Water exchange, rate

Table 4. Water Exchange Rates and Activation Parameters of Hexaaqua Complexes at 25°C, ...

P56 0.58 Computed from the P content of surface layer given in Table 14-3 and a water exchange rate between surface and deep ocean of 2 m/yr (Broecker, 1971)... 

HzPO . The values are 0.24, 0.21, 0.16 and 0.13, respectively. The values span a range of a factor of two which must be admitted to be a little larger than the experimental uncertainty and also easily within the differences among the anions in their probability of occupancy of the crucial outer sphere site adjacent to the leaving water molecule. All are nearly a factor of five below the water exchange rate. These results conform neatly to the predictions. 

Fig. 1. Mean lifetimes of a single water molecule in the first coordination sphere of a given metal ion, th2o> and the corresponding water exchange rate constants, h2o- The tall bars indicate directly determined values, and the short bars indicate values deduced from ligand substitution studies. References to the plotted values appear in the text.

Fig. 13. Absolute partial molar volumes, Vab8°, of [Ln(H20) P in aqueous LnCl3 solutions (301) (closed circles), compared with the calculated Vabs° values (4, 42) for [Ln(H20)8]3+ and [Ln(H20)9]3 indicated by the upper and lower solid curves, respectively. Interchange rate constants, kj (298 K) (310), for the substitution of S042 on [Ln(H20) ]3+ are shown as open squares, and water exchange rate constants, fcn2o (298 K) (311, 312), for [Ln(H20)8]3+ are shown as open circles. 

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...

Table 1 Water-exchange rates and activation volumes determined on small molecular weight Gd111 chelates...

The water exchange rate is hardly affected by substituents which do not directly interfere in the inner coordination sphere. Different bisamide DTPA derivatives have similar exchange rates.28-30 Substituents on the carbon backbone of DTPA also have little influence on the water-exchange kinetics.31 38 39... 

Table 2 Water exchange rates and activation volumes measured on macromolecular Gd111 chelates and on the corresponding monomer units by 170 NMR.

Figure 9 pH-sensitive paramagnetic liposomes at physiological pH and above, Gd(DTPA-BMA) is entrapped in the liposome and the relaxivity is low due to the low water exchange rate (/cex) through the liposome membrane. At lower pH, the liposome is disrupted and the relaxivity increases.172... 

Water Exchange Rates of Selected Monohydrated Gdln Complexes... 

Fig. 2. Calculated relaxivities as a function of the water exchange rate for various proton Larmor frequencies and rotational correlation times, tr. The simulations have been performed by using the common Solomon-Bloembergen-Morgan theory of paramagnetic relaxation.

We should note that the water exchange rate remains relatively constant on introducing different substituents on the ligand, which do not directly interfere in the inner coordination shell. Similar rates have been reported for the different bisamide DTPA derivatives (57,59,60,68). Even bulky substituents on the carbon backbone of the DTPA had limited influence on the water... 

Charge density is known to influence water exchange rates at a metal center (160). Since OH- is a stronger donor than OH2, this... 

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