A Two-Step Binding Process of Eu-Containing Polyoxometalates to Bovine Serum Albumin

Polyoxometalates (POMs) represent a type of typical polyanionic nanoclusters that can be utilized as inorganic bioactive materials; however, the detailed interactions of them with many target biomolecules such as peptides and proteins were not well clarified due to the complexity of the binding process. In the present study, the binding-induced physiochemical phenomena of a highly charged Eu-containing polyoxometalate, K13[Eu(SiW9Mo2O39)2] (EuSiWMo), with a model protein, bovine serum albumin (BSA), was identified upon the examination of luminescence of both the components during the titration. The large emission enhancement and subsequent quenching of the EuSiWMo were found in close relation to the amount of added BSA. Being different from the known binding type of less charged POMs, a distinct two-step binding process was concluded, and the possible mechanism was proposed through the analysis on the time-resolved fluorescence spectra, isothermal titration calorimetry (ITC), transmission electron microscope (TEM), and two-dimensional correlation spectroscopy (2D COS). The present results directed a new understanding for the charge numbers and existing state of POMs affecting the interaction with proteins, which is important to exploit the biological functionalities of POMs in related systems and the development of POMs as potential inorganic drugs.