Solvent Trapping Research Articles

Qualitative assessment of ultra-fast non-Grotthuss proton dynamics in S1 excited state of liquid H2O from ab initio time-dependent density functional theory

We study qualitatively ultra-fast proton transfer (PT) in the first singlet (S1) state of liquid water (absorption onset) through excited-state dynamics by means of time-dependent density functional theory and ab initio Born-Oppenheimer molecular dynamics. We find that after the initial excitation, a PT occurs in S1 in form of a rapid jump to a neighboring water molecule, on which the proton either may rest for a relatively long period of time (as a consequence of possible defect in the hydrogen bond network) followed by back and forth hops to its neighboring water molecule or from which it further moves to the next water molecule accompanied by back and forth movements. In this way, the proton may become delocalized over a long water wire branch, followed again by back and forth jumps or short localization on a water molecule for some femtoseconds. As a result, the mechanism of PT in S1 is in most cases highly non-Grotthuss-like, delayed and discrete. Furthermore, upon PT an excess charge is ejected to the solvent trap, the so-called solvated electron. The spatial extent of the ejected solvated electron is mainly localized within one solvent shell with overlappings on the nearest neighbor water molecules and delocalizing (diffuse) tails extending beyond the first solvent sphere. During the entire ultra-short excited-state dynamics the remaining OH radical from the initially excited water molecule exhibits an extremely low mobility and is non-reactive.

Direct Detection of the Open-Shell Singlet Phenyloxenium Ion: An Atom-Centered Diradical Reacts as an Electrophile.

A new photoprecursor to the phenyloxenium ion, 4-methoxyphenoxypyridinium tetrafluoroborate, was investigated using trapping studies, product analysis, computational investigations, and laser flash photolysis experiments ranging from the femtosecond to the millisecond time scale. These experiments allowed us to trace the complete arc of the photophysics and photochemistry of this photoprecursor beginning with the initially populated excited states to its sequential formation of transient intermediates and ultimate formation of stable photoproducts. We find that the excited state of the photoprecursor undergoes heterolysis to generate the phenyloxenium ion in ∼2 ps but surprisingly generates the ion in its open-shell singlet diradical configuration (1A2), permitting an unexpected look at the reactivity of an atom-centered open-shell singlet diradical. The open-shell phenyloxenium ion (1A2) has a much shorter lifetime (τ ∼ 0.2 ns) in acetonitrile than the previously observed closed-shell singlet (1A1) phenyloxenium ion (τ ∼ 5 ns). Remarkably, despite possessing no empty valence orbitals, this open-shell singlet oxenium ion behaves as an even more powerful electrophile than the closed-shell singlet oxenium ion, undergoing solvent trapping by weakly nucleophilic solvents such as water and acetonitrile or externally added nucleophiles (e.g., azide) rather than engaging in typical diradical chemistry, such as H atom abstraction, which we have previously observed for a triplet oxenium ion. In acetonitrile, the open-shell singlet oxenium ion is trapped to generate ortho and para Ritter intermediates, one of which (para) is directly observed as a longer-lived species (τ ∼ 0.1 ms) in time-resolved resonance Raman experiments. The Ritter intermediates are ultimately trapped by either the 4-methoxypyridine leaving group (in the case of para addition) or trapped internally via an essentially barrierless rearrangement (in the case of ortho addition) to generate a cyclized product. The expectation that singlet diradicals react similarly to triplet or uncoupled diradicals needs to be reconsidered, as a recent study by Perrin and Reyes-Rodríguez (J. Am. Chem. Soc. 2014, 136, 15263) suggested the unsettling possibility that singlet p-benzyne could suffer nucleophilic attack to generate a naked phenyl anion. Now, this study provides direct spectroscopic observation of this phenomenon, with an atom-centered open-shell singlet diradical reacting as a powerful electrophile. To the question of whether a nucleophile can attack a singly occupied molecular orbital, the answer is apparently yes, at least if another partially occupied orbital is available to avoid violation of the rules of valence.

