Standard electrode potentials involving radicals in aqueous solution: inorganic radicals

Inorganic radicals, such as superoxide and hydroxyl, play an important role in biology. Their tendency to oxidize or to reduce other compounds has been studied by pulse radiolysis; electrode potentials can be derived when equilibrium is established with a well-known reference compound. An IUPAC Task Group has evaluated the literature and produced the recommended standard electrode potentials for such couples as (O2/O2·-), (HO·, H+/H2O), (O3/O3·-), (Cl2/Cl2·-), (Br2·-/2Br-), (NO2·/NO2-), and (CO3·-/CO32-

Quality Control for Building Libraries from Electrospray Ionization Tandem Mass Spectra

Electrospray ionization (ESI) tandem mass spectrometry coupled with liquid chromatography is a routine technique for identifying and quantifying compounds in complex mixtures. The identification step can be aided by matching acquired tandem mass spectra (MS²) against reference library spectra as is routine for electron ionization (EI) spectra from gas chromatography/mass spectrometry (GC/MS). However, unlike the latter spectra, ESI MS² spectra are likely to originate from various precursor ions for a given target molecule and may be acquired at varying energies and resolutions and have characteristic noise signatures, requiring processing methods very different from EI to obtain complete and high quality reference spectra for individual analytes. This paper presents procedures developed for creating a tandem mass spectral library that addresses these factors. Library building begins by acquiring MS² spectra for all major MS¹ peaks in an infusion run, followed by assigning MS² spectra to clusters and creating a consensus spectrum for each. Intensity-based constraints for cluster membership were developed, as well as peak testing to recognize and eliminate suspect peaks and reduce noise. Consensus spectra were then examined by a human evaluator using a number of criteria, including a fraction of annotated peaks and consistency of spectra for a given ion at different energies. These methods have been developed and used to build a library from >9000 compounds, yielding 230,000 spectra

Dehydration Versus Deamination of N-Terminal Glutamine in Collision-Induced Dissociation of Protonated Peptides

Some of the most prominent “neutral losses” in peptide ion fragmentation are the loss of ammonia and water from N-terminal glutamine. These processes are studied by electrospray ionization mass spectrometry in singly- and doubly-protonated peptide ions undergoing collision-induced dissociation in a triple quadrupole and in an ion trap instrument. For this study, four sets of peptides were synthesized: (1) QLLLPLLLK and similar peptides with K replaced by R, H, or L, and Q replaced by a number of amino acids, (2) QLnK (n = 0, 1, 3, 5, 7, 9, 11), (3) QLnR (n = 0, 1, 3, 5, 7, 9), and (4) QLn (n = 1, 2, 3, 4, 8). The results for QLLLPLLLK and QLLLPLLLR show that the singly protonated ions undergo loss of ammonia and to a smaller extent loss of water, whereas the doubly protonated ions undergo predominant loss of water. The fast fragmentation next to P (forming the y5 ion) occurs to a larger extent than the neutral losses from the singly protonated ions but much less than the water loss from the doubly protonated ions. The results from these and other peptides show that, in general, when N-terminal glutamine peptides have no “mobile protons”, that is, the number of charges on the peptide is no greater than the number of basic amino acids (K, R, H), deamination is the predominant neutral loss fragmentation, but when mobile protons are present the predominant process is the loss of water. Both of these processes are faster than backbone fragmentation at the proline. These results are rationalized on the basis of resonance stabilization of the two types of five-membered ring products that would be formed in the neutral loss processes; the singly protonated ion yields the more stable neutral pyrrolidinone ring whereas the doubly protonated ion yields the protonated aminopyrroline ring (see Schemes). The generality of these trends is confirmed by analyzing an MS/MS spectra library of peptides derived from tryptic digests of yeast. In the absence of mobile protons, glutamine deamination is the most rapid neutral loss process. For peptides with mobile protons, dehydration from glutamine is far more rapid than from any other amino acid. Most strikingly, end terminal glutamine is by far the most labile source of neutral loss in excess-proton peptides, but not highly exceptional when mobile protons are not available. In addition, rates of deamination are faster in lysine versus arginine C-terminus peptides and 20 times faster in positively charged than negatively charged peptides, demonstrating that these formal neutral loss reactions are not “neutral reactions” but depend on charge state and stability