Characterizing synthetic polymers by MALDI MS

Disentangle a Complex MALDI TOF Mass Spectrum of Polyethylene Glycols into Three Separate Spectra via Selective Formation of Protonated Ions and Sodium or Potassium Adducts

In MALDI TOF MS analysis, complicated mass spectra can usually be recorded for polymers with high affinities to protons and alkali metal ions. For these polymers, protonated ions and sodium and potassium adducts can often be formed concomitantly. By distributing these ions into three separate spectra of protonated ions, sodium adducts, and potassium adducts, significantly simplified spectra can be acquired. Mass spectra consisting of only sodium or potassium adducts can often be obtained by simply adding sodium salt and potassium salt, respectively. We report here a method to selectively generate protonated ions. A polyethylene glycol (PEG) sample with amino end groups was selected as the model polymer and α-cyano-4-hydroxycinnamic acid (CHCA) as the matrix. Octadecylamine (ODA) or a mixture of a tetrabutylammonium (TBA) salt and an ammonium salt was used as the co-matrix to inhibit the release of sodium and potassium ions and their related adducts into the MALDI gas phase plume. By depositing the polymer sample on top of a preloaded layer of CHCA with a co-matrix, the generation of Na⁺ and K⁺ adducts is suppressed, while [ODA + H]⁺ and NH₄⁺ released from the preloaded matrix layer can serve as protonation reagents to protonate the polymer molecules via proton transfer reactions. It is clearly demonstrated that disentangling a complex mass spectrum filled densely with various series of ions into three separate spectra, with each one consisting of only one type of ions, allows unambiguous identification of mass peaks and greatly helps the interpretation of MS results.

Chemical modification for improving catalytic performance of lipase B from Candida antarctica with hydrophobic proline ionic liquid

In this study, a series of proline ionic liquids with different lengths of hydrophobic alkyl on the side chain were used to modify the Candida Antarctic lipase B (CALB). The catalytic activity, thermal stability and tolerance to methanol and DMSO of the modified enzyme were all improved simultaneously. The optimum temperature changed from 55 to 60 ℃. The hydrophobicity and anion type of the modifier have important influence on the catalytic performance of CALB. CALB modified by [ProC₁₂][H₂PO₄] has a better effect. Under the optimal conditions, its hydrolysis activity was 3.0 times than that of the native enzyme, the catalytic efficiency Kcat/Km improved 2.8 times in aqueous phase, and the tolerance to organic solvent with strong polarity (50% methanol 2 h) was increased by 6.8 times. Fluorescence spectra and circular dichroism (CD) spectroscopy showed that the introduction of ionic liquids changed the microenvironment near the fluorophores of the enzyme protein, the α-helix decreased and β-sheet increased in the secondary structure of the modified enzymes. The root mean square deviation (RMSD), residue root mean square fluctuation (RMSF), radius of gyration (Rg), and solution accessible surface area (SASA) of [ProC₂][Br]-CALB, [ProC₁₂][Br]-CALB and native CALB were obtained for comparison by molecular dynamics simulation. The results of dynamics simulation were in good agreement with enzymology experiment. The introduction of ionic liquids can keep CALB in a better active conformation, and proline ionic liquids with long hydrophobic chains can significantly improve the surface hydrophobicity and overall rigidity of CALB. This research offers a new idea for rapid screening of efficient modifiers and provision of enzymes with high stability and activity for industrial application.

Cell and Tissue Imaging by TOF-SIMS and MALDI-TOF: An Overview for Biological and Pharmaceutical Analysis

The potential of mass spectrometry imaging (MSI) has been demonstrated in cell and tissue research since 1970. MSI can reveal the spatial distribution of a wide range of atomic and molecular ions detected from biological sample surfaces, it is a powerful and valuable technique used to monitor and detect diverse chemical and biological compounds, such as drugs, lipids, proteins, and DNA. MSI techniques, notably matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) and time of flight secondary ion mass spectrometry (TOF-SIMS), witnessed a dramatic upsurge in studying and investigating biological samples especially, cells and tissue sections. This advancement is attributed to the submicron lateral resolution, the high sensitivity, the good precision, and the accurate chemical specificity, which make these techniques suitable for decoding and understanding complex mechanisms of certain diseases, as well as monitoring the spatial distribution of specific elements, and compounds. While the application of both techniques for the analysis of cells and tissues is thoroughly discussed, a briefing of MALDI-TOF and TOF-SIMS basis and the adequate sampling before analysis are briefly covered. The importance of MALDI-TOF and TOF-SIMS as diagnostic tools and robust analytical techniques in the medicinal, pharmaceutical, and toxicology fields is highlighted through representative published studies.

