Polyoxometalates (POMs) are early-transition metal-oxygen anionic clusters. Specifically, their promising anti-bacterial, antiviral, anti-tumor, anti-cancer and anti-HIV activities  arouse our interest. Two representative molecules of this class of chemicals, the wheel-shaped [NaP₅W₃₀O₁₁₀]^14- (P5W30) and the Keggin-type anion [H₂W₁₂O₄₀]^6- (H2W12) are shown to interact with the fatty acid-free human serum albumin (HSA). The excited state lifetime of the single tryptophan molecule of this protein is dramatically decreased by the binding. The quenching mechanism is found to constitute the first example of energy transfer between HSA and POMs. Circular Dichroism (CD) was used to assess the structural effects of POM binding on HSA and to confirm the interaction revealed by fluorescence studies. CD experiments showed that the two POMs have different effects on the secondary structure of the protein. Binding the P5W30 partially unfolds the protein whereas the H2W12 has no remarkable effect on the structure of the protein.

The results constitute a helpful guide in an attempt to unravel the mechanism of POM-protein interactions. They might also be meaningful in subsequent design of polyoxometalate-based inorganic drugs.

Acknowledgment: This work was supported by the CNRS (UMR 8000 and 8113), the Université Paris-Sud 11, Ecole Normale Supérieure de Cachan and INSERM U759/Institut Curie-Recherche and European Community for Marie Curie Incoming Fellowship (Contract NO.040487)