Characterization of Proteins Present in Isolated Senile Plaques from Alzheimer’s Diseased Brains by MALDI-TOF MS with MS/MS

The increase of insoluble senile plaques in the brain is a primary hallmark of Alzheimer’s disease. The usefulness of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) with tandem MS for the characterization of senile plaques from AD brains and the relevance of the components identified to furthering AD research using MS is discussed. Thirty-three components were reproducibly observed within tryptic aliquots of senile plaques from two different AD brains after sample preparation optimization. Additionally, this is one of the first accounts of LIFT being utilized for the direct sequencing of peptides from isolated senile plaques. While many of the species observed coisolated within senile plaques have been linked to AD etiology, if only speculatively, this is the first instance that many of them have been demonstrated to be a part of the plaques themselves. This work is the first step in determining the potential roles that the species may have in the aggregation or proliferation of the plaques

Capillary Liquid Chromatography Mass Spectrometry Analysis of Intact Monolayer-Protected Gold Clusters in Complex Mixtures

In some respects, large noble-metal clusters protected by thiolate ligands behave as giant molecules of definite composition and structure; however, their rigorous analysis continues to be quite challenging. Analysis of complex mixtures of intact monolayer-protected clusters (MPCs) by liquid chromatography mass spectrometry (LC-MS) could provide quantitative identification of the various components present. This advance is critical for biomedical and toxicological research, as well as in fundamental studies that rely on the identification of selected compositions. This work expands upon the separate LC and MS results previously achieved, by interfacing the capillary liquid chromatograph directly to the electrospray source of the mass spectrometer, in order to provide an extremely sensitive, quantitative, and rapid means to characterize MPCs and their derivatives far beyond that of earlier reports. Here, we show that nonaqueous reversed-phase chromatography can be coupled to mass-spectrometry detection to resolve complex mixtures in minute (∼100 ng) samples of gold MPCs, of molecular masses up to ∼40 kDa, and with single-species sensitivity easily demonstrated for components on the level of sub-10 ng or picomole (1 pmol)

Collision-Induced Dissociation of Monolayer Protected Clusters Au₁₄₄ and Au₁₃₀ in an Electrospray Time-of-Flight Mass Spectrometer

Gas-phase reactions of larger gold clusters are poorly known because generation of the intact parent species for mass spectrometric analysis remains quite challenging. Herein we report in-source collision-induced dissociation (CID) results for the monolayer protected clusters (MPCs) Au₁₄₄(SR)₆₀ and Au₁₃₀(SR)₅₀, where R- = PhCH₂CH₂-, in a Bruker micrOTOF time-of-flight mass spectrometer. A sample mixture of the two clusters was introduced into the mass spectrometer by positive mode electrospray ionization. Standard source conditions were used to acquire a reference mass spectrum, exhibiting negligible fragmentation, and then the capillary-skimmer potential difference was increased to induce in-source CID within this low-pressure region (∼4 mbar). Remarkably, distinctive fragmentation patterns are observed for each MPC­[3+] parent ion. An assignment of all the major dissociation products (ions and neutrals) is deduced and interpreted by using the distinguishing characteristics in the standard structure-models for the respective MPCs. Also, we propose a ring-forming elimination mechanism to explain R-H neutral loss, as separate from the channels leading to RS-SR or (AuSR)₄ neutrals

Gold Nanocluster Prospecting via Capillary Liquid Chromatography-Mass Spectrometry: Discovery of Three Quantized Gold Clusters in a Product Mixture of “2 nm Gold Nanoparticles”

A nonaqueous reversed phase liquid chromatography-mass spectrometry (LC-MS) method has been developed for extremely hydrophobic MPCs (monolayer-protected clusters), and has been applied to the efficient separation of gold–dodecanethiolate (ddt) assemblies, leading to the identification of three dodecanethiolate-protected gold clusters, Au₁₃₀(ddt)₅₀, Au₁₃₇(ddt)₅₆, and Au₁₄₄(ddt)₆₀, as prominent components of a commercial product of nominally 2 nm (core-diameter) protected gold nanoparticles obtained from nanoComposix, Inc. Various components were separated, according to hydrophobic character, using a linear gradient of methanol–dichloromethane mobile phases, on a C₁₈ HPLC column. Varying concentrations of mobile-phase modifier (triethylammonium acetate) were compared for effect on chromatographic peak shape and cluster retention. Positive electrospray ionization (ESI) was used to ionize all components in the sample. LC separation prior to inline + ESI-MS detection facilitated sample analysis via production of simplified mass spectra for each eluting cluster species and provided insight into the relative polarity of the clusters shown here. UV–vis detection facilitated method development and allowed determination of nonionizing, and/or polydisperse components

Liquid Chromatography Separation and Mass Spectrometry Detection of Silver-Lipoate Ag₂₉(LA)₁₂ Nanoclusters: Evidence of Isomerism in the Solution Phase

Evidence for the existence of condensed-phase isomers of silver-lipoate clusters, Ag₂₉(LA)₁₂, where LA = (<i>R</i>)-α lipoic acid, was obtained by reversed-phase ion-pair liquid chromatography with in-line UV–vis and electrospray ionization (ESI)-MS detection. All components of a raw mixture were separated according to surface chemistry and increasing size via reversed-phase gradient HPLC methods and identified by their corresponding <i>m</i>/<i>z</i> ratio by ESI in the negative ionization mode. Aqueous and methanol mobile-phase mixtures, each containing 400 mM hexafluoroisopropanol (HFIP)–15 mM triethylamine (TEA), were employed to facilitate the interaction between the clusters and stationary phase via formation of ion-pairs. TEA-HFIP (triethylammonium-<i>hexafluoroisopropoxide</i>) had been shown to provide superior chromatographic peak shape and mass spectral signal compared with alternative modifiers such as TEAA (triethylammonium-<i>acetate</i>) for analysis of oligonucleotide samples. Liquid chromatographic separation prior to mass spectrometry detection facilitated sample analysis by production of simplified mass spectra for each eluting cluster species and provided insight into the existence of at least two major solution-phase isomers of Ag₂₉(LA)₁₂. UV–vis detection in-line with ESI analysis provided independent confirmation of the existence of the isomers and their similar electronic structure as judged from their identical optical spectra in the 300–500 nm range. [Diastereomerism provides a possible interpretation for the near-equal abundance of the two forms, based on a structurally defined nonaqueous homologue.

Triethylamine Solution for the Intractability of Aqueous Gold–Thiolate Cluster Anions: How Ion Pairing Enhances ESI-MS and HPLC of <i>aq</i>-Au_<i>n</i>(pMBA)_<i>p</i>

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 = <i>p</i>-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 reversed-phase ion-pairing HPLC-ESI-MS approach to analyzing complex mixtures of Au-pMBA oligomers and clusters. Previously anticipated but never established compounds, including Au₃₆(pMBA)₂₄, are thereby demonstrated. These results are in accord with recent theoretical simulations of ion pairing of model Au₁₄₄(pMBA)₆₀ clusters in aqueous solutions. This advance complements our recent work on their <i>nonaqueous</i> (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