Spectroscopic Study of the Interaction between Horse Heart Myoglobin and Zirconium(IV)-Substituted Polyoxometalates as Artificial Proteases

A recent study [Angew. Chem. Int. Ed. 2015, 54, 7391-7394] has shown that horse heart myoglobin (HHM) is selectively hydrolyzed by a range of zirconium(IV)-substituted polyoxometalates (POMs) under mild conditions. In this study, the molecular interactions between the Zr-POM catalysts and HHM are investigated by using a range of complementary techniques, including circular dichroism (CD), UV/Vis spectroscopy, tryptophan fluorescence spectroscopy, and H-1 and (PNMR)-P-31 spectroscopy. A tryptophan fluorescence quenching study reveals that, among all examined Zr-POMs, the most reactive POM, 2:2 Zr-IV-Keggin, exhibits the strongest interaction with HHM. (PNMR)-P-31 spectroscopy studies show that this POM dissociates in solution, resulting in the formation of a monomeric 1:1 Zr-IV-Keggin structure, which is likely to be a catalytically active species. In the presence of Zr-IV-POMs, HHM does not undergo complete denaturation, as evidenced by CD, UV/Vis, tryptophan fluorescence, and (HNMR)-H-1 spectroscopy. CD spectroscopy shows a gradual decrease in the -helical content of HHM upon addition of Zr-IV-POMs. The largest effect is observed in the presence of a large Zr-IV-Wells-Dawson structure, whereas small Zr-IV-Lindqvist POM has the least influence on the decrease in the -helical content of HHM. In all cases, the Soret band at =409nm is maintained in the presence of all examined Zr-POMs, which indicates that no conformational changes in the protein occur near the heme group.