Isotope enrichment, cost

Solid-phase peptide synthesis offers a fast and convenient route for many peptides when isotope-enriched compounds are not required. Classical synthesis additionally permits the use of non-natural amino acids and allows site-specific isotope labeling. Although Fmoc protected 15N-labeled amino adds are commercially available, the cost of such synthesis is usually prohibitive, and the peptides from chemical synthesis require perdeuterated detergents and unfortunately exclude investigation of internal dynamics through measurement of 15N relaxation. 

In reality, the ideal isotope labeling pattern (one component 100% labeled, the other one 0%) is almost never fulfilled first, a 100% isotopic enrichment will be generally very hard to reach (and very costly ) for chemically synthesized as well as for overexpressed compounds. On the other hand, even the nonenriched components will always contain NMR-active isotopes at natural abundance unless they are derived from specially isotope-depleted (and again very expensive) starting material. 

Considering the various approaches of solid-state NMR spectroscopy, contrasting advantages and limitations must be mentioned for batch and flow techniques. MAS NMR spectroscopy under batch reaction conditions with glass inserts for the preparation of the catalyst samples has the advantage that all the materials and equipment are commercially available. Because the amounts of reactants necessary for these experiments are small, only low costs for isotopically enriched materials... 

A significant limitation of NMR experiments of working catalysts in flow systems is the necessity of using isotopically enriched materials as reactants, which leads to high costs. Furthermore, most of the flow approaches described in Section III.B are based on homemade equipments requiring large efforts to make the techniques feasible. 

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... 

All metal isotopes having a magnetic moment can, in principle, be observed by NMR, but nuclei with low y values (such as Rh), low natural abundance (such as " Sn) or both drawbacks (such as Fe) are difficult to observe Erectly. It is possible to circumvent the second problem by isotopic enrichment but this leads to costly experiments and sometimes to difficult syntheses. 

The main disadvantages of SSNMR are related to the relatively high cost of the spectrometers and to the fact that the technique is nonroutine and requires a highly trained operator. Furthermore, because of the low sensitivity of the most interesting nuclei such as 13C or 1SN, a reasonable signal-to-noise ratio can only be achieved with a long acquisition time. Selective isotopic enrichment of certain sites in the molecule shortens the acquisition time by increasing the sensitivity however, the expense of the analysis increases dramatically. 

Tracer studies in which chemically similar species are studied on the basis of containing a radioisotope are discusssed in chapter 10. It is fairly obvious that, with detection techniques readily available for the measurement of non-radioactive isotopes, the principle can be extended to noh-radioactive systems. Where in viva studies are concerned there are clear safety reasons for so doing. Although some progress is being made in this direction, it is being limited by the high cost and poor availability of isotopically enriched tracers. 

The ground-state isotope should ideally be stable and have a high natural abundance, otherwise it may become necessary to use artificially enriched compounds at greatly increased cost and inconvenience. Isotopic enrichment is, however, a very important method of improving resolution of spectra and may become essential in work with biological materials or in the study of doped solids where the actual concentration of the element of interest is extremely small. 

Performing chemistry with a very low bias to determine the exact amount of spike Ny would, however, be very difficult and require a large (and hence costly) amount of isotopically enriched material. The solution to this can be another isotope dilution i.e. assaying the spike Y (in solution for example) with accurately assayed material Z of... 

Mass spectrometric approaches are also very useful for the measurement of stable isotopes in drug metabolism studies. The application of MS to the quantitative measurement of stable isotope has been limited due to the high cost and sophistication of the instruments necessary for stable isotope enrichment studies. Nonetheless, recent improvements in instrument design and performance, as well as computer software for instrument control, data acquisition, and analysis, have increased the sensitivity and reliability of stable isotopic enrichment studies. These new MS instruments, including continuous-flow isotope ratio mass spectrometry (CF-IRMS) and HPLC-chemical reaction interface mass spectrometry (HPLC-CRIMS) are increasingly less expensive, easier to operate, and accessible for mass balance/ metabolite identification studies with stable isotopes. 

It is interesting to compare the economics of gas centrifugation to gaseous diffusion, albeit crudely. London (1961) divides enrichment cost into two principal categories (1) specific investment, i.e., capital cost per separative work unit (SWU) amortized over the plant life, and (2) power cost per SWU. (SWU, which is a function of quantities and concentrations of feed, product, and waste provides a quantitative measure of the isotope separation task for any conceivable process.) The latter comprises the bulk of operating costs. The estimates, which are crude, clearly demonstrate an advantage for gas centrifugation. 

The Papers have shown that there is a considerable choice of design for heavy water reactors. Moreover, it is claimed that natural uranium can be used as fuel, albeit with some penalty in fuel bum-up and in extra capital cost for the plant. Isotopic enrichment of uranium is thus not essential to a heavy water reactor programme. Typically the Inventory of heavy water in these reactors is about 0.5-1 ton/MWe output for power plant of a few hundred megawatts capacity, depending upon the type of system selected. 

Isotopic enrichment, which is common for and NMR, is much rarer for Si because of the higher costs involved and the limitation on chemical species which can be purchased in the enriched form (in effect, only Si02 is available). However, Harris et al. have made considerable use (21-24) of the enrichment technique for unraveling the chemical complexities of aaueous silicate solutions. Isotopic enrichment for Sn or Pb NMR is unnecessary and has never been used. 

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