A general purpose MALDI matrix for the analyses of small organic, peptide and protein molecules

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS) has been widely applied for the analysis of large biomolecules. The emergence of inorganic material substrates and new organic matrices extends the use of MALDI MS for small molecule analyses. However, there are usually preferred matrices for different types of analytes. Here, an organic compound, 4-hydroxy-3-nitrobenzonitrile, was found to be a general purpose matrix for the analyses of small organic, peptide and protein molecules. In particular, 4-hydroxy-3-nitrobenzonitrile has a strong UV absorption property, and it provides a clean background in the low mass range. Its analytical performances as a UV-laser matrix were demonstrated for different types of analytes, including organic drugs, peptides, proteins, mouse brain tissue and bacteria. Compared with commercial matrices, this new matrix has better performances when analyzing small molecules, such as drugs, peptides and lipids, while it has similar performances when analyzing proteins.

Advances in glycosaminoglycan detection

BACKGROUND

Glycosaminoglycans (GAGs) are negatively charged long linear (highly sulfated) polysaccharides consisting of repeating disaccharide units that are expressed on the surfaces of all nucleated cells. The expression of GAGs is required for embryogenesis, regulation of cell growth and proliferation, maintenance of tissue hydration, and interactions of the cells via receptors. Mucopolysaccharidoses (MPS) are caused by deficiency of specific lysosomal enzymes that result in the accumulation of GAGs in multiple tissues leading to organ dysfunction. Therefore, GAGs are important biomarkers for MPS. Without any treatment, patients with severe forms of MPS die within the first two decades of life.

SCOPE OF REVIEW

Accurate measurement of GAGs is important to understand the diagnosis and pathogenesis of MPS and to monitor therapeutic efficacy before, during, and after treatment of the disease. This review covers various qualitative and quantitative methods for measurement of GAGs, including dye specific, thin layer chromatography (TLC), capillary electrophoresis, high-performance liquid chromatography (HPLC), liquid chromatography-tandem mass spectrometry (LC-MS/MS), gas chromatography, ELISA, and automated high-throughput mass spectrometry. Major conclusion: There are several methods for GAG detection however, specific GAG detection in the various biological systems requires rapid, sensitive, specific, and cost-effective methods such as LC-MS/MS.

GENERAL SIGNIFICANCE

This review will describe different methods for GAG detection and analysis, including their advantages and limitation.

Hydrogels with multiple clickable anchor points: synthesis and characterization of poly(furfuryl glycidyl ether)-block-poly(ethylene glycol) macromonomers

Functional polyacrylamide hydrogels containing multiple furfuryl anchor points for Diels–Alder reactions were prepared employing new macromonomers.

Synthesis of Polystyrene-Coated Superparamagnetic and Ferromagnetic Cobalt Nanoparticles

Polystyrene-coated cobalt nanoparticles (NPs) were synthesized through a dual-stage thermolysis of cobalt carbonyl (Co₂(CO)₈). The amine end-functionalized polystyrene surfactants with varying molecular weight were prepared via atom-transfer radical polymerization technique. By changing the concentration of these polymeric surfactants, Co NPs with different size, size distribution, and magnetic properties were obtained. Transmission electron microscopy characterization showed that the size of Co NPs stabilized with lower molecular weight polystyrene surfactants (M_n = 2300 g/mol) varied from 12–22 nm, while the size of Co NPs coated with polystyrene of middle (M_n = 4500 g/mol) and higher molecular weight (M_n = 10,500 g/mol) showed little change around 20 nm. Magnetic measurements revealed that the small cobalt particles were superparamagnetic, while larger particles were ferromagnetic and self-assembled into 1-D chain structures. Thermogravimetric analysis revealed that the grafting density of polystyrene with lower molecular weight is high. To the best of our knowledge, this is the first study to obtain both superparamagnetic and ferromagnetic Co NPs by changing the molecular weight and concentration of polystyrene through the dual-stage decomposition method.

