Bottom-up proteomics typically involves enzymatic digestion of proteins, generating a complex peptide mixture. These peptides are separated using reversed-phase ultrahigh-performance liquid chromatography (UHPLC) and analyzed using electrospray ionization (ESI) tandem mass spectrometry (MS/MS) in positive ion mode. Despite its widespread use, this approach has limitations, particularly in ionizing highly acidic or hydrophobic peptides and detecting certain post-translational modifications (PTMs). To overcome these challenges, alternative ionization methods, such as vacuum ultraviolet (VUV) atmospheric pressure photoionization (APPI), have been explored. In this study, we propose peptide analysis using a novel prototype APPI source employing soft X-ray photons. Soft X-ray photons possess orders of magnitude higher energy than VUV photons, enabling additional ionization pathways. Here, we present peptide ionization data using soft X-ray and VUV APPI in both positive and negative ion modes. Notably, soft X-ray photons exhibited a remarkable capacity to generate deprotonated peptides and hydrogen-deficient peptide radical anions ([M - 2H]•-), outperforming conventional VUV photons. Furthermore, collision-induced dissociation (CID) of [M - 2H]•- provided unique structural insight, facilitating PTM characterization. Our findings emphasize the significant potential of soft X-ray APPI in advancing peptide analysis and highlight the utility of negative ion mode for proteomic applications.

Department of Analytical Chemistry Faculty of Science Charles University Prague Hlavova 2030 8 CZ 128 43 Prague 2 Czech Republic

Drug Research Program and Division of Pharmaceutical Chemistry and Technology Faculty of Pharmacy University of Helsinki P O Box 56 FI 00014 Helsinki Finland

Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences Flemingovo nám 2 CZ 166 10 Prague 6 Czech Republic

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Fischer F., Poetsch A.. Protein cleavage strategies for an improved analysis of the membrane proteome. Proteome Sci. 2006;4:2. doi: 10.1186/1477-5956-4-2. PubMed DOI PMC

Siepen J. A., Keevil E. J., Knight D., Hubbard S. J.. Prediction of missed cleavage sites in tryptic peptides aids protein identification in proteomics. J. Proteome Res. 2007;6(1):399–408. doi: 10.1021/pr060507u. PubMed DOI PMC

Benevento M., Di Palma S., Snijder J., Moyer C. L., Reddy V. S., Nemerow G. R., Heck A. J. R.. Adenovirus composition, proteolysis, and disassembly studied by in-depth qualitative and quantitative proteomics. J. Biol. Chem. 2014;289(16):11421–11430. doi: 10.1074/jbc.M113.537498. PubMed DOI PMC

Guo X., Trudgian D. C., Lemoff A., Yadavalli S., Mirzaei H.. Confetti: A multiprotease map of the HeLa proteome for comprehensive proteomics. Mol. Cell Proteomics. 2014;13(6):1573–1584. doi: 10.1074/mcp.M113.035170. PubMed DOI PMC

Aye T. T., Scholten A., Taouatas N., Varro A., Van Veen T. A., Vos M. A., Heck A. J.. Proteome-wide protein concentrations in the human heart. Mol. Biosyst. 2010;6(10):1917–1927. doi: 10.1039/c004495d. PubMed DOI

Biringer R. G., Amato H., Harrington M. G., Fonteh A. N., Riggins J. N., Huhmer A. F.. Enhanced sequence coverage of proteins in human cerebrospinal fluid using multiple enzymatic digestion and linear ion trap LC-MS/MS. Brief Funct Genomic Proteomic. 2006;5(2):144–153. doi: 10.1093/bfgp/ell026. PubMed DOI

Bigwarfe P. M., Wood T. D.. Effect of ionization mode in the analysis of proteolytic protein digests. Int. J. Mass Spectrom. 2004;234(1–3):185–202. doi: 10.1016/j.ijms.2004.02.015. DOI

Penanes P. A., Gorshkov V., Ivanov M. V., Gorshkov M. V., Kjeldsen F.. Potential of negative-ion-mode proteomics: sn mS1-only approach. J. Proteome Res. 2023;22(8):2734–2742. doi: 10.1021/acs.jproteome.3c00307. PubMed DOI PMC

Robinson M. R., Taliaferro J. M., Dalby K. N., Brodbelt J. S.. 193 nm ultraviolet photodissociation mass spectrometry for phosphopeptide characterization in the positive and negative ion modes. J. Proteome Res. 2016;15(8):2739–2748. doi: 10.1021/acs.jproteome.6b00289. PubMed DOI PMC

