Molecular interactions between serum albumin proteins and Keggin type polyoxometalates studied using luminescence spectroscopy

Abstract

The interaction between the plenary Keggin H3PW12O40, lacunary Keggin K7PW11O39 and the Eu(III)-substituted Keggin K4EuPW11O39 (Eu-Keggin) type polyoxometalates (POMs), and the proteins human and bovine serum albumin (HSA and BSA) was studied using steady state and time-resolved Eu(III) luminescence and tryptophan (Trp) fluorescence spectroscopy. The excitation spectrum of the Eu-Keggin POM is dominated by a ligand-to-metal charge transfer band at 291 nm. In the absence of proteins, the number of water molecules coordinated in the first coordination sphere of the Eu(III) center of Eu-Keggin was determined to be 4, indicating that Eu(III) occurs as a 1 : 1 isomer in solution. In the presence of HSA or BSA, the number of coordinated water molecules decreased to 0 and 1, respectively, suggesting interaction between the Eu-Keggin POM and the protein surface. As a result of this interaction, a five-fold increase of the hypersensitive (5)D0 → (7)F2 transition in the luminescence intensity was observed for the Eu-Keggin-HSA complex. The association constants were calculated to be 1.5 × 10(2) M(-1) and 2.0 × 10(3) M(-1) for the Eu-Keggin-HSA and Eu-Keggin-BSA complexes, respectively. Tryptophan fluorescence quenching studies were performed and the quenching constants were calculated using a Stern-Volmer analysis. The obtained values of the quenching constants were 6.1 × 10(4) M(-1) and 2.0 × 10(6) M(-1) for the Eu-Keggin-HSA and Eu-Keggin-BSA complexes, respectively. The surface map of both proteins shows that the cavity containing the tryptophan has a positive surface potential, providing a specific binding site at the surface of albumin proteins for the negatively charged POM.

Eu(III) luminescence and tryptophan fluorescence spectroscopy as a tool for understanding interactions between hen egg white lysozyme and metal-substituted Keggin type polyoxometalates

“…This resulted in a value of 2.2 × 10 2 M −1 for the Eu-Keggin POM/HEWL complex. This value is comparable to previously obtained values of association constants for the Eu-Keggin/HSA and Eu-Keggin/BSA complexes at slightly lower pH values (pH = 7.37 and 6.62, respectively) [51], which were 1.5 × 10 2 M −1 and 2.0 × 10 3 M −1 , respectively. When compared to the reported values for the Eu-Lindqvist POM, lower association constants were obtained for the Eu-Keggin POM [26,49].…”

“…1). This was also found in previous experiments [51]. Remarkably, q(Eu) decreases linearly when the concentration of Eu-Keggin POM is increased (Fig.…”

“…As previously reported [51], metal-substituted Keggin POMs can form different complexes in solution. Factors that influence the solution speciation of these complexes are for instance the pH and ionic strength of the solution and the concentration of the POM.…”

“…Factors that influence the solution speciation of these complexes are for instance the pH and ionic strength of the solution and the concentration of the POM. In our previous work, the characterization of the Eu-Keggin POM in solution was performed at a fixed concentration (100 μM) in the absence of buffer (pH = 6.25) [51]. Under these conditions the formation of a 1:1 Eu-Keggin complex was observed (Fig.…”

Gadolinium polytungstate nanoclusters: a new theranostic with ultrasmall size and versatile properties for dual-modal MR/CT imaging and photothermal therapy/radiotherapy of cancer

“…The resulting GdW 10 @BSA NCs were stored at room temperature for further experiments. [79][80][81] Characterizations of GdW 10 nanoclusters and GdW 10 -BSA The size distributions of the GdW 10 NCs and the GdW 10 @BSA NCs were measured on a Nicomp380 ZLS plus ZETADi. The X-ray photoelectron spectra of the GdW 10 NCs were obtained on a Kratos AXIS-165 surface analysis system.…”