Interaction of Cm(III) with human serum albumin studied by time-resolved laser fluorescence spectroscopy and NMR

The complexation of Cm(III) with human serum albumin (HSA) was investigated using time-resolved laser fluorescence spectroscopy (TRLFS). The Cm(III) HSA species is dominating the speciation between pH 7.0 and 9.3. The first coordination sphere is composed by three to four H2O molecules and five to six coordinating ligands from the protein. For the complex formation at pH 8.0 a conditional stability constant of logK = 6.16 +/- 0.50 was determined. Furthermore, information on the Cm(III) HSA binding site were obtained. With increasing Cu(II) concentration the Cm(III) HSA complexation is suppressed whereas the addition of Zn(II) has no effect. This points to the complexation of Cm(III) at the N-terminal binding site (NTS) which is the primary Cu(II) binding site. NMR experiments with Cu(II), Eu(III) and Am(III) HSA show a decrease of the peak assigned to the His C2 proton of His 3, which is part of the NTS, with increasing metal ion concentration. This confirms the complexation of Eu(III) and Am(III) at the Cu(II) binding site NTS. The results presented in this study contribute to a better understanding of relevant biochemical reactions of incorporated actinides.