Form Conversion and Solvent Entrapment during API Drying

An active pharmaceutical ingredient (API) was isolated from solution as a crystalline hydrate and then converted to the desired weakly crystalline form during drying. Incomplete solvent removal was observed at the factory scale. It was suspected that differences in the processing equipment and drying conditions led to the rapid loss of water from the solids, early form conversion, and subsequent trapping of the process solvent above specification levels. Experiments were conducted to gain a better understanding of the drying process and test theories for solvent trapping. The practical methods and techniques described here are considered broadly applicable to API drying process development; findings associated with solvent entrapment may be common across compounds undergoing a form transition during drying. This article presents experimental results that led to an increased process understanding and identification of conditions for successful drying of the API.

Anodic Cyclization Reactions and the Mechanistic Strategies That Enable Optimization.

Oxidation reactions are powerful tools for synthesis because they allow us to reverse the polarity of electron-rich functional groups, generate highly reactive intermediates, and increase the functionality of molecules. For this reason, oxidation reactions have been and continue to be the subject of intense study. Central to these efforts is the development of mechanism-based strategies that allow us to think about the reactive intermediates that are frequently central to the success of the reactions and the mechanistic pathways that those intermediates trigger. For example, consider oxidative cyclization reactions that are triggered by the removal of an electron from an electron-rich olefin and lead to cyclic products that are functionalized for further elaboration. For these reactions to be successful, the radical cation intermediate must first be generated using conditions that limit its polymerization and then channeled down a productive desired pathway. Following the cyclization, a second oxidation step is necessary for product formation, after which the resulting cation must be quenched in a controlled fashion to avoid undesired elimination reactions. Problems can arise at any one or all of these steps, a fact that frequently complicates reaction optimization and can discourage the development of new transformations. Fortunately, anodic electrochemistry offers an outstanding opportunity to systematically probe the mechanism of oxidative cyclization reactions. The use of electrochemical methods allows for the generation of radical cations under neutral conditions in an environment that helps prevent polymerization of the intermediate. Once the intermediates have been generated, a series of "telltale indicators" can be used to diagnose which step in an oxidative cyclization is problematic for less successful transformation. A set of potential solutions to address each type of problem encountered has been developed. For example, problems with the initial cyclization reaction leading to either polymerization of the radical cation, elimination of a proton from or solvent trapping of that intermediate, or solvent trapping of the radical cation can be identified in the proton NMR spectrum of the crude reaction material. Such an NMR spectrum shows retention of the trapping group. The problems can be addressed by tuning the radical cation, altering the trapping group, or channeling the reactive intermediate down a radical pathway. Specific examples each are shown in this Account. Problems with the second oxidation step can be identified by poor current efficiency or general decomposition in spite of cyclic voltammetry evidence for a rapid cyclization. Solutions involve improving the oxidation conditions for the radical after cyclization by either the addition of a properly placed electron-donating group in the substrate or an increase in the concentration of electrolyte in the reaction (a change that stabilizes the cation generated from the second oxidation step). Problems with the final cation typically lead to overoxidation. Solutions to this problem require an approach that either slows down elimination side reactions or changes the reaction conditions so that the cation can be quickly trapped in an irreversible fashion. Again, this Account highlights these strategies along with the specific experimental protocols utilized.

Chemoselective Modification of Vinyl DNA by Triazolinediones.

A new method for the post-synthetic modification of nucleic acids was developed that involves mixing a phenyl triazolinedione (PTAD) derivative with DNA containing a vinyl nucleobase. The resulting reactions proceeded through step-wise mechanisms, giving either a formal [4+2] cycloaddition product, or, depending on the context of nucleobase, PTAD addition along with solvent trapping to give a secondary alcohol in water. Catalyst-free addition between PTAD and the terminal alkene of 5-vinyl-2'-deoxyuridine (VdU) was exceptionally fast, with a second-order rate constant of 2×103 m-1 s-1 . PTAD derivatives selectively reacted with VdU-containing oligonucleotides in a conformation-selective manner, with higher yields observed for G-quadruplex versus duplex DNA. These results demonstrate a new strategy for copper-free bioconjugation of DNA that can potentially be used to probe nucleic acid conformations in cells.

