Speciation of Transition-Metal-Substituted Keggin-Type Silicotungstates Affected by the Co-crystallization Conditions with Proteinase K

[Image: see text] We report on the synthesis of the tetrasubstituted sandwich-type Keggin silicotungstates as the pure Na salts Na(14)[(A-α-SiW(10)O(37))(2){Co(4)(OH)(2)(H(2)O)(2)}]·37H(2)O (Na{SiW(10)Co(2)}(2)) and Na(14)[(A-α-SiW(10)O(37))(2){Ni(4)(OH)(2)(H(2)O)(2)}]·77.5H(2)O (Na{SiW(10)Ni(2)}(2)), which were prepared by applying a new synthesis protocol and characterized thoroughly in the solid state by single-crystal and powder X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Proteinase K was applied as a model protein and the polyoxotungstate (POT)–protein interactions of Na{SiW(10)Co(2)}(2) and Na{SiW(10)Ni(2)}(2) were studied side by side with the literature-known K(5)Na(3)[A-α-SiW(9)O(34)(OH)(3){Co(4)(OAc)(3)}]·28.5H(2)O ({SiW(9)Co(4)}) featuring the same number of transition metals. Testing the solution behavior of applied POTs under the crystallization conditions (sodium acetate buffer, pH 5.5) by time-dependent UV/vis spectroscopy and electrospray ionization mass spectrometry speciation studies revealed an initial dissociation of the sandwich POTs to the disubstituted Keggin anions H(x)Na(5–x)[SiW(10)Co(2)O(38)](3–) and H(x)Na(5–x)[SiW(10)Ni(2)O(38)](3–) ({SiW(10)M(2)}, M = Co(II) and Ni(II)) followed by partial rearrangement to the monosubstituted compounds (α-{SiW(11)Co} and α-{SiW(11)Ni}) after 1 week of aging. The protein crystal structure analysis revealed monosubstituted α-Keggin POTs in two conserved binding positions for all three investigated compounds, with one of these positions featuring a covalent attachment of the POT anion to an aspartate carboxylate. Despite the presence of both mono- and disubstituted anions in a crystallization mixture, proteinase K selectively binds to monosubstituted anions because of their preferred charge density for POT–protein interaction.