Synthesis of Polypyrrole Induced by [Fe(CN)₆]3- and Redox Cycling of [Fe(CN)₆]4-/[Fe(CN)₆]3

Chemical synthesis of the conducting polymer polypyrrole induced by [Fe(CN)₆]^3- is reported. Reaction kinetics were characterized spectrophotometrically. Reaction rate was evaluated at several different pH levels in the presence of [Fe(CN)₆]^3- and [Fe(CN)₆]^4- ions. The formation of polypyrrole at aerobic and anaerobic conditions was evaluated. We report that at anaerobic conditions [Fe(CN)₆]^4- cannot initiate oxidative polymerization, while its oxidized form [Fe(CN)₆]^3- successfully initiates and maintains the pyrrole polymerization reaction. The formation of polypyrrole was also observed in the solution containing a pyrrole monomer, [Fe(CN)₆]^4- and dissolved oxygen due to re-oxidation (redox cycling) of [Fe(CN)₆]^4- into [Fe(CN)₆]^3- by dissolved oxygen. Experiments to determine the polymerization reaction rate were performed and showed the highest rate in the presence of 0.5 mM of [Fe(CN)₆]^3- at pH 9.0, while the polymerization reaction performed at pH 7.0 was determined as the slowest. This investigation opens new horizons for the application of [Fe(CN)₆]^4-/[Fe(CN)₆]^3--based redox cycling reactions in the synthesis of the conducting polymer polypyrrole and potentially in the formation of other conducting polymers which can be formed by oxidative polymerization.

Characterization of Synthetic Polymers via Matrix Assisted Laser Desorption Ionization Time of Flight (MALDI-TOF) Mass Spectrometry

There are many techniques that can be employed in the characterization of synthetic homopolymers, but few provide as useful of information for end group analysis as matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS). This tutorial demonstrates methods for optimization of the sample preparation, spectral acquisition, and data analysis of synthetic polymers using MALDI-TOF MS. Critical parameters during sample preparation include the selection of the matrix, identification of an appropriate cationization salt, and tuning the relative proportions of the matrix, cation, and analyte. The acquisition parameters, such as mode (linear or reflector), polarization (positive or negative), acceleration voltage, and delay time, are also important. Given some knowledge of the chemistry involved to synthesize the polymer and optimizing both the data acquisition parameters and the sample preparation conditions, spectra should be obtained with sufficient resolution and mass accuracy to enable the unambiguous determination of the end groups of most homopolymers (masses below 10,000) in addition to the repeat unit mass and the overall molecular weight distribution. Though demonstrated on a limited set of polymers, these general techniques are applicable to a much wider range of synthetic polymers for determining mass distributions, though end group determination is only possible for homopolymers with narrow dispersity.

Use of MALDI-TOF mass spectrometry fingerprinting to characterize Enterococcus spp. and Escherichia coli isolates

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) is a faster and more accurate method to identify intact bacteria than conventional microbiology and/or molecular biology methods. The MALDI-TOF MS method is potentially applicable in diagnostic laboratories to characterize commensal bacterial species, some of which are major pathogens, from human or animal gastrointestinal tracts. The aim of this study was to analyze at the cluster and statistical level the capacity of MALDI-TOF MS to distinguish between previously characterized enterococci and Escherichia coli isolated from wild birds of the Azores archipelago. Soluble proteins were extracted from intact cell cultures of 60 isolates of Enterococcus spp. and 60 isolates of E. coli by an expedient method. MALDI-TOF MS was used to obtain 1200 mass spectra that were statistically analyzed and compared. A total of 215 distinct mass-to-charge (m/z) peaks were obtained, including a peak at m/z 4428±3, which is exclusively found in spectra from Enterococcus isolates, and peaks at m/z 5379±3 and m/z 6253±3, which are only detected in spectra from E. coli isolates. By processing mass spectra and analyzing them statistically with MassUp software, including principal component analysis (PCA) and clustering, it was possible to correctly distinguish between isolates of Enterococcus and Escherichia genera. The results of the proteomic analysis confirm that these tools could be used to characterize whole bacterial cells. In the future, with an optimized protocol for obtaining plasmid-associated proteins and the further development of bioinformatics methods, it is likely that mass peak characteristic of antimicrobial resistance will be detected, increasing the potential usefulness of MALDI-TOF in routine clinical assays.

BIOLOGICAL SIGNIFICANCE

This study highlights the importance of MALDI-TOF MS in the rapid and reliable identification of bacteria by whole-cell analysis. The mass spectrometry approach performed in this study further contributes for the microbial biomarker discovery culminating in a preferable bacteria identification and antimicrobial resistance tool for the future of clinical microbiology. This article is part of a Special Issue entitled: HUPO 2014.