Chemoselective Modification of Vinyl DNA by Triazolinediones

AbstractA new method for the post‐synthetic modification of nucleic acids was developed that involves mixing a phenyl triazolinedione (PTAD) derivative with DNA containing a vinyl nucleobase. The resulting reactions proceeded through step‐wise mechanisms, giving either a formal [4+2] cycloaddition product, or, depending on the context of nucleobase, PTAD addition along with solvent trapping to give a secondary alcohol in water. Catalyst‐free addition between PTAD and the terminal alkene of 5‐vinyl‐2′‐deoxyuridine (VdU) was exceptionally fast, with a second‐order rate constant of 2×103 m−1 s−1. PTAD derivatives selectively reacted with VdU‐containing oligonucleotides in a conformation‐selective manner, with higher yields observed for G‐quadruplex versus duplex DNA. These results demonstrate a new strategy for copper‐free bioconjugation of DNA that can potentially be used to probe nucleic acid conformations in cells.

Theoretical illumination of highly original photoreactive 3MC states and the mechanism of the photochemistry of Ru(ii) tris(bidentate) complexes.

We have identified highly novel photoreactive 3MC states of ruthenium(ii) 4,4'-bi-1,2,3-triazolyl (btz) complexes of the form [Ru(N^N)(btz)2]2+ and have elucidated the mechanism of the highly unusual experimental observations of photochemical ligand dechelation and concomitant ligand rearrangement reactivity to form unusual photoproducts trans-[Ru(N^N)(κ2-btz)(κ1-btz)(solvent)]2+. The triplet metal-to-ligand charge-transfer (3MLCT) states and classical Jahn-Teller type triplet metal-centred (3MC) states of the series of complexes [Ru(N^N)3-n(btz)n]2+ (btz = 4,4'-bi-1,2,3-triazolyl; N^N = 2,2'-bipyridyl (bpy), n = 0 (1), 1 (2), 2 (3), 3 (5); N^N = 4-(pyrid-2-yl)-1,2,3-triazole (pytz), n = 1 (4)) have been optimised by density functional theory (DFT) and characterised. There is a gradual and significant destabilisation of the 3MLCT states as the triazole content of the complexes increases, which occurs with a slight stabilisation of the 3MC states. Whilst consistent with the promotion of photochemical reactivity in the heteroleptic complexes of the series relative to 1, these classical 3MC states fail to account for the extraordinary ligand rearrangement processes that accompany ligand ejection. Thorough theoretical exploration of the lowest excited triplet potential energy surface (3PES) here reveals the existence of a new type of 3MC state and the role it plays in the photochemical reactivity of the complexes. This newly discovered state, called MC(F), displays a flattened geometry (indicated by the 'F' in the parentheses) which makes it clearly on the path to achieving the coplanarity of the bidentate ligands in the experimentally observed trans-photoproduct. Further novel 'pentacoordinate' 3MC states with coplanar bidentate ligands, called MC(P) (where the 'P' in the parentheses denotes the pentacoordinate character), were then identified and optimised. The energy barriers between the different triplet states were confirmed to be small which makes all triplet states accessible. Solvent trapping, which occurs on the singlet PES according to Wigner's rules, is finally achieved by a singlet pentacoordinate species to yield the monosolvento photoproduct. Thus, our calculations not only reveal highly novel 3MC states but more significantly demonstrate their crucial role in the formation of the experimentally observed photoproducts.

Competition Studies and the Use of Mechanistic Insight to Overcome Synthetic Barriers

