Direct assessment of water exchange on a Gd(III) chelate bound to a protein

The Gd(III) complex of 4-pentylbicyclo[2.2.2]octane-1-carboxyl-di-L-aspartyl-lysine-derived DTPA, [GdL(H2O)](2-), binds to serum albumin in vivo, through hydrophobic interaction. A variable temperature O-17 NMR? EPR, and Nuclear Magnetic Relaxation Dispersion (NMRD) study resulted in a water exchange rate of k(ex)(298) = 4.2 x 10(6) s(-1), and let us conclude that the GdL complex is identical to [Gd(DTPA)(H2O)](2-) in respect to water exchange and electronic relaxation. The effect of albumin binding on the water exchange rate has been directly evaluated by O-17 NMR. Contrary to expectations, the water exchange rate on GdL does not decrease considerably when bound to bovine serum albumin (BSA); the lowest limit can be given as k(ex,GdL-BSA) = k(ex,GdL)/2. In the knowledge of the water exchange rate for the BSA-bound GdL complex, the analysis of its NMRD profile at 35 degrees C yielded a rotational correlation time of 1.0 ns, one order of magnitude shorter than that of the whole protein. This value is supported by the longitudinal O-17 relaxation rates. This indicates a remarkable internal flexibility, probably due to the relatively large distance between the protein- and metal-binding moieties of the ligand.