SAXS data for free Gd-HPDO3A. SAXS curves at variable concentrations, normalized against concentration. The continuous lines represent fits with a modified hard-sphere model (equation S4): Gd-HPDO3A molecules are represented by homogeneous spheres of radius R that move freely in solution but cannot approach each other closer than a distance d between their centers. Insets: table with fit parameters and representations of an atomic Gd-HPDO3A model (PyMOL). 4.2 Å corresponds to the distance between opposite N atoms in the macrocycle.