Integrating the governing chemistry with the genomics and phenotypes of microbial colonies has been a "holy grail" in microbiology. This work describes a highly sensitive, broadly applicable, and costeffective approach that allows metabolic profiling of live microbial colonies directly from a Petri dish without any sample preparation. Nanospray desorption electrospray ionization mass spectrometry (MS), combined with alignment of MS data and molecular networking, enabled monitoring of metabolite production from live microbial colonies from diverse bacterial genera, including Bacillus subtilis, Streptomyces coelicolor, Mycobacterium smegmatis, and Pseudomonas aeruginosa. This work demonstrates that, by using these tools to visualize small molecular changes within bacterial interactions, insights can be gained into bacterial developmental processes as a result of the improved organization of MS/MS data. To validate this experimental platform, metabolic profiling was performed on Pseudomonas sp. SH-C52, which protects sugar beet plants from infections by specific soil-borne fungi [R. Mendes et al. (2011) Science 332:1097-1100]. The antifungal effect of strain SH-C52 was attributed to thanamycin, a predicted lipopeptide encoded by a nonribosomal peptide synthetase gene cluster. Our technology, in combination with our recently developed peptidogenomics strategy, enabled the detection and partial characterization of thanamycin and showed that it is a monochlorinated lipopeptide that belongs to the syringomycin family of antifungal agents. In conclusion, the platform presented here provides a significant advancement in our ability to understand the spatiotemporal dynamics of metabolite production in live microbial colonies and communities. ambient mass spectrometry | microbial ecology | natural products M icrobes use secreted factors to interact, communicate with, and manipulate their local environment and neighboring cell populations in a process known as metabolic exchange (1-5). By using a wide breadth of molecules ranging from signaling compounds to defensive metabolites, metabolic exchange dictates not only basic microbial behavior, such as biofilm formation, sporulation, and motility, but also social interactions, such as syntrophy and quorum sensing, which enables microbes to establish communities (1-5). Despite these secreted factors, also known as the parvome, having a major impact on the phenotypic development of microbial populations, there is a lack of tools that enable scientists to probe the chemistry of microbial colonies in a direct manner. Currently, the chemistry of microbes is usually studied by monitoring individual molecular species and requires a significant time and monetary investment. Our laboratories are interested in the development of tools that make this process more efficient as well as making it easier for nonchemists to study the chemistry of microbes and nonmicrobe cell populations. Ideally, these tools should be easy to implement, compatible with existing infrastructure, and easily inco...

Nanospray desorption electrospray ionization (nano-DESI) mass spectrometry is presented as an ambient pressure liquid extraction-ionization technique for analysis of organic and biological molecules on substrates. Analyte is desorbed into a solvent bridge formed between two capillaries and the analysis surface. One capillary supplies solvent to create and maintain the bridge, while the second capillary transports the dissolved analyte from the bridge to the mass spectrometer. A high voltage applied between the inlet of mass spectrometer and the primary capillary creates a self-aspirating nanospray. This approach enables the separation of desorption and ionization events, thus providing independent control of desorption, ionization, and transport of the analyte. We present analytical capabilities of the method and discuss its potential for imaging applications.

Two classes of gas-phase aluminum-iodine clusters have been identified whose stability and reactivity can be understood in terms of the spherical shell jellium model. Experimental reactivity studies show that the Al13I-x clusters exhibit pronounced stability for even numbers of I atoms. Theoretical investigations reveal that the enhanced stability is associated with complementary pairs of I atoms occupying the on-top sites on the opposing Al atoms of the Al13- core. We also report the existence of another series, Al14I-x, that exhibits stability for odd numbers of I atoms. This series can be described as consisting of an Al14I-3 core upon which the I atoms occupy on-top locations around the Al atoms. The potential synthetic utility of superatom chemistry built upon these motifs is addressed.

The reactions of metal clusters with small molecules often depend on cluster size. The selectivity of oxygen reactions with aluminum cluster anions can be well described within an electronic shell model; however, not all reactions are subject to the same fundamental constraints. We observed the size selectivity of aluminum cluster anion reactions with water, which can be attributed to the dissociative chemisorption of water at specific surface sites. The reactivity depends on geometric rather than electronic shell structure. Identical arrangements of multiple active sites in Al16-, Al17-, and Al18- result in the production of H2 from water.

Ambient ionization imaging mass spectrometry is uniquely suited for detailed spatially-resolved chemical characterization of biological samples in their native environment. However, the spatial resolution attainable using existing approaches is limited by the ion transfer efficiency from the ionization region into the mass spectrometer. Here we present a first study of ambient imaging of biological samples using nanospray desorption ionization (nano-DESI). Nano-DESI is a new ambient pressure ionization technique that uses minute amounts of solvent confined between two capillaries comprising the nano-DESI probe and the solid analyte for controlled desorption of molecules present on the substrate followed by ionization through self-aspirating nanospray. We demonstrate highly sensitive spatially resolved analysis of tissue samples without sample preparation. Our first proof-of-principle experiments indicate the potential of nano-DESI for ambient imaging with a spatial resolution of better than 12 μm. The significant improvement of the spatial resolution offered by nano-DESI imaging combined with high detection efficiency will enable new imaging mass spectrometry applications in clinical diagnostics, drug discovery, molecular biology, and biochemistry.

