Assignment of the NMR spectra of a potent ribonucleotide reductase inactivator - utilization of HMQC-TOCSY to establish connectivities in exchange broadened spectra

Inverted and Suppressed Direct Response (IDR and SDR, respectively) HMQC-TOCSY experiments are evaluated relative to the conventional HMQC-TOCSY experiment in the assignment of the congested proton and carbon spectra of a 2-acetylpyridine thiocarbonohydrazone used to potentiate the antiviral drug acyclovir. Spectra with moderate overlap can be most expeditiously interpreted using the inverted direct response HMQC-TOCSY variant; spectra with severe overlap can be more readily interpreted if direct responses are suppressed.     

Simplification of protein NOESY spectra using bioorganic precursor synthesis and NMR spectral editing

A novel method is proposed for the analysis of protein NOEs in solution. In this approach, chemically synthesized precursor compounds for the amino acids valine, leucine, and isoleucine are used for amino acid specific labeling of these hydrophobic residues. The methodology is based on a novel synthetic route to 12C,1H,2H Val, Leu, and Ile side chains selectively labeled with 13CH3 only at the terminal methyl group. In an otherwise 12C,1H labeled protein, discrimination between protons bound to 12C and 13C (or 15N) can be achieved using standard isotope-editing NMR pulse schemes. This strategy significantly relieves problems with spectral overlap through selective observation of interresidue methyl NOEs and will thus be a powerful extension of existing biomolecular NMR methodology.     

Emission spectra of a direct current plasma in the vacuum ultraviolet spectral region from 100 to 200 nm

DCP emission spectra have been recorded over the spectral range, extending from 100 to 200 nm. These spectra were acquired by coupling the DCP to a 0.5-m Seya-Namioka vacuum monochromator through a specially designed, helium purged, glass/quartz side arm. DCP background lines and emission lines of P, S and Cl have been identified and tabulated.     

A convenient isotopic dilution method for the direct determination of oxygen in fluorinated organic materials

Summary A method is described for the microdetermination of oxygen in fluorinated organic compounds. The unknown material is pyrolyzed together with¹⁸O-containing succinic acid, in a sealed, evacuated, specially-purified copper tube. After equilibration at 850°C, the tube is punctured and the pyrolysis gases are admitted to a mass spectrometer. Comparison of the m/e-46/44 ratio (C¹⁶O¹⁸O/C¹⁶O¹⁶O) to the ratio obtained from similar pyrolysis of the oxygen-18 compound only, provides a means of calculating the oxygen content of the unknown.The copper tubes are made from ordinary, rigid, 1/4 o. d. tubing, which is purified by reduction in hydrogen and heating under vacuum. They are sealed by induction welding.The procedure has been used successfully for a variety of fluorinated compounds including several metal salts of fluorinated acids.

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Zusammenfassung Eine Methode für die Mikrobestimmung von Sauerstoff in fluorhaltigen organischen Verbindungen wurde beschrieben. Das Untersuchungsmaterial wird mit¹⁸O-markierter Bernsteinsäure in einem evakuierten, verschlossenen, besonders gereinigten Kupferrohr pyrolysiert. Nach Gleichgewichtsemstellung bei 850° C wird dieses Rohr punktiert und die Pyrolysegase in ein Massenspektrometer geleitet. Der Vergleich des m/e-46/44-Verhältnisses (C¹⁶O¹⁸O/ C¹⁶O₂) mit dem nach Pyrolyse der¹⁸O-markierten Verbindung allein bietet die Möglichkeit, den Sauerstoffgehalt der Probe zu berechnen.Die Kupferrohre werden aus gewöhnlichem, hartem Rohrmaterial mit 1/4 Zoll äußerem Durchmesser hergestellt, mit Wasserstoff reduziert, im Vakuum erhitzt und schließlich durch Induktion verschweißt.Das Verfahren wurde erfolgreich zur Analyse verschiedener fluorhaltiger Verbindungen einschließlich Metallsalzen fluorhaltiger Säuren verwendet.

