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 (Et(2)NH(2))(10)[Zr(PW(11)O(39))(2)]∙7H(2)O (Zr-K 1:2), (Et(2)NH(2))(8)[{α-PW(11)O(39)Zr-(μ-OH) (H(2)O)}(2)]∙7H(2)O (Zr-K 2:2), Wells-Dawson K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O (Zr-WD 1:2), Na(14)[Zr(4)(α-P(2)W(16)O(59))(2)(μ(3)-O)(2)(μ-OH)(2)(H(2)O)(4)]·57H(2)O (Zr-WD 4:2) and Lindqvist (Me(4)N)(2)[ZrW(5)O(18)(H(2)O)(3)] (Zr-L 1:1), (nBu(4)N)(6)[(ZrW(5)O(18)(μ–OH))(2)]∙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), (31)P-NMR spectroscopy, in order to gain better understanding of different efficiency of investigated Zr-POMs. A tryptophan fluorescence quenching study revealed that the most reactive Zr-WD species 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.