title: Eu(III) luminescence and tryptophan fluorescence spectroscopy as a tool for understanding interactions between hen egg white lysozyme and metal-substituted Keggin type polyoxometalates.
abstract: The interaction between the lacunary Keggin K7PW11O39, the Eu(III)-substituted Keggin K4EuPW11O39 (Eu-Keggin) and the Ce(IV)-substituted Keggin [Me2NH2]10[Ce(PW11O39)2] (Ce-Keggin) polyoxometalates (POMs), and the proteins hen egg white lysozyme (HEWL) and the structurally homologous α-lactalbumin (α-LA) was studied by steady state and time-resolved Eu(III) luminescence and tryptophan (Trp) fluorescence spectroscopy. The excitation spectrum of Eu-Keggin at lower concentrations ([Eu-Keggin]<100 μM) is dominated by a ligand-to-metal charge transfer band (291 nm). For higher concentrations ([Eu-Keggin]>250 μM) the (5)L6←(7)F0 transition becomes the most intense peak. In the absence of protein, the number of coordinated water molecules to the Eu(III) centre of Eu-Keggin is 4, indicating a 1:1 Eu(III):POM species. In the presence of phosphate buffer this number linearly decreases from 4 to 2 upon increasing phosphate buffer concentration. Upon addition of HEWL, there are no coordinated water molecules, suggesting interaction between Eu-Keggin and the protein surface. In addition, this interaction results in a more than threefold increase of the hypersensitive (5)D0→(7)F2 transition for the Eu-Keggin/HEWL mixture. The calculated association constant amounted to 2.2×10(2) M(-1) for the Eu-Keggin/HEWL complex. Tryptophan fluorescence quenching studies were performed and the quenching constants were calculated to be 9.1×10(4) M(-1), 4×10(4) M(-1) and 4.1×10(5) M(-1) for the lacunary Keggin/HEWL, the Eu-Keggin/HEWL and the Ce-Keggin/HEWL complexes, respectively. The number of bound POM molecules to HEWL was 1.04 for the lacunary Keggin POM, and 1.0 for Eu-Keggin, indicating the formation of a 1:1 POM/HEWL complex. The value of 1.38 for Ce-Keggin might indicate a transition from 1:1 to 1:2 interaction.
authors: Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium.
authors: Vincent
authors: V
authors: Goovaerts
authors: Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium.
authors: Karen
authors: K
authors: Stroobants
authors: Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium.
authors: Gregory
authors: G
authors: Absillis
authors: Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Leuven, Belgium. Electronic address: tatjana.vogt@chem.kuleuven.be.
authors: Tatjana N
authors: TN
authors: Parac-Vogt
mesh_terms: D000818:Animals; D002021:Buffers; D002645:Chickens; D003903:Deuterium; D005063:Europium / Q000737:chemistry*; D005260:Female; D006868:Hydrolysis; D007768:Lactalbumin / Q000737:chemistry*; D049449:Luminescence; D009113:Muramidase / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry; D011485:Protein Binding; D013050:Spectrometry, Fluorescence; D014364:Tryptophan / Q000737:chemistry*; D017973:Tungsten Compounds / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D002021:Buffers; D010710:Phosphates; D017973:Tungsten Compounds; D014867:Water; D005063:Europium; D014364:Tryptophan; D007768:Lactalbumin; D003903:Deuterium; C421864:hen egg lysozyme; D009113:Muramidase
keywords: Artificial metalloproteases; Luminescence spectroscopy; Polyoxometalates; Tryptophan fluorescence