Tris-functionalized hybrid Anderson polyoxometalates: synthesis, characterization, hydrolytic stability and inversion of protein surface charge

Abstract

Single- and double-sided functionalized hybrid organic-inorganic Anderson polyoxomolybdates with Ga(III) and Fe(III) positioned as central heteroatoms have been synthesized in a mild, two-step synthesis in an aqueous medium. Compounds 1-4 were isolated as hydrated salts, [TBA]3[GaMo6O18(OH)3{(OCH2)3CCH2OH}]×12 H2O (1) (TBA = tetrabutylammonium), Na3[FeMo6O18{(OCH2)3CCH2OH}2]×11 H2O (2), [TMA]2[GaMo6O18(OH)3{(OCH2)3CNH3}]×7 H2O (3) (TMA = tetramethylammonium), and Na[TMA]2[FeMo6O18(OH)3{(OCH2)3CNH3}](OH)×6 H2O (4). All the compounds were characterized based on single-crystal X-ray diffraction (SXRD), FTIR, UV/Vis, thermogravimetric, ESI-MS, NMR, and elemental analyses. Compound 1 was also crystallized with two smaller organic cations, giving [TMA]3[GaMo6O18(OH)3{(OCH2)3CCH2OH}]×n H2O (5) and [GDM]3[GaMo6O18(OH)3{(OCH2)3CCH2OH}]×n H2O (6) (GDM = guanidinium) and were characterized based on UV/Vis, NMR, FTIR, and elemental analyses. The use of these compounds as additives in macromolecular crystallography was investigated by examining their hydrolytic stability by using ESI-MS in a pH range of 4 to 9. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed that BSA remains intact in a solution containing up to 100 equivalents of 1 or 4 over more than four days at 20 °C. Zeta potential measurements demonstrate that 1-4 induce charge inversions on the positively charged surface of BSA (1 mg mL(-1)) with concentrations starting as low as 1.29 mM for compounds 1 and 2, which have the highest negative surface charge.

Keywords: POM-protein interactions; crystal growth; organic-inorganic hybrid composites; polyoxometalates.