Water proton relaxation rates

The efficiency of a paramagnetic chelate to act as a contrast agent is expressed by its proton relaxivity, ri or r2, referring to the paramagnetic enhancement of the longitudinal or transverse water proton relaxation rate, 1/T1 and 1/T2, respectively, by a unity concentration of the agent (ImM) ... 

The water proton NMRD of the pseudooctahedral Co(H20)g (reported in Fig. 13) shows almost field-independent water proton relaxation rate values in the 0.01-60 MHz region (47). Therefore, the (Os c = 1 a nd of course the co/Cc = l dispersions must occur at fields higher than 60 MHz. This provides an upper limit value for Tig equal to 4 x 10 s. Such a low Tig value is consistent with the low water proton relaxation rate values. By using the SBM theory, Tie at 298 K can be estimated to be about 10 s. It can be larger, if the presence of a probable static ZFS is taken into account (47). When measurements are performed in highly viscous ethyleneglycol the observed rates are similar to those obtained in water. This suggests that Tig is also similar and, therefore, it is rotation-independent (47). 

Fig. 25. Outer-sphere contribution to the water proton relaxation rate, calculated for a distance of closest approach, d, of 3 A and different values of S (1/2,1, 3/2, 2, 5/2).

Fig. 17. Plot of the pOi dependence of the longitudinal water proton relaxation rate of a 0.125 mM solution of MnTPPS (Chart 13) (20 MHz, 25°C and pH 7).

The central metal ion has nine coordination sites. It is attached to the three nitrogen atoms and to five carboxylate moieties (oxygen atoms). A single water molecule is able to coordinate at the vacant ninth site resulting in a strong enhancement of the water proton relaxation rate. The chelate can be described as a distorted capped square antiprism according to X-ray analysis [6]. 

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... 

Relaxation Measurements. Measurements of the magnetic fieldz dependence of the solvent water proton relaxation rate (T] l), i.e., nuclear magnetic relaxation dispersion (NMRD), were made by the field cycling method previously described (9,10). 

The binding of Gd3+ to Ca2+-ATPase was also examined using water proton nuclear relaxation rates. Figure 11 shows the behavior of the observed enhancement of the longitudinal water proton relaxation rate when Gd3+ is used to titrate a solution of the Ca2" "-ATPase. At the lower concentrations of Gd3" " the large observed enhancement of the water proton relaxation rate suggests the formation of a tight binary Gd3+-ATPase complex. 

Figure 11. Effect of Gd3 on the enhancement of the longitudinal water proton relaxation rate in solutions containing Ca2 -ATPase (24). The solutions contained 0.05M TMA-Pipes, pH 7.0, 0.24mM Ca2 -ATPase, and the noted concentrations of GdCl.,. T = 23°C. The theoretical curve was fitted to the points by assuming that the sites were filled in the manner of the data of Figure 10 and assuming the enhancements described in the text.

In detailed studies of the frequency dependence of water proton relaxation rates in solutions of Cu(II)-transferrin and Fe(III)-transferrin, a water molecule was thought to be close enough to the metal ion (about 2 A for the proton—Fe distance) to be considered a ligand 46, 47). 

Water proton relaxation rate enhancement at 24.3 MHz for Gd3+-protein complexes. 

Aime, S., Ascenzi, R, Comoglio, E., et al. (1995) Molecular recognition of R- and T-states of human adult hemoglobin by a paramagnetic Gd(II 1) complex by means of the measurement of solvent water proton relaxation rate. Journal of the American Chemical Society, 117, 9365-9366. 

The interactions of Mn2+ with the membrane-bound (Na++K+)-ATPase from sheep kidney medulla have been examined by kinetic and magnetic resonance techniques (80,81). EPR and water proton relaxation rate studies show that the enzyme binds Mn2+ at one tight binding site (Kd=0.88 /tM). Kinetic studies yield an activator constant for Mn2+ of 0.88 /M, identifying the one tight Mn2+ binding site as the active site of the ATPase. 

The measurements of water proton relaxation rates (PRR) as an NMR appli-... 

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