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.