Quantitative analysis of polyhexamethylene guanidine (PHMG) oligomers via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with an ionic-liquid matrix

RATIONALE

Quantifying polymers by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) with a conventional crystalline matrix generally suffers from poor sample-to-sample or shot-to-shot reproducibility. An ionic-liquid matrix has been demonstrated to mitigate these reproducibility issues by providing a homogeneous sample surface, which is useful for quantifying polymers. In the present study, we evaluated the use of an ionic liquid matrix, i.e., 1-methylimidazolium α-cyano-4-hydroxycinnamate (1-MeIm-CHCA), to quantify polyhexamethylene guanidine (PHMG) samples that impose a critical health hazard when inhaled in the form of droplets.

METHODS

MALDI-TOF mass spectra were acquired for PHMG oligomers using a variety of ionic-liquid matrices including 1-MeIm-CHCA. Calibration curves were constructed by plotting the sum of the PHMG oligomer peak areas versus PHMG sample concentration with a variety of peptide internal standards.

RESULTS

Compared with the conventional crystalline matrix, the 1-MeIm-CHCA ionic-liquid matrix had much better reproducibility (lower standard deviations). Furthermore, by using an internal peptide standard, good linear calibration plots could be obtained over a range of PMHG concentrations of at least 4 orders of magnitude.

CONCLUSIONS

This study successfully demonstrated that PHMG samples can be quantitatively characterized by MALDI-TOFMS with an ionic-liquid matrix and an internal standard.

Enantioselectivity and catalysis improvements of Pseudomonas cepacia lipase with Tyr and Asp modification

A concise strategy to improve the p-nitrophenyl palmitate catalytic activity and enantioselectivity towards secondary alcohols of PcL is described.

PAS-cal: a generic recombinant peptide calibration standard for mass spectrometry

We describe the design, preparation, and mass-spectrometric characterization of a new recombinant peptide calibration standard with uniform biophysical and ionization characteristics for mass spectrometry. "PAS-cal" is an artificial polypeptide concatamer of peptide cassettes with varying lengths, each composed of the three small, chemically stable amino acids Pro, Ala, and Ser, which are interspersed by Arg residues to allow site-specific cleavage with trypsin. PAS-cal is expressed at high yields in Escherichia coli as a Small Ubiquitin-like MOdifier (SUMO) fusion protein, which is easily purified and allows isolation of the PAS-cal moiety after SUMO protease cleavage. Upon subsequent in situ treatment with trypsin, the PAS-cal polypeptide yields a set of four defined homogeneous peptides in the range from 2 to 8 kDa with equal mass spacing. ESI-MS analysis revealed a conveniently interpretable raw spectrum, which after deconvolution resulted in a very simple pattern of four peaks with similar ionization signals. MALDI-MS analysis of a PAS-cal peptide mixture comprising both the intact polypeptide and its tryptic fragments revealed not only the four standard peptides but also the singly and doubly charged states of the intact concatamer as well as di- and trimeric adduct ion species between the peptides, thus augmenting the observable m/z range. The advantageous properties of PAS-cal are most likely a result of the strongly hydrophilic and conformationally disordered PEG-like properties of the PAS sequences. Therefore, PAS-cal offers an inexpensive and versatile recombinant peptide calibration standard for mass spectrometry in protein/peptide bioanalytics and proteomics research, the composition of which may be further adapted to fit individual needs.

Quantitative analysis of polysorbates 20 and 40 by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

RATIONALE

Polysorbates are nonionic surfactants that consist primarily of fatty acid esters of polyethoxy sorbitan. This study proved that polysorbates can be quantitatively analyzed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Using MALDI-TOF MS, relative intensity and concentration ratios were correlated, and extensive research was conducted to understand the influencing factors.

METHODS

Polysorbate 20 and 40 were mixed in the desired ratios and irradiated with a N2 laser. MALDI-TOF mass spectra were recorded in positive ion mode to test the linearity. All commercial polysorbates were analyzed to determine the relative concentration of the components using the same method.

RESULTS

The relative peak intensity ratio as a function of the relative concentration ratio was analyzed, and a reasonably good linearity (R(2) = 0.987 for polysorbate 20) was obtained. This study illustrates the process of converting the analyte signal response into the concentration, supporting the notion that quantitative MALDI-TOF MS can be used to analyze polymers. MALDI-TOF MS analysis of commercial polysorbate formulations revealed a complex mixture of oligomers that was related to the fatty acid composition.

