Combined elemental and biomolecular mass spectrometry imaging for probing the inventory of tissue at a micrometer scale.

Several complementary mass spectrometric imaging techniques allow mapping of various analytes within biological tissue sections. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) quantitatively detects elements and isotopes with very high sensitivity and a particularly high dynamical range. Matrix-assisted laser desorption/ionization ion mobility mass spectrometry (MALDI-IM-MS) allows a pixel-by-pixel classification and identification of biomolecules. In order to dispose of the healthy hemisphere as an internal calibrant in addition to routinely used external standards, adjacent brain sections of mice with a unilateral 6-OHDA lesion of the medial forebrain bundle were chosen as exemplary samples. We demonstrate a comprehensive way of data acquisition and analysis by coregistering mass spectrometric data on photomicrographs as common reference space and thus providing trimodal spatial information. Registering subsequent planar element maps yielded continuous 3-dimensional data sets. Furthermore, we introduce a correction of MSI data for variable slice thickness applicable to all MSI techniques. In the present case, we observed increased concentrations of iron, manganese, and copper in the lesioned substantia nigra while monounsaturated lipid levels were decreased in the identical region of interest. Our techniques provide new insights into the intricate spatial relationship of morphology and chemistry within tissue.

Structural mass spectrometry: rapid methods for separation and analysis of peptide natural products.

A significant challenge in natural product discovery is the initial discrimination of discrete secondary metabolites alongside functionally similar primary metabolic cellular components within complex biological samples. A property that has yet to be fully exploited for natural product identification and characterization is the gas-phase collision cross section, or, more generally, the mobility-mass correlation. Peptide natural products possess many of the properties that distinguish natural products, as they are frequently characterized by a high degree of intramolecular bonding and possess extended and compact conformations among other structural modifications. This report describes a rapid structural mass spectrometry technique based on ion mobility-mass spectrometry for the comparison of peptide natural products to their primary metabolic congeners using mobility-mass correlation. This property is empirically determined using ion mobility-mass spectrometry, applied to the analysis of linear versus modified peptides, and used to discriminate peptide natural products in a crude microbial extract. Complementary computational approaches are utilized to understand the structural basis for the separation of primary metabolism derived linear peptides from secondary metabolite cyclic and modified cyclic species. These findings provide a platform for enhancing the identification of secondary metabolic peptides with distinct mobility-mass ratios within complex biological samples.

Structural resolution of carbohydrate positional and structural isomers based on gas-phase ion mobility-mass spectrometry.

This report describes the rapid characterization of positional and structural carbohydrate isomers based on structural separations provided by ion mobility-mass spectrometry (IM-MS). Many of the diseases associated with glycoprotein variation can be more effectively treated with earlier detection substantiating the need for high-throughput methodologies for glycan characterization. This remains particularly difficult due to heterogeneity, branching, and large size of carbohydrate moieties which creates the potential for numerous isobaric positional and structural isomers that are difficult to characterize using conventional MS methods. IM-MS provides rapid (µs to ms) structural separations by IM and subsequent identification by MS which presents a means for characterization of positional and structural carbohydrate isomers. To chart the structural variation observed in IM-MS, the ion-neutral collision cross sections for over 300 carbohydrates are reported. This diversity can also be varied through the utility of using different alkali metals to tune separation selectivity via alkali metal-carbohydrate coordination. Furthermore, the advantages of combining either pre- and/or post-IM fragmentation prior to MS analysis is demonstrated for enhanced confidence in carbohydrate identification.

Surface fragmentation of complexes from thiolate protected gold nanoparticles by ion mobility-mass spectrometry.

Matrix-assisted laser desorption/ionization-ion mobility-mass spectrometry (MALDI-IM-MS) was used to analyze low mass gold-thiolate fragments generated from thiolate-protected gold nanoparticles (AuNPs). This is the first report of using gas-phase structural separations by IM-MS for the characterization of AuNPs, revealing significant structural variation between organic and gold-thiolate ionic species. Through the separation of background chemical noise, gold-thiolate ion species corresponding to fragments from the AuNP surface can be isolated. In the negative ion mode, many of these fragments correlate to capping structural motifs observed in the literature. In the positive ion mode, the fragment ions do not correlate to predicted structural motifs, but are nearly identical to the positive ions generated from the gold-thiolate AuNP precursor complexes. This suggests that energetic processes during laser desorption/ionization induce a structural rearrangement in the capping gold-thiolate structure of the AuNP, resulting in the generation of positively charged gold-thiolate complexes similar to the precursors of AuNP formation by reduction and negatively charged complexes more representative of the AuNP surface.

A structural mass spectrometry strategy for the relative quantitation of ligands on mixed monolayer-protected gold nanoparticles.

