Water proton relaxivity

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) ... 

Carbon nanotubes have been also used as a macromolecular scaffold for Gdm complexes. An amphiphilic gadolinium(III) chelate bearing a C16 chain was adsorbed on multiwalled carbon nanotubes (264). The resulting suspensions were stable for several days. Longitudinal water proton relaxivities, r] showed a strong dependence on the GdL concentration, particularly at low field. The relaxivities decreased with increasing field as predicted by the SBM theory. Transverse water proton relaxation times, T2, were practically independent of both the frequency and the GdL concentration. An in vivo feasibility MRI study has been... 

Relaxivity is the term given to a contrast agent s characteristic potency to decrease the nuclear spin relaxation times of water protons. Relaxivity is defined by the following relationship ... 

Lauffer RB (1987) Paramagnetic metal complexes as water proton relaxation agents for NMR imaging theory and design. Chem. Rev. 87 901-927. 

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).

Fig. 7. H water proton relaxivity i.e., the nuclear spin-lattice relaxation rate per mM of metal, plotted as a function of the magnetic field strength expressed as the proton Larmor frequency for aqueous solutions of manganese(H) and iron(HI) ions at 298 K. (A) 0.10 mM manganese(II) chloride in 2.80 M perchloric acid (B) 0.1 mM aqueous manganese(H) chloride at pH 6.6 (C) 0.5 mM iron(HI) perchlorate in 2.80 M perchloric acid (D) 0.5 mM iron(IH) perchlorate in water at pH 3.1 (F) 2.0 mM Fe(HI) in 2.0 M ammonium fluoride at pH 7, which causes a distribution of species dominated by [FeFe]"-.

Fig. 5.6. Water H NMRD profiles for a solution of methemoglobin ( ) and fluoro-methe-moglobin ( ) at 279 K. In the latter case, fast exchange is responsible for water proton relaxation enhancements which are quenched by slow exchange in the former case [12].

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]. 

In the 9-coordinate TTHA complexes of the heavier Ln3+ ions, the situation is more complex, since there also the terminal N-atom bearing the uncoordinated acetate moiety is chiral. 170 NMR [49, 50], luminescence [47, 51] and NMRD measurements [46] have shown that, for both 9- and 10-coordinate Ln(TTHA)3 complexes, the inner coordination sphere of the metal ion is fully occupied by donating groups of the ligand, leaving no space for the coordination of water. Consequently, the water proton relaxation enhancement has no inner sphere contribution and the [Gd(TTHA)]3 complex is not very suitable for application... 

NMRD studies of Gd3+ complexes of DOTPME and DOTPMB indicate q< 1 suggesting that the inner coordination sphere of these complexes is obstructed due to the steric encumbrance of the alkoxy substituents [ 104]. In a multinuclear NMR study of Ln3+ complexes (Ln = La, Gd, Dy, Tm and Yb) with a fluorinated ethyl ester analog of DOTP (F-DOTPME), the 19F NMR spectra reveal up to 16 resonances, which demonstrate that these complexes exist in aqueous solution as a mixture of stereoisomers [105]. [Gd(F-DOTPME)] afforded a water proton relaxivity typical of non-hydrated complexes. 170 NMR of the Dy 1 complex confirmed the lack of a bound water molecule. 

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