Because intramolecular anodic olefin coupling reactions involve a reversal in polarity of a normally nucleophilic olefin, they can be used to develop entirely new pathways to a variety of molecules. The reactions are compatible with a wide variety of functional groups, the generation of fused, bridged, and spirocyclic ring skeletons, and the formation of quaternary carbons. However, not all anodic olefin coupling reactions work. The reactions are triggered by the formation of a highly reactive radical cation intermediate. If the desired reaction pathway is not fast enough, then the radical cation will initiate undesirable reaction pathways, often involving elimination, fragmentation, or solvent trapping. When this occurs, the question quickly becomes what can be done to solve the problem and channel the reaction down a more productive pathway? The best way to address this question has been to carefully probe the mechanism of the reactions, identify the nature of the undesirable reactions, and then manipulate the relative rates of the reactions available to the radical cation to favor the process that is wanted. Several steps in the reaction can be important, but two play an especially important role (Scheme 1). First, the rate of the cyclization is important. If the cyclization is too slow, then side reactions involving the reactive radical cation will begin to dominate. The rate of the second oxidation (k2) is also critical for the success of the reaction. Many radical cyclizations are reversible and in those cases the loss of the second electron helps to drive the product to completion. Understanding these principles is essential for the design of more complex reactions, and in the talk to be presented, a series of new anodic cyclization reactions (Scheme 2) will be used to highlight the important interplay between mechanistic considerations and synthetic design. Each of the reactions discussed will involve a transformation that was originally a failure but is now a cornerstone of ongoing synthetic efforts. Figure 1

One-step production of multiple emulsions: microfluidic, polymer-stabilized and particle-stabilized approaches.

Multiple emulsions have great potential for application in food science as a means to reduce fat content or for controlled encapsulation and release of actives. However, neither production nor stability is straightforward. Typically, multiple emulsions are prepared via two emulsification steps and a variety of approaches have been deployed to give long-term stability. It is well known that multiple emulsions can be prepared in a single step by harnessing emulsion inversion, although the resulting emulsions are usually short lived. Recently, several contrasting methods have been demonstrated which give rise to stable multiple emulsions via one-step production processes. Here we review the current state of microfluidic, polymer-stabilized and particle-stabilized approaches; these rely on phase separation, the role of electrolyte and the trapping of solvent with particles respectively.

Measurement of breakthrough volumes of volatile chemical warfare agents on a poly(2,6-diphenylphenylene oxide)-based adsorbent and application to thermal desorption–gas chromatography/mass spectrometric analysis

To establish adequate on-site solvent trapping of volatile chemical warfare agents (CWAs) from air samples, we measured the breakthrough volumes of CWAs on three adsorbent resins by an elution technique using direct electron ionization mass spectrometry. The trapping characteristics of Tenax® TA were better than those of Tenax® GR and Carboxen® 1016. The latter two adsorbents showed non-reproducible breakthrough behavior and low VX recovery. The specific breakthrough values were more than 44 (sarin) L/g Tenax® TA resin at 20°C. Logarithmic values of specific breakthrough volume for four nerve agents (sarin, soman, tabun, and VX) showed a nearly linear correlation with the reciprocals of their boiling points, but the data point of sulfur mustard deviated from this linear curve. Next, we developed a method to determine volatile CWAs in ambient air by thermal desorption–gas chromatography (TD–GC/MS). CWA solutions that were spiked into the Tenax TA® adsorbent tubes were analyzed by a two-stage TD–GC/MS using a Tenax® TA-packed cold trap tube. Linear calibration curves for CWAs retained in the resin tubes were obtained in the range between 0.2pL and 100pL for sarin, soman, tabun, cyclohexylsarin, and sulfur mustard; and between 2pL and 100pL for VX and Russian VX. We also examined the stability of CWAs in Tenax® TA tubes purged with either dry or 50% relative humidity air under storage conditions at room temperature or 4°C. More than 80% sarin, soman, tabun, cyclohexylsarin, and sulfur mustard were recovered from the tubes within 2 weeks. In contrast, the recoveries of VX and Russian VX drastically reduced with storage time at room temperature, resulting in a drop to 10–30% after 2 weeks. Moreover, we examined the trapping efficiency of Tenax TA® adsorbent tubes for vaporized CWA samples (100mL) prepared in a 500mL gas sampling cylinder. In the concentration range of 0.2–2.5mg/m3, >50% of sarin, soman, tabun, cyclohexylsarin, and HD were recovered, whereas <1% of VX and Russian VX were recovered in the same concentration range. The results indicate that CWA vapors, with the exception of VX and Russian VX, can be measured by an on-site collection procedure using the Tenax® TA resin tubes, followed by a subsequent TD–GC/MS analysis.