It is becoming increasingly common to use gold nanoparticles (AuNPs) protected by a heterogeneous mixture of thiolate ligands, but many ligand mixtures on AuNPs cannot be properly characterized due to the inherent limitations of commonly used spectroscopic techniques. Using ion mobility-mass spectrometry (IM-MS), we have developed a strategy that allows measurement of the relative quantity of ligands on AuNP surfaces. This strategy is used for the characterization of three samples of mixed-ligand AuNPs: tiopronin:glutathione (av diameter 2.5 nm), octanethiol:decanethiol (av diameter 3.6 nm), and tiopronin:11-mercaptoundecyl(poly ethylene glycol) (av diameter 2.5 nm). For validation purposes, the results obtained for tiopronin:glutathione AuNPs were compared to parallel measurements using nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry (MS) without ion mobility separation. Relative quantitation measurements for NMR and IM-MS were in excellent agreement, with an average difference of less than 1% relative abundance. IM-MS and MS without ion mobility separation were not comparable, due to a lack of ion signals for MS. The other two mixed-ligand AuNPs provide examples of measurements that cannot be performed using NMR spectroscopy.

Characterizing ion mobility-mass spectrometry conformation space for the analysis of complex biological samples.

The conformation space occupied by different classes of biomolecules measured by ion mobility-mass spectrometry (IM-MS) is described for utility in the characterization of complex biological samples. Although the qualitative separation of different classes of biomolecules on the basis of structure or collision cross section is known, there is relatively little quantitative cross-section information available for species apart from peptides. In this report, collision cross sections are measured for a large suite of biologically salient species, including oligonucleotides (n = 96), carbohydrates (n = 192), and lipids (n = 53), which are compared to reported values for peptides (n = 610). In general, signals for each class are highly correlated, and at a given mass, these correlations result in predicted collision cross sections that increase in the order oligonucleotides < carbohydrates < peptides < lipids. The specific correlations are described by logarithmic regressions, which best approximate the theoretical trend of increasing collision cross section as a function of increasing mass. A statistical treatment of the signals observed within each molecular class suggests that the breadth of conformation space occupied by each class increases in the order lipids < oligonucleotides < peptides < carbohydrates. The utility of conformation space analysis in the direct analysis of complex biological samples is described, both in the context of qualitative molecular class identification and in fine structure examination within a class. The latter is demonstrated in IM-MS separations of isobaric oligonucleotides, which are interpreted by molecular dynamics simulations.

Enhanced carbohydrate structural selectivity in ion mobility-mass spectrometry analyses by boronic acid derivatization.

The boronic acid derivatization of carbohydrates is demonstrated as an ion mobility shift strategy to improve confidence in the identification and characterization of carbohydrate assignments using ion mobility-mass spectrometry.

Structural separations by ion mobility-MS for glycomics and glycoproteomics.

This chapter describes the utility of ion mobility-mass spectrometry (IM-MS) for the detection and characterization of glycoproteins and associated glycoconjugates. IM-MS provides separations in two dimensions; one on the basis of molecular surface area or structure, and the other on molecular mass which creates the ability to differentiate biomolecular classes and isobaric species. When applied to the characterization of glycoproteins, IM-MS separates peptides from the associated glycans in the same digest without purification, and can also be used to separate different isomeric glycans which is a significant challenge in current glycomic studies. The chapter details the methodologies to use IM-MS for the study of glycans and glycoproteins for an audience ranging from new and potential practitioners to those already utilizing the technique.

Structurally selective imaging mass spectrometry by imaging ion mobility-mass spectrometry.

This chapter describes the utility of structurally based separations combined with imaging mass spectrometry (MS) by ion mobility-MS (IM-MS) approaches. The unique capabilities of combining rapid (mus-ms) IM separations with imaging MS are detailed for an audience ranging from new to potential practitioners in IM-MS technology. Importantly, imaging IM-MS provides the ability to rapidly separate and elucidate various types of endogenous and exogenous biomolecules (e.g., nucleotides, carbohydrates, peptides, and lipids), including isobaric species. Drift tube and traveling wave IM-MS instrumentation are described and specific protocols are presented for calculating ion-neutral collision cross sections (i.e., apparent ion surface area or structure) from experimentally obtained IM-MS data. Special emphasis is placed on the use of imaging IM-MS for the analysis of samples in life sciences research (e.g., thin tissue sections), including selective imaging for peptide/protein and lipid distributions. Future directions for rapid and multiplexed imaging IM-MS/MS are detailed.

Simultaneous glycoproteomics on the basis of structure using ion mobility-mass spectrometry.

Simultaneous glycoproteomic characterization using rapid (mus to ms) structural separations provided by ion mobility-mass spectrometry (IM-MS) is described. Advantages from using both ESI and MALDI ion sources are presented with future implications toward high throughput glycan and glycoconjugate characterization.