Isotopic enrichment schemes

Os-191 is produced by neutron irradiation of isotopically enriched 0s-190 (isotopic composition 0s-190, 97.8 o Os-188, 0.47 o Os-192, 1. 02 o). Irradiations are currrently performed at the Oak Ridge National Laboratory in the High Flux Isotope Reactor (HFIR) at a neutron flux of 2.5 x 10 n/cm -s. The routes to the various nuclides produced during irradiation of the 0s-190 target and the neutron cross-section values (2 ) are summarized below (Scheme I). 

Fyfe et al. (354) have combined 31P and 13C CP/MAS NMR studies first to identify the polymer-immobilized catalyst (Scheme 4, compound ii) formed from the precursor i by treatment with Pd(PPh3)4 and, second, to monitor the carbonyl insertion reaction using 13C-enriched CO to yield iii. The use of isotopically enriched CO was required so as to record meaningful signals above those emerging from the carbon-rich polymer background. 

The introduction and implementation of heteronuclear-based multidimensional techniques have revolutionized the protein NMR field. Large proteins (> 100 residues) are now amenable to detailed NMR studies and structure determination. These techniques, however, necessarily require a scheme by which and isotopes can be incorporated into the protein to yield a uniformly labeled sample. Additional complications, such as extensive covalent post-translational modifications, can seriously limit the ability to efficiently and cost effectively express a protein in isotope enriched media - the c-type cytochromes are an example of such a limitation. In the absence of an effective labeling protocol, one must therefore rely on more traditional proton homonuclear NMR methods. These include two-dimensional (1) and, more recently, three-dimensional H experiments (2,3). Cytochrome c has become a paradigm for protein folding and electron transfer studies because of its stability, solubility and ease of preparation. As a result, several high-resolution X-ray crystal structure models for c-type cytochromes, in both redox states, have emerged. Although only subtle structural differences between redox states have been observed in these... 

The relative merits of different isotopic enrichment strategies that might be used in solid state NMR protein structure determinations have been discussed by Gehman et aV The basis for comparison of these merits is the determination of the relative uncertainties in rates measured by a generalised dipolar recoupling experiment. The different schemes considered use N and labelling of... 

Several syntheses of carotenoids isotopically labelled with deuterium have been reported [65-68]. The total synthesis of spheroidenes (97) specifically labelled with deuterium in the central part is based on the synthetic scheme discussed above for the C-labelled spheroidenes [68]. When deuterium-enriched compounds are used, a few modifications are necessary to avoid scrambling and isotope dilution (Scheme 28). 

Improvements to protein expression. Whilst many of the advances in studying biomolecules by NMR are as a direct result of developments in sample labelling schemes, such as isotope enrichment at specific sites or unnatural amino add incorporation (yide infra), there are still great improvements that can be made to routine protein expression methods. For conventional and perdeuteration, Li et al. describe optimisation of... 

The first experimental proof of the mechanism of the silylation in the McKenna reaction has been presented by Blazewska. The reaction of a model compound isotopically enriched diethyl phenylphosphonates (644) (P= 0 or P= 0) showed that reaction in acetonitrile proceeded via an attack of P=0 ojgrgen on the silicon atom in hromotrimethylsilane (Scheme 188). The change in the phosphorus-o g gen hond order was directly detected using P and NMR spectroscopy. 

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