NMR samples, preparation

Although alternative expression systems have been successfully adapted for the production of isotope-labeled proteins (see Sect. 1.5), heterologous expression in E. coli often remains the method of choice for NMR sample preparation. There is a fundamental difference, however, with respect to the kind of medium in which the cells are cultivated. In a so-called chemically defined or minimal medium only one or a very limited number of carbon sources is provided, e. g. glucose or glycerol. All bacterial metabolites have to be biosynthesized by the cells through the various, sometimes lengthy and energy-de-... 

The 31P NMR of the Li+ salt (see Properties under Section A for details of this NMR sample preparation) gives a single P(2) resonance at 8 — 22.6 ppm for the product recrystallized twice from water. If the crude material is recrystallized only once an unassigned resonance (<5%) is observed at 8 — 34.0 ppm. (Note the importance of the recrystallization step(s) prior to... 

NMR Sample Preparation of Yeast Ubiquitin Purified From E. coli... 

Except for extraction, no separation methods have been used in the NMR sample preparations in the interlaboratory comparison tests (7 15). Thus, the chemicals of interest, impurities, and background present in the original sample and in the extract were all present in the NMR sample. The advantage of this... 

The analysis of CAD and alkamides has been completed using a variety of analytical techniques that include high-performance liquid chromatography (HPLC), capillary electrophoresis, gas chromatography (GC), GC-mass spectrometry (GC-MS), liquid chromatography-mass spectrometry (LC-MS), and nuclear magnetic resonance (NMR). Sample preparation methodologies utilize hexane, methanol and ethanol as the primary extraction solvents. 

The details of NMR sample preparation for sealed samples as well as those investigated via in situ techniques are described previously[l,4-7]. A home-built spectrometer with resonance frequency of 220 MHz was used for the NMR experiments on the sealed samples. All NMR measurements on the sealed samples were performed at 294 1K. 

The NMR sample prepared by the above method will contain lithium chloride," but this does not usually prevent a satisfactory NMR spectrum being obtained. 

It is the job of the isolation chemist to make available isolated materials solubility and solution stability learnings to the NMR spectroscopist. Identifying which solvents the isolated impurity material is soluble in will assist in solvent choice for the NMR analysis. Identifying solvents that could be cause for concern with regards to solution degradation is very important. It is not uncommon to have an impurity that is pH sensitive or labile in protic solvents. NMR sample preparation will avoid DMSO for impurities sensitive to high pH as DMSO is basic in nature. NMR sample preparation will avoid chloroform for impurities sensitive to low pH as chloroform is acidic in nature. 

Interestingly, complex 21 underwent essentially instantaneous deutera-tion, which was already complete during the NMR sample preparation. Isotope exchange involved the heterocyclic proton at C4 exclusively. Complex 23 was less reactive and H/D exchange occurred first at the C2-bound CHj group (one hour). Subsequent deuteration of the heterocyclic carbon C4 position was significantly slower and required two days to reach completion. These observations point to a more electrophilic character of the triazolylidene heterocycle (fast reactivity towards OH"), while the imidazolylidene is rather nucleophilic (fast reaction with H ), hence highlighting a remarkable difference between the two types of abnormal carbene. 

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