CONCLUSIONS

Polysorbates 20 and 40 were characterized, and the simultaneous quantitative analysis of polysorbate 20 was reported. This method requires no tedious sample pretreatment. Therefore, it is a promising method for the rapid simultaneous quantitation of polysorbates 20 and 40.

GRGDS-Functionalized Poly(lactide)-graft-poly(ethylene glycol) Copolymers: Combining Thiol-Ene Chemistry with Staudinger Ligation

A tri(ethylene glycol)-containing lactide analogue was synthesized via thiol-ene chemistry between a bi-functional triethylene glycol and allyl lactide. Subsequent tin-octoate-catalyzed ring-opening polymerization yielded well-defined poly(lactide)-graft-poly(ethylene glycol) copolymers with molecular weights of 6000 g/mol and polydispersity indices of 1.6. The tri(ethylene glycol) chains along the copolymers contain azide termini that are capable of 'click'-type postpolymerization functionalization. The utility of this strategy was demonstrated via successful Staudinger ligation to install the Gly-Arg-Gly-Asp-Ser (GRGDS) peptide.

Possible evidence of amide bond formation between sinapinic acid and lysine-containing bacterial proteins by matrix-assisted laser desorption/ionization (MALDI) at 355 nm

We previously reported the apparent formation of matrix adducts of 3,5-dimethoxy-4-hydroxy-cinnamic acid (sinapinic acid or SA) via covalent attachment to disulfide bond-containing proteins (HdeA, Hde, and YbgS) from bacterial cell lysates ionized by matrix-assisted laser desorption/ionization (MALDI) time-of-flight-time-of-flight tandem mass spectrometry (TOF-TOF-MS/MS) and post-source decay (PSD). We also reported the absence of adduct formation when using α-cyano-4-hydroxycinnamic acid (CHCA) matrix. Further mass spectrometric analysis of disulfide-intact and disulfide-reduced over-expressed HdeA and HdeB proteins from lysates of gene-inserted E. coli plasmids suggests covalent attachment of SA occurs not at cysteine residues but at lysine residues. In this revised hypothesis, the attachment of SA is preceded by formation of a solid phase ammonium carboxylate salt between SA and accessible lysine residues of the protein during sample preparation under acidic conditions. Laser irradiation at 355 nm of the dried sample spot results in equilibrium retrogradation followed by nucleophilic attack by the amine group of lysine at the carbonyl group of SA and subsequent amide bond formation and loss of water. The absence of CHCA adducts suggests that the electron-withdrawing effect of the α-cyano group of this matrix may inhibit salt formation and/or amide bond formation. This revised hypothesis is supported by dissociative loss of SA (-224 Da) and the amide-bound SA (-206 Da) from SA-adducted HdeA and HdeB ions by MS/MS (PSD). It is proposed that cleavage of the amide-bound SA from the lysine side-chain occurs via rearrangement involving a pentacyclic transition state followed by hydrogen abstraction/migration and loss of 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-ynal (-206 Da).

Serum tolerance and endosomal escape capacity of histidine-modified pDNA-loaded complexes based on polyamidoamine dendrimer derivatives

Aiming to aid polyamidoamine (PAMAM, generation 4, PG4) to overcome gene delivery barriers like extrinsic serum inhibition, intrinsic cytotoxicity and lysosome digestion, histidine motifs modified PAMAM was prepared. The histidine activated PAMAM generation 4 (HPG4) was synthesized via aminolysis reaction and characterized by 1H NMR spectrum and MALDI-TOF-MS. Cytotoxicity profiles of HPG4 on MD-MB-231 cells were significantly improved in the form of polymer and polymer/DNA complexes comparing to PG4. The luciferase protein expression level of HPG4 was 20-, 2.7- and 1.2- fold higher than that of PG4, SuperFect and PEI 25k. Most importantly, flow cytometry and gene transfection studies showed that histidine motifs of HPG4 not only acted as enhancer for faster cellular uptake, but also played an important role on enhancing serum tolerance of the system on cellular uptake and transfection. Among the serum concentrations of 10%-50%, HPG4 showed 10-100 folds higher transfection efficiency than PG4. Intracellular fate observation conducted by confocal microscope provided visual and quantitative evidence that endsomal escape efficiency of HPG4 system was higher than that of PG4. Lastly, the endosomal escape mechanism of HPG4 system was analyzed by endosome destabilization and proton pump inhibition treatment. Collectively, compared to PG4/pDNA, HPG4/pDNA showed improvement on cellular uptake, serum tolerance, cytotoxicity profile, and endosomal escape.

