Water-soluble, isolable gold clusters protected by tiopronin and coenzyme a monolayers

Isolable, water-soluble gold clusters protected by monolayers of tiopronin (tiopronin-MPCs) or coenzyme A (CoA-MPCs) were synthesized by a procedure of comparable simplicity to the Brust synthesis for alkanethiolate monolayer-protected gold clusters. High-resolution transmission electron microscopy shows that, like their alkanethiolate-MPC counterparts, the average core diameters of tiopronin-MPCs can be systematically controlled by varying the tiopronin:Au mole ratio employed in the reaction. The UV-vis spectra of tiopronin-MPCs exhibit pH and core size dependency of the surface plasmon band position and intensity, respectively. Thermogravimetric analysis of the tiopronin-MPCs gave average numbers of tiopronin ligands per cluster; for example, tiopronin-MPCs with an average core size of 1.8 nm (∼Au₂₀₁) are protected with an average of 85 tiopronin ligands. ¹H NMR reveals a size-dependent evolution of spectral features interpreted as reflecting differences in attachment sites (terrace, defects) and/or restriction in ligand mobility. Infrared spectroscopy reveals strong hydrogen bonding in the monolayer and provides evidence for intercluster association, and acid/base titrations produce pK_A values similar to the free ligand in the presence of a charge-screening supporting electrolyte, but higher in its absence. The same analytical methods were also applied to CoA-MPCs.