New methodologies in trace analysis of volatile organic compounds

Two methods for sampling and concentration of volatile organic compounds are reported. In the first method, traps coated with a very thick film (ca. 100 μm) of cross-linked silicone stationary phase are employed. Such thick films can be prepared with a modified dynamic coating procedure, which is briefly described. The low phase ratio traps can be utilized for enrichment of volatiles from gaseous as well as aqueous matrices. The second technique is based on chromatographic evaporation of a solvent in a capillary tube, where the process is sustained by a repeated sample injection and a cyclic flow reversal. In this way, large solvent volumes can be handled by a small volume system. Under optimal conditions, when using a solvent barrier, quantitative recovery is possible even for compounds of comparatively high volatility. Another important application of the technique is extraction of trace components from gases such as headspace samples, polluted air, etc.     

Cationic poly(methacryl oxyethyl trimethylammonium) and its poly(ethylene glycol)‐grafted analogue as capillary coating materials in electrophoresis

Cationic polyelectrolytes were synthesized and used as semipermanent coating materials for capillaries in electrophoresis. The polyelectrolytes used were a homopolymer of poly(methacryl oxyethyl trimethylammonium chloride) (PMOTAC) and its poly(ethylene glycol) (PEG)‐grafted analogue. Two PMOTAC polyelectrolytes, with molar masses of 85,000 and 300,000 g/mol, and PEG‐grafted PMOTAC with a molar mass of 280,000 g/mol were synthesized and then characterized by size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR) spectroscopy. Attachment of the polyelectrolytes to the wall of the fused silica capillary for electrophoresis caused the electroosmotic flow (EOF) to reverse. The polyelectrolyte coatings were tested over the pH range 2–11 at different buffer ionic strengths, and the most stable and strongest anodic EOFs were obtained at acidic pH values with low ionic strength buffers. Between runs the capillary is merely rinsed for 2 or 3 min with the background electrolyte solution. With the PMOTAC coatings at pH values ≤5, the RSDs of the EOFs were less than 2.9% after 60 injections. The effects of the molar mass of the polycation and of PEGylation of PMOTAC on the interactions between the polycations and basic proteins were studied at acidic pH values. The differences in the effective electrophoretic mobilities, resolution values, and plate numbers of the proteins with the different coatings were due to the EOF, as demonstrated through calculations of reduced mobilities, relative resolution values, and relative plate numbers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2655–2663, 2007     

Enantio- and Diastereoselective Aldol-Reaction of 2,6-Dimethylphenyl Propionate Using Titanium-Carbohydrate Complexes

Chloro(cyclopentadienyl)bis(1,2:5,6-di-O-isopropylidene-α-D -glucofuranos-3-O-yl)titanium ( 1 ) is used for the transmetallation of Li-enolates obtained from propionyl derivatives. While such Ti-enolates of ketones and hydrazones appear to be unreactive, the (E)enolate 13 of 2,6-dimethylphenyl propionate ( 11 ) adds to the re-side of aldehydes, affording various syn-aldols 14 with high dia- and enantioselectivity (92–97% ds, 91–97% ee, cf. Scheme 2 and Table 1). Racemic syn-aldols (±)- 14 are obtained analogously from the achiral bis(2-propyloxy)-Ti-enolate 15 (Scheme 2 and Table 2). In contrast to the unstable Li-enolate 10 , the Ti-enolates 13 and 15 isomerize at ?30°, presumably to the thermodynamically more stable (Z)-enolates (Scheme 4), While the diastereoselectivity of the achiral enolate 15 is lost upon this equilibration, the chiral (Z)-enolate 27 quite unexpectedly affords anti-aldols 12 of high optical purity (94–98% ec) and, in most cases, with acceptable-to-good diastereoselectivity (82–90% ds). Notable exceptions are branched unsaturated and aromatic aldehydes which form a greater proportion of synepimers of moderate optical purity (Scheme 5 and Table 3). Consistent with these findings, re-facial-and ami -selective aldol-addition is also exhibited by the (Z)-configurated Ti-enolate 22 of N-propionyl-oxazolidi-none 19 (Scheme 3).     

