Matches function isotope

It is possible to specify the isotope of any atom in a SMILES string. This is generally not necessary because the most common isotope is simply assumed. But if, for example, a database contains information about 13C, this can be readily encoded into the SMILES using [13C] instead of simply C. The [13C] atom is considered different from the normal C atom in a SMILES. A direct lookup using canonical SMILES will not locate isotopes of the same structure. A substructure search using the matches function will locate isotopes. This is because the match function uses SMARTS to specify the desired substructure. 

Thermal ionization has three distinct advantages the ability to produce mass spectra free from background interference, the ability to regulate the flow of ions by altering the filament temperature, and the possibility of changing the filament material to obtain a work function matching ionization energies. This flexibility makes thermal ionization a useful technique for the precise measurement of isotope ratios in a variety of substrates. 

Figure 15 Isotope-ratio measurement precision as a function of the number of accumulated counts. Each measured value (circles) represents the relative standard deviation of 10 repetitions. The dotted line represents the precision predicted by counting statistics for a given number of accumulated counts. Because the measured values match the predicted values closely, it is confidently expected that longer integration times will yield even better precision (<0.01 %). (From Ref. 47.)...

Additional density functional calculations on orotate derivatives by Singleton et al. indicated that the major factor favoring decarboxylation via 04 protonation is likely an inherent preference for 04 protonation in uracil derivatives, rather than strong selective stabilization of the 04-protonated decarboxylated product and the transition state for its formation [28]. The authors also compared experimental (in solution without enzyme present) and theoretical kinetic isotope effects, finding that those computed for the 04 protonation pathway matched best the experimentally determined values. It was also noted, though, that differences between isotope effects measured for the carboxylate carbon in the uncatalyzed [28] and enzyme-catalyzed [29] decarboxylations may indicate that the mechanisms in these two environments differ considerably. More recent isotope effect calculations performed by Phillips and Lee [30], however, indicate that protonation of either 04 or 02 is consistent with the reported experimental isotope effects for the ODCase-catalyzed reaction [31]. 

A typical Mossbauer experiment thus involves an oscillating radioactive source that contains a parent isotope (e.g., "Co for Fe), a stationary absorber that is usually the sample, and a detector. The Mossbauer spectrum consists of a plot of y-ray counts (relative absorption) as a function of the velocity of the source. In the source the radioactive isotope feeds the excited state of the Mossbauer isotope, which decays to the ground state. The energy of the recoil-free emitted radiation is Doppler modulated. Resonant absorption occurs when the energy of the y-ray just matches the nuclear transition energy for a Mossbauer atom in the absorber. This is detected by the decreased... 

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