Thorsten W. Jaskolla

Compelling Evidence for Lucky Survivor and Gas Phase Protonation: The Unified MALDI Analyte Protonation Mechanism

This work experimentally verifies and proves the two long since postulated matrix-assisted laser desorption/ionization (MALDI) analyte protonation pathways known as the Lucky Survivor and the gas phase protonation model. Experimental differentiation between the predicted mechanisms becomes possible by the use of deuterated matrix esters as MALDI matrices, which are stable under typical sample preparation conditions and generate deuteronated reagent ions, including the deuterated and deuteronated free matrix acid, only upon laser irradiation in the MALDI process. While the generation of deuteronated analyte ions proves the gas phase protonation model, the detection of protonated analytes by application of deuterated matrix compounds without acidic hydrogens proves the survival of analytes precharged from solution in accordance with the predictions from the Lucky Survivor model. The observed ratio of the two analyte ionization processes depends on the applied experimental parameters as well as the nature of analyte and matrix. Increasing laser fluences and lower matrix proton affinities favor gas phase protonation, whereas more quantitative analyte protonation in solution and intramolecular ion stabilization leads to more Lucky Survivors. The presented results allow for a deeper understanding of the fundamental processes causing analyte ionization in MALDI and may alleviate future efforts for increasing the analyte ion yield.

4-Chloro-alpha-cyanocinnamic acid is an advanced, rationally designed MALDI matrix.

Matrix-assisted laser desorption ionization (MALDI) has become an enabling technology for the fields of protein mass spectrometry (MS) and proteomics. Despite its widespread use, for example, in protein identification via peptide mass fingerprinting, a comprehensive model for the generation of free gas-phase ions has not yet been developed. All matrices in use today, such as α-cyano-4-hydroxycinnamic acid (CHCA), have been found empirically and stem from the early days of MALDI. By systematic and targeted variation of the functional groups of the α-cyanocinnamic acid core unit, 4-chloro-α-cyanocinnamic acid (Cl-CCA) was selected and synthesized, and it exhibited outstanding matrix properties. Key features are a substantial increase in sensitivity and a considerably enhanced peptide recovery in proteomic analyses because of a much more uniform response to peptides of different basicity. Using Cl-CCA as a matrix for a 1 fmol bovine serum albumin (BSA) in-solution digest, the sequence coverage is raised to 48%, compared with 4% for CHCA. For a gel band containing 25 fmol of BSA, unambiguous protein identification becomes possible with Cl-CCA. These findings also imply ion formation via a chemical ionization mechanism with proton transfer from a reactive protonated matrix species to the peptide analytes. The considerable increase in performance promises to have a strong impact on future analytical applications of MALDI, because current sensitivity limits are overcome and more comprehensive analyses come into reach.

Ion Yields in UV-MALDI Mass Spectrometry As a Function of Excitation Laser Wavelength and Optical and Physico-Chemical Properties of Classical and Halogen-Substituted MALDI Matrixes

The laser wavelength constitutes a key parameter in ultraviolet-matrix-assisted laser desorption ionization-mass spectrometry (UV-MALDI-MS). Optimal analytical results are only achieved at laser wavelengths that correspond to a high optical absorption of the matrix. In the presented work, the wavelength dependence and the contribution of matrix proton affinity to the MALDI process were investigated. A tunable dye laser was used to examine the wavelength range between 280 and 355 nm. The peptide and matrix ion signals recorded as a function of these irradiation parameters are displayed in the form of heat maps, a data representation that furnishes multidimensional data interpretation. Matrixes with a range of proton affinities from 809 to 866 kJ/mol were investigated. Among those selected are the standard matrixes 2,5-dihydroxybenzoic acid (DHB) and α-cyano-4-hydroxycinnamic acid (HCCA) as well as five halogen-substituted cinnamic acid derivatives, including the recently introduced 4-chloro-α-cyanocinnamic acid (ClCCA) and α-cyano-2,4-difluorocinnamic acid (DiFCCA) matrixes. With the exception of DHB, the highest analyte ion signals were obtained toward the red side of the peak optical absorption in the solid state. A stronger decline of the molecular analyte ion signals generated from the matrixes was consistently observed at the low wavelength side of the peak absorption. This effect is mainly the result of increased fragmentation of both analyte and matrix ions. Optimal use of multiply halogenated matrixes requires adjustment of the excitation wavelength to values below that of the standard MALDI lasers emitting at 355 (Nd:YAG) or 337 nm (N(2) laser). The combined data provide new insights into the UV-MALDI desorption/ionization processes and indicate ways to improve the analytical sensitivity.

