Tris-Functionalized Hybrid Anderson Polyoxometalates: Synthesis, Characterization, Hydrolytic Stability and Inversion of Protein Surface Charge

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)(3)CCH2OH}]x12H(2)O (1) (TBA=tetrabutylammonium), Na-3[FeMo6O18{(OCH2)(3)CCH2OH}(2)]x11H(2)O (2), [TMA](2)[GaMo6O18(OH)(3){(OCH2)(3)CNH3}]x7H(2)O (3) (TMA=tetramethylammonium), and Na[TMA](2)[FeMo6O18(OH)(3){(OCH2)(3)CNH3}](OH)x6H(2)O (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)(3)CCH2OH}]xnH(2)O (5) and [GDM](3)[GaMo6O18(OH)(3){(OCH2)(3)CCH2OH}]xnH(2)O (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 100equivalents of 1 or 4 over more than four days at 20 degrees C. Zeta potential measurements demonstrate that 1-4 induce charge inversions on the positively charged surface of BSA (1mgmL(-1)) with concentrations starting as low as 1.29mM for compounds 1 and 2, which have the highest negative surface charge.