Structure and Reactivity of the N -Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

The radical ion chemistry of a suite of S-nitrosopeptides has been investigated. Doubly and triply-protonated ions of peptides NYCGLPGEYWLGNDK, NYCGLPGEYWLGNDR, NYCGLP-GERWLGNDR, NACGAPGEKWAGNDK, NYCGLPGEKYLGNDK, NYGLPGCEKWYGNDK and NYGLPGEKWYGCNDK were subjected to electron capture dissociation (ECD), and collisioninduced dissociation (CID). The peptide sequences were selected such that the effect of the site of S-nitrosylation, the nature and position of the basic amino acid residues, and the nature of the other amino acid side chains, could be interrogated. The ECD mass spectra were dominated by a peak corresponding to loss of• NO from the charge-reduced precursor, which can be explained by a modified Utah-Washington mechanism. Some backbone fragmentation in which the nitrosyl modification was preserved was also observed in the ECD of some peptides. Molecular dynamics simulations of peptide ion structure suggest that the ECD behavior was dependent on the surface accessibility of the protonated residue. CID of the S-nitrosylated peptides resulted in homolysis of the S-N bond to form a long-lived radical with loss of • NO. The radical peptide ions were isolated and subjected to ECD and CID. ECD of the radical peptide ions provided an interesting comparison to ECD of the unmodified peptides. The dominant process was electron capture without further dissociation (ECnoD). CID of the radical peptide ions resulted in cysteine, leucine, and asparagine side chain losses, and radical-induced backbone fragmentation at tryptophan, tyrosine, and asparagine residues, in addition to charge-directed backbone fragmentation.

Section: Ms3: Cid-ecd Of S-nitrosylated Peptidesmentioning

confidence: 99%

“…• NO neutral loss is observed following CID of Snitrosopeptides [5,22,[26][27][28], which might suggest a thermal process is occurring; however, no loss of + ions were observed when the energy of the electrons was reduced (Supplemental Figure 3).…”

Section: Electron Capture Dissociation Of S-nitrosylated Peptidesmentioning

confidence: 99%

The Radical Ion Chemistry of S-Nitrosylated Peptides

Jones

Winn

Cooper

2012

“…This observation indicates hydrogen transfer from the first two amino acid residues to the thiyl radical. In fact, experimental and theoretical studies have shown that intramolecular hydrogen transfer to the thiyl radical is a facile process within polypeptides and cysteine ions [30][31][32][33]. [34] or CID of Nterminus amidated peptides [35].…”

Section: Formation Of C Z Ionsmentioning

confidence: 99%

Electron Transfer Dissociation (ETD) of Peptides Containing Intrachain Disulfide Bonds

Cole

Zhang

et al. 2011

The fragmentation chemistry of peptides containing intrachain disulfide bonds was investigated under electron transfer dissociation (ETD) conditions. Fragments within the cyclic region of the peptide backbone due to intrachain disulfide bond formation were observed, including: c (odd electron), z (even electron), c-33 Da, z+33 Da, c+32 Da, and z-32 Da types of ions. The presence of these ions indicated cleavages both at the disulfide bond and the N-Cα backbone from a single electron transfer event. Mechanistic studies supported a mechanism whereby the N-Cα bond was cleaved first, and radical-driven reactions caused cleavage at either an S-S bond or an S-C bond within cysteinyl residues. Direct ETD at the disulfide linkage was also observed, correlating with signature loss of 33 Da (SH) from the charge-reduced peptide ions. Initial ETD cleavage at the disulfide bond was found to be promoted amongst peptides ions of lower charge states, while backbone fragmentation was more abundant for higher charge states. The capability of inducing both backbone and disulfide bond cleavages from ETD could be particularly useful for sequencing peptides containing intact intrachain disulfide bonds. ETD of the 13 peptides studied herein all showed substantial sequence coverage, accounting for 75%-100% of possible backbone fragmentation.

“…Dissociation of the hydrogen rich radical cations often gives rise to c and z• ions, along with side-chain losses, through which they can convert quickly to hydrogen deficient radical ions [19][20][21]. Hydrogen deficient cations can be generated by a variety of methods: laser ablation followed by ultraviolet (UV) photoionization [22,23]; collision-induced dissociation (CID) of metal-ligand-peptide complexes [24][25][26]; CID of peptide derivatives with labile bonds such as S-nitrosylation [27,28], serine/homoserine nitrate esters [29], peroxycarbamates [30], 2,2,6,6-tetramethylpiperidine-1-oxy (TEMPO) [31,32], and 4,4'-azobis(4-cyanopentanoic acid) (Vazo 68) [33]; UV photolysis of iodinated tyrosine containing peptides [34]; or noncovalent complexes with photolabile precursor [35]; electroninduced dissociation of multiply charged ions [36,37]. Hydrogen deficient radical anions can be formed by electron detachment [38] or photodetachment [39] from multiply deprotonated molecules, CID of peptide-metal complexes [40,41], and photodissociation of iodinated peptide [42].…”

mentioning

confidence: 99%

“…Several groups have utilized both experimental approaches (i.e., ion/molecule reactions [14,15,27] and ion spectroscopy [43][44][45]) and theoretical calculations [21,[46][47][48][49] to investigate the structures of amino acid or small peptide radical ions. Radical ion chemistry, such as intramolecular radical migration [27,50] and competition between charge-and radical-directed dissociation upon collisional activation [49][50][51] have also been explored with different chemical systems. The capability of controlling the radical site upon its formation is highly desirable in studying the fundamental aspects as mentioned above.…”

mentioning

confidence: 99%

Gas-Phase Peptide Sulfinyl Radical Ions: Formation and Unimolecular Dissociation

Tan

Xia

2012

A variety of peptide sulfinyl radical (RSO•) ions with a well-defined radical site at the cysteine side chain were formed at atmospheric pressure (AP), sampled into a mass spectrometer, and investigated via collision-induced dissociation (CID). The radical ion formation was based on AP reactions between oxidative radicals and peptide ions containing single inter-chain disulfide bond or free thiol group generated from nanoelectrospray ionization (nanoESI). The radical induced reactions allowed large flexibility in forming peptide radical ions independent of ion polarity (protonated or deprotonated) or charge state (singly or multiply charged). More than 20 peptide sulfinyl radical ions in either positive or negative ion mode were subjected to low energy collisional activation on a triple-quadrupole/linear ion trap mass spectrometer. The competition between radical-and charge-directed fragmentation pathways was largely affected by the presence of mobile protons. For peptide sulfinyl radical ions with reduced proton mobility (i.e., singly protonated, containing basic amino acid residues), loss of 62 Da (CH 2 SO), a radicalinitiated dissociation channel, was dominant. For systems with mobile protons, this channel was suppressed, while charge-directed amide bond cleavages were preferred. The polarity of charge was found to significantly alter the radical-initiated dissociation channels, which might be related to the difference in stability of the product ions in different ion charge polarities.