Slow water exchange

The low-spin t g [Ru(H20)6]3+ is four orders of magnitude more labile than the t g [Ru(H20)6]2+ and exchanges water by an Ia mechanism (125). The slow water exchange on both complexes allowed the direct measurement of the electron exchange of the [Ru(H20)e]3+/2+ couple in acidic solution (Eq. (10)) (133). 

In order to visualize the effects of water exchange, rotation and electronic relaxation as well as of magnetic field on proton relaxivity, we have calculated proton relaxivities as a function of these parameters (Fig. 2). The relaxivity maximum is attained when the correlation time, tc1, equals the inverse proton Lar-mor frequency (l/rcl = l/rR + l/rm + l/Tle = a>j). The most important message of Fig. 2 is that the rotational correlation time, proton exchange and electronic relaxation rates have to be optimized simultaneously in order to attain maximum relaxivities. If one or two of them have already an optimal value, the remaining parameter starts to become more limitative. The marketed contrast agents have relaxivities around 4-5 mM1 s 1 contrary to the theoretically attainable values over 100 mM 1 s1, which is mainly due to their fast rotation and slow water exchange. 

Gd(III) chelates, the relatively low relaxivity is the consequence of the flexibility of the linker group between the Gd(III) chelate and the rigid dendrimer molecule (slow water exchange is also limitative). Internal flexibility has been also proved for certain non-covalently bound Gd(III) chelate - protein adducts. The tr value determined for MP-2269 bound to bovine serum albumin is 1.0 ns, one order of magnitude lower than the rotational correlation time of the protein molecule [50]. 

The analysis of 170 NMR and NMRD data on Gd(D03A-monoamide) functionalized dendrimers (generations 5,4 and 3) as well as on Gadomer 17 has shown that, beside fast rotation, slow water exchange also limits proton relaxivity [43,106]. These results show that high molecular weight complexes like these... 

The tetrahedrally [77-81] coordinated beryllium(II) and the octahedrally [82-87] coordinated magnesium] 11) show relatively slow water exchange rates from the first coordination sphere and can be measured directly by NMR techniques (Table 4.5). The water exchange reaction on beryllium(ll) is characterized by the most negative activation volume observed for a water exchange process (AV = -13.6... 

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