Significant sensitivity improvements by matrix optimization: a MALDI-TOF mass spectrometric study of lipids from hen egg yolk

Due to its sensitivity, the tolerance of impurities and the simplicity of performance, matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) mass spectrometry (MS) is increasingly used to analyze lipids from biological sources. Although its detailed role is not understood so far, the applied matrix has a pronounced effect on the achievable spectrum quality and particularly how sensitive the individual lipid classes are detectable. Different matrix compounds were recently established in the lipid field including 2,5-dihydroxybenzoic acid (DHB), 9-aminoacridine (9-AA), para-nitroaniline (PNA), 2-mercaptobenzothiazole (MBT), and 2-(2-aminoethylamino)-5-nitropyridine (AAN). It is the aim of this paper to compare the properties of these matrices with the newly synthesized matrix, alpha-cyano-2,4-difluorocinnamic acid (Di-FCCA). An organic extract from hen egg yolk was used as a simple and easily available test system. It will be shown that Di-FCCA is the matrix of choice to detect lipids in the positive-ion mode due to an achievable sensitivity gain of more than one order of magnitude compared to alternative matrices. In contrast, Di-FCCA is not suitable for negative-ion detection of phospholipids. Here, 9-AA is unequivocally the matrix of choice.

Entianin, a Novel Subtilin-Like Lantibiotic from Bacillus subtilis subsp. spizizenii DSM 15029T with High Antimicrobial Activity

ABSTRACT Lantibiotics, such as nisin and subtilin, are lanthionine-containing peptides that exhibit antimicrobial as well as pheromone-like autoinducing activity. Autoinduction is specific for each lantibiotic, and reporter systems for nisin and subtilin autoinduction are available. In this report, we used the previously reported subtilin autoinduction bioassay in combination with mass spectrometric analyses to identify the novel subtilin-like lantibiotic entianin from Bacillus subtilis subsp. spizizenii DSM 15029T. Linearization of entianin using Raney nickel-catalyzed reductive cleavage enabled, for the first time, the use of tandem mass spectrometry for the fast and efficient determination of an entire lantibiotic primary structure, including posttranslational modifications. The amino acid sequence determined was verified by DNA sequencing of the etnS structural gene, which confirmed that entianin differs from subtilin at 3 amino acid positions. In contrast to B. subtilis ATCC 6633, which produces only small amounts of unsuccinylated subtilin, B. subtilis DSM 15029T secretes considerable amounts of unsuccinylated entianin. Entianin was very active against several Gram-positive pathogens, such as Staphylococcus aureus and Enterococcus faecalis. The growth-inhibiting activity of succinylated entianin (S-entianin) was much lower than that of unsuccinylated entianin: a 40-fold higher concentration was required for inhibition. For succinylated subtilin (S-subtilin), a concentration 100-fold higher than that of unsuccinylated entianin was required to inhibit the growth of a B. subtilis test strain. This finding was in accordance with a strongly reduced sensing of cellular envelope stress provided by S-entianin relative to that of entianin. Remarkably, S-entianin and S-subtilin showed considerable autoinduction activity, clearly demonstrating that autoinduction and antibiotic activity underlie different molecular mechanisms.

Comparison between vacuum sublimed matrices and conventional dried droplet preparation in MALDI-TOF mass spectrometry

The properties of several cinnamic acid compounds used as matrices for matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) were investigated as standard dried droplet (DD) and vacuum sublimed preparations. The differences between both preparation methods were analyzed with regard to matrix grain size, internal ion energy, initial velocity, analyte intensity, and analyte incorporation depth. Some of the used cinnamic acid derivatives exhibit clearly reduced grain sizes as sublimed preparations compared with standard DD approaches. In these cases higher effective temperatures could be measured accompanied by increased analyte intensities, which can be explained by stronger volatilization processes caused by a hindered heat dissipation resulting in a raised analyte transfer into the gas phase. For all sublimed compounds, a strong increase of the initial ion velocity compared with DD preparations could be measured. Higher initial ion velocities correlate with a decrease in internal ion energy which might be attributed to the very uniform crystal morphology exhibited by sublimed compounds. For sublimed matrices without reduced grain size, at least slightly higher analyte intensities could be detected at raised laser fluences. Analyte accumulation in the uppermost matrix layers or the detected higher ion stability can be explanations for these results.

