Cation identity dependence of crown ether photonic crystal Pb<Superscript>2+</Superscript> sensing

We quantitatively modeled the volume phase transition of a hydrogel containing a crystalline colloidal array with a crown ether ligand which binds Pb²⁺. The hydrogel volume response and the wavelength diffracted depend on the Pb²⁺ concentration and on both the ionic strength and the valence of the nonbinding ionic species. We successfully modeled the response of this hydrogel Pb²⁺ sensor to ionic strength and the cation valence of the added salts. Figure Cation identity dependence of crown ether photonic crystal Pb⁺ sensing     

Pattern recognition analysis for <Superscript>1</Superscript>H NMR spectra of plasma from hemodialysis patients

¹H NMR spectroscopic and pattern recognition-based methods (NMR-PR) were applied to the metabolic profiling studies on hemodialysis (HD). Plasma samples were collected from 37 patients before and after HD and measured by 600 MHz NMR spectroscopy. Each spectrum was data-processed and subjected to principal component analysis for pattern recognition. Spectral patterns of plasma between pre- and post-dialyses were clearly discriminated, together with significant fluctuations in the levels of creatinine, trimethylamine-N-oxide, glucose, lactate, and acetate, which were quantitated. We have first observed the significant elevation of lactate levels in post-dialysis plasma. The present study has demonstrated the high feasibility of NMR-PR method for monitoring the dialysis condition and comprehensive profiling of the change of low-molecular-weight metabolites in HD. Figure PCA for 1H NMR spectra of plasma from HD patients     

Total selenium and selenomethionine in pharmaceutical yeast tablets: assessment of the state of the art of measurement capabilities through international intercomparison CCQM-P86

Results of an international intercomparison study (CCQM-P86) to assess the analytical capabilities of national metrology institutes (NMIs) and selected expert laboratories worldwide to accurately quantitate the mass fraction of selenomethionine (SeMet) and total Se in pharmaceutical tablets of selenised-yeast supplements (produced by Pharma Nord, Denmark) are presented. The study, jointly coordinated by LGC Ltd., UK, and the Institute for National Measurement Standards, National Research Council of Canada (NRCC), was conducted under the auspices of the Comité Consultatif pour la Quantité de Matière (CCQM) Inorganic Analysis Working Group and involved 15 laboratories (from 12 countries), of which ten were NMIs. Apart from a protocol for determination of moisture content and the provision of the certified reference material (CRM) SELM-1 to be used as the quality control sample, no sample preparation/extraction method was prescribed. A variety of approaches was thus used, including single-step and multiple-step enzymatic hydrolysis, enzymatic probe sonication and hydrolysis with methanesulfonic acid for SeMet, as well as microwave-assisted acid digestion and enzymatic probe sonication for total Se. For total Se, detection techniques included inductively coupled plasma (ICP) mass spectrometry (MS) with external calibration, standard additions or isotope dilution MS (IDMS), inductively coupled plasma optical emission spectrometry , flame atomic absorption spectrometry and instrumental neutron activation analysis. For determination of SeMet in the tablets, five NMIs and three academic/institute laboratories (of a total of five) relied upon measurements using IDMS. For species-specific IDMS measurements, an isotopically enriched standard of SeMet (⁷⁶Se-enriched SeMet) was made available. A novel aspect of this study relies on the approach used to distinguish any errors which arise during analysis of a SeMet calibration solution from those which occur during analysis of the matrix. To help those participants undertaking SeMet analysis to do this, a blind sample in the form of a standard solution of natural abundance SeMet in 0.1 M HCl (with an expected value of 956 mg kg⁻¹ SeMet) was provided. Both high-performance liquid chromatography (HPLC)–ICP-MS or gas chromatography (GC)–ICP-MS and GC-MS techniques were used for quantitation of SeMet. Several advances in analytical methods for determination of SeMet were identified, including the combined use of double IDMS with HPLC-ICP-MS following extraction with methanesulfonic acid and simplified two-step enzymatic hydrolysis with protease/lipase/driselase followed by HPLC-ICP-IDMS, both using a species-specific IDMS approach. Overall, satisfactory agreement amongst participants was achieved; results averaged 337.6 mg kg⁻¹ (n = 13, with a standard deviation of 9.7 mg kg⁻¹) and 561.5 mg kg⁻¹(n = 11, with a standard deviation of 44.3 mg kg⁻¹) with median values of 337.6 and 575.0 mg kg⁻¹ for total Se and SeMet, respectively. Recovery of SeMet from SELM-1 averaged 95.0% (n = 9). The ability of NMIs and expert laboratories worldwide to deliver accurate results for total Se and SeMet in such materials (selensied-yeast tablets containing approximately 300 mg kg⁻¹ Se) with 10% expanded uncertainty was demonstrated. The problems addressed in achieving accurate quantitation of SeMet in this product are representative of those encountered with a wide range of organometallic species in a number of common matrices. Figure Looking into the quantitative speciation of selenium in pharmaceutical supplements Photo courtesy of LGC.     

