An alternative model that can be used to fit these profiles is considering the presence of solvent molecules that can be subjected to translational diffusion on the macromolecule surface. The contribution from the two-dimensional translational diffusion of these water molecules thus sums up to the contribution from water molecules rigidly reorienting by macromolecular tumbling (the contribution from three-dimensional translational diffusion of water molecules is expected to be negligible, due to its field dependence and the short diffusional correlation time, of the order of 30 ps). The profiles can thus be fitted using Eq. ( )