Complexation of Cm(iii) with the recombinant N-lobe of human serum transferrin studied by time-resolved laser fluorescence spectroscopy (TRLFS)

Abstract

The complexation of Cm(III) with the recombinant N-lobe of human serum transferrin (hTf/2N) is investigated in the pH range from 4.0 to 11.0 using TRLFS. At pH ≥ 7.4 a Cm(III) hTf/2N species is formed with Cm(III) bound at the Fe(III) binding site. The results are compared with Cm(III) transferrin interaction at the C-lobe and indicate the similarity of the coordination environment of the C- and N-terminal binding sites with four amino acid residues of the protein, two H₂O molecules and three additional ligands (e.g. synergistic anions such as carbonate) in the first coordination sphere. Measurements at c(carbonate)_tot = 0.23 mM (ambient carbonate concentration) and c(carbonate)_tot = 25 mM (physiological carbonate concentration) show that an increase of the total carbonate concentration suppresses the formation of the Cm(III) hTf/2N species significantly. Additionally, the three Cm(III) carbonate species Cm(CO₃)⁺, Cm(CO₃)₂⁻ and Cm(CO₃)₃³⁻ are formed successively with increasing pH. In general, carbonate complexation is a competing reaction for both Cm(III) complexation with transferrin and hTf/2N but the effect is significantly higher for the half molecule. At c(carbonate)_tot = 0.23 mM the complexation of Cm(III) with transferrin and hTf/2N starts at pH ≥ 7.4. At physiological carbonate concentration the Cm(III) transferrin species II forms at pH ≥ 7.0 whereas the Cm(III) hTf/2N species is not formed until pH > 10.0. Hence, our results reveal significant differences in the complexation behavior of the C-terminal site of transferrin and the recombinant N-lobe (hTf/2N) towards trivalent actinides.