A technique for coating capillary columns with a very thick film of cross-linked stationary phase

A method for preparation of capillary columns and traps with a very thick film (up to 100 μm) of stationary phase is described. The principle of this method is based on an immediate fixation of a film of prepolymer, formed during dynamic coating. Thus, the development of film irregularities, such as are caused by Rayleigh instability is avoided. Fixation of the film is conveniently accomplished by heat-accelerated crosslinking as was demonstrated in this work, where a commercially available silicone prepolymer (Sylgard 184, Dow Corning) was employed. The low phase ratio columns which thus can be prepared are interesting both in chromatography with dense mobile phases and as enrichment devices. Examples of the latter application are shown, where trace organic components from air and water were concentrated.     

GnRH binding RNA and DNA Spiegelmers: a novel approach toward GnRH antagonism

Mirror-image oligonucleotide ligands (Spiegelmers) that bind to the pharmacologically relevant target gonadotropin-releasing hormone I (GnRH) with high affinity and high specificity have been identified using the Spiegelmer technology. GnRH is a decapeptide that plays an important role in mammalian reproduction and sexual maturation and is associated with several benign and malignant diseases. First, aptamers that bind to D-GnRH with dissociation constants of 50-100 nM were isolated out of RNA and DNA libraries. The respective enantiomers of the DNA and RNA aptamers were synthesized, and their binding to L-GnRH was shown. These Spiegelmers bind to L-GnRH with similar affinity to that of the corresponding aptamers that bind to D-GnRH. We further demonstrated dose-dependent inhibition of GnRH-induced Ca(2+) release in Chinese hamster ovary cells that were stably transfected with the human GnRH receptor.     

A mechanism from quantum chemical studies for methane formation in methanogenesis

The mechanism for methane formation in methyl-coenzyme M reductase (MCR) has been investigated using the B3LYP hybrid density functional method and chemical models consisting of 107 atoms. The experimental X-ray crystal structure of the enzyme in the inactive MCR(ox1)(-)(silent) state was used to set up the initial model structure. The calculations suggest a mechanism not previously proposed, in which the most remarkable feature is the formation of an essentially free methyl radical at the transition state. The reaction cycle suggested starts from a Michaelis complex with CoB and methyl-CoM coenzymes bound and with a squareplanar coordination of the Ni(I) center in the tetrapyrrole F(430) prosthetic group. In the rate-limiting step the methyl radical is released from methyl-CoM, induced by the attack of Ni(I) on the methyl-CoM thioether sulfur. In this step, the metal center is oxidized from Ni(I) to Ni(II). The resulting methyl radical is rapidly quenched by hydrogen-atom transfer from the CoB thiol group, yielding the methane molecule and the CoB radical. The estimated activation energy is around 20 kcal/mol, which includes a significant contribution from entropy due to the formation of the free methyl. The mechanism implies an inversion of configuration at the reactive carbon. The size of the inversion barrier is used to explain the fact that CF(3)-S-CoM is an inactive substrate. Heterodisulfide CoB-S-S-CoM formation is proposed in the final step in which nickel is reduced back to Ni(I). The suggested mechanism agrees well with experimental observations.     

A density functional study on a biomimetic non-heme iron catalyst: insights into alkane hydroxylation by a formally HO-FeV=O oxidant

The reactivity of [HO-(tpa)Fe(V)=O] (TPA=tris(2-pyridylmethyl)amine), derived from O-O bond heterolysis of its [H(2)O-(tpa)Fe(III)-OOH] precursor, was explored by means of hybrid density functional theory. The mechanism for alkane hydroxylation by the high-valent iron-oxo species invoked as an intermediate in Fe(tpa)/H(2)O(2) catalysis was investigated. Hydroxylation of methane and propane by HO-Fe(V)=O was studied by following the rebound mechanism associated with the heme center of cytochrome P450, and it is demonstrated that this species is capable of stereospecific alkane hydroxylation. The mechanism proposed for alkane hydroxylation by HO-Fe(V)=O accounts for the experimentally observed incorporation of solvent water into the products. An investigation of the possible hydroxylation of acetonitrile (i.e., the solvent used in the experiments) shows that the activation energy for hydrogen-atom abstraction by HO-Fe(V)=O is rather high and, in fact, rather similar to that of methane, despite the similarity of the H-CH(2)CN bond strength to that of the secondary C-H bond in propane. This result indicates that the kinetics of hydrogen-atom abstraction are strongly affected by the cyano group and rationalizes the lack of experimental evidence for solvent hydroxylation in competition with that of substrates such as cyclohexane.