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

Abstract

The ubiquitous Au 144 (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) Rgroups, 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 Au 144 (SR) 60 clustersaccompanied by small quantities of its Au 137 and Au 130 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.

Basic [Au25(SCH2CH2Py)18]−⋅Na+ Clusters: Synthesis, Layered Crystallographic Arrangement, and Unique Surface Protonation

“…These few works seemed to confirm a statement made in 2005, namely that small positively charged ligands do not support the production of monolayer protected clusters (MPCs) in the Brust synthesis . However, the amino‐terminated Au clusters would offer great advantages in 1) making MPCs behave like proteins, which envisions the possibility to analyze them using modern standard techniques utilized in molecular biology developed steadily over more than half a century, and 2) biological applications in which the amino moiety incorporated on the nanoparticle surface caters a long‐held belief that it yields intimate interactions with the negatively charged contents . In our recent works, the modified Brust method at an optimized reduction rate could successfully synthesize Au 25 clusters utilizing a SR ligand with the pendant quaternary ammonium group, −[N(CH 3 ) 3 ] + .…”

“…However, cystamine interacts with mercaptocarboxylic acids for stabilizing the Au 25 clusters and therefore pure basic cystamine‐capped Au 25 was not obtained. The latest reports by Whetten et al . released the earlier evidence of polydisperse captamino (−S(CH 2 ) 2 N(CH 3 ) 2 )‐capped Au clusters with atom numbers ranging from 25–144.…”

New Evidence of the Bidentate Binding Mode in 3-MBA Protected Gold Clusters: Analysis of Aqueous 13&ndash;18 kDa Gold-Thiolate Clusters by HPLC-ESI-MS Reveals Special Compositions Au<em>_n</em>(3-MBA)<em>_p</em>, (<em>n</em> = 48 &ndash; 67, <em>p</em> = 26-30)

“…If the procedure is varied--even slightly--to reduce this time period, a wider range of cluster sizes is formed, that is supported by the recent reported "captamino", a base side, thiolated gold clusters in the size range 25-144 number of Au, or even larger. 17,18 In the Figure 3…”

“…); for their facile modification via ligand exchange reactions; [10][11][12] for high-contrast detection, whether visual/optical or in X-ray and electron scattering; 13 and for the fascination and potential utility of their strongly size-dependent optical, electrical and structure-bonding properties. [14][15][16] By now, much evidence has accumulated to suggest that many of these MPCs may be obtained in high yield as pure macromolecular substances of definite composition [17][18] and structure-bonding characteristics, [19][20][21] as opposed to the more usual metal colloidal or nanoparticle 9 substances that often show heterogeneity. Such proven structural uniformity of MPCs is essential to precision-intensive applications, as well as to all fundamental physicochemical understanding.…”

New Evidence of the Bidentate Binding Mode in 3-MBA Protected Gold Clusters: Analysis of Aqueous 13–18 kDa Gold-Thiolate Clusters by HPLC-ESI-MS Reveals Special Compositions Aun(3-MBA)p, (n = 48–67, p = 26–30)

“…By now, much evidence has accumulated to suggest that many of these MPCs may be obtained in high yield as pure macromolecular substances of definite composition [17,18] and structure-bonding characteristics [19,20,21], as opposed to the more usual metal colloidal or nanoparticle [9] substances that often show heterogeneity. Such a proven structural uniformity of MPCs is essential to precision-intensive applications, as well as to all fundamental physicochemical understanding.…”