A novel tryptamine-appended rhodamine-based chemosensor for selective detection of Hg2+ present in aqueous medium and its biological applications

A novel rhodamine–tryptamine conjugate–based fluorescent and chromogenic chemosensor (RTS) for detection of Hg²⁺ present in water was reported. After gradual addition of Hg²⁺ in aqueous methanol solution of RTS, a strong orange fluorescence and deep-pink coloration were observed. The probe showed high selectivity towards Hg²⁺ compared to other competitive metal ions. The 1:1 binding stoichiometry between RTS and Hg²⁺ was established by Job’s plot analysis and mass spectroscopy. Initial studies showed that the synthesized probe RTS possessed fair non-toxicity and effectively passed through cell walls of model cell systems, viz., human neuroblastoma (SHSY5Y) cells and cervical cells (HeLa) to detect intercellular Hg²⁺ ions, signifying its utility in biological system. The limit of detection (LOD) was found to be 2.1 nM or 0.42 ppb by fluorescence titration. Additionally, the potential relevance of synthesized chemosensor for detecting Hg²⁺ ions in environmental water samples has been demonstrated.

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Analysis of palladium concentrations in airborne particulate matter with reductive co-precipitation, He collision gas, and ID-ICP-Q-MS

The concentration of platinum group elements (PGE) in the environment has increased significantly in the last 20 years mainly due to their use as catalysts in automotive catalytic converters. The quantitation of these metals in different environmental compartments is, however, challenging due to their very low concentrations and the presence of interfering matrix constituents when inductively coupled plasma-mass spectrometry (ICP-MS) is used for analysis. Previously, the research focus was on the analysis of platinum (Pt) and rhodium (Rh). However, due to the increasing use of palladium (Pd) in automotive catalytic converters, quantitation of this element in airborne particulate matter (PM) is also needed. Compared to Pt and Rh, measurements of Pd using ICP-MS are plagued by greater molecular interferences arising from elements such as copper (Cu), zinc (Zn) strontium (Sr), yttrium (Y), and zirconium (Zr). The aim of this study was to evaluate the applicability of reductive co-precipitation procedures using both mercury (Hg) and tellurium (Te) for the pre-concentration of Pd from airborne PM. Furthermore, helium (He) was tested as a collision gas for isotope dilution-inductively coupled plasma-quadrupole-mass spectrometry (ID-ICP-Q-MS) to measure Pd in the Hg and Te precipitates. Airborne PM samples (PM₁₀) were collected from Neuglobsow (Brandenburg, north-eastern Germany) and Deuselbach (Rhineland-Palatinate, south-western Germany), considered to represent background levels, and from the city Frankfurt am Main (Hesse, Germany), a high-traffic area. Samples were first digested with aqua regia in a high-pressure asher (HPA) at 320 °C and 130 bar prior to the application of reductive co-precipitation procedures. The method was validated with road dust reference material BCR-723 and the CANMET-CCRMP reference material TDB-1 and WPR-1. In airborne PM collected at the background areas Neuglobsow and Deuselbach, Pd was detected with median concentrations values of 0.5 and 0.6 pg/m³, respectively. Much higher median concentration values of 14.8 pg Pd/m³ (detection limit = 0.01 pg Pd/m³) were detected in samples collected in the city of Frankfurt am Main. Results have shown that Hg co-precipitation depletes the concentrations of interfering matrix constituents by at least one order of magnitude more, compared to Te co-precipitation, making it a more effective method for the isolation and pre-enrichment of Pd from airborne PM prior to analysis. The use of a He gas flow of 120 ml/min in the plasma further minimized interferences, particularly those arising from CuAr⁺, YO⁺, and ZrO⁺ during the determination of Pd. The results demonstrate that Hg co-precipitation and the use of He collision gas, in combination with isotope dilution, are highly effective methods for the quantitation of Pd in airborne PM using ICP-MS.      

