The binding of Sc3+ to chicken ovotransferrin has been investigated by Sc-45 and C-13 NMR spectroscopy. In the presence of carbonate, one observes two Sc-45 and C-13 signals which can be assigned using the proteolytic half-molecules of ovotransferrin to bound Sc3+ and (CO32-)-C-13 in both metal ion binding sites of the protein. When the synergistic anion is changed to oxalate, two overlapping Sc-45 resonances are again detected. Several properties of the transferrin-bound Sc-45 signals, such as their dependence on pulse length, magnetic field, protein size, and temperature, are consistent with the detection of only the central (m = 1/2 --> -1/2) transition of a quadrupolar nucleus under far from extreme narrowing conditions. From Sc-45 chemical shift and line width data for the Sc3+/carbonate form of ovotransferrin at four magnetic fields, we have calculated values for the quadrupole coupling constant ((chi)) and rotational correlation time (tau(c)) for the bound metal ion in each site of the protein. In addition, from chemical shift information at two fields, we have obtained estimates of chi for the Sc3+/oxalate form of ovotransferrin, as well as for the Sc3+/carbonate derivative of human serotransferrin. The results in each case are comparable to the chi values we have determined for two octahedral Sc3+ organometallic complexes. From the chi data, we have calculated values for the electric field gradient (eq(ionic)) at the metal nucleus for transferrin-bound Sc3+, by taking into account the nuclear quadrupole moment for Sc-45 and the Sternheimer antishielding factor for Sc3+. These results are compared to our previous Al-27 NMR data for the analogous Al3+ forms of the transferrins [Aramini, J. M.; Vogel, H. J. J. Am. Chem. Sec. 1993, 115, 245-252. Aramini, J. M.; Germann, M. W.; Vogel, H. J. J. Am. Chem. Sec. 1993, 115, 9750-9753]. This report represents the first Sc-45 NMR study of a metalloprotein and is another example of the applicability of quadrupolar metal ion NMR to the investigation of metal ion binding sites in large proteins.