Gadolinium complexes relaxation rates

Outer sphere relaxation arises from the dipolar intermolecular interaction between the water proton nuclear spins and the gadolinium electron spin whose fluctuations are governed by random translational motion of the molecules (106). The outer sphere relaxation rate depends on several parameters, such as the closest approach of the solvent water protons and the Gdm complex, their relative diffusion coefficient, and the electron spin relaxation rate (107-109). Freed and others (110-112) developed an analytical expression for the outer sphere longitudinal relaxation rate, (l/Ti)os, for the simplest case of a force-free model. The force-free model is only a rough approximation for the interaction of outer sphere water molecules with Gdm complexes. 

Relaxation times measured with isotropic Gd(III) chelates can also be incorporated into the fitting process . Changes in relaxation times vary only with the distance (1/r ) from the gadolinium ion. Therefore, relative relaxation rates of the nuclei of the substrate provide relative distances from the gadolinium ion. Incorporation of relaxation data into the fitting process requires that the geometry of the gadolinium-substrate complex be identical to that of the other lanthanide-substrate complex used to measure shift data, which must be assumed with care . ... 

D-xy/o-5-Hexulosonic acid has been shown by n.m.r. to exist predominantly in the P-furanose form in solution longitudinal relaxation rate data suggest that in the gadolinium(III) complex, the Gd(III) binds preferentially to the keto tautomer in a bidentate fashion involving carboxylate oxygen... 

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