Inorganic mass spectrometry as a tool for characterisation at the nanoscale

Inorganic mass spectrometry techniques may offer great potential for the characterisation at the nanoscale, because they provide unique elemental information of great value for a better understanding of processes occurring at nanometre-length dimensions. Two main groups of techniques are reviewed: those allowing direct solid analysis with spatial resolution capabilities, i.e. lateral (imaging) and/or in-depth profile, and those for the analysis of liquids containing colloids. In this context, the present capabilities of widespread elemental mass spectrometry techniques such as laser ablation coupled with inductively coupled plasma mass spectrometry (ICP-MS), glow discharge mass spectrometry and secondary ion/neutral mass spectrometry are described and compared through selected examples from various scientific fields. On the other hand, approaches for the characterisation (i.e. size, composition, presence of impurities, etc.) of colloidal solutions containing nanoparticles by the well-established ICP-MS technique are described. In this latter case, the capabilities derived from the on-line coupling of separation techniques such as field-flow fractionation and liquid chromatography with ICP-MS are also assessed. Finally, appealing trends using ICP-MS for bioassays with biomolecules labelled with nanoparticles are delineated.      

Bio-inspired colorimetric detection of Hg<Superscript>2+</Superscript> and Pb<Superscript>2+</Superscript> heavy metal ions using Au nanoparticles

Heavy metal ions are highly toxic species which can cause long-term damage to biological systems. These species are known to disrupt biological events at the cellular level, cause significant oxidative damage, and are carcinogens. The production of simple, in-field detection methods that are highly sensitive for these cations is highly desirable in response to global pollution. In that regard, bio-inspired colorimetric sensing systems have been developed to detect Hg²⁺ and Pb²⁺, and other cations, down to nmol L⁻¹ concentrations. The benefits of these systems, which are reviewed herein, include cost-effective production, facile usage, and a visual color change for the detection method. Such advantages are significant positive steps for heavy metal ion detection, especially in regions where sophisticated laboratory studies are prohibited. Figure Biological-based colorimetric detection of heavy metal cations. The materials on the left are independent Au nanoparticles in solution, functionalized with heavy metal binding biomolecules, which, upon metal addition, aggregate to evolve a detectable solution color change.

Rapid response behavior,at room temperature,of a nanofiber-structured TiO<Subscript>2</Subscript> sensor to selected simulant chemical-warfare agents

A chemical prototype sensor was constructed based on nanofiber-structured TiO₂ and highly sensitive quartz resonators. The gas-sensing behavior of this new sensor to selected simulant warfare agents was investigated at room temperature. Results showed rapid response and good reversibility of this sensor when used with high-purity nitrogen. This provides a simple approach to preparation of materials needed as chemical sensors for selected organic volatiles or warfare agents. Figure Sensing behavior of TiO₂ nanofiber sensor to chemical vapors     

Proof of principle of a generalized fuzzy Hough transform approach to peak alignment of one-dimensional <Superscript>1</Superscript>H NMR data

In metabolic profiling, multivariate data analysis techniques are used to interpret one-dimensional (1D) ¹H NMR data. Multivariate data analysis techniques require that peaks are characterised by the same variables in every spectrum. This location constraint is essential for correct comparison of the intensities of several NMR spectra. However, variations in physicochemical factors can cause the locations of the peaks to shift. The location prerequisite may thus not be met, and so, to solve this problem, alignment methods have been developed. However, current state-of-the-art algorithms for data alignment cannot resolve the inherent problems encountered when analysing NMR data of biological origin, because they are unable to align peaks when the spatial order of the peaks changes—a commonly occurring phenomenon. In this paper a new algorithm is proposed, based on the Hough transform operating on an image representation of the NMR dataset that is capable of correctly aligning peaks when existing methods fail. The proposed algorithm was compared with current state-of-the-art algorithms operating on a selected plasma dataset to demonstrate its potential. A urine dataset was also processed using the algorithm as a further demonstration. The method is capable of successfully aligning the plasma data but further development is needed to address more challenging applications, for example urine data. Figure Traces of NMR peaks visualizing the Generalized Fuzzy Hough Transform (GFHT) method for elucidating peak correspondence between samples. The spectra are sorted according to one shift sensitive peak and reveals that other peaks exhibit a similar shift pattern. This pattern(s) can now be searched for using the GFHT. The red and black spectra in the figure are the most shifting spectra (top and bottom), by following the GFHT traces peak correspondence is easily established although peaks change spatial location Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

