The magnetic field dependence of the water proton longitudinal relaxation rate for the paramagnetic complex Gd-BMA-DTPA has been measured and analyzed at different temperatures. Interestingly we have found that the solvent relaxivity is largely affected by the residence lifetime (ZM) of the water molecule in the coordination site on the metal chelate. The calculated values for ZM at 298 K is 0.81 ps, i.e. 4-fold longer than the values recently reported for Gd-DTPA and much longer than the values commonly assumed in the analysis of NMRD profiles of poly(amino carboxylate) complexes. This result has been also confirmed by an alternative approach for the structurally analogous Dy(III) complex. In this latter case, from the measurement of the paramagnetic contribution to 'H NMR chemical shift and transverse relaxation rate of the solvent water protons, we evaluated the residence lifetime ZM by solving iteratively the Swift-Connick equations.

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The x-ray crystal structure of the fluoride derivative of ferric sperm whale (Physeter catodon) myoglobin (Mb) has been determined at 2.5 A resolution (R = 0.187) by difference Fourier techniques. The fluoride anion, sitting in the central part of the heme distal site and coordinated to the heme iron, is hydrogen bonded to the distal His(64)E7 NE2 atom and to the W195 solvent water molecule. This water molecule also significantly interacts with the same HisE7 residue, which stabilizes the coordinated fluoride ion. Moreover, fluoride and formate binding to ferric Aplysia limacina Mb, sperm whale (Physeter catodon) Mb, horse (Caballus caballus) Mb, loggerhead sea turtle (Caretta caretta) Mb, and human hemoglobin has been investigated by 1H-NMR relaxometry. A strong solvent proton relaxation enhancement is observed for the fluoride derivatives of hemoproteins containing HisE7. Conversely, only a small outer-sphere contribution to the solvent relaxation rate has been observed for all of the formate derivatives considered and for the A. limacina Mb:fluoride derivative, where HisE7 is replaced by Val.

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Thc exchange rate of the coordinated water molecule in the neutral complex [Gd(DTPA-BBA)(H,O)] (DTPA-BBA = 1,7-bis[ (N-benzylcarbamoy1)methyll-I ,4,7-triazaheptane-l,4,7-triacetate or diethylenetriaminopentaacetate N,N'-bis(benzy1amide)) is slower than in the parent complex [Gd(DTPA)(H,0)I2-. From the analysis of the temperature dependence of the solvent 7O NMR transverse relaxation time in a n aqueous solution of the paramagnetic complex, a value of 4.5 x lo5 s -' (at 298 K) is obtained for the exchange rate of the coordinated water molecule. This rate constant does not vary in the pH range 7-12. Conversely, over the same pH range and a t 298K and 20 MHz, the longitudinal water proton relaxivity increases from 4.8 to 6.5 s-nlM-'. The analysis of the dependence of the longitudinal water proton relaxation rate on magnetic field and temperature at pH 7 and p H 12 shows that the increase in relaxivity at basic p H has Keywords gadolinium . NMR spectroscopyprototropic exchange -rare earth compounds water exchange rate