Hersberger K. E., Hakansson K.. Characterization of O-sulfopeptides by negative ion mode tandem mass spectrometry: superior performance of negative ion electron capture dissociation. Anal. Chem. 2012;84(15):6370–6377. doi: 10.1021/ac301536r. PubMed DOI

Nwosu C. C., Strum J. S., An H. J., Lebrilla C. B.. Enhanced detection and identification of glycopeptides in negative ion mode mass spectrometry. Anal. Chem. 2010;82(23):9654–9662. doi: 10.1021/ac101856r. PubMed DOI PMC

Ewing N. P., Cassady C. J.. Dissociation of multiply charged negative ions for hirudin (54–65), fibrinopeptide B, and insulin A (oxidized) J. Am. Soc. Mass Spectrom. 2001;12(1):105–116. doi: 10.1016/S1044-0305(00)00195-1. PubMed DOI

Bilusich D., Bowie J. H.. Fragmentations of (M-H)- anions of underivatised peptides. Part 2: Characteristic cleavages of Ser and Cys and of disulfides and other post-translational modifications, together with some unusual internal processes. Mass Spectrom Rev. 2009;28(1):20–34. doi: 10.1002/mas.20206. PubMed DOI

Bowie J. H., Brinkworth C. S., Dua S.. Collision-induced fragmentations of the (M-H)- parent anions of underivatized peptides: an aid to structure determination and some unusual negative ion cleavages. Mass Spectrom Rev. 2002;21(2):87–107. doi: 10.1002/mas.10022. PubMed DOI

Waugh R. J., Eckersley M., Bowie J. H., Hayes R. N.. Collision-induced dissociations of deprotonated peptides - dipeptides containing serine or threonine. International Journal of Mass Spectrometry and Ion Processes. 1990;98(2):135–145. doi: 10.1016/0168-1176(90)85013-R. DOI

Waugh R. J., Bowie J. H., Hayes R. N.. Collision-induced dissociations of deprotonated peptides - dipeptides containing aspartic or glutamic acids. Org. Mass Spectrom. 1991;26(4):250–256. doi: 10.1002/oms.1210260413. DOI

Waugh R. J., Bowie J. H., Hayes R. N.. Collision-induced dissociations of deprotonated peptides - dipeptides containing phenylalanine, tyrosine, histidine and tryptophan. International Journal of Mass Spectrometry and Ion Processes. 1991;107(2):333–347. doi: 10.1016/0168-1176(91)80068-X. DOI

Waugh R. J., Bowie J. H., Gross M. L.. Collision-induced dissociations of deprotonated peptides - dipeptides containing Asn, Arg and Lys. Aust. J. Chem. 1993;46(5):693–702. doi: 10.1071/CH9930693. DOI

Waugh R. J., Bowie J. H., Gross M. L.. Collision-induced dissociations of deprotonated peptides - dipeptides containing methionine or cysteine. Rapid Commun. Mass Sp. 1993;7(7):623–625. doi: 10.1002/rcm.1290070714. DOI

Steinborner S. T., Bowie J. H.. The negative ion mass spectra of [M-H](−) ions derived from caeridin and dynastin peptides. Internal backbone cleavages directed through Asp and Asn residues. Rapid Commun. Mass Sp. 1997;11(3):253–258. doi: 10.1002/(SICI)1097-0231(19970215)11:3<253::AID-RCM825>3.0.CO;2-K. DOI

Waugh R. J., Bowie J. H.. A review of the collision-induced dissociations of deprotonated dipeptides and tripeptides - an aid to structure determination. Rapid Commun. Mass Sp. 1994;8(2):169–173. doi: 10.1002/rcm.1290080209. DOI

Pu D., Cassady C. J.. Negative ion dissociation of peptides containing hydroxyl side chains. Rapid Commun. Mass Spectrom. 2008;22(2):91–100. doi: 10.1002/rcm.3337. PubMed DOI

Zuo M. Q., Sun R. X., Fang R. Q., He S. M., Dong M. Q.. Characterization of collision-induced dissociation of deprotonated peptides of 4–16 amino acids using high-resolution mass spectrometry. Int. J. Mass Spectrom. 2019;445:116186. doi: 10.1016/j.ijms.2019.116186. DOI