B<Subscript>24</Subscript>N<Subscript>24</Subscript> nanocages: a GIAO density functional theory study of <Superscript>14</Superscript>N and <Superscript>11</Superscript>B nuclear magnetic shielding and electric field gradient tensors

Abstract  Density functional theory (DFT) calculations were performed to determine boron-11 and nitrogen-14 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) spectroscopy parameters in the three most stable B₂₄N₂₄ fullerenes for the first time. The considered samples were first allowed to relax entirely, and then the NMR and NQR calculations were performed on the geometrically optimized models. The calculations of the ¹¹B and ¹⁴N nuclear magnetic shielding tensors and electric field gradient tensors employed the Gaussian 98 software implementation of the gauge-including atomic orbital (GIAO) method using the Becke3, Lee-Yang-Parr (B3LYP) DFT level and 6-311G** and 6-311++G** standard basis sets in each of the three optimized forms, and converted the results to experimentally measurable NMR parameters.The calculated NMR chemical shieldings of the three cages show significant differences, providing a way to identify these clusters. The evaluated NQR parameters of the ¹¹B and ¹⁴N nuclei in the clusters are also reported and discussed. Graphical abstract        

Studies on toxic oil syndrome: development of an enzyme-linked immunosorbent assay for 3-(<Emphasis Type="Italic">N</Emphasis>-phenylamino)propane-1,2-diol in human urine

The fatty acid esters of 3-(N-phenylamino)propane-1,2-diol (PAP) are biomarkers of toxic oil batches that caused toxic oil syndrome (TOS), an intoxication that caused over 400 deaths and affected 20,000 people in Spain in 1981. PAP esters are converted into PAP by human pancreatic lipase. The in vivo biotransformation of PAP in two mouse strains generated potentially toxic metabolites. Here we report an enzyme-linked immunosorbent assay (ELISA) for PAP detection incorporating antibodies generated using PAP-hapten derivatives 1 and 2. The immunizing haptens were designed to recognize the phenylamino and hydroxymethylene moieties of the PAP structure. The antisera raised against 1-HCH showed greater affinity for free PAP, as demonstrated in competitive experiments using either 1-BSA or 2-BSA as coating antigens. The developed ELISA detects PAP at a threshold of 130 μg L⁻¹ and can be used over a wide range of pH and ionic strength values. The assay can be applied to human urine samples, after a simple treatment method, with good recovery according to the correlation obtained when analyzing blind spiked urine samples. Figure Development of an ELISA for PAP in human urine     

Real-time measurement of endosomal acidification by a novel genetically encoded biosensor

Genetically encoded fluorescent proteins are optimal reporters when used to monitor cellular processes as they can be targeted to any subcellular region by fusion to a protein of interest. Here, we present the pH-sensitive fluorescent protein E¹GFP which is ideally suited to monitor pH changes in dynamic intracellular structures in real time with high spatio temporal resolution. E¹GFP is a ratiometric pH indicator by emission with a pK close to 6.0. We describe an application of this novel pH reporter in the measurement of pH changes along the endo-lysosomal pathway. By fusing E¹GFP to the HIV-Tat protein which is endowed with cell-penetrating properties, we were able to monitor multi-step endocytosis from the initial cell-surface binding through to the intracellular endocytic network in real time. This represents a framework for the application of E¹GFP to the in situ detection of pH changes involved in dynamic biological phenomena. Figure The green fluorecent protein variant, E1GFP, is a ratiometric pH-indicator by emission with a pK close to 6.0 and is therefore particularly suitable for pH detection below neutrality. Upon excitation of the neutral state of the chromophore (~400-410 nm), E1GFP emission properties are strongly dependent on the environmental pH. We describe an application of this novel pH-reporter in the measurement of pH changes along the endo-lysosomal pathway. By fusing E1GFP to the HIV-Tat protein, which is endowed with cell-penetrating properties, we were able to monitor in real-time multi-step endocytosis from the initial cell-surface binding through to the intracellular endocytic network. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Colorimetric detection of immunoglobulin G by use of functionalized gold nanoparticles on polyethylenimine film

A method based on use of functionalized gold nanoparticles on polyethylenimine film has been developed for colorimetric detection of immunoglobulin G (IgG). The immunogold nanoparticles were immobilized on quartz slides by recognition between antibody and antigen, with the antigen chemically adsorbed on the polyethylenimine film. By measurement of the UV–visible spectra of the immobilized immunogold, detection of h-IgG was achieved. The detection limit for h-IgG by use of this method can be as low as 0.01 μg mL⁻¹. This method is quite promising for numerous applications in immunoassay. Figure     