Matrix-assisted laser desorption/ionization mass spectrometric analysis of aliphatic biodegradable photoluminescent polymers using new ionic liquid matrices

In this study, two novel ionic liquid matrices (ILMs), N,N-diisopropylethylammonium 3-oxocoumarate and N,N-diisopropylethylammonium dihydroxymonooxoacetophenoate, were tested for the structural elucidation of recently developed aliphatic biodegradable polymers by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). The polymers, formed by a condensation reaction of three components, citric acid, octane diol, and an amino acid, are fluorescent, but the exact mechanism behind their luminescent properties has not been fully elucidated. In the original studies, which introduced the polymer class (J. Yang et al., Proc. Natl. Acad. Sci. USA 2009, 106, 10086-10091), a hyper-conjugated cyclic structure was proposed as the source for the photoluminescent behavior. With the use of the two new ILMs, we present evidence that supports the presence of the proposed cyclization product. In addition, the new ILMs, when compared with a previously established ILM, N,N-diisopropylethylammonium α-cyano-3-hydroxycinnimate, provided similar signal intensities and maintained similar spectral profiles. This research also established that the new ILMs provided good spot-to-spot reproducibility and high ionization efficiency compared with corresponding crystalline matrix preparations. Many polymer features revealed through the use of the ILMs could not be observed with crystalline matrices. Ultimately, the new ILMs highlighted the composition of the synthetic polymers, as well as the loss of water that was expected for the formation of the proposed cyclic structure on the polymer backbone.

Observation of poly(ethylene glycol) clusters with the chlorine anion in the gas phase under electrospray conditions

It is demonstrated herein that poly(ethylene glycol) (PEG) oligomers can form stable complexes with the chlorine anion in the gas phase as evidenced by results from electrospray ionization mass spectrometry (ESI-MS) and molecular dynamics simulation. While the formation of crown-ether-like structures by acyclic polyethers in their complexes with alkali metal cations coordinated by the ether oxygen atoms has been extensively studied, the possibility of forming 'inversed' quasi-cyclic structures able to bind a monoatomic anion has not been proved till now. We have observed the formation of stable gas-phase complexes of oligomers of PEG-400 with the Cl(-) anion experimentally by ESI-MS for the first time. It is suggested that a necessary precondition for obtaining the polyether-chlorine anion clusters is the prevention of the formation of neutral ion pairs. Molecular dynamics simulation has demonstrated the wrapping of the Cl(-) anion by the PEG chain, to stabilize the PEG(n)•Cl(-) clusters in the gas phase. The conformation of the polyether chain in such quasi-cyclic or quasi-helical complexes is 'inversed' compared with that in the complexes with cations: that is its hydrogen atoms are turned towards the central anion. Awareness of the possibility of the Cl(-) anion being trapped in quasi-cyclic PEG structures may be of practical importance when considering the intermolecular interactions of PEGs.

Covalent attachment and dissociative loss of sinapinic acid to/from cysteine-containing proteins from bacterial cell lysates analyzed by MALDI-TOF-TOF mass spectrometry

We report covalent attachment via a thiol ester linkage of 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid or SA) to cysteine-containing protein biomarkers from bacterial cell lysates of E. coli analyzed by matrix-assisted laser desorption/ionization (MALDI) mass spectrometry when using SA as the matrix. Evidence to support this conclusion is the appearance of additional peaks in the MS spectra when using SA, which are absent when using alpha-cyano-4-hydroxycinnamic acid (HCCA). The additional peaks appear at a mass-to-charge (m/z) approximately 208 greater to the m/z of a more abundant protein ion peak. Protein biomarkers were identified by tandem mass spectrometry (MS/MS) using a MALDI time-of-flight/time-of-flight (TOF-TOF) mass spectrometer and top-down proteomics. Three protein biomarkers, HdeA, HdeB, and homeobox or YbgS (each containing two cysteine residues) were identified as having reactivity to SA. Non-cysteine-containing protein biomarkers showed no evidence of reactivity to SA. MS ions and MS/MS fragment ions were consistent with covalent attachment of SA via a thiol ester linkage to the side-chain of cysteine residues. MS/MS of a protein biomarker ion with a covalently attached SA revealed fragment ion peaks suggesting dissociative loss SA. We propose dissociative loss of SA is facilitated by a pentacyclic transition-state followed by proton abstraction of the beta-hydrogen of the bound SA by a sulfur lone pair followed by dissociative loss of 3-(4-hydroxy-3,5-dimethoxyphenyl)prop-2-ynal. The apparent reactivity of SA to cysteine/disulfide-containing proteins may complicate identification of such proteins, however the apparent differential reactivity of SA and HCCA toward cysteine/disulfide-containing proteins may be exploited for identification of unknown cysteine-containing proteins.

