Protein-Like Large Gold Clusters Based on the ω-Aminothiolate DMAET: Precision Thermal and Reaction Control Leading to Selective Formation of Cationic Gold Clusters in the Critical Size Range, n = 130-144 Gold Atoms

The ubiquitous Au₁₄₄(SR)₆₀ and closely related compounds of -29 kDa (core mass) are large monolayer protected clusters (MPC) comprising 144 gold atoms and 60 thiolate ligands (RS groups). Since this cluster family was first identified in 1996, it has served innumerable investigations from physicochemical fundamentals in the quantum "critical size" regime to practical applications in biomedicine, using various phases appropriate to the R-group properties. Very recently, Wu and co-workers achieved an unambiguous determination of the virus-like chiral-icosahedral symmetry of its Au-S core- and surface-structure. Despite these enormous advances, certain practical considerations restricted its applicability. An extraordinary recent development is that ESI-MS allows all the coexisting species in solution to be precisely detected, revolutionizing research on smaller MPCs. The difficult electrospray ionization properties of the critical-size MPCs have so far precluded the full application of ESI-MS methods. Building on the advances by Ishida and co-workers in cationized (quat-terminated) R-groups, we have recently found a path to tertiary-amino thiolate-protected clusters with characteristic basic properties including pH-controlled ionization of terminal amino groups. Here we report the first extension of these methods beyond the small-cluster domain and into the critical-size regime. In particular, we disclose a precision route to captamine-protected Au₁₄₄(SR)₆₀ clusters - accompanied by small quantities of its Au₁₃₇ and Au₁₃₀ byproducts and indications of somewhat larger (plasmonic) species - that promises to overcome these challenges due to its amphiphilic nature and solubility in both aqueous and organic phase upon contact with acid or base in the solution. The ESI-MS analysis generates up to a 8+ charge state without any special counterions or ion-pairing agents.