Novel gel-based rapid test for non-instrumental detection of ochratoxin A in beer

A rapid easy-to-use immunoassay was optimised for the non-instrumental detection of ochratoxin A (OTA) in beer. The analytical method involves preconcentration on the immunoaffinity layer inside a column followed by direct competitive ELISA detection in the same layer. The visual cut-off value, i.e. the lowest OTA concentration resulting in no colour development, was 0.2 μg L^-1. Assay validation was performed using samples spiked with OTA. Thirty-seven naturally contaminated samples were screened with the gel-based method developed and no false-negative results were obtained. The method described offers a simple, rapid and cost-effective screening tool, thus contributing to better health protection of consumers. Figure Gel-based immunoassay of spiked beer samples.     

Prediction of retention times of polycyclic aromatic hydrocarbons and <Emphasis Type="Italic">n</Emphasis>-alkanes in temperature-programmed gas chromatography

We have developed an iterative procedure for predicting the retention times of polycyclic aromatic hydrocarbons (PAHs) and n-alkanes during separations by temperature-programmed gas chromatography. The procedure is based on estimates of two thermodynamic properties for each analyte (the differences in enthalpy and entropy associated with movements between the stationary and mobile phases) derived from data acquired experimentally in separations under isothermal conditions at temperatures spanning the range covered by the temperature programs in ten-degree increments. The columns used for this purpose were capillary columns containing polydimethylsiloxane-based stationary phases with three degrees of phenyl substitution (0%, 5%, and 50%). Predicted values were mostly within 1% of experimentally determined values, implying that the method is stable and precise. Figure Predicted values were mostly within 1 % of experimentally determined values, thus implying that the method is stable and precise     

A combined experimental and theoretical study on the complexation of Li<Superscript>+</Superscript> with valinomycin

Abstract  From extraction experiments in the two-phase water–nitrobenzene system and γ-activity measurements, the stability constant of the valinomycin–lithium complex in nitrobenzene saturated with water was determined. Further, the structure of the resulting complex was indicated by means of the density functional level of theory (DFT) calculations. Graphical abstract        

Imidazolium-based ionic liquid derivative/Cu<Superscript>II</Superscript> complexes as efficient catalysts of the lucigenin chemiluminescence system and its application to H<Subscript>2</Subscript>O<Subscript>2</Subscript> and glucose detection

The effects of six synthetic imidazolium-based ionic liquids (ILs) on the Cu^II-catalyzed chemiluminescence of lucigenin (Luc-CL) in the pH range 6.0–11 were investigated. Preliminary experiments found that the CL emission was strongly enhanced or inhibited in the presence of the ILs. The degree of enhancement or inhibition of the CL intensity in the presence of each IL was related to the molecular structure of the IL, the medium used, and the pH. The maximum enhancement of the CL intensity was observed at pH 9.0 (amplification factor?=?443). This decrease in the pH at which maximum CL enhancement occurred and the substantial signal amplification of the Luc-CL may be related to a strong interaction between Cu^II and the imidazolium ring of superior ILs at this pH. Additionally, the formation of IL microdomains in semi-aqueous media permitted more solubility of the product yielded by the Luc-CL reaction (N-methylacridone), which could increase the CL intensity. To obtain consistent data on the catalytic efficiency of Cu^II in the presence of various ILs as well as the corresponding CL emission intensities, fluorescence quantum yields (Φ _F) of lucigenin were measured under the same conditions. Comparison of the data pointed to the mechanism that controls the properties of Luc-CL in the presence of the Cu^II/IL complexes. Based on the catalytic effect of the Cu^II/IL complex and the measurement of the enzymatically generated H₂O₂, a novel, simple, and sensitive CL method for determining glucose with a detection limit (LoD) of 6.5 μM was developed. Moreover, this method was satisfactorily applied to the determination of glucose in human serum and urine samples.