Ion Trap Mass Spectrometric Effect Leading to Peak Suppression and Solvent-Free Peak Focusing due to Co-Chromatography

During the enantioselective analysis of whisky lactones after headspace in-tube extraction and gas chromatography with mass spectrometric detection using an ion trap mass analyzer (ITMS), an unusual effect led to peak suppression for one of the four enantiomers of whisky lactones. Using the ITMS in the selective ion monitoring (SIM) mode, one of the four targeted whisky lactone enantiomers could not be detected due to an abundant co-eluting matrix compound. For troubleshooting, detection with a quadrupole MS proved the presence of the missing enantiomer peak. A hypothetical explanation of this phenomenon is at present based on the different ion scanning mechanism of the two MS instruments. Considering the irregular sampling during the discontinuous scanning process of the ITMS being dictated by the ion current, this might have led to the suppression of the analyte. Furthermore, a solvent trapping effect of the target analyte with a co-chromatographing abundant matrix compound created a peak distortion (focusing) of the minor whisky lactone enantiomer. In our case, the inexplicably missing enantiomer peak triggered this investigation, but such matrix effects could easily be overseen with ITMS (SIM) detection, then eventually causing false-negative results.

Development of low-molecular-weight gelators and polymer-based gelators

In this review, the development of low-molecular-weight gelators and polymer-based gelators is described. The driving forces for physical gelation are non-covalent bonds, such as hydrogen bonds, electrostatic interactions, van der Waals interactions and π–π interactions. When gelation occurs, the gelator molecules self-assemble into macromolecule-like aggregates through non-covalent intermolecular interactions. The close relationship between crystallization and gelation is discussed. Crystallization is a phenomenon in which crystals are separated from solution by the formation of a three-dimensional arrangement of solute through intermolecular interactions. Conversely, physical gelation is caused by the trapping of solvent in fibrous networks that are formed by gelator molecules through intermolecular interactions. Amino acid derivatives and cyclic dipeptides are introduced as typical gelators. To develop gelators that form semipermanent stable gels, a new concept termed ‘gelation-driving segments’ is proposed. Polymer-based gelators that can form semipermanent stable gels are synthesized by connecting gelation-driving segments to polymers or oligomers. Low-molecular-weight compounds, which form physical gels, are called ‘gelators’ and have received a great amount of scientific and technological interest. The physical gelation by gelator results from non-covalent bonds, represented by hydrogen bond. Molecules of gelator are first self-assembled in cooling process, producing fibrous assemblies. Then, these fibrous assemblies form a three-dimensional network structure, and gelation occurs by trapping solvent in the networks. Fibrous assemblies can be observed by electron microscope. This is a transmission electron microscopy image of tetrachloromethane gel formed by N-octadecylamide of N-benzyloxycarbonyl-L-isoleucine.

A generic approach to post-column refocusing in liquid chromatography

To increase detection sensitivity in liquid chromatography, a generic post-column refocusing strategy has been developed to enrich (target) analytes prior to detection. In this strategy, after separation on the analytical column, the analytes are led to a trap column preferably containing a stationary phase with strong retentive properties (e.g. silica C30). They are then eluted using a strong solvent in a backward-elution mode. A first key element of the proposed strategy is that the trapping time should be at least equal to the time the front of the remobilization solvent needs to cover the entire length of the trap column, divided by the ratio of the flow rates used for trapping and remobilization. This condition is independent of the retention properties of the analytes in the trapping and remobilization solvent. Another essential element is the addition of a third solvent (isopropanol in the present case) to the remobilization solvent to overcome viscous-fingering effects caused by the viscosity difference between the trap and the remobilization solvents. The potential of the proposed post-column refocusing strategy is demonstrated for an isocratic separation of KI (t0 marker), an antibiotic (sulfamethazine), and acetophenone as a case study. Using optimized remobilization conditions a maximum signal-enhancement factor of 8 was achieved. Higher enhancement factors using a remobilization solvent with slightly higher elution strength were prohibited by disturbances of the UV background signal.