The suitability of different DHB isomers as matrices for the MALDI-TOF MS analysis of phospholipids: which isomer for what purpose?

Although the analysis of large biomolecules is the prime application of matrix-assisted laser desorption and ionization time-of-flight mass spectrometry (MALDI-TOF MS), there is also increasing interest in lipid analysis. Since lipids possess relatively small molecular weights, matrix signals should be as small as possible to avoid overlap with lipid peaks. Although 2,5-dihydroxybenzoic acid (DHB) is an established MALDI matrix, the question whether just this isomer is ideal for lipid analysis was not yet addressed. UV absorptions of all six DHB isomers were determined and their laser desorption spectra recorded. In addition, all isomers were used as matrices to record positive and negative ion mass spectra of selected phospholipids (phosphatidylcholine and -serine): In the order 2,5-, 2,6-, 2,3- and 2,4-DHB, the quality of the positive ion lipid spectra decreases. This correlates well with the decreasing acidity of the applied DHB isomers. The 3,4- and 3,5- isomers give only very weak positive ion signals especially of acidic lipids. In contrast, the most suitable matrices in the negative ion mode are 2,5-, 2,4- and 3,5-DHB. 2,6-DHB does not provide any signal in the negative ion mode due to its marked acidity. Finally, differences in the crystallization behavior of the pure matrix and the matrix/lipid co-crystals were also monitored by atomic force microscopy (AFM): 2,5-DHB gave the smallest crystals and the skinniest layer. It is concluded that basically all DHB isomers can be used as MALDI matrices but the 2,5-isomer represents the most versatile compound.

A paramagnetic contrast agent with myeloperoxidase-sensing properties

Myeloperoxidase (MPO) is an essential component of inflammatory response in norm and pathology. With an ultimate goal of non-invasive imaging of MPO we used a gadolinium-chelating bis(5-hydroxytrytamide) derivative of diethylenetetraamine pentaacetic acid (L1-Gd3+ salt) as a paramagnetic sensor of enzymatic activity. We tested whether L1-Gd3+ is active in reducing the oxidized form of myeloperoxidase, generated as a result of hydrogen peroxide reduction by the enzyme. We expected that upon activation by MPO/H2O2 L1-Gd3+ would not only oligomerise but also bind to other macromolecules present in the media and that the overall process will give rise to a net T1-weighted MRI signal increase.

Quantitation of synthetic polymers using an internal standard by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

MALDI-TOF MS is utilized to perform quantitative analysis on synthetic polymers. Despite the inherent limitations of MALDI, good quantitative results have been obtained in the three sets of experiments described here. An internal standard with similar molecular properties as the analytes is introduced. Plots of relative integrated intensity ratios as a function of theoretical ratios of stoichiometry are drawn based on the results. The satisfactory slopes and correlation coefficients illustrated the practicality of quantitative measurement by MALDI-TOF MS.

Practical quantitative biomedical applications of MALDI-TOF mass spectrometry

Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOFMS) is used to obtain fast and accurate determinations of molecular mass, but quantitative determinations are generally made by other techniques. In this study we illustrate the practical utility of automated MALDI-TOFMS as a tool for quantifying a diverse array of biomolecules covering an extensive molecular weight range, and present in biological extracts and fluids. Growth hormone was measured in rat pituitary tissue; insulin in human pancreatic tissue; homovanillic acid in human urine; and LVV-hemorphin-7, epinephrine and norepinephrine in human adrenal and pheochromocytoma tissues. Internal standards including compounds of similar molecular weight, structural analogs or isotopomers were incorporated into each analysis. We report on the current practical limitations of quantitative MALDI-TOFMS and highlight some of the potential benefits of this technique as a quantitative tool.

