Abstract
We report on the synthesis of the tetrasubstituted sandwich-type Keggin silicotungstates as the pure Na salts Na14[(A-α-SiW10O37)2{Co4(OH)2(H2O)2}]·37H2O (Na{SiW10Co2}2) and Na14[(A-α-SiW10O37)2{Ni4(OH)2(H2O)2}]·77.5H2O (Na{SiW10Ni2}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{SiW10Co2}2 and Na{SiW10Ni2}2 were studied side by side with the literature-known K5Na3[A-α-SiW9O34(OH)3{Co4(OAc)3}]·28.5H2O ({SiW9Co4}) 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 HxNa5–x[SiW10Co2O38]3– and HxNa5–x[SiW10Ni2O38]3– ({SiW10M2}, M = CoII and NiII) followed by partial rearrangement to the monosubstituted compounds (α-{SiW11Co} and α-{SiW11Ni}) 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.
Highlights
We report on the synthesis of the tetrasubstituted sandwich-type Keggin silicotungstates as the pure Na salts
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−
The speciation remained unchanged in water for Na{SiW10Ni2}2 and Na{SiW10Co2}2 after 1 week (Figure S13B and S14B), while additional signals attributed to monosubstituted anions HxNa3−x[SiW11MO39]3− (M = CoII and NiII, x = 1−3) at m/ z 912.1, 919.4, and 926.8 for the CoII representative and at m/ z 912.0, 919.3, and 926.7 for the NiII representative were detected in 100 mM NaOAc/AcOH
Summary
HxNa5−x[SiW10Co2O38]3− and the monosubstituted compounds (α-{SiW11Co} and α-{SiW11Ni}) after 1 week of aging. The speciation remained unchanged in water for Na{SiW10Ni2}2 and Na{SiW10Co2}2 after 1 week (Figure S13B and S14B), while additional signals attributed to monosubstituted anions HxNa3−x[SiW11MO39]3− (M = CoII and NiII, x = 1−3) at m/ z 912.1, 919.4, and 926.8 for the CoII representative and at m/ z 912.0, 919.3, and 926.7 for the NiII representative were detected in 100 mM NaOAc/AcOH (pH 5.5; Figures S13D and S14D) This is another indication of how a buffer affects the POM chemistry that is often overlooked.[20] ESI-MS studies indicate probable complete dissociation of the sandwich-type Na{SiW10M2}2 to the disubstituted monomeric species {SiW10M2}, followed by further rearrangement to the monosubstituted Keggin representatives α-{SiW11M} (M = CoII and NiII) in acetate buffer over 1 week (Figure 1B,C and Scheme S1). IR, TGA, SXRD, PXRD, UV/vis spectroscopy, ESI-MS, and protein crystallization (PDF)
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