Molecular factors that determine Curie spin relaxation in dysprosium complexes

Dysprosium complexes can serve as transverse relaxation (T-2) agents for wa ter protons through chemical exchange and the Curie spin relaxation mechani sm. Using a pair of matched dysprosium(ill) complexes, Dy-L1 (contains one inner-sphere water) and Dy-L2 (no inner-sphere water), it is shown that the transverse relaxation of bulk water is predominantly an inner-sphere effec t. The kinetics of water exchange at Dy-L1 were determined by O-17 NMR. Pro ton transverse relaxation by Dy-L1 at high fields is governed primarily thr ough a large chemical shift difference between free and bound water. Dy-L1 forms a noncovalent adduct with human serum albumin which dramatically leng thens the rotational correlation time, tau (R), causing the dipole-dipole c omponent of the Curie spin mechanism to become significant and transverse r elaxivity to increase by 3-8 times that of the unbound chelate. These findi ngs aid in the design of new molecular species as efficient r(2) agents. Pu blished 2001 Wiley-Liss, Inc.(dagger).