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Chiu KY, Ai Y, Tanim-Ai Hassan M, Li X, Gunawardena HP, Chen H. Standards-Free Absolute Quantitation of Oxidizable Glycopeptides by Coulometric Mass Spectrometry. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2024; 35:1441-1450. [PMID: 38815255 DOI: 10.1021/jasms.4c00052] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/01/2024]
Abstract
Currently, glycopeptide quantitation is mainly based on relative quantitation due to absolute quantitation requiring isotope-labeled or standard glycopeptides which may not be commercially available or are very costly and time consuming to synthesize. To address this grand challenge, coulometric mass spectrometry (CMS), based on the combination of electrochemistry (EC) and mass spectrometry (MS), was utilized to quantify electrochemically active glycopeptides without the need of using standard materials. In this study, we studied tyrosine-containing glycopeptides, NYIVGQPSS(β-GlcNAc)TGNL-OH and NYSVPSS(β-GlcNAc)TGNL-OH, and successfully quantified them directly with CMS with a discrepancy of less than 5% between the CMS measured amount and the theoretical amount. Taking one step further, we applied this approach to quantify glycopeptides generated from the digestion of NIST mAb, a monoclonal antibody reference material. Through HILIC column separation, five N297 glycopeptides resulting from NIST mAb tryptic digestion were successfully separated and quantified by CMS for an absolute amount without the use of any standard materials. This study indicates the potential utility of CMS for quantitative proteomics research.
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Affiliation(s)
- Kai-Yuan Chiu
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Yongling Ai
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Md Tanim-Ai Hassan
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Xuanwen Li
- Analytical Research & Development, Merck & Co., Inc., Rahway, New Jersey 07065, United States
| | - Harsha P Gunawardena
- The Janssen Pharmaceutical Companies of Johnson & Johnson, Springhouse, Pennsylvania 19002, United States
| | - Hao Chen
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
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2
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FNU PIJ, Tanim-Al-Hassan M, Yaroshuk T, Ai Y, Chen H. Absolute Quantitation of Peptides and Proteins by Coulometric Mass Spectrometry After Derivatization. INTERNATIONAL JOURNAL OF MASS SPECTROMETRY 2024; 495:117153. [PMID: 38009161 PMCID: PMC10673616 DOI: 10.1016/j.ijms.2023.117153] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/28/2023]
Abstract
Peptide/protein quantitation using mass spectrometry (MS) is advantageous due to its high sensitivity. Traditional absolute peptide quantitation methods rely on making calibration curves using peptide standards or isotope-labelled peptide standards, which are expensive and take time to synthesize. A method which can eliminate the need for using standards would be beneficial. Recently, we developed coulometric mass spectrometry (CMS) which can be used to quantify peptides that are oxidizable (e.g., those containing tyrosine or tryptophan), without using peptide standard. The method is based on electrochemical oxidation of peptides followed by MS to measure the oxidation yield. However, it cannot be directly used to quantify peptides without oxidizable residues. To extend this method for quantifying peptides/proteins in general, in this study, we adopted a derivatization strategy, in which a target peptide is first tagged with an electroactive reagent such as monocarboxymethylene blue NHS ester (MCMB-NHS ester), followed with quantitation by CMS. To illustrate the power of this method, we have analyzed peptides MG and RPPGFSPFR. The quantification error was less than 5%. Using RPPGFSPFR as an example, the quantitation sensitivity of the technique was found to be 0.25 pmol. Furthermore, we also used the strategy to quantify proteins cytochrome C and β-casein with an error of 2-26%.