The New Matrix 4-Chloro-α-Cyanocinnamic Acid Allows the Detection of Phosphatidylethanolamine Chloramines by MALDI-TOF Mass Spectrometry

Phosphatidylethanolamines (PEs) are abundant lipid constituents of the cellular membrane. The amino group of PEs exhibits high reactivity with hypochlorous acid that is generated under inflammatory conditions in vivo. The analysis of the resulting PE mono- and dichloramines is of significant interest since these species represent important mediators of lipid peroxidation. We have shown in a previous communication that mass spectrometric detection of PE chloramines is only possible with ESI MS, whereas MALDI-TOF MS fails to detect these products if standard matrices are used.In this work we demonstrate that the detection of PE chloramines is also possible by MALDI-TOF MS if 4-chloro-α-cyanocinnamic acid is used as matrix. The underlying processes leading to ionization of these species will be discussed in detail. Both, experimental and theoretical studies taking into account possible intramolecular rearrangements were performed to clarify these aspects.

Structure elucidation and biosynthesis of lysine-rich cyclic peptides in Xenorhabdus nematophila

Thirteen novel PAX (peptide-antimicrobial-Xenorhabdus) peptides were identified in Xenorhabdus nematophila HGB081. Their structures including the absolute configuration were elucidated using a combination of labeling experiments, detailed MS/MS experiments, the advanced Marfeys method, and a detailed analysis of the biosynthesis gene cluster, which was identified as well.

High-reactivity matrices increase the sensitivity of matrix enhanced secondary ion mass spectrometry.

Secondary ion mass spectrometry (SIMS) is a desorption/ionization method in which ions are generated by the impact of a primary ion beam on a sample. Classic matrix assisted laser desorption and ionization (MALDI) matrices can be used to increase secondary ion yields and decrease fragmentation in a SIMS experiment, which is referred to as matrix enhanced SIMS (ME-SIMS). Contrary to MALDI, the choice of matrices for ME-SIMS is not constrained by their photon absorption characteristics. This implies that matrix compounds that exhibit an insufficient photon absorption coefficient have the potential of working well with ME-SIMS. Here, we evaluate a set of novel derivatives of the classical MALDI matrices α-cyano-4-hydroxycinnamic acid (CHCA) and 2,5-dihydroxybenzoic acid (DHB) for usability in ME-SIMS. This evaluation was carried out using peptide mixtures of different complexity and demonstrates significant improvements in signal intensity for several compounds with insufficient UV absorption at the standard MALDI laser wavelengths. Our study confirms that the gas-phase proton affinity of a matrix compound is a key physicochemical characteristic that determines its performance in a ME-SIMS experiment. As a result, these novel matrices improve the performance of matrix enhanced secondary ion mass spectrometry experiments on complex peptide mixtures.

Comparison between the Matrices α-Cyano-4-hydroxycinnamic Acid and 4-Chloro-α-cyanocinnamic Acid for Trypsin, Chymotrypsin, and Pepsin Digestions by MALDI-TOF Mass Spectrometry

The performance of the recently developed 4-chloro-alpha-cyanocinnamic acid (Cl-CCA) matrix-assisted laser desorption ionization mass spectrometry (MALDI MS) matrix was investigated in comparison to the most widely used matrix alpha-cyano-4-hydroxycinnamic acid (CHCA). For this purpose, in-solution digestions of standard proteins in the low femtomole range with the proteases trypsin, chymotrypsin, and pepsin were used as analytes. For all protein-protease combinations, Cl-CCA revealed to be highly superior in terms of number of identified peptides, obtained sequence coverages and peptide detection reproducibility. A deeper inspection of the detected peptide signals with regard to both physicochemical peptide properties (their isoelectric point) and mass spectrometric performance (signal-to-noise ratios and mass accuracies) showed that the progress achieved with Cl-CCA is due to the detection of numerous acidic to neutral peptides. Moreover, the higher Cl-CCA sensitivity allowed for the detection of numerous additional phosphopeptides, all of which were verified by means of MS/MS investigations. The occurrence of strong signals of doubly charged peptides which is exclusively observed for the Cl-CCA matrix can be traced back to the peptide amino-acid composition, that is, the presence of a high number of basic amino acids (Arg, Lys, and His) and is thus more pronounced for nontryptic protein digests. These observed improvements well agree with an increased protonation reactivity of Cl-CCA and are more pronounced with a decreasing level of protease specificity and decreasing sample amounts.