Characterization of the chemical composition and chemical transformations of secondary organic aerosol (SOA) is both a major challenge and the area of greatest uncertainty in current aerosol research. This study presents the first application of desorption electrospray ionization combined with high-resolution mass spectrometry (DESI-MS) for detailed chemical characterization and studies of chemical aging of organic aerosol (OA) samples collected on Teflon substrates. DESI-MS offers unique advantages both for detailed characterization of chemically labile components in OA that cannot be detected using traditional electrospray ionization mass spectrometry (ESI-MS) and for studying chemical aging of OA. DESI-MS enables rapid characterization of OA samples collected on substrates by eliminating the sample preparation stage. In addition, it enables detection and structural characterization of chemically labile molecules in OA samples by minimizing the residence time of analyte in the solvent. In this study, DESI-MS and tandem mass spectrometry experiments (MS/MS) were used to examine chemical aging of SOA produced by the ozonolysis of limonene (LSOA) in the presence of gaseous ammonia. Exposure of LSOA to ammonia resulted in measurable changes in the optical properties of the sample observed using ultraviolet (UV)-visible spectroscopy. High-resolution DESI-MS analysis demonstrated that chemical aging results in formation of highly conjugated nitrogen-containing species that are most likely responsible for light-absorbing properties of the aged LSOA. Detailed analysis of the experimental data allowed us to identify several key aging reactions, including the transformation of carbonyls to imines, intramolecular dimerization of imines with other carbonyl compounds in SOA, and intermolecular cyclization of imines. This study presents an important step toward understanding the formation of light-absorbing OA (brown carbon) in the atmosphere.

The effect of relative humidity (RH) on the composition and concentrations of gas-phase products and secondary organic aerosol (SOA) generated from the photooxidation of isoprene under high-NO_x conditions was investigated. Experiments were performed with hydrogen peroxide as the OH precursor and in the absence of seed aerosol. The relative yields of most gas-phase products were the same regardless of initial water vapor concentration with exception of hydroxyacetone and glycolaldehyde, which were considerably affected by RH. A significant change was observed in the SOA composition, with many unique condensed-phase products formed under humid (90 % RH) vs. dry (<2 % RH) conditions, without any detectable effect on the rate and extent of the SOA mass growth. There is a 40 % reduction in the number and relative abundance of distinct particle-phase nitrogen-containing organic compounds (NOC) detected by high resolution mass spectrometry. The suppression of condensation reactions, which produce water as a product, is the most important chemical effect of the increased RH. For example, the total signal from oligomeric esters of 2-methylglyceric acid was reduced by about 60 % under humid conditions and the maximum oligomer chain lengths were reduced by 7–11 carbons. Oligomers formed by addition mechanisms, without direct involvement of water, also decreased at elevated RH but to a much smaller extent. The observed reduction in the extent of condensation-type oligomerization at high RH may have substantial impact on the phase characteristics and hygroscopicity of the isoprene aerosol. The reduction in the amount of organic nitrates in the particle phase has implications for understanding the budget of NOC compounds

We recently demonstrated that, in gas phase clusters containing aluminum and iodine atoms, an Al 13 cluster behaves like a halogen atom, whereas an Al 14 cluster exhibits properties analogous to an alkaline earth atom. These observations, together with our findings that Al 13 ؊ is inert like a rare gas atom, have reinforced the idea that chosen clusters can exhibit chemical behaviors reminiscent of atoms in the periodic T he formation of materials with properties different from those of the constituent atoms is a known phenomenon in nature. For example, the formation of NaCl molecules/solid with characteristics different from its constituent elements, Na and Cl, is a classic example. One of the objectives of the research on superatoms (1-3) is to explore if one can carry out, what nature does, in a more facile and controlled manner. Developing an understanding of the factors governing the chemical behavior of clusters (4-10) and demonstrating that this knowledge can be used to design stable building blocks for new materials is critical for translating this concept into practice. For metal clusters, a simple electronic shell model called jellium (11) is routinely used to describe the global features of the electronic structure. In this model, the nuclei together with the innermost electrons form a positive-charged background, whereupon the valence electrons coming from individual atoms are then subjected to this potential. For pure metal clusters, within a spherical jellium background, this approach results in a shell structure where the electrons are arranged in electronic shells 1s 2 , 1p 6 , 1d 10 , 2s 2 , 1f 14 , 2p 6 . . . compared with 1s 2 , 2s 2 , 2p 6 , 3s 2 , 3p 6 , 4s 2 , 3d 10 . . . in individual atoms. Similar shell structure is also obtained for square well and harmonic forms of background potential (12), indicating that the shells derived within a jellium picture represent generic features of electronic states in a confined free electron gas. Clusters containing 2, 8, 18, 20, 34, 40 . . . electrons correspond to filled electronic shells and exhibit enhanced stability as seen via abundances in mass spectra of simple metal clusters, higher ionization potential, lower electron affinity, and chemical inertness seen in reactivity experiments. In this respect, an Al 13 cluster with 39 valence electrons and an electronic structure of 1s 2 , 1p 6 , 1d 10 , 2s 2 , 1f 14 , 2p 5 lacks a single electron as do halogen atoms, which, upon addition of a single electron, acquire a filled shell status (13). Indeed, previous studies (14,15) have shown that Al 13 has an electron affinity comparable to halogen atoms, indicating a chemical behavior reminiscent of halogen atoms. In a similar vein, we had recently shown that in cluster compounds with iodine, an Al 14 cluster exhibits behavior analogous to alkaline earth atoms (3). We had shown that Al 14 I 3 Ϫ is a stable species and that its stability can be reconciled by considering Al 14 in a ϩ 2 valence state (3). The electronic shell structure, outlined above, does ...