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Affiliation(s)
- Praneeth Ivan Joel FNU
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102, USA
| | - Md. Tanim-Al-Hassan
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102, USA
| | - Timothy Yaroshuk
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102, USA
| | - Yongling Ai
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102, USA
| | - Hao Chen
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102, USA
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3
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Ai Y, Gunawardena HP, Li X, Kim YI, Dewald HD, Chen H. Standard-Free Absolute Quantitation of Antibody Deamidation Degradation and Host Cell Proteins by Coulometric Mass Spectrometry. Anal Chem 2022; 94:12490-12499. [PMID: 36018377 PMCID: PMC10492508 DOI: 10.1021/acs.analchem.2c02709] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Proteomic absolute quantitation strategies mainly rely on the use of synthetic stable isotope-labeled peptides or proteins as internal standards, which are highly costly and time-consuming to synthesize. To circumvent this limitation, we recently developed a coulometric mass spectrometry (CMS) approach for absolute quantitation of proteins without the use of standards, based on the electrochemical oxidation of oxidizable surrogate peptides, followed by mass spectrometry measurement of the peptide oxidation yield. Previously, CMS was only applied for single-protein quantitation. In this study, first, we demonstrated absolute quantitation of multiple proteins in a mixture (e.g., β-lactoglobulin B, α-lactalbumin, and carbonic anhydrase) by CMS in one run, without using any standards. The CMS quantitation result was validated with a traditional isotope dilution method. Second, CMS can be used for absolute quantitation of a low-level target protein in a mixture; for instance, 500 ppm of PLBL2, a problematic host cell protein (HCP), in the presence of a highly abundant monoclonal antibody (mAb) was successfully quantified by CMS with no use of standards. Third, taking one step further, this study demonstrated the unprecedented quantitative analysis of deamidated peptide products arising from the mAb heavy chain deamidation reaction. In particular, absolute quantitation of the deamidation succinimide intermediate which had not been performed before due to the lack of standard was conducted by CMS, for the first time. Overall, our data suggest that CMS has potential utilities for quantitative proteomics and biotherapeutic drug discovery.
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Affiliation(s)
- Yongling Ai
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Harsha P Gunawardena
- Janssen Research & Development, The Janssen Pharmaceutical Companies of Johnson & Johnson, Spring House, Pennsylvania 19477, United States
| | - Xuanwen Li
- Analytical Research & Development, Merck & Co., Inc., 2000 Galloping Hill Road, Kenilworth, New Jersey 07033, United States
| | - Yong-Ick Kim
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Howard D Dewald
- Department of Chemistry and Biochemistry, Ohio University, Chemistry Building, 133 University Terrace, Athens, Ohio 45701, United States
| | - Hao Chen
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
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Wang Q, Liu Z, Liu Y, Chen H. Absolute Quantitation of N-Nitrosamines by Coulometric Mass Spectrometry without Using Standards. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2022; 33:875-884. [PMID: 35446584 PMCID: PMC9119692 DOI: 10.1021/jasms.2c00064] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Carcinogenic N-nitrosamines were recently found in the sartan family of drugs and caused many drug recalls. Both of their detection and quantification are therefore important. Methods reported for N-nitrosamine quantitation rely on the use of standards and are just applicable to simple N-nitrosamines. There is an urgent need to quantify N-nitrosamines derived from drugs with a complicated structure that lack standards. To tackle the issue, this study describes a novel absolute quantitation strategy for N-nitrosamines using coulometric mass spectrometry (CMS) without standards. In our approach, N-nitrosamine is first converted into electrochemically active hydrazine via zinc reduction under acidic condition and the resulting hydrazine can then be easily quantified using CMS. To validate our method, six simple N-nitrosamines, N-nitrosodiethylamine (NDEA), N-nitroso-4-phenylpiperidine (NPhPIP), N-nitrosodiphenylamine (NDPhA), N-nitrosodibutylamine (NDBA), N-nitrosodipropylamine (NDPA), and N-nitrosopiperidine (NPIP), were chosen as test samples, and they all were quantified with excellent measurement accuracy (quantitation error ≤1.1%). Taking this one step further, as a demonstration of the method utility, a drug-like N-nitrosamine, (R)-N-(2-(6-chloro-5-methyl-1'-nitroso-2,3-dihydrospiro[indene-1,4'-piperidin]-3-yl)propan-2-yl)acetamide (VII), was also synthesized and successfully quantified using our method at 15 ppb level in a complex formulation matrix, following solvent extraction, N-nitrosamine isolation, and reductive conversion. Because of the feature of requiring no standards, CMS provides a simple and powerful approach for N-nitrosamine absolute quantitation and has great potential for analysis of other drug impurities or metabolites.