Halim M. A., Girod M., MacAleese L., Lemoine J., Antoine R., Dugourd P.. 213 nm ultraviolet photodissociation on peptide anions: radical-directed fragmentation patterns. J. Am. Soc. Mass Spectrom. 2016;27(3):474–486. doi: 10.1007/s13361-015-1297-5. PubMed DOI

Borotto N. B., Ileka K. M., Tom C., Martin B. R., Hakansson K.. Free radical initiated peptide sequencing for direct site localization of sulfation and phosphorylation with negative Ion mode mass spectrometry. Anal. Chem. 2018;90(16):9682–9686. doi: 10.1021/acs.analchem.8b02707. PubMed DOI PMC

Kjeldsen F., Silivra O. A., Ivonin I. A., Haselmann K. F., Gorshkov M., Zubarev R. A.. C alpha-C backbone fragmentation dominates in electron detachment dissociation of gas-phase polypeptide polyanions. Chemistry. 2005;11(6):1803–1812. doi: 10.1002/chem.200400806. PubMed DOI

Lam C. N., Chu I. K.. Formation of anionic peptide radicals in vacuo. J. Am. Soc. Mass Spectrom. 2006;17(9):1249–1257. doi: 10.1016/j.jasms.2006.05.008. PubMed DOI

Yoo H. J., Wang N., Zhuang S., Song H., Hakansson K.. Negative-ion electron capture dissociation: radical-driven fragmentation of charge-increased gaseous peptide anions. J. Am. Chem. Soc. 2011;133(42):16790–16793. doi: 10.1021/ja207736y. PubMed DOI

Bilusich D., Bowie J. H.. Determination of disulfide functionality in underivatised peptides using negative ion mass spectrometry: An aid to structure determination. Curr. Anal Chem. 2006;2(4):341–352. doi: 10.2174/157341106778520490. DOI

Xu N. X., Lin Y. H., Hofstadler S. A., Matson D., Call C. J., Smith R. D.. A microfabricated dialysis device for sample cleanup in electrospray ionization mass spectrometry. Anal. Chem. 1998;70(17):3553–3556. doi: 10.1021/ac980233x. PubMed DOI

Bagag A., Jault J. M., Sidahmed-Adrar N., Refregiers M., Giuliani A., Le Naour F.. Characterization of hydrophobic peptides in the presence of detergent by photoionization mass spectrometry. PLoS One. 2013;8(11):e79033. doi: 10.1371/journal.pone.0079033. PubMed DOI PMC

Sedlácková S., Hubálek M., Vrkoslav V., Blechová M., Kozlík P., Cvacka J.. Positive effect of acetylation on proteomic analysis based on liquid chromatography with atmospheric pressure chemical ionization and photoionization mass spectrometry. Molecules. 2023;28(9):3711. doi: 10.3390/molecules28093711. PubMed DOI PMC

Sedlácková S., Hubálek M., Vrkoslav V., Blechová M., Cvacka J.. Utility of atmospheric-pressure chemical ionization and photoionization mass spectrometry in bottom-up proteomics. Separations. 2022;9(2):42. doi: 10.3390/separations9020042. PubMed DOI PMC

Zhou W., Yang S., Wang P. G.. Matrix effects and application of matrix effect factor. Bioanalysis. 2017;9(23):1839–1844. doi: 10.4155/bio-2017-0214. PubMed DOI

Bagag A., Giuliani A., Laprévote O.. Atmospheric pressure photoionization of peptides. Int. J. Mass Spectrom. 2011;299(1):1–4. doi: 10.1016/j.ijms.2010.08.010. PubMed DOI

Kloudova B., Strmen T., Vrkoslav V., Chara Z., Paces O., Cvacka J.. Gas dynamic virtual nozzle sprayer for an introduction of liquid samples in atmospheric pressure ionization mass spectrometry. Anal. Chem. 2023;95(8):4196–4203. doi: 10.1021/acs.analchem.2c05349. PubMed DOI PMC

Tan S., Yin X., Feng L., Wang J., Xue Z., Jiang Y., Dai X., Gong X., Fang X.. Nanoliter atmospheric pressure photoionization-mass spectrometry for direct bioanalysis of polycyclic aromatic hydrocarbons. Analyst. 2023;148(16):3730–3739. doi: 10.1039/D3AN00442B. PubMed DOI

Allegrand J., Touboul D., Giuliani A., Brunelle A., Laprévote O.. Atmospheric pressure photoionization mass spectrometry of guanine using tunable synchrotron VUV radiation. Int. J. Mass Spectrom. 2012;321–322:14–18. doi: 10.1016/j.ijms.2012.05.009. DOI

