Relaxivity metal-centered dependence

The H NMRD profiles of Mn(OH2)g+ in water solution show two dispersions (Fig. 5.43). The first (at ca. 0.05 MHz, at 298 K) is attributed to the contact relaxation and the second (at ca. 7 MHz, at 298 K) to the dipolar relaxation. From the best fit procedure, the electron relaxation time, given by rso = 3.5 x 10 9 and r = 5.3 x 10 12 s, is consistent with the position of the first dispersion, the rotational correlation time xr = 3.2 x 10 11 s is consistent with the position of the second dispersion and is in accordance with the value expected for hexaaquametal(II) complexes, the water proton-metal center distance is 2.7 A and the constant of contact interaction is 0.65 MHz (see Table 5.6). The impressive increase of / 2 at high fields is due to the field dependence of the electron relaxation time and to the presence of a non-dispersive zs term in the equation for contact relaxation (see Section 3.7.2). If it were not for the finite residence time, xm, of the water molecules in the coordination sphere, the increase in Ri could continue as long as the electron relaxation time increases. 

As important as calcium is probably iron [122]. Iron is the metal center of many essential proteins and enzymes, such as hemoglobin, an oxygen carrier, or peroxidase, that oxidizes hydrogen peroxide, or even the large family of cytochromes, which act as electron transfer proteins in many important biochemical processes [85]. New families of MRI contrast agents have been designed such that their relaxivity is iron concentration dependent [128-130]. The two latest are based on Gd(III) chelates (Fig. 20) but differ by the mechanism responsible for their iron sensitivity and will be described further. 

Interactions of water protons account for an alteration of the local magnetic field surrounding a paramagnetic center. The observed relaxivity depends on the distance and time of these interactions and the translational diffusion. The interactions are classified as either inner-sphere, which describes protons of water molecules that are bound to the metal center, or outer-sphere, which describes bulk solvent molecules that experience a paramagnetic effect when they diffuse around the metal center. The diamagnetic contribution has a linear relationship... 

This mechanism has been modified later, proposing two main kinds of propagating species differing in whether the last formed double bond is still coordinated to the metal center or not. Furthermore, the stereochemistry of this coordinated bond must be considered. Hence species like the chiral Pc respectively Pt, P and the more relaxed achiral P are predicted (structure 35). Depending on the microstructure of the particularly formed polymer various of these species must be taken into account as chain carriers and different reaction pathways were proposed [284,290,291,302-304]. This proposal of such kinetically distinct propagating species provides also a rationale for the often recognized blocky distribution of cis and trans double bonds in ROM polymers. 

In addition to the standard constraints introduced previously, structural constraints obtainable from the effects of the paramagnetic center(s) on the NMR properties of the nuclei of the protein can be used (24, 103). In iron-sulfur proteins, both nuclear relaxation rates and hyperfine shifts can be employed for this purpose. The paramagnetic enhancement of nuclear relaxation rates [Eqs. (1) and (2)] depends on the sixth power of the nucleus-metal distance (note that this is analogous to the case of NOEs, where there is a dependence on the sixth power of the nucleus-nucleus distance). It is thus possible to estimate such distances from nuclear relaxation rate measurements, which can be converted into upper (and lower) distance limits. When there is more than one metal ion, the individual contributions of all metal ions must be summed up (101, 104-108). If all the metal ions are equivalent (as in reduced HiPIPs), the global paramagnetic contribution to the 7th nuclear relaxation rate is given by... 

Finally, two potential models for the Mo and Fe sites of sulfite oxidase have been prepared in which a hydrotris(3,5-dimethylpyrazolyl)borate complex of oxomolybdenum(V) is covalently attached to one of the phenyl rings of a tetraphenylporphinatoiron(III) derivative in which three phenyl rings carry / -Me groups and the other is either a 2,3- or 3,4-catecholate. Although the metals are not directly bridged in these complexes, they are held at fixed Mo - Fe distances. EPR spectroscopy at 4K shows distance-dependent static dipolar coupling between the Mo and Fe centers. At 77 K the dipolar coupling is modulated by rapid relaxation of the Fe center. Coordination of two A-methylimidazoles to the Fe centers produce S = l/2)Fe-(5 = 1/2)mo coupled systems that are not only... 

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