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Affiliation(s)
- Qi Wang
- Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Zhijian Liu
- Department of Process Research and Development, Merck Research Laboratories, Merck & Co., Inc., Rahway, New Jersey 07065, United States
| | - Yong Liu
- Analytical Research and Development, Merck Research Laboratories, Merck & Co., Inc., Rahway, New Jersey 07065, United States
| | - Hao Chen
- Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
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Ai Y, Zhao P, FNU PIJ, Chen H. Absolute Quantitation of Tryptophan-Containing Peptides and Amyloid β-Peptide Fragments by Coulometric Mass Spectrometry. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2021; 32:1771-1779. [PMID: 34101439 PMCID: PMC8925997 DOI: 10.1021/jasms.1c00121] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/13/2023]
Abstract
Isotope-labeled internal standards are routinely used for mass spectrometry (MS)-based absolute quantitation. However, syntheses of isotope-labeled peptides are time-consuming and costly. To tackle this issue, we recently developed a coulometric mass spectrometric (CMS) approach for absolute quantitation without the use of standards, based on the electrochemical oxidation of cysteine or tyrosine-containing peptides followed by mass spectrometric measurement of the oxidation yield. To further expand the utility of this method, herein we present the CMS method for absolute quantitation of peptides based on tryptophan electrochemical oxidation. Several tryptophan-containing peptides, such as WGG, WQPPRARI, WAGGDASGE, RTRPLWVRME, and KVPRNQDWL, were successfully quantified with a quantification error ranging from -4.5 to +4.3%. Furthermore, this quantitation approach is also applicable to protein, in which protein can be digested and a surrogate peptide can be selected for quantification to reflect the amount of the parent protein, as exemplified by CMS analysis of peptide GITWK from cytochrome c. The CMS result agreed well with the traditional isotope dilution method, with only a small difference of 3.5%. In addition, CMS was used to successfully quantify amyloid beta (Aβ) peptide fragments (up to 28 amino acid residues) based on tyrosine oxidation. The validity of the CMS method for peptide and protein absolute quantitation without using isotope-labeled peptide standards would greatly facilitate proteomics research.
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Affiliation(s)
| | | | | | - Hao Chen
- Corresponding Author: Hao Chen - Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey, USA 07102
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6
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Kang A, Ryu J, Lee J, Kim S, Lee CY, Yun WS. Calibration curve-free electrochemical quantitation by micro-nano multi-scale gap devices. Mikrochim Acta 2021; 188:200. [PMID: 34041606 DOI: 10.1007/s00604-021-04861-7] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2021] [Accepted: 05/13/2021] [Indexed: 12/25/2022]
Abstract
Quantitation without relying on the calibration curve has long been an issue of overcoming analytical problems accompanied with the inherent limitations of the calibration curve fitting errors. Here, we report on a calibration curve-free method for electrochemical quantitation based on a multi-scale gap device (MGD). The MGD is an integrated device having a series of interdigitated electrodes (IDE) with micro-to-nano gap distances. The device shows a gap-dependent redox current of the analyte when subjected to the electrochemical cycling between the two facing electrodes of its componential IDEs. Based on the fact that the current increases as the gap distance decreases, the analyte concentration could be directly estimated: the rate of increase in the current was directly proportional to the analyte concentration. The calibration curve was not necessary for the quantitation. The accuracy of this MGD approach was better than that of an IDE collection of the same gap distance, which was deteriorated at the larger gap distances particularly. The MGD-based quantitation of dopamine, potassium ferricyanide, and aminophenol was demonstrated in a relatively broad range of concentrations (100 nM-5 mM).
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Affiliation(s)
- Aeyeon Kang
- Department of Chemistry, Sungkyunkwan University, Suwon, 16419, South Korea
| | - Jiho Ryu
- Department of Chemistry, Sungkyunkwan University, Suwon, 16419, South Korea
| | - Jisu Lee
- Department of Chemistry, Sungkyunkwan University, Suwon, 16419, South Korea
| | - Seunghun Kim
- Department of Chemistry, Sungkyunkwan University, Suwon, 16419, South Korea
| | - Cho Yeon Lee
- Nano/Bio Fusion Technology Research Center, Sungkyunkwan University, Suwon, 16419, Republic of Korea
| | - Wan Soo Yun
- Department of Chemistry, Sungkyunkwan University, Suwon, 16419, South Korea.
- Nano/Bio Fusion Technology Research Center, Sungkyunkwan University, Suwon, 16419, Republic of Korea.
