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

 The Gd(III) complex of 4-pentylbicyclo2.2.2]octane-1-carboxyl-di-l-aspartyl-lysine-derived DTPA, GdL(H₂O)]^2–, binds to serum albumin in vivo, through hydrophobic interaction. A variable temperature ¹⁷O NMR, EPR, and Nuclear Magnetic Relaxation Dispersion (NMRD) study resulted in a water exchange rate of k ²⁹⁸ _ex=4.2×10⁶ 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 ¹⁷O 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  °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 ¹⁷O 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. Received: 25 June 1998 / Accepted: 11 August 1998