Measuring salty samples without adducts with MALDI MS

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) has been used for the discovery of hundreds of novel cell to cell signaling peptides. Beyond its advantages of sensitivity and minimal sample preparation requirements, MALDI MS is attractive for biological analyses as high quality mass spectra may be obtained directly from specific locations within prepared tissue sections. However, due to the large quantity of salts present in physiological tissues, these mass spectra often contain many adducts of cationic salts such as sodium and potassium, in addition to the molecular ion [M + H]⁺. To reduce the presence of cation adducts in MALDI mass spectra obtained directly from tissues, we present a methodology that uses a slow condensation procedure to enable the formation of distinct regions of matrix/analyte crystals and cation (salt) crystals. Secondary ion mass spectrometric imaging suggests that the salts and MALDI matrix undergo a mutually exclusive crystallization process that results in the separation of the salts and matrix in the sample.     

A Probe Design for the Acquisition of Homonuclear, Heteronuclear, and Inverse Detected NMR Spectra from Multiple Samples

A new probe design is presented for obtaining homonuclear, heteronuclear, and inverse detected NMR spectra from more than one sample in the same total data acquisition time as for a single sample, thus increasing data acquisition efficiency. Specifically, a two-coil system, with each solenoidal coil impedance matched to 50 Omega at both proton and nitrogen frequencies, has been designed for operation at 11.7 T with an observe volume of 15 microL for each coil. Isolation between the two frequencies for each individual coil, and at each frequency between coils, was greater than 30 dB. Two-dimensional COSY and HMQC spectra were obtained with negligible NMR cross-talk between the two coils.     

Fluidic communication between multiple vertically segregated microfluidic channels connected by nanocapillary array membranes

Hybrid microfluidic/nanofluidic devices offer unique capabilities for manipulating and analyzing minute volumes of expensive or hard-to-obtain samples. Here, multilayer poly-(methyl methacrylate) microchips, with multiple spatially isolated microfluidic channels interconnected by nanocapillary array membranes (NCAMs), are fabricated using an adhesive contact printing process. The NCAMs, positioned between the microfluidic channel layers, add functionality to the inter-microchannel fluid transfer unit operation. They do so because the transport of specific analytes through the NCAM can be controlled by adjusting the ionic strength, the polarity of the applied bias, the surface charge density, and the pore size. A simplified, floating injection technique for NCAM-coupled nanofluidic devices is described and compared with conventional biased injection. In the floating injection approach, a voltage is applied across the injection channel and the slight electric field extension at the cross-section is used to transfer analytes through the nanopores to the separation channel. Floating injection excels in plug reproducibility, separation resolution, and operation simplicity, although it decreases assay throughput relative to biased injection. Floating injection can avoid the uneven distribution of analytes in the microfluidic channel that sometimes results from biased injection because of the volume mismatch between NCAM nanopore transport capacity and the supply of fluid. Moreover, the pressure-driven flow caused by the mismatch of the EOFs in the microfluidic channels connected by an NCAM must be considered when using NCAMs with pore diameters below 50 nm.     

Relative Quantitation of Neuropeptides over a Thousand-fold Concentration Range

Neuropeptides are essential cell-to-cell signaling molecules that influence diverse regulatory and behavioral functions within biological systems. Differing in their amino acid sequences and post-translational modifications, hundreds of neuropeptides are produced via a series of enzymatic processing steps, and their levels vary with location, time, and physiological condition. Due to their wide range of endogenous concentrations and inherent chemical complexity, using mass spectrometry (MS) to accurately quantify changes in peptide levels can be challenging. Here we evaluate three different MS systems for their ability to accurately measure neuropeptide levels: capillary liquid chromatography-electrospray ionization-ion trap (CapLC-ESI-IT) MS, ultraperformance liquid chromatography-electrospray ionization-quadrupole-time-of-flight (UPLC-LC-ESI-Q-TOF) MS, and matrix-assisted laser desorption/ionization-time-of-flight (MALDI-TOF) MS. Specifically, eight sample mixtures composed of five neuropeptide standards, with four technical replicates of each, were labeled with H₄/D₄-succinic anhydride, followed by relative peptide quantitation using the three MS platforms. For these samples, the CapLC-ESI-IT MS platform offered the most robust ability to accurately quantify peptides over a concentration range of 1200-fold, although it required larger sample sizes than the other two platforms. Both the UPLC-ESI-Q-TOF MS and the MALDI-TOF MS systems had lower limits of quantification, with the MALDI-TOF having the lowest. By implementing several data acquisition schemes and optimizing the data analysis approaches, we were able to accurately quantify peptides over a three orders of magnitude concentration range using either the UPLC or MALDI-TOF platforms. Overall these results increase our understanding of both the capabilities and limits of using MS-based approaches to measure peptides.     

