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]∙7H₂O (Zr-K 1:2), (Et₂NH₂)₈[{α-PW₁₁O₃₉Zr-(μ-OH) (H₂O)}₂]∙7H₂O (Zr-K 2:2), Wells-Dawson K₁₅H[Zr(α₂-P₂W₁₇O₆₁)₂]·25H₂O (Zr-WD 1:2), Na₁₄[Zr₄(α-P₂W₁₆O₅₉)₂(μ₃-O)₂(μ-OH)₂(H₂O)₄]·57H₂O (Zr-WD 4:2) and Lindqvist (Me₄N)₂[ZrW₅O₁₈(H₂O)₃] (Zr-L 1:1), (nBu₄N)₆[(ZrW₅O₁₈(μ-OH))₂]∙2H₂O (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), ³¹P-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 ³¹P-NMR spectroscopy in the presence and absence of transferrin, providing insight into stability of the catalysts under reaction condition.