Zubarev R. A., Kruger N. A., Fridriksson E. K., Lewis M. A., Horn D. M., Carpenter B. K., McLafferty F. W.. Electron capture dissociation of gaseous multiply-charged proteins is favored at disulfide bonds and other sites of high hydrogen atom affinity. J. Am. Chem. Soc. 1999;121(12):2857–2862. doi: 10.1021/ja981948k. DOI

Sawicka A., Skurski P., Hudgins R. R., Simons J.. Model calculations relevant to disulfide bond cleavage via electron capture influenced by positively charged groups. J. Phys. Chem. B. 2003;107(48):13505–13511. doi: 10.1021/jp035675d. DOI

Syrstad E. A., Turecček F.. Toward a general mechanism of electron capture dissociation. J. Am. Soc. Mass Spectrom. 2005;16(2):208–224. doi: 10.1016/j.jasms.2004.11.001. PubMed DOI

Madsen J. A., Cheng R. R., Kaoud T. S., Dalby K. N., Makarov D. E., Brodbelt J. S.. Charge-site-dependent dissociation of hydrogen-rich radical peptide cations upon vacuum UV photoexcitation. Chemistry. 2012;18(17):5374–5383. doi: 10.1002/chem.201103534. PubMed DOI PMC

Fort K. L., Dyachenko A., Potel C. M., Corradini E., Marino F., Barendregt A., Makarov A. A., Scheltema R. A., Heck A. J.. Implementation of ultraviolet photodissociation on a benchtop q exactive mass spectrometer and its application to phosphoproteomics. Anal. Chem. 2016;88(4):2303–2310. doi: 10.1021/acs.analchem.5b04162. PubMed DOI

Brodbelt J. S., Morrison L. J., Santos I.. Ultraviolet photodissociation mass spectrometry for analysis of biological molecules. Chem. Rev. 2020;120(7):3328–3380. doi: 10.1021/acs.chemrev.9b00440. PubMed DOI PMC

Yin H., Chacon A., Porter N. A., Masterson D. S.. Free radical-induced site-specific peptide cleavage in the gas phase: low-energy collision-induced dissociation in ESI- and MALDI mass spectrometry. J. Am. Soc. Mass Spectrom. 2007;18(5):807–816. doi: 10.1016/j.jasms.2007.01.004. PubMed DOI

Moore B., Sun Q., Hsu J. C., Lee A. H., Yoo G. C., Ly T., Julian R. R.. Dissociation chemistry of hydrogen-deficient radical peptide anions. J. Am. Soc. Mass Spectrom. 2012;23(3):460–468. doi: 10.1007/s13361-011-0318-2. PubMed DOI

Hieta J. P., Vesander R., Sipila M., Sarnela N., Kostiainen R.. Soft X-ray atmospheric pressure photoionization in liquid chromatography-mass spectrometry. Anal. Chem. 2021;93(27):9309–9313. doi: 10.1021/acs.analchem.1c01127. PubMed DOI PMC

Chu I. K., Siu C. K., Lau J. K. C., Tang W. K., Mu X. Y., Lai C. K., Guo X. H., Wang X., Li N., Xia Y.. et al. Proposed nomenclature for peptide ion fragmentation. Int. J. Mass Spectrom. 2015;390:24–27. doi: 10.1016/j.ijms.2015.07.021. DOI

Biemann K., Martin S. A.. Mass-spectrometric determination of the amino-acid-sequence of peptides and proteins. Mass Spectrom. Rev. 1987;6:1–76. doi: 10.1002/mas.1280060102. DOI

Robb D. B., Blades M. W.. State-of-the-art in atmospheric pressure photoionization for LC/MS. Anal. Chim. Acta. 2008;627(1):34–49. doi: 10.1016/j.aca.2008.05.077. PubMed DOI

Papayannopoulos I. A.. The interpretation of collision-induced dissociation tandem mass-spectra of peptides. Mass Spectrom. Rev. 1995;14(1):49–73. doi: 10.1002/mas.1280140104. DOI

Biemann K.. Contributions of mass spectrometry to peptide and protein structure. Biomed Environ. Mass Spectrom. 1988;16(1–12):99–111. doi: 10.1002/bms.1200160119. PubMed DOI

Laskin J., Yang Z. B., Lam C., Chu I. K.. Energy and entropy effects in dissociation of peptide radical anions. Int. J. Mass Spectrom. 2012;316–318:251–258. doi: 10.1016/j.ijms.2012.01.006. DOI