Characterization of oligomeric polypropyleneglycol acrylate by GC, SFC and MALDI-TOF-MS

Polypropyleneglycol acrylate (PGA), one of the typical acrylic oligomers manufactured industrially, was comprehensively characterized by gas chromatography (GC), supercritical fluid chromatography (SFC) and matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). The homologous series of polypropyleneglycol diacrylate (DA), polypropyleneglycol monoacrylate (MA), and unreacted polypropyleneglycol (PG) were observed as Na adducts in the MALDI-MS spectra of the PGA samples. The relative intensities of these peaks reflect the distributions of the homologues, although their accurate quantification was generally difficult because of change in the ionization efficiency depending on the chemical structure and the molecular weight of the species. On the other hand, the DA and the MA homologues were observed in the chromatograms obtained by SFC in a temperature-programming mode, while the PG homologues were not detected under the given SFC conditions using UV detection. Here, the determination of the degree of polymerization of each component in the chromatograms was accomplished through SFC fractionation for the corresponding peaks, followed again by MALDI-TOF-MS measurement. Furthermore, most of the components in the PGA samples were almost completely separated in the resulting gas chromatograms, and their unequivocal assignments were made also using the retention data on the gas chromatograms of the SFC fractions. As for the quantitative analysis, the relative abundances among DA, MA and PG for lower degrees of polymerization can be easily calculated based on the observed gas chromatograms, whereas the distribution of DA and MA can be estimated from the observed SFC data even for the relatively higher molecular weight fractions, which are generally difficult to determine accurately by GC because of their lower volatility. These results demonstrated that even the complex PGA samples were able to be characterized in detail by using GC, SFC and MALDI-TOF-MS complementarily.

Detailed analysis of alpha,omega-bis(4-hydroxybutyl) poly(dimethylsiloxane) using GPC-MALDI TOF mass spectrometry

In this study the prepolymer alpha,omega-bis(4-hydroxybutyl) poly(dimethylsiloxane), used in the formulation of oxygen permeable films, is evaluated by gel permeation chromatography (GPC) combined with matrix assisted laser desorption ionization (MALDI) time of flight (TOF) mass spectrometry (MS). Two unexpected mass distributions are observed in the mass spectra. Reaction schemes for the formation of these distributions are proposed. A solution phase trimethylsilane end group modification was performed on the prepolymer to determine whether the unexpected mass distributions occur as impurities from synthesis or as artifacts from the MS process. Evaluation of the TMS modified prepolymer indicates the unexpected mass distributions indeed occur as impurities from the synthetic procedure. Average molecular weight values are determined by traditional GPC, direct MALDI-TOF MS, and GPC-MALDI-TOF MS methods and the results are compared.

Identification of the modifying sites of mono-PEGylated salmon calcitonins by capillary electrophoresis and MALDI-TOF mass spectrometry

A capillary electrophoretic method (CE) was developed for the determination of the PEG-modification sites of three positional isomers of mono-PEG modified salmon calcitonins (mono-PEG-sCTs). Resistance to proteolytic degradation on the PEG modification sites resulted in different patterns of CE electropherograms for the tryptic digested mono-PEG-sCTs isomers, and the PEG modification sites were assigned accordingly. The PEG-modification sites were also confirmed directly by determining the molecular masses of the tryptic digested PEG-modified fragments of respective mono-PEG-sCT by the matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry.

Separation and quantification of the linear and cyclic structures of polyamide-6 at the critical point of adsorption

The linear and cyclic structures of polyamide-6 were separated by liquid chromatography at critical conditions (LCCC) and identified with different mass spectrometric (MS) techniques and quantitated by LCCC with evaporative light-scattering detection (ELSD). Electrospray ionization MS was not suitable to identify the higher cyclic structures. For this purpose, matrix-assisted laser desorption ionization time-of-flight MS performed better and cyclic and linear structures were oligomerically resolved and separately identified in the mass spectrometer. The highest cyclic structure present and detected was the cyclic pentacontamer. It could be demonstrated that cyclic and linear oligomers follow different ionization and fragmentation routes/patterns. Quantification with ELSD of the components separated by LCCC using a universal calibration curve or an iterative procedure was developed. An area correction to account for different peak widths of coeluting components improves precision and accuracy of the calibration curve and improves quantitation accuracy for the samples analyzed. With these corrected values, no molecular mass dependency was observed for the cyclic and linear structures. Under critical conditions, the linear and cyclic structures of polyamide-6 were separated, identified and quantified.