Targeted Single-Cell Microchemical Analysis: MS-Based Peptidomics of Individual Paraformaldehyde-Fixed and Immunolabeled Neurons

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Highlights? Investigation of archived biological tissue at the single-cell level ? Screening of neuropeptides from immunolabeled tissue preserved up to 42 weeks ? Validation of antibody specificity using mass spectrometric approaches ? Standardized protocols for comprehensive characterization of selected cells     

Characterization of peptides from Aplysia using microbore liquid chromatography with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry guided purification

Liquid chromatography (LC) has been used extensively for the separation and isolation of peptides due to its high selectivity and peak capacity. An approach combining microbore LC with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-MS) detection is described to identify peptides in cells and guide the purification of peptides from the marine mollusc Aplysia californica. Direct MALDI-MS of neurons and processes provides molecular mass information for unknown peptides with almost no sample preparation, and LC-MALDI-MS allows the isolation and purification of these peptides from pooled samples, thus enabling new putative neuropeptides to be isolated from complex cellular samples. Both direct MALDI-MS and LC-MALDI-MS are compared in terms of detecting peptides from neuronal samples. Using both approaches, two peaks from Aplysia californica connectives having molecular masses of 5013 and 5021 have been isolated, partially sequenced and identified as novel collagen-like peptides.     

Sampling techniques for single-cell electrophoresis

Cells are extraordinarily complex, containing thousands of different analytes with concentrations spanning at least nine orders of magnitude. Analyzing single cells instead of tissue homogenates provides unique insights into cell-to-cell heterogeneity and aids in distinguishing normal cells from pathological ones. The high sensitivity and low sample consumption of capillary and on-chip electrophoresis, when integrated with fluorescence, electrochemical, and mass spectrometric detection methods, offer an ideal toolset for examining single cells and even subcellular organelles; however, the isolation and loading of such small samples into these devices is challenging. Recent advances have addressed this issue by interfacing a variety of enhanced mechanical, microfluidic, and optical sampling techniques to capillary and on-chip electrophoresis instruments for single-cell analyses.     

Determining sequences and post-translational modifications of novel conotoxins in Conus victoriae using cDNA sequencing and mass spectrometry

A combination of cDNA cloning and detailed mass spectrometric analyses was employed to identify novel conotoxins from Conus victoriae. Eleven conotoxin sequences were determined using molecular methods: one belonging to the A superfamily (Vc1.1), six belonging to the O superfamily (Vc6.1-Vc6.6) and four members of the T superfamily (Vc5.1-Vc5.4). In order to verify the sequences and identify the post-translational modifications (excluding the disulfide connectivity) of three Conus victoriae conotoxins, vc1a, vc5a and vc6a, deduced from sequences Vc1.1, Vc5.1, and Vc6.1, respectively, liquid chromatography/electrospray ionization ion trap mass spectrometry, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and nanospray ionization ion trap mass spectrometry with collisionally induced dissociation were performed on reduced and alkylated venom fractions. We report that vc1a, the native form of alpha-conotoxin Vc1.1 (an unmodified 16 amino acid residue peptide that has notable pain-relieving capabilities), includes a hydroxyproline and a gamma-carboxyglutamate residue. Conotoxin vc5a is a 10-residue peptide with two disulfide bonds and a hydroxyproline and vc6a is a 25 amino acid peptide with three disulfide bonds.