Analysis of Cigarette Smoke by Headspace Solid Phase Microextraction Gas Chromatography–Mass Spectrometry

This paper reports the analysis of the combustion products from moderate and low-tar cigarettes. The compounds were collected by a solvent trap coupled with automatic headspace solid phase micro-extraction and gas chromatography–mass spectrometry (GC–MS). The fiber, solvent system, extraction temperature, and extraction time were investigated by orthogonal experimental design. The optimized conditions included the use of an 85 µm carboxen/polydimethylsiloxane fiber, tert-butyl methyl ether as the solvent, an extraction temperature of 110°C, and an extraction time of 30 min. Thirty compounds were identified by GC–MS, and twelve aromatic hydrocarbons (51.08%) and 11 hydrocarbons (36.38%) were the major components. The 1,2,3-Trimethylbenzene (79.60%) and furfural (60.03%) had the largest reduction in low-tar cigarettes compared to moderate-tar cigarettes. This method was shown to be fast, simple, accurate, and precise.

Photochemical Behaviour of Carbamates Structurally Related to Herbicides in Aqueous Media: Nucleophilic Solvent Trapping versus Radical Reactions

Irradiation ofN-arylO-aryl carbamates has been carried out in H2O/CH3CN (1 : 1 v/v) solutions atλ&gt;290 nm. When chlorine is on theN-aryl ring, halogen-substituted products are found. These photoproducts derive from the trapping of the intermediate radical cation by water and, even, by acetonitrile leading to phenols andN-arylacetamides (photo-Ritter products), respectively. UnsubstitutedN-aryl carbamates slowly undergo photo-Fries reaction.

Modification of Ozone Deposition and I2 Emissions at the Air–Aqueous Interface by Dissolved Organic Carbon of Marine Origin

The reaction between gaseous ozone (O3) and aqueous iodide (I(-)) at the surface microlayer (SML) is believed to be a major chemical contributor to the oceanic dry deposition of O3 over open ocean waters and has also recently been shown to produce environmentally significant quantities of gaseous molecular iodine (I2). Here we investigate how this reaction is affected by the presence of dissolved organic carbon (DOC) of marine origin, using a heterogeneous flow reactor and detection of gaseous I2 by solvent trapping and UV/vis spectroscopy. Ozone deposition measurements over coastal seawater implied an O3 reactivity (λ) toward coastal marine DOC of ∼500 (420-580) s(-1), 2-5 times higher than that toward iodide at typical ocean concentrations (∼0.5-1 × 10(-7) M). We added varying amounts of highly concentrated DOC extracted from coastal seawater to I(-) solutions (1 × 10(-5) M) such that the relative reactivities of DOC and I(-) toward O3 (λDOC/λI) were in the expected range for natural seawater. The evolution of gaseous I2 and the loss of aqueous I(-) both reduced as DOC concentrations increased, with an overall suppression of I2 emissions of about a factor of 2 under conditions of λDOC/λI representative of open ocean waters (0.5-1). A kinetic model of the SML suggested that neither competition of DOC with I(-) for reaction with interfacial O3, nor direct loss of I2 and hypoiodous acid (HOI) through reaction with increasing quantities of DOC, can fully explain these results. We conclude that the suppression of I2 emissions by DOC is largely a physical effect arising from a decrease in the net transfer of I2 from the aqueous to gas phase, as suggested by recent laboratory studies.

A new solvent-based enhanced heavy oil recovery method: Cyclic production with continuous solvent injection