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7
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Freitas D, Chen X, Cheng H, Davis A, Fallon B, Yan X. Recent Advances of In-Source Electrochemical Mass Spectrometry. Chempluschem 2021; 86:434-445. [PMID: 33689239 DOI: 10.1002/cplu.202100030] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/16/2021] [Revised: 03/03/2021] [Indexed: 12/16/2022]
Abstract
Hyphenation of electrochemistry (EC) and mass spectrometry has become a powerful tool to study redox processes. Approaches that can achieve this hyphenation include integrating chromatography/electrophoresis between electroinduced redox reactions and detection of products, coupling an EC flow cell to a mass spectrometer, and performing electrochemical reactions inside the ion source of a mass spectrometer. The first two approaches have been well reviewed elsewhere. This Minireview highlights the inherent electrochemical properties of many mass spectrometry ion sources and their roles in the coupling of electrochemistry and mass spectrometric analysis. Development of modified ion sources that allow the compatibility of electrochemistry with ionization processes is also surveyed. Applications of different in-source electrochemical devices are provided including intermediate capturing, bioanalytical studies, nanoparticle formation, electrosynthesis, and electrode imaging.
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Affiliation(s)
- Dallas Freitas
- Department of Chemistry, Texas A&M University, 580 Ross St., College Station, TX 77843, USA
| | - Xi Chen
- Department of Chemistry, Texas A&M University, 580 Ross St., College Station, TX 77843, USA
| | - Heyong Cheng
- Department of Chemistry, Texas A&M University, 580 Ross St., College Station, TX 77843, USA
| | - Austin Davis
- Department of Chemistry, Texas A&M University, 580 Ross St., College Station, TX 77843, USA
| | - Blake Fallon
- Department of Chemistry, Texas A&M University, 580 Ross St., College Station, TX 77843, USA
| | - Xin Yan
- Department of Chemistry, Texas A&M University, 580 Ross St., College Station, TX 77843, USA
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Liu C, Wang Q, Hivick BE, Ai Y, Champagne PA, Pan Y, Chen H. Capture of Electrochemically Generated Fleeting Carbazole Radical Cations and Elucidation of Carbazole Dimerization Mechanism by Mass Spectrometry. Anal Chem 2020; 92:15291-15296. [PMID: 33084312 DOI: 10.1021/acs.analchem.0c01223] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The capture of reactive intermediates is important for the elucidation of reaction mechanisms. We report the first observation of electrochemically generated, short-lived radical cations of carbazole (t1/2 ≈ 97 μs) and two N-substituted carbazole derivatives by mass spectrometry. In addition, online investigation of the reactivity of electrochemically generated carbazole radical cations supports that the carbazole dimerization mechanism involves the reaction of one radical cation with one neutral molecule rather than the previously proposed coupling of two radical cations.
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Affiliation(s)
- Chengyuan Liu
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States.,National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
| | - Qi Wang
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Brian E Hivick
- Department of Chemistry & Biochemistry, Ohio University, Athens, Ohio 45701, United States
| | - Yongling Ai
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Pier Alexandre Champagne
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States
| | - Yang Pan
- National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China
| | - Hao Chen
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States.,Department of Chemistry & Biochemistry, Ohio University, Athens, Ohio 45701, United States
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Song X, Chen H, Zare RN. Coulometry-assisted quantitation in spray ionization mass spectrometry. JOURNAL OF MASS SPECTROMETRY : JMS 2020; 56:e4628. [PMID: 33245185 DOI: 10.1002/jms.4628] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/10/2020] [Revised: 06/16/2020] [Accepted: 06/23/2020] [Indexed: 06/11/2023]
Abstract
The concentration of target analyte in a mixture can be quantified by combining coulometric measurements with spray ionization mass spectrometry. A three-electrode system screen printed on the polymer support acts both as the coulometry platform for electrochemical oxidation and the sample loading tip for spray ionization. After loading a droplet of the analyte solution onto the tip, two steps were taken to implement quantitation. First, the electrochemical oxidation potential was optimized with cyclic voltammetry followed by coulometric measurements to calculate the amount of oxidized analyte under a constant low voltage within a fixed period of time (5 s). Then, a high voltage (+4.5 kV) was applied to the tip to trigger spray ionization for measuring the oxidation yield from the native analyte ion and its oxidized product ion intensities by mass spectrometry. The analyte's native concentration is quantified by dividing the oxidized product's concentration (based on Coulomb's law) and the oxidation yield (estimated from mass spectrometry [MS] assuming that the parent and oxidation product have nearly the same ionization efficiencies). The workflow has an advantage in being free of any standard for constructing the quantitation curve. Several model compounds (tyrosine, dopamine, and angiotensin II) were selected for method validation. It was demonstrated that this strategy was feasible with an accuracy of ~15% for a wide coverage of different species including endogenous metabolites and peptides. As an example of its possible practical use, it was initially employed to make a bilirubin assay in urine.
