Selective Hydrolysis of Transferrin Promoted by Zr-Substituted Polyoxometalates

The hydrolysis of the iron-binding blood plasma glycoprotein transferrin (Tf) has been examined at pH = 7.4 in the presence of a series of Zr-substituted polyoxometalates (Zr-POMs) including Keggin (Et2NH2)(10)[Zr(PW11O39)(2)]center dot 7H(2)O (Zr-K 1:2), (Et2NH2)(8)[{alpha-PW11O39Zr-(mu-OH) (H2O)}(2)]center dot 7H(2)O (Zr-K 2:2), Wells-Dawson K15H[Zr(alpha(2)-P2W17O61)(2)]center dot 25H(2)O (Zr-WD 1:2), Na-14[Zr-4(alpha-P2W16O59)(2)(mu(3)-O)(2)(mu-OH)(2)(H2O)(4)]center dot 57H(2)O (Zr-WD 4:2) and Lindqvist (Me4N)(2)[ZrW5O18(H2O)(3)] (Zr-L 1:1), (nBu(4)N)(6)[(ZrW5O18(mu-OH))(2)]center dot 2H(2)O (Zr-L 2:2)) type POMs. Incubation of transferrin with Zr-POMs resulted in formation of 13 polypeptide fragments that were observed on sodium dodecyl sulfate poly(acrylamide) gel electrophoresis (SDS-PAGE), but the hydrolysis efficiency varied depending on the nature of Zr-POMs. Molecular interactions between Zr-POMs and transferrin were investigated by using a range of complementary techniques such as tryptophan fluorescence, circular dichroism (CD),P-31-NMR spectroscopy, in order to gain better understanding of different efficiency of investigated Zr-POMs. A tryptophan fluorescence quenching study revealed that the most reactiveZr-WDspecies show the strongest interaction toward transferrin. The CD results demonstrated that interaction of Zr-POMs and transferrin in buffer solution result in significant secondary structure changes. The speciation of Zr-POMs has been followed by(31)P-NMR spectroscopy in the presence and absence of transferrin, providing insight into stability of the catalysts under reaction condition.