H/D exchange experiments

The experiment was carried out with (i )-(-)-2[l-(methoxycarbonyl)ethyl]benzene-diazonium chloride (6.80). The product, methyl 3-methyl-3-//-indazole-3-carboxylate (6.81), was racemic. With regard to the inconclusive H/D exchange experiments one therefore has to conclude that the cyclization of the diazo-methylene intermediate 6.75 is faster than the rate of deuterium incorporation. 

The application of H/D exchange experiments in the identification of small molecules is of great importance as well, and has also been reported. The experiment is usually carried out using deuterium oxide... 

TABLE 8 MS Results of H/D Exchange Experiment Mass Spectrometer Was Operated at Negative Ion Mode... 

More recently, the reactivity of SZ has been assigned to its oxidizing ability,129-131 which should not be surprising because it has often been considered as SO3 adsorbed on zirconium oxide. However, that sulfated zirconia is not only an oxidant but also a strong protic acid has been demonstrated by Sommer, Walspurger, and co-workers132 on the basis H/D exchange experiments with neopentane. 

Details of the H/D exchange experiments are described elsewhere19. A downflow fixed-bed reactor (diameter of 4 mm) was loaded with -5-10 mg of the catalyst and dilluted with 60 mg of SiC>2, both with a sieve fraction of 90 < dp < 150 im. The basic catalysts (Pt/HT and Pt/LTL [K/A1>1] showed extremely low activities, and the reactor was loaded with 70-90 mg pure catalyst. 

Bleijlevens B, Faber BW, Albracht SP (1997) The [NiFe] hydrogenase from Allochromatium vinosum studied in EPR-detectable states H/D exchange experiments that yield new information about die structure of the active site. J. Bio. Inorg. Chem. 6 763—769... 

If the bases are in anti orientation, the C(8)H of purine and C(6)H of pyrimidine are located over the sugar and close enough to 0(50 that a (base) C-H 0(5 ) hydrogen bond can form, Fig. 17.6. Because the C(8)H and C(6)H hydrogen atoms have slightly acidic character as shown by H/D exchange experi-... 

A case of special interest in the trypsin H/D exchange experiment is the Ser54 hydroxyl bridged N-H 0 contact between peptide groups 55N-H and 43C=0 ... 

No reactions of 269 with correspondingly strong and sterically hindered bases and no H-D exchange experiments have so far been reported.165a In the paper published in 1926 by Hurtley and Smiles,m however, the authors described results that could be explained by deprotonation of the benzo-l,3-dithiolium ion (284) (which should be a weaker acid than 269, owing to the fused aromatic ring) to the benzodithiocarbene (285). 

To avoid the expensive use of deuterated solvents for H/D exchange experiments, Tolonen et al. [21] have described the postcolumn infusion of D2O to facilitate the LC-MS detection and identification of labile protons in a column eluant. Whilst acknowledging the potential limitations with respect to a reduced level of exchange, and hence sensitivity, compared to the use of deuterated mobile-phase solvents, they optimized the column effluent flow rate (via a splitting connector) with the infused D2O flow rate to enable the very useful determination of up to four labile protons. The method was exemplified by the differentiation of hydroxylated metabolites of the alkaloidal drugs imipramine and omeprazole (Figure 13.5) from the N-oxide and sulfone metabolites, respectively [21]. This was a differentiation that could not be achieved by high-resolution mass measurements. 

Tian, Z. Pawlow, A. Poutsma, J. C. Kass, S. R. Are carboxyl groups the most acidic sites in amino acids Gas-phase acidity, H/D exchange experiments, and computations on cysteine and its conjugate base, J. Am. Chem. Soc. 2007,129, 5403-5407. 

Base-catalysed H/D exchange experiments for a series of haloforms [80] (Table 4.10) demonstrate that carbanion formation is stabilised by halogen in the order 1 > Br > Cl > F. When these results are combined with acidity measurements which show that CF3H (pKa 31) is little more acidic than methane (pKa 40) [81], we can conclude that, in these systems, fluorine attached to a carbanion centre is stabilising with respect to hydrogen but destabilising compared with the effects of other halogens. Similar conclusions can be drawn from pKa measurements of a number of halobis(trifluoromethyl) methanes [82]. 

Koyanag and coworkers have determined the relative acidities of the cis/trans isomeric phenylthiocyclopropanecarboxylates 245 and 246, respectively, as compared to phenylcyclopropane carboxylate (247). H/D exchange experiments have shown that the ester 245 is 18.4 and 246 is 8.0 times more acidic than 247. Thus, a cis-phenylthio group stabilizes a negative charge much better than, for example, a phenyl group. 

Determination of the relative kinetic acidities of the 2- and 3-protons in thiophene (H/D exchange experiments) shows the 2-proton to be 500 times more acidic than the 3-proton [127]. However, when a sufficient amount of HMPT is added to a solution of 2-lithiothiophene, ring opening to LiC=C—CH=CH—SLi occurs, which means that under these conditions exchange between Li and the 3-proton can take place [128]. Selenophene [129] and tellurophene [130] have been lithiated with satisfactory results. We have the impression however, that a more careful control of the reaction conditions is required, and bases with a more moderate reactivity, such as EtLi LiBr in Et20, seem to give better results, especially in the case of tellurophene [9]. 

Taking into consideration the results of H/D exchange experiments, it is assumed that the adsorbate formed from CH3-CCI3 is an ethylidyne species C-CHj). 

Alkylpyridines undergo reactions analogous to benzene such as side-chain halogenation and oxidative functionalization (cf. p 291). In addition, C-H bonds directly attached to the heterocycle display a kinetic acidity which is greater by a factor of > 10 compared to the corresponding benzene derivatives. This is more pronounced in the 2- and 4-positions than in the 3-position. H/D-Exchange experiments of 2-, 3- and 4-picoline with a relative exchange rate of 130 1 1810 (MeOD/MeONa at 20°C, cf toluene 10" ) demonstrate this point. 

In some instances, with MS alone it is not possible to distinguish isomers or identify which part of the molecule has undergone modification in the transformation of the parent compound. Prior to MS analysis, derivatization of functional groups or H/D-exchange experiments can be performed. 

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