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Affiliation(s)
- Xiaowei Song
- Department of Chemistry, Fudan University, Shanghai, 200438, China
| | - Hao Chen
- Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102, USA
| | - Richard N Zare
- Department of Chemistry, Fudan University, Shanghai, 200438, China
- Department of Chemistry, Stanford University, Stanford, CA, 94305, USA
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10
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Affiliation(s)
- Thomas Herl
- Institute of Analytical Chemistry, Chemo- and BiosensorsUniversity of Regensburg Universitätsstraße 31 93053 Regensburg Germany
| | - Frank‐Michael Matysik
- Institute of Analytical Chemistry, Chemo- and BiosensorsUniversity of Regensburg Universitätsstraße 31 93053 Regensburg Germany
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11
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Zhao P, Wang Q, Kaur M, Kim YI, Dewald HD, Mozziconacci O, Liu Y, Chen H. Absolute Quantitation of Proteins by Coulometric Mass Spectrometry. Anal Chem 2020; 92:7877-7883. [DOI: 10.1021/acs.analchem.0c01151] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022]
Affiliation(s)
- Pengyi Zhao
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States,
| | - Qi Wang
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States,
| | - Manpreet Kaur
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States,
| | - Yong-Ick Kim
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States,
| | - Howard D. Dewald
- Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States
| | - Olivier Mozziconacci
- Department of Analytical Sciences, Merck Research Laboratories, Merck &Co., Inc., Rahway, New Jersey 07065, United States
| | - Yong Liu
- Department of Analytical Sciences, Merck Research Laboratories, Merck &Co., Inc., Rahway, New Jersey 07065, United States
| | - Hao Chen
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, United States,
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12
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Zhao P, Zare RN, Chen H. Absolute Quantitation of Oxidizable Peptides by Coulometric Mass Spectrometry. JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY 2019; 30:2398-2407. [PMID: 31429055 DOI: 10.1007/s13361-019-02299-z] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/10/2019] [Revised: 07/15/2019] [Accepted: 07/21/2019] [Indexed: 06/10/2023]
Abstract
Quantitation methods for peptides using mass spectrometry have advanced rapidly. These methods rely on using standard and/or isotope-labeled peptides, which might be difficult or expensive to synthesize. To tackle this challenge, we present a new approach for absolute quantitation without the use of standards or calibration curves based on coulometry combined with mass spectrometry (MS). In this approach, which we call coulometric mass spectrometry (CMS), the mass spectrum of a target peptide containing one or more tyrosine residues is recorded before and after undergoing electrochemical oxidation. We record the total integrated oxidation current from the electrochemical measurement, which according to the Faraday's Law of coulometry, provides the number of moles of oxidized peptide. The ion intensity ratio of the target peptide before and after oxidation provides an excellent estimate of the fraction of the peptide that has been oxidized, from which the total amount of peptide is calculated. The striking strength of CMS is that it needs no standard peptide, but CMS does require the peptide to contain a known number of oxidizable groups. To illustrate the power of this method, we analyzed various tyrosine-containing peptides such as GGYR, DRVY, oxytocin, [Arg8]-vasotocin and angiotensinogen 1-14 with a quantification error ranging from - 7.5 to + 2.4%. This approach is also applicable to quantifying phosphopeptides and could be useful in proteomics research.
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Affiliation(s)
- Pengyi Zhao
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102, USA
| | - Richard N Zare
- Department of Chemistry, Stanford University, Stanford, CA, 94305-5080, USA.
| | - Hao Chen
- Department of Chemistry & Environmental Science, New Jersey Institute of Technology, Newark, NJ, 07102, USA.
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