Proton exchange rates effect

It is clear from the above equations that numerous parameters (proton exchange rate, kcx = l/rm rotational correlation time, tr electronic relaxation times, 1 /rlj2e Gd proton distance, rGdH hydration number, q) all influence the inner-sphere proton relaxivity. Simulated proton relaxivity curves, like that in Figure 3, are often used to visualize better the effect of the... 

The four examples given above illustrate the effect of water exchange or more exactly proton exchange rate and of the number of water molecules in the first coordination sphere of the paramagnetic metal ion on the water proton relaxation rate. Nonetheless, the relaxivity of a contrast agent can also be strongly... 

Ekiel, I., Banville D., Shen, S.H., and Gehring, K. (1998) Effect of peptide binding on amide proton exchange rates in the PDZ2 domain from human phosphatase hPTPlE, Biochemistry and Cell Biology 76, 334-340. 

Porphyrin inner proton exchange. Ribo and co-workersreported a study of the effect of steric factors due to bridge substitution on the modulation of proton exchange rates in a water soluble porphyrin, 161. The authors utilized both HSQC and... 

Woessner [56] also considered the effects of proton exchange on the dipolar oscillations arising from oriented water in chabazite and vermiculite. This paper involves a nice application of the density matrix for calculation of the effect of exchange on the FID. Comparisons are made between the proton exchange rate measured for interlayer water and pure water, and it was found that the rate was very dependent on the clay structure and cation involved. Fripiat reviewed the area of proton exchange on acid catalysts in 1976 [59]. The findings of Woessner [56] are not in complete agreement with conclusions reached in this review, which assert that dissociation of water is more pronounced in layered materials. Woessner s results seem to indicate that this is not necessarily the case. 

This possibility is supported by the effect on the proton exchange rate of the base remaining in solution after 1 is formed (BUjP makes exchange faster than does EtjN). A similar effect (of counterion on H/D exchange rate) was reported by Berke and coworkers with Re(NO)(CO)L2Hj in ref 15. 

The hydrogen-deuterium exchange rates for 1,2-dimethylpyrazolium cation (protons 3 and 5 exchange faster than proton 4 Section 4.04.2.1.7(iii)) have been examined theoretically within the framework of the CNDO/2 approximation (73T3469). The final conclusion is that the relative reactivities of isomeric positions in the pyrazolium series are determined essentially by inductive and hybridization effects. 

The enol can be observed by NMR spectroscopy and at —20°C has a half-life of several hours. At -1-20°C the half-life is only 10 minutes. The presence of bases causes very r id isomerization to acetaldehyde via the enolate. Solvents have a significant effect on the lifetime of such unstable enols. Solvents such as DMF and DMSO, which are known to slow rates of proton exchange by hydrogen bonding, increase the lifetime of unstable enols. ... 

Complete exchange of protons in a sterically unhindered position a to a carbonyl group can be achieved by heating a solution of the ketone in O-deuterated solvents in the presence of an acid or base catalyst, the latter being the more effective. The most commonly used solvents are methanol-OD, ethanol-OD, and the aprotic solvent anhydrous tetrahydrofuran or dioxane mixed with deuterium oxide. Under alkaline conditions the exchange rate in 153 2 14,164 stcroids, for example, is usually... 

Figure 3 Effect of the water exchange rate, kex, and the rotational correlation time, rR, on inner-sphere proton relaxivity. The plot was simulated for a particular value of the longitudinal electron spin relaxation rate, 1/Tie — 5.28xlOss 1. The marketed contrast agents all have relaxivities around 4—5mM 1s 1 in contrast to the theoretically attainable values over lOOrnM-1 s 1, and this is mainly due to their fast rotation...

---