrodynamic sequential injection. The system overcomes the problems of the conventional DCIP test such as long analysis time and imprecision, which are due to temperature fluctuation and imprecise detection timing. The flow based DCIP system can shorten the incubation time (usually 25 min to 1 h) to 3 min by using a higher temperature of 50°C instead of 37–40°C as in the conventional method. The reagents used were very simple and the whole process was done online with a microcontroller for controlling time. The clearness of the mixture was monitored with a spectrophotometer to improve precision as compared to the bare eyes observation method used in conventional DCIP test. The system could differentiate the HbE carriers (positive group) from negative group which agreed with the routine screening method using anion exchange micro-column.