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-asparty l-lysine-derived DTPA, [GdL(H2O)](2-), binds to serum albumin in vivo, thro ugh hydrophobic interaction. A variable temperature O-17 NMR? EPR, and Nucl ear Magnetic Relaxation Dispersion (NMRD) study resulted in a water exchang e 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 an d electronic relaxation. The effect of albumin binding on the water exchang e rate has been directly evaluated by O-17 NMR. Contrary to expectations, t he 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-boun d GdL complex, the analysis of its NMRD profile at 35 degrees C yielded a r otational correlation time of 1.0 ns, one order of magnitude shorter than t hat of the whole protein. This value is supported by the longitudinal O-17 relaxation rates. This indicates a remarkable internal flexibility, probabl y due to the relatively large distance between the protein- and metal-bindi ng moieties of the ligand.