Abstract
The interaction between human serum albumin (HSA) and a series of Zr(IV)-substituted polyoxometalates (POMs) (Lindqvist type POM ((nBu4N)6[{W5O18Zr (μ-OH)}2]·2H2O, Zr2-L2), two Keggin type POMs ((Et2NH2)10[Zr(PW11O39)2]·7H2O, Zr1-K2 and (Et2NH2)8[{α-PW11O39Zr(μ-OH)(H2O)}2]·7H2O, Zr2-K2), and two Wells-Dawson type POMs (K15H[Zr(α2-P2W17O61)2]·25H2O, Zr1-WD2 and Na14[Zr4(P2W16O59)2(μ3-O)2(OH)2(H2O)4]·10H2O, Zr4-WD2) was investigated by tryptophan (Trp) fluorescence spectroscopy. The fluorescence data were analyzed using the Tachiya model, ideally suited for multiple binding site analysis. The obtained quenching constants have the same order of magnitude for all the measured POM:protein complexes, ranging from 1.9 × 105 M−1 to 5.1 × 105 M−1. The number of bound POM molecules to HSA was in the range of 1.5 up to 3.5. The influence of the ionic strength was studied for the Zr1-WD2:HSA complex in the presence of NaClO4. The calculated quenching constant decreases upon increasing the ionic strength of the solution from 0.0004 M to 0.5004 M, indicating the electrostatic nature of the interaction. The number of POM molecules bound to HSA increases from 1.0 to 4.8. 31P NMR spectroscopy provided evidence for the stability of all investigated POM structures during the interaction with HSA.
Highlights
Amino acids, the individual building blocks of proteins, are linked via peptide bonds, which have an extremely high stability towards hydrolysis
Protein hydrolysis is of prime importance for a broad range of proteomics techniques ranging from protein sequencing to protein identification [3,4,5]
A series of Zr(IV)-substituted POMs, which were previously proven to be hydrolytically active towards human serum albumin (HSA), were investigated with tryptophan (Trp) fluorescence spectroscopy in order to gain insight into the interaction between the POMs and HSA, which precedes the actual protein hydrolysis
Summary
The individual building blocks of proteins, are linked via peptide bonds, which have an extremely high stability towards hydrolysis. Key components that influence the interaction between POMs and proteins include the size, shape and charge of the POM as well as the kind of incorporated metal ion [30,32,33,34,35,36,37,38,39] It was previously reported by our research group that isopolyoxomolybdates [40,41,42,43], isopolyoxovandates [44,45] and metal-substituted POMs [46] are able to efficiently hydrolyze the phopho(di)ester bonds in DNA and RNA model compounds. A series of Zr(IV)-substituted POMs, which were previously proven to be hydrolytically active towards HSA, were investigated with tryptophan (Trp) fluorescence spectroscopy in order to gain insight into the interaction between the POMs and HSA, which precedes the actual protein hydrolysis. The quenching of Trp214 fluorescence by Zr1-K2, Zr2-K2, Zr2-L2, Zr1-WD2, and Zr4-WD2 was investigated at physiological pH (pH 7.4) and room temperature to quantify the strength of the binding and identify the stoichiometry of this interaction
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