Since the paramagnetic enhancements are linear with respect to the concentration of the paramagnetic centers, in the absence of concentration dependent changes in molecular structure and dynamics, it is common to define a quantity called relaxivity 𝑟 1 as the enhancement due to 0.001 mol dm -3 (1 mM) of paramagnetic centers in solution. Fig. shows the relaxivity profile of the protein L-asparaginase II, where a gadolinium(III)-DOTA derivative has been attached via amide bond formation. The relaxivity of this paramagnetic macromolecule is much larger than that measured for the free gadolinium complex and the profile shows a peak in the high-field region. These effects are due to a reorientation time 𝜏 𝑅 that is much longer for the macromolecule than for the Gd-DOTA complex, so that the correlation time for the electron-nucleus dipole-dipole interaction (Eq. 7), is largely determined by 𝜏 𝑅 for the gadolinium(III) complex, and mostly given by the electron relaxation time for the biomolecule. Contributions from both the water molecule coordinated to the gadolinium(III) ion and from freely diffusing water molecules are considered. The SBM model appeared however insufficient for a satisfactory fit of the relaxivity profile, which required to consider the presence of static zero-field splitting. The temperature dependence of the relaxivity profiles also showed that the coordinated water molecule is in slow exchange. )