Delobel A., Halgand F., Laffranchise-Gosse B., Snijders H., Laprevote O.. Characterization of hydrophobic peptides by atmospheric pressure photoionization-mass spectrometry and tandem mass spectrometry. Anal. Chem. 2003;75(21):5961–5968. doi: 10.1021/ac034532k. PubMed DOI

Méjean M., Giuliani A., Brunelle A., Touboul D.. Exploring the peptide fragmentation mechanisms under atmospheric pressure photoionization using tunable VUV synchrotron radiation. Int. J. Mass Spectrom. 2015;379:80–86. doi: 10.1016/j.ijms.2014.12.011. DOI

Debois D., Giuliani A., Laprévote O.. Fragmentation induced in atmospheric pressure photoionization of peptides. J. Mass Spectrom. 2006;41(12):1554–1560. doi: 10.1002/jms.1122. PubMed DOI

Budnik B. A., Zubarev R. A.. MH2+* ion production from protonated polypeptides by electron impact:: observation and determination of ionization energies and a cross-section. Chem. Phys. Lett. 2000;316(1–2):19–23. doi: 10.1016/S0009-2614(99)01256-7. DOI

Takayama M.. MALDI in-source decay of protein: the mechanism of c-ion formation. Mass Spectrom (Tokyo) 2016;5(1):A0044. doi: 10.5702/massspectrometry.A0044. PubMed DOI PMC

Vrkoslav V., Muck A., Brown J. M., Hubalek M., Cvacka J.. The matrix-assisted laser desorption/ionisation in-source decay of peptides using ion mobility enabled quadrupole-time-of-flight mass spectrometry. Rapid Commun. Mass Spectrom. 2018;32(24):2099–2105. doi: 10.1002/rcm.8284. PubMed DOI

Kauppila T. J., Kotiaho T., Kostiainen R., Bruins A. P.. Negative ion-atmospheric pressure photoionization-mass spectrometry. J. Am. Soc. Mass Spectrom. 2004;15(2):203–211. doi: 10.1016/j.jasms.2003.10.012. PubMed DOI

Dzidic I., Carroll D. I., Stillwell R. N., Horning E. C.. Gas-phase reactions - ionization by proton-transfer to superoxide anions. J. Am. Chem. Soc. 1974;96(16):5258–5259. doi: 10.1021/ja00823a045. DOI

Muftakhov M. V., Shchukin P. V.. Dissociative electron attachment to glycyl-glycine, glycyl-alanine and alanyl-alanine. Phys. Chem. Chem. Phys. 2011;13(10):4600–4606. doi: 10.1039/c0cp00940g. PubMed DOI

Geddes S., Zahardis J., Eisenhauer J., Petrucci G. A.. Low energy photoelectron resonance capture ionization aerosol mass spectrometry of small peptides with cysteine residues: Cys-Gly, γ-Glu-Cys, and glutathione (γ-Glu-Cys-Gly) Int. J. Mass Spectrom. 2009;282(1–2):13–20. doi: 10.1016/j.ijms.2009.01.020. DOI

Vasil’ev Y. V., Figard B. J., Morre J., Deinzer M. L.. Fragmentation of peptide negative molecular ions induced by resonance electron capture. J. Chem. Phys. 2009;131(4):044317. doi: 10.1063/1.3186747. PubMed DOI PMC

Gschliesser D., Vizcaino V., Probst M., Scheier P., Denifl S.. Formation and decay of the dehydrogenated parent anion upon electron attachment to dialanine. Chemistry. 2012;18(15):4613–4619. doi: 10.1002/chem.201102433. PubMed DOI PMC

Sekimoto K., Sakakura M., Kawamukai T., Hike H., Shiota T., Usui F., Bando Y., Takayama M.. Ionization characteristics of amino acids in direct analysis in real time mass spectrometry. Analyst. 2014;139(10):2589–2599. doi: 10.1039/C3AN02193A. PubMed DOI

LeLacheur R. M., Glaze W. H.. Reactions of ozone and hydroxyl radicals with serine. Environ. Sci. Technol. 1996;30(4):1072–1080. doi: 10.1021/es940544z. DOI

Vasil’ev Y. V., Figard B. J., Voinov V. G., Barofsky D. F., Deinzer M. L.. Resonant electron capture by some amino acids and their methyl esters. J. Am. Chem. Soc. 2006;128(16):5506–5515. doi: 10.1021/ja058464q. PubMed DOI

