Triethylamine Solution for the Intractability of Aqueous Gold–Thiolate Cluster Anions: How Ion Pairing Enhances ESI-MS and HPLC of aq-Au_n(pMBA)_p

Abstract

Herein we disclose methods that greatly improve the solution-and gas-phase handling properties of larger aqueous-phase gold−thiolate clusters, which previously presented extreme technical obstacles to molecular analysis and size control, even as they have enjoyed ever-wider applications in materials science and biomedicine. The methods are based upon an analogy between the polyacidic surface structure of the pMBA-protected clusters (pMBA = p-mercaptobenzoic acid) and that of oligonucleotides. A volatile ion-pairing reagent, TEA = triethylamine, greatly improves solution-phase stability near neutral pH and thus facilitates both electrospray generation of the gas-phase ions and the in-line reversedphase ion-pairing HPLC-ESI-MS approach to analyzing complex mixtures of Au-pMBA oligomers and clusters. Previously anticipated but never established compounds, including Au 36 (pMBA) 24 , are thereby demonstrated. These results are in accord with recent theoretical simulations of ion pairing of model Au 144 (pMBA) 60 clusters in aqueous solutions. This advance complements our recent work on their nonaqueous (hydrophobic) counterparts, in which redox electrochemistry is sufficient to support the efficient LC-ESI processes, enabling various precise measurements on the intact molecular ions. Here, we report (i) novel conditions for enhanced ESI generation of polyanions of the aqueous clusters and by extension (ii) a notably improved method by which mixtures of these clusters may be successfully separated and detected by ion-pair reversed-phase HPLC-MS.

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)

“…In the present work discussed here, efforts were focused to determine whether the 3-MBA/Au systems were amenable to analysis by HPLC-ESI-MS in the same way as previously observed for the analogous 4-MBA/Au systems (aka p-MBA/Au). Specifically of interest was the possibility of ion-pairing with triethylammonium cations (TEAH+) 27 for retention and separation of these poly-acid clusters via reversed phase chromatography. Also, of interest was an understanding of the effectiveness of this ion-pairing strategy for electrospray ionization (ESI) and if the necessary conditions could be implemented to supported determination of cluster compositions with some degree of clarity by minimizing fragmentation.…”

“…By use of a suitable ion-pairing agent (TEA+), well resolved Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 26 August 2019 doi:10.20944/preprints201908.0263.v1 mass spectra could be obtained under gentler conditions (to reduce poly-anion fragmentation in electrospray ionization) and nearly free from alkali-ion and solvent adducts. 27 In a related work, silver-lipoate clusters [29,12] (3-) have been effectively resolved, where lipoate acts as a bidentate (dithiolate) ligand with a terminal carboxylate ("thioctic acid"). [37][38]…”

“…[25][26] Our approach here has been to adapt a method-electrospray ionization (ESI)-coupled high performance liquid chromatography mass spectrometry (HPLC-MS)established earlier for bio-macromolecules of a similar size (or mass) and surface chemistry as the MPCs under investigation. 27 Specifically, the larger Au/MBA clusters have many (~24-60) acidterminated ligands, 3 and so are presumed to exist in aqueous solution at normal (or higher) pH as poly-anions (plus respective counter-cations). For this case, long experience with oligonucleotides (DNA or RNA), composed of similar number, ~24 -60 base-sugar-phosphate repeats) seem most instructive.…”

“…Our aims in the present work have been (i) to determine whether the unusual solution-phase characteristics of the Au/3-MBA clusters will permit them to yield to analyze by the ESI-coupled LC-MS methods that have recently improved the analysis of Au-pMBA clusters ranging from small oligomers and clusters (25,18) and (36,24) to the larger species (102, 44), (130, 50) and (144, 60); 27 (ii) to examine whether ion-pairing agents will work similarly to enable both high-resolution LC separations and reduced-fragmentation ESI-ToF (time-of-flight) mass spectra; (iii) to provide some insight into the powerful selection principles underlying the results in refs. 1-4; (iv) to search for minor or hidden components (new compositions) as semi-stable or transition MPCs; (v) to provide additional evidence pertaining to the 'bidentate' or dynamical carboxyl-gold interactions described in ref.…”

Deepening the Understanding of Thiolate-Protected Metal Clusters Using High-Performance Liquid Chromatography

“…They used a reversed-phase column (Figure 24(b)) to separate the hydrophilic gold clusters along with an ion-pair reagent (ion-pair chromatography; Table 2). 910 If a hydrophilic interaction liquid chromatography (HILIC) column (Table 2) can be used for cluster separation, an ion-pair reagent becomes unnecessary and the mass distribution evaluation should be further facilitated ( Figure 24(c)). Then, we attempted to find HILIC columns 965 for HPLC suitable for the high-resolution separation of hydrophilic metal clusters and thereby to determine the chemical compositions of hydrophilic metal clusters.…”