Abstract
Gold clusters protected by 3-MBA ligands (MBA = mercaptobenzoic acid, –SPhCO2H) have attracted recent interest due to their unusual structures and their advantageous ligand-exchange and bioconjugation properties. Azubel et al. first determined the core structure of an Au68-complex, which was estimated to have 32 ligands (3-MBA groups). To explain the exceptional structure-composition and reaction properties of this complex, and its larger homologs, Tero et al. proposed a “dynamic stabilization” via carboxyl O–H––Au interactions. Herein, we report the first results of an integrated liquid chromatography/mass spectrometer (LC/MS) analysis of unfractionated samples of gold/3-MBA clusters, spanning a narrow size range 13.4 to 18.1 kDa. Using high-throughput procedures adapted from bio-macromolecule analyses, we show that integrated capillary high performance liquid chromatography electrospray ionization mass spectrometer (HPLC-ESI-MS), based on aqueous-methanol mobile phases and ion-pairing reverse-phase chromatography, can separate several major components from the nanoclusters mixture that may be difficult to resolve by standard native gel electrophoresis due to their similar size and charge. For each component, one obtains a well-resolved mass spectrum, nearly free of adducts or signs of fragmentation. A consistent set of molecular mass determinations is calculated from detected charge-states tunable from 3− (or lower), to 2+ (or higher). One thus arrives at a series of new compositions (n, p) specific to the Au/3-MBA system. The smallest major component is assigned to the previously unknown (48, 26); the largest one is evidently (67, 30), vs. the anticipated (68, 32). Various explanations for this discrepancy are considered. A prospective is given for the various members of this novel series, along with a summary of the advantages and present limitations of the micro-scale integrated LC/MS approach in characterizing such metallic-core macro-molecules, and their derivatives.
Highlights
Using procedures adapted from oligonucleotide analyses, we show that integrated capillary HPLC-electrospray ionization (ESI)-MS, based on aqueous-methanol mobile phases and ion-pairing reverse-phase chromatography, can separate at least two major components that are present in all sources
The size-uniform samples prepared at the University of Texas at San Antonio by Germán Placencia-Villa (GPV) for this work are synthesized according to a modified Brust-Schiffrin approach described elsewhere [1]
Efforts were focused to determine whether the 3-mercaptobenzoic acid (MBA)/Au systems were amenable to analysis by HPLC-ESI-MS in the same way as previously observed for the analogous
Summary
This work on the 3-MBA protected gold clusters, or cluster compounds, has, in brief, been motivated by the following circumstances:. (iv) According to our best evidence, the main component is (67, 30), rather than the previously published (68, 32), and the smaller main component is (48, 26) These unusual numbers, and the entire graph of the observed composition number (p vs n), are consistent with the theoretical and experimental (nuclear magnetic resonance, NMR) proposition of a special (bidentate) mode of 3-MBA binding. Especially of gold and its intermetallic compounds, form highly stable complexes with thiolate and other pseudo-halide ligands These are often called “monolayer protected clusters” (MPCs), because of their relation to the analogous self-assembled monolayers (SAMs) on planar or extended electrodes [6,7,8]. The smaller major component is assigned to the previously unknown (48, 26); the larger one is assigned to (67, 30), vs. the anticipated (68, 32)
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