     

Towards a universal product ion mass spectral library - reproducibility of product ion spectra across eleven different mass spectrometers

Product ion spectra produced by collision-induced dissociation (CID) in tandem mass spectrometry experiments can differ markedly between instruments. There have been a number of attempts to standardise the production of product ion spectra; however, a consensus on the most appropriate approach to the reproducible production of spectra has yet to be reached. We have previously reported the comparison of product ion spectra on a number of different types of instruments - a triple quadrupole, two ion traps and a Fourier transform ion cyclotron resonance mass spectrometer (Bristow AWT, Webb KS, Lubben AT, Halket JM. Rapid Commun. Mass Spectrom. 2004; 18: 1). The study showed that a high degree of reproducibility was achievable. The goal of this study was to improve the comparability and reproducibility of CID product ion mass spectra produced in different laboratories and using different instruments. This was carried out experimentally by defining a spectral calibration point on each mass spectrometer for product ion formation. The long-term goal is the development of a universal (instrument independent) product ion mass spectral library for the identification of unknowns.The spectra of 48 compounds have been recorded on eleven mass spectrometers: six ion traps, two triple quadrupoles, a hybrid triple quadrupole, and two quadrupole time-of-flight instruments. Initially, 4371 spectral comparisons were carried out using the data from eleven instruments and the degree of reproducibility was evaluated. A blind trial has also been carried out to assess the reproducibility of spectra obtained during LC/MS/MS.The results suggest a degree of reproducibility across all instrument types using the tuning point technique. The reproducibility of the product ion spectra is increased when comparing the tandem in time type instruments and the tandem in space instruments as two separate groups. This may allow the production of a more limited, yet useful, screening library for LC/MS/MS identification using instruments of the same type from different manufacturers.     

Assignments of the 1H and 13C NMR spectra of pseudo-symmetric heterocyles using the HMQC-TOCSY experiment to differentiate overlapping spin systems

The ¹H nmr spectra of phenanthro[9′,10′:4,5]thieno[2,3-c]quinoline, benzo[f]phenanthro-[9′,10′:4,5]thieno[2,3-c]quinoline and benzo[h]phenanthro[9′,10′:4,5]thieno[2,3-c]quinoline are highly congested. For each compound, all protons abide in an aromatic environment complicated by pseudo-symmetric regions which result in multiple overlap of the different spin systems these molecules contain. We illustrate here the utility of the HMQC-TOCSY experiment to identify spin systems when the proton spectrum is highly congested. To complete the assignment of the ¹H and ¹³C nmr spectra of each compound the HMBC experiment is used to assign the quaternary carbons.     

A convenient method for the preparation of 20-[18O]-labeled ingenol

A short and efficient protecting group-free synthesis of isotopically labeled 20-[¹⁸O]-ingenol has been developed. Based on a highly selective (only one out of four hydroxy groups) Mitsunobu reaction of ingenol with ¹⁸O₂-acetic acid and subsequent methanolysis, this route yielded the desired 20-[¹⁸O]-ingenol in high yield and chemical and isotopic purity.     

A convenient method for the preparation of [1-2H]- and [1-3H]-terminal acetylenes

Conclusions A convenient one-step method has been proposed for the preparation of 1-isotope-substituted acetylenes based on the use of a phase transfer catalyst.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No, 8, pp. 1921–1922, August, 1985.     

A local pre-processing method for near-infrared spectra,combined with spectral segmentation and standard normal variate transformation

Pre-processing of near-infrared (NIR) spectral data has become a necessary part of chemometrics modeling and is widely used in many practical applications. The objective of the pre-processing is to remove physical phenomena in the spectra in order to improve subsequent qualitative or quantitative analysis. Herein, a localized version of standard normal variate (SNV) is proposed, in which the correction parameters are estimated from local spectral areas. The method of determining the optimal spectral segmentation is also presented. Compared with full range methods, the local method demonstrates advantages in spectral linearity correction, model interpretation and prediction accuracy. Several benchmark NIR data sets were studied in our experiments; the proposed method achieved comparable performance against proven full range methods, with the reduction of prediction errors being statistically significant in many cases.     

