Active isotopes

The structures of doubly bonded compounds containing tin readily lend themselves to study by multinuclear NMR spectroscopy, as tin has two NMR active isotopes ll7Sn and ll9Sn. The chemical shifts of doubly bonded tin species in the 119Sn NMR spectrum cover a broad range from 6 = -150 to +835 (Table VI) however, the majority of the signals occur at low field S = 400 or above. 

The experiments we have so far described have been used to study nuclei with spin I = Vi ( ll, 13C, 31P). Our model compounds 1 and 2 contain two further atoms (oxygen and chlorine), which have no NMR-active isotope with spin Vi. Oxygen does however have an NMR-active isotope with spin I = 5/2 but very low natural abundance (0.037%) this is 170. Chlorine has two NMR-active isotopes 35C1 (I = 3/2,75.53%) and 37C1 (I = 3/2,24.47%). 

Tin is an unusual element in that it has three magnetically active isotopes, all spin-Vi. However, tin-115 has a natural abundance of only 0.35%, and is never studied. The other two, tin-117 and tin-119, occur in similar amounts (7.61 and 8.58% respectively). Spectra of the latter are normally recorded, as it is about 25% stronger. Tetramethyltin is taken as the zero-point, and the total chemical shift range is about 3000 ppm. 

Platinum-195 is the only magnetically active isotope of platinum, the natural abundance being 33.8%. The shift of a saturated solution of K2PtCl6 is in D20 defined as zero ppm. The total chemical shift range is huge, about 13,000 ppm (from -6000 to +7000 ppm ). 

The semiconductors that have been the subject of numerous investigations in bulk, alloyed, or nanocrystalline form include Si, Ge, doped diamond, SiC, (B, Al, Ga, In)(N, P, As, Sb), and (Zn, Cd, Hg, Pb)(0, S, Se, Te). Nature has been exceptionally benign in providing NMR-active isotopes at natural abundances exceeding 4% for all of the preceding elements except in the cases of 13C, 33S, and 170, and enrichment with isotopic-labels has become more common. 

An important group of analytical methods is based on measurements of the change in isotopic ratio when active and non-active isotopes are mixed. In the simplest case, a known amount w1 of labelled analyte of known specific activity at is added to the sample. After isotopic mixing has been established sufficient of the analyte is separated (nor normally 100%) to allow the new specific activity a2 to be measured. Measurements of activity and the amount of the analyte separated are thus required. Subsequently the amount w2 of analyte in the sample may be calculated from equation (10.17). 

The employment of NMR-active isotopes permits to access experimental parameters which are intrinsically difficult to measure, unless a significant concentration of the sugar is present in the NMR tube. For instance, aqueous solutions of N-acetyIncuraminic acid, labeled with 13C at Cl, C2, and/or C3, were analyzed to detect and quantify the various chemical species present in equilibrium at different pHs. In fact, in addition to the expected a and (3 pyranose forms, acyclic keto, keto hydrate and enol forms were identified on the basis of 13C NMR spectroscopic data. Besides, DFT methods were employed to predict the effect of enol and hydrate structure on the coupling constant values Jc,u and /c c involving C2 and C3, finding that 2/c2,h3 can be safely used to differentiate the cis and tram isomers of the enol forms.9... 

NMR active isotope Natural abundance Gyromagnetic ratio y fH = 100) Corresponding NMR inactive" isotope Natural abundance Gyromagnetic ratio y ( H=T00)... 

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. 

One component is practically completely devoid of a specific NMR active isotope, while the other component is enriched with this isotope to a reasonable level, or... 

Abstract In recent years NMR methods have been developed that enable the observation of proteins inside living bacterial cells. Because of the sensitivity of the chemical shift to environmental changes these in-ceU NMR experiments can be used to study protein conformation, molecular interaction or dynamics in a protein s natmal surrounding. Detection of proteins in the bacterial cytoplasm relies on labeling of the protein of interest with NMR active isotopes. This review describes different labeling techniques based on either imiform i5n or labeling as well as amino acid specific labeling schemes. In addition potential applications of these in-cell NMR experiments and their limitations are discussed. 

BIOMINERALIZATION SPECIFIC ACTIVITY ISOTOPE DILUTION SPECIFIC CATALYSIS... 

Magnesium has one NMR-active isotope, Mg, which is only 10.0% naturally abundant. This is a spin 5/2 nucleus with a magnetogyric ratio of —1.639 X 10 rads T , which means it has a Larmor frequency that is 6% of that of protons in the same magnetic field and a receptivity of 72.9% of that of The relatively low Larmor frequency was one of the key factors... 

Of the radio-active isotopes of chlorine, only chlorine-36 could be used in polymer research. Its half-life is very long it gives /8-rays of maximum energy of 0.71 MeV and of considerably greater penetrating power than those from the isotopes just considered. The available specific activities are not very high but are adequate for the applications which can be foreseen. 

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