This paper presents a new enhanced heavy oil recovery (EHOR) process, Cyclic Production with Continuous Solvent Injection (CPCSI). In this process, a vapourized solvent near its dew point is continuously injected into the reservoir to maintain reservoir pressure and also supply extra gas drive to flush the diluted oil out through an injector that is located on the top of the reservoir; while a producer, which is located at the bottom of the reservoir, is operated in a shut-in/open cyclic way. A series of experiments have been conducted to evaluate the CPCSI performance. The recovery factors (RFs) are up to 85% of original oil in place (OOIP) in 1-D tests, and the RF is improved by 11% by using the 2-D lateral CPCSI, compared with the traditional 2-D lateral VAPEX. Well configurations and the producer shut-in/open scenarios are key optimization factors that affect the CPCSI performance. Experimental results show that the foamy oil flow and solvent trap are the two major EHOR mechanisms for enhancing the oil production rate during the production period. In comparison with continuous injection process, such as vapour extraction (VAPEX), and cyclic injection process, such as cyclic solvent injection (CSI), CPCSI offers free gas driving, and the reservoir pressure is maintained during the producer opening period so that the diluted oil viscosity is kept low. This work shows that CPCSI could be an alternative optimization production scenario for applying solvent based in situ EHOR techniques for heavy oil reservoirs in Western Canada.

Metabolic Engineering of Synechocystis sp. Strain PCC 6803 for Isobutanol Production

Global warming and decreasing fossil fuel reserves have prompted great interest in the synthesis of advanced biofuels from renewable resources. In an effort to address these concerns, we performed metabolic engineering of the cyanobacterium Synechocystis sp. strain PCC 6803 to develop a strain that can synthesize isobutanol under both autotrophic and mixotrophic conditions. With the expression of two heterologous genes from the Ehrlich pathway, the engineered strain can accumulate 90 mg/liter of isobutanol from 50 mM bicarbonate in a gas-tight shaking flask. The strain does not require any inducer (i.e., isopropyl β-d-1-thiogalactopyranoside [IPTG]) or antibiotics to maintain its isobutanol production. In the presence of glucose, isobutanol synthesis is only moderately promoted (titer = 114 mg/liter). Based on isotopomer analysis, we found that, compared to the wild-type strain, the mutant significantly reduced its glucose utilization and mainly employed autotrophic metabolism for biomass growth and isobutanol production. Since isobutanol is toxic to the cells and may also be degraded photochemically by hydroxyl radicals during the cultivation process, we employed in situ removal of the isobutanol using oleyl alcohol as a solvent trap. This resulted in a final net concentration of 298 mg/liter of isobutanol under mixotrophic culture conditions.

Interaction of organic solvents with protein structures at protein-solvent interface

The effect of non-denaturing concentrations of three different organic solvents, formamide, acetone and isopropanol, on the structure of haloalkane dehalogenases DhaA, LinB, and DbjA at the protein-solvent interface was studied using molecular dynamics simulations. Analysis of B-factors revealed that the presence of a given organic solvent mainly affects the dynamical behavior of the specificity-determining cap domain, with the exception of DbjA in acetone. Orientation of organic solvent molecules on the protein surface during the simulations was clearly dependent on their interaction with hydrophobic or hydrophilic surface patches, and the simulations suggest that the behavior of studied organic solvents in the vicinity of hyrophobic patches on the surface is similar to the air/water interface. DbjA was the only dimeric enzyme among studied haloalkane dehalogenases and provided an opportunity to explore effects of organic solvents on the quaternary structure. Penetration and trapping of organic solvents in the network of interactions between both monomers depends on the physico-chemical properties of the organic solvents. Consequently, both monomers of this enzyme oscillate differently in different organic solvents. With the exception of LinB in acetone, the structures of studied enzymes were stabilized in water-miscible organic solvents.

Comparison of efficiency of two methods for tar sampling in the syngas

In the present work, a cold solvent trapping (CST) and solid-phase adsorption (SPA) methods for determining concentration of tar compounds have been chosen for comparison. When the cold solvent trapping method is used, the producer gas flows through a series of impingers containing 2-propanol, whereas in a solid-phase adsorption method it passes through two adsorbent cartridges loaded with 500mg of aminopropyl-bonded silica, and 100mg of activated coconut charcoal. During the experiment, 52 compounds were identified by the cold solvent trapping method and 48 compounds by the solid-phase adsorption method. The SPA method is more accurate than those using impingers, especially for determining such volatile organic compounds as benzene, toluene, and xylenes, due to the use of a second sorbent, activated coconut charcoal. By contrast, the CST method proves to be more accurate for determining components of heavy tar due to a much larger volume of the sampled gas.