Thermodecarbonylation of α-substituted cycloalkanones: a convenient one-carbon ring contraction method

A new and very convenient one-carbon ring contraction method is reported. Pyrolysis of α-substituted cycloalkanones at 600-650 °C under flow conditions produces the ring contracted compounds under loss of carbon monoxide. Substrates varying in ring size and nature of the α-substituent have been investigated.     

Simple and convenient chemiluminescence method for the determination of melatonin

A simple and convenient chemiluminescence (CL) method for the determination of melatonin is reported. It is based on the observation that melatonin can greatly enhance the ultra-weak CL between H₂O₂ and acetonitrile in alkaline solution. Light emission is intense and long-lived, and even with a simple setup, a high sensitivity could be achieved. The log-log linear range was 250 nM-250 μM with a 3σ detection limit of 100 nM. Singlet oxygen is suggested to be produced by the reaction between H₂O₂ and acetonitrile and is responsible for the CL of melatonin. As a preliminary application, this simple method has been successfully applied into the determination of melatonin in health-foods.     

A convenient method for the synthesis of chlorocymantrene and bromocymantrene

Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2046–2047, November, 1994.     

Synthesis of the axially substituted titanium Pc-C60 dyad with a convenient method

[structure: see text] We successfully synthesized the axially substituted titanium Pc-C(60) dyad with a convenient method that improves on the traditional asymmetrical phthalocyanine routine to covalently linked phthalocyanines with other functional molecules. The intramolecular photoinduced process between phthalocyanine donor and fullerene acceptor was preliminarily studied.     

Simple and convenient analytical method for the direct determination of chlorine species by ETV-ICP-AES using tungsten boat furnace vaporiser and exchangeable small sample cuvettes

A tungsten boat furnace vaporiser and an exchangeable sample cuvette system were applied for the direct determination of chlorine in plastic samples by electrothermal vaporisation inductively coupled plasma atomic emission spectrometry. Each piece of plastic samples was moulded into a thin film, and weighed into small sample cuvettes. An ethanolic solution of the modifier potassium hydroxide and an organic solvent were added to each sample cuvette. The contents including the plastic piece were spread over the bottom of each sample cuvette by warming on a hotplate. After the preparation, one of the cuvettes was placed in the tungsten boat furnace equipped with the electrothermal vaporiser. The temperature was gradually elevated. At the flash vaporisation step of 1800 °C, the analyte was vaporised and introduced into the ICP to measure an atomic emission assignable to Cl I 134.724 nm. A widely spread thin layer of the plastic facilitated its rapid evaporation and introduction into an ICP atomic emission spectrometer. Since all the chlorine species were on-furnace decomposed to form potassium chloride during the pretreatment, the solid samples were analysed using a calibration curve prepared from the aqueous standard solutions. By using this system, a detection limit of 1.5 μg g⁻¹ of chlorine in solid plastic samples was established when a sample amount taken of 2 mg was studied. The exchangeable sample cuvette technique makes it possible to measure a number of sample sequentially by preparing a lot of sample cuvettes prior to analysis. Approximately 20 batches could be vaporised per hour. Analytical results of some plastic samples that were analysed by this technique are given in this report.     

A new simple and convenient method for preparation of oligonucleotides containing a pyrene or a cholesterol moiety

Russian Chemical Bulletin - A new simple and convenient approach to the synthesis of oligonucleotides containing pyrene or cholesterol moieties is described. The approach does not require a special...     

Total assignment of the proton and carbon NMR spectra of the alkaloid quindoline — utilization of HMQC-TOCSY to indirectly establish protonated carbon-protonated carbon connectivities

The reisolation of the indoloquinoline alkaloid quindoline (also known as norcryptolepine) from Cryptolepis sanguinolenta is reported. The structure was unequivocally confirmed by two-dimensional nmr methods; the proton and carbon spectra were assigned for the first time. Because of congestion in the proton spectrum HMQC-TOCSY was used as an alternative to the more familiar COSY experiment. In addition to establishing proton-proton connectivities, HMQC-TOCSY affords the added benefit of providing, in an indirect sense, connectivity information between protonated carbons.