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Graphical Abstract The lucigenin chemiluminescence assay for H₂O₂ and glucose using imidazolium–based ionic liquid derivatives/Cu^II complexes as efficient catalysts at pH 9.0

     

Use of the bromine isotope ratio in HPLC-ICP-MS and HPLC-ESI-MS analysis of a new drug in development

A combination of inductively coupled plasma mass spectrometry (ICP-MS) and electrospray ionization mass spectrometry (ESI-MS) was deployed for the metabolite profiling and metabolite identification of a new antituberculosis compound (R207910, also known as TMC207) that is currently in drug development. R207910 contains one bromine atom, allowing the detection by ICP-MS. Fluctuations in the Br sensitivity caused by the HPLC gradient were counteracted by the use of species-unspecific isotope dilution. In order to evaluate the method developed, the results obtained were compared with those acquired via radioactivity detection. HPLC-ESI-MS was used for the structural identification of R207910 and its metabolites. The ⁷⁹Br/⁸¹Br isotope ratio is also valuable in the search for metabolites in the complex background of endogenous compounds obtained using HPLC-ESI-MS analyses. Data-dependent scanning using isotope recognition with an ion trap mass spectrometer or processing of Q-Tof data provides HPLC-ICP-MS-like “bromatograms”. The combination of accurate mass measurements and the fragmentation behavior in the MS² spectra obtained using the Q-Tof Ultima mass spectrometer or MSⁿ spectra acquired using the LTQ-Orbitrap allowed structural characterization of the main metabolites of R207910 in methanolic dog and rat faeces extracts taken 0–24 h post-dose. Figure Analyses of a rat faeces extract taken 0–24 h post-dose: a HPLC-ICP-MS using isotope dilution, b corresponding Br mass flow chromatogram, c radio-HPLC, d Q-Tof ESI-MS TIC, e Q-Tof ESI-MS bromatogram after Br stripping, f LTQ-Orbitrap ESI-MS² TIC obtained with isotopic-data-dependent scanning     

Difference of Electron Capture and Transfer Dissociation Mass Spectrometry on Ni<Superscript>2+</Superscript>-, Cu<Superscript>2+</Superscript>-, and Zn<Superscript>2+</Superscript>-Polyhistidine Complexes in the Absence of Remote Protons

Electron capture dissociation (ECD) and electron transfer dissociation (ETD) in metal-peptide complexes are dependent on the metal cation in the complex. The divalent transition metals Ni²⁺, Cu²⁺, and Zn²⁺ were used as charge carriers to produce metal-polyhistidine complexes in the absence of remote protons, since these metal cations strongly bind to neutral histidine residues in peptides. In the case of the ECD and ETD of Cu²⁺-polyhistidine complexes, the metal cation in the complex was reduced and the recombination energy was redistributed throughout the peptide to lead a zwitterionic peptide form having a protonated histidine residue and a deprotonated amide nitrogen. The zwitterion then underwent peptide bond cleavage, producing a and b fragment ions. In contrast, ECD and ETD induced different fragmentation processes in Zn²⁺-polyhistidine complexes. Although the N–C_α bond in the Zn²⁺-polyhistidine complex was cleaved by ETD, ECD of Zn²⁺-polyhistidine induced peptide bond cleavage accompanied with hydrogen atom release. The different fragmentation modes by ECD and ETD originated from the different electronic states of the charge-reduced complexes resulting from these processes. The details of the fragmentation processes were investigated by density functional theory.

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Graphical Abstract ?

     

The role of sulfur and sulfur isotope dilution analysis in quantitative protein analysis

The element sulfur is almost omnipresent in all natural proteomes and plays a key role in protein quantification. Incorporated in the amino acids cysteine and methionine, it has been served as target for many protein-labeling reactions in classic quantitative proteomic approaches based on electrospray or MALDI mass spectrometry. This critical review discusses the potential and limitations of sulfur isotope dilution analysis (IDA) by inductively coupled plasma—mass spectrometry (ICP-MS) for absolute protein quantification. The development of this approach was made possible due to the improved sensitivity and accuracy of sulfur isotope ratio measurement by ICP-MS in recent years. The unique feature of ICP-MS, compound-independent ionization, enables compound (species)-unspecific sulfur IDA. This has the main advantage that only one generic sulfur standard (i.e., one isotopically labeled sulfur spike) is required to quantify each peptide or protein in a sample provided that they are completely separated in chromatography or electrophoresis and that their identities are known. The principles of this approach are illustrated with selected examples from the literature. The discussion includes also related fields of P/S and metal/S ratio measurements for the determination of phosphorylation degrees of proteins and stoichiometries in metalloproteins, respectively. Emerging new areas and future trends such as protein derivatization with metal tags for improved sensitivity of protein detection in ICP-MS are discussed. Figure The key role of sulfur in protein quantification