Denifl S., Flosadóttir H. D., Edtbauer A., Ingólfsson O., Märk T. D., Scheier P.. A detailed study on the decomposition pathways of the amino acid valine upon dissociative electron attachment. Eur. Phys. J. D. 2010;60(1):37–44. doi: 10.1140/epjd/e2010-00060-5. DOI

Takahashi H., Sekiya S., Nishikaze T., Kodera K., Iwamoto S., Wada M., Tanaka K.. Hydrogen attachment/abstraction dissociation (HAD) of gas-phase peptide ions for tandem mass spectrometry. Anal. Chem. 2016;88(7):3810–3816. doi: 10.1021/acs.analchem.5b04888. PubMed DOI

Asakawa D., Takahashi H., Sekiya S., Iwamoto S., Tanaka K.. Sequencing of sulfopeptides using negative-ion tandem mass spectrometry with hydrogen attachment/abstraction dissociation. Anal. Chem. 2019;91(16):10549–10556. doi: 10.1021/acs.analchem.9b01568. PubMed DOI

Goshe M. B., Chen Y. H., Anderson V. E.. Identification of the sites of hydroxyl radical reaction with peptides by hydrogen/deuterium exchange: prevalence of reactions with the side chains. Biochemistry. 2000;39(7):1761–1770. doi: 10.1021/bi991569j. PubMed DOI

Shaw J. B., Madsen J. A., Xu H., Brodbelt J. S.. Systematic comparison of ultraviolet photodissociation and electron transfer dissociation for peptide anion characterization. J. Am. Soc. Mass Spectrom. 2012;23(10):1707–1715. doi: 10.1007/s13361-012-0424-9. PubMed DOI PMC

Zubarev R. A., Nielsen M. L., Budnik B. A.. Tandem ionization mass spectrometry of biomolecules. Eur. J. Mass Spectrom. 2000;6(3):235–240. doi: 10.1255/ejms.351. DOI

Brunet C., Antoine R., Allouche A. R., Dugourd P., Canon F., Giuliani A., Nahon L.. Gas phase photo-formation and vacuum UV photofragmentation spectroscopy of tryptophan and tyrosine radical-containing peptides. J. Phys. Chem. A. 2011;115(32):8933–8939. doi: 10.1021/jp205617x. PubMed DOI

Brunet C., Antoine R., Dugourd P., Canon F., Giuliani A., Nahon L.. Formation and fragmentation of radical peptide anions: insights from vacuum ultra violet spectroscopy. J. Am. Soc. Mass Spectrom. 2012;23(2):274–281. doi: 10.1007/s13361-011-0285-7. PubMed DOI

Joly L., Antoine R., Broyer M., Lemoine J., Dugourd P.. Electron photodetachment from gas phase peptide dianions. Relation with optical absorption properties. J. Phys. Chem. A. 2008;112(5):898–903. doi: 10.1021/jp0752365. PubMed DOI

Antoine R., Joly L., Tabarin T., Broyer M., Dugourd P., Lemoine J.. Photo-induced formation of radical anion peptides. Electron photodetachment dissociation experiments. Rapid Commun. Mass Spectrom. 2007;21(2):265–268. doi: 10.1002/rcm.2810. PubMed DOI

Matheis K., Joly L., Antoine R., Lepine F., Bordas C., Ehrler O. T., Allouche A. R., Kappes M. M., Dugourd P.. Photoelectron spectroscopy of gramicidin polyanions: competition between delayed and direct emission. J. Am. Chem. Soc. 2008;130(47):15903–15906. doi: 10.1021/ja803758w. PubMed DOI

Dixon D. A., Lipscomb W. N.. Electronic structure and bonding of the amino acids containing first row atoms. J. Biol. Chem. 1976;251(19):5992–5600. doi: 10.1016/S0021-9258(17)33049-1. PubMed DOI

Wood G. P., Moran D., Jacob R., Radom L.. Bond dissociation energies and radical stabilization energies associated with model peptide-backbone radicals. J. Phys. Chem. A. 2005;109(28):6318–6325. doi: 10.1021/jp051860a. PubMed DOI

Anusiewicz I., Jasionowski M., Skurski P., Simons J.. Backbone and side-chain cleavages in electron detachment dissociation (EDD) J. Phys. Chem. A. 2005;109(49):11332–11337. doi: 10.1021/jp055018g. PubMed DOI