Understanding the Regioselective Hydrolysis of Human Serum Albumin by Zr(IV)-Substituted Polyoxotungstates Using Tryptophan Fluorescence Spectroscopy

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.