Abstract
The Keggin-type polyoxometalate [γ-SiW10O36]8− was covalently modified to obtain a bis-biotinylated conjugate able to bind avidin. Spectroscopic studies such as UV-vis, fluorimetry, circular dichroism, coupled to surface plasmon resonance technique were used to highlight the unique interplay of supramolecular interactions between the homotetrameric protein and the bis-functionalized polyanion. In particular, the dual recognition mechanism of the avidin encompasses (i) a complementary electrostatic association between the anionic surface of the polyoxotungstate and each positively charged avidin subunit and (ii) specific host-guest interactions between each biotinylated arm and a corresponding pocket on the tetramer subunits. The assembly exhibits peroxidase-like reactivity and it was used in aqueous solution for L-methionine methyl ester oxidation by H2O2. The recognition phenomenon was then exploited for the preparation of layer-by-layer films, whose structural evolution was monitored in situ by ATR-FTIR spectroscopy. Finally, cell tracking studies were performed by exploiting the specific interactions with a labeled streptavidin.
Highlights
The preparation of bio-inorganic conjugates is currently investigated for the preparation of biosensors, metal-based antibiotics, radiopharmaceuticals, anti-cancer drugs, and imaging contrast agents (Orvig and Abrams, 1999; Barry and Sadler, 2013; Albada and Metzler-Nolte, 2016; Liu et al, 2016)
The bis-biotinylated POM has been obtained starting from the divacant decatungstosilicate K8[γ-SiW10O36] K-POM, which has been initially treated with aminopropyl triethoxysilane (APTES) and tetrabutylammonium bromide to yield the doubly functionalized amino-derivative3H[γ-SiW10O36 {NH2(CH2)3Si}2O] TBA-POM-NH2 (Scheme S1) (Carraro et al, 2006)
TBA-POM-biot2 has been characterized by 1H, 13C, 29Si, 183W NMR (CD3CN), FT-IR, ESI-MS (Figures S1–S6) and elemental analysis. 1H and 13C NMR signals confirm the presence of biotin signals, while heteronuclear (29Si and 183W, Figures S3, S4) NMR yield the typical signal patterns expected for a divacant Keggin structure decorated with a R-Si-O-Si-R tweezer-like motif,11 confirming the integrity of the POM scaffold after the post-functionalization with biotin
Summary
The preparation of bio-inorganic conjugates is currently investigated for the preparation of biosensors, metal-based antibiotics, radiopharmaceuticals, anti-cancer drugs, and imaging contrast agents (Orvig and Abrams, 1999; Barry and Sadler, 2013; Albada and Metzler-Nolte, 2016; Liu et al, 2016). The addition of dicyclohexylcarbodiimide (DCC) to a CH3CN/DMF solution of POM-NH2, biotin and N,Ndiisopropylethylamine at 0◦C, results in the formation of the biotinylated conjugate (nBu4N)3H[γ-SiW10O36{(C5H7N2OS) (CH2)4CONH(CH2)3Si}2O] (TBA-POM-biot2) (68% yield, Figure 1).
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