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Proteins are biological molecules built up of amino acids, linked to each other by peptide bonds. Since proteins are large structures, it is often needed to cleave them into smaller fragments in order to be used in biochemical analysis techniques. In nature, the cleavage of the peptide bond (or hydrolysis) has a half-life of 350 to 600 years. Because of this slow cleavage reaction, enzymes and chemicals are used to accelerate this reaction. However because of some drawbacks, an alternative for peptide bond hydrolysis has to be found. The use of metal ions was often proposed although none of the artificial metalloproteases were able to compete with the existing proteases due to a lack of selectivity. Polyoxometalates (POMs), a tuneable and versatile class of metal-oxygen clusters, were proposed by our research group as ligands which can incorporate hydrolytically active metal ions. Some POMs are shown to interact electrostatically to the protein surface. Therefore, when a metal ion is incorporated into the structure, it can come closer to the peptide bonds inducing a more efficient hydrolysis reaction. A number of metal-substituted polyoxometalates, including CeIV-Keggin and ZrIV-Wells-Dawson, were investigated in the past and found to show reactivity towards peptide bond hydrolysis in oligopeptides and proteins. The electrostatic basis of the interaction was further examined in this study. The hydrolytic activity of two polyoxometalates (POMs) toward the hydrolysis of proteins was investigated at different pH and ionic strength values. A difference in pH is known to alter the charges on the protein surface. A lower pH induces the protein surface to carry more positive charges, while at a higher pH the protein surface becomes more negatively charged. Since the interaction of the negatively charged POM to the protein is electrostatic, the strength of the interaction alters upon altering protein charges. The addition of salt is known to interrupt electrostatic inter...