Metastable fragmentation of somatostatin-14 (SS-14) and a series of SS-14 analogs formed with liquid secondary ion mass spectrometry: Observation of fragment ions which involve unsymmetric disulfide bridge cleavage concomitant with peptide chain cleavage

Thiol-disulfide redox equilibria of glutathione metaboloma compounds investigated by tandem mass spectrometry

The thiol group of cysteine plays a pivotal role in structural and functional biology. We use mass spectrometry to study glutathione-related homo- and heterodimeric disulfides, aiming at understanding the factors affecting the redox potentials of different disulfide/thiol pairs. Several electrospray ionization (ESI)-protonated disulfides of cysteamine, cysteine, penicillamine, N-acetylcysteine, N-acetylpenicillamine, gammaGluCySH, HSCyGly, and glutathione were analyzed on a triple quadrupole instrument to measure their energy-resolved tandem mass spectra. Fission of the disulfide bond yields RSH*H(+) and RS(+) ions. The logarithm of the intensity ratio of the RS(+)/RSH*H(+) fragments in homodimeric disulfides is proportional to the normal reduction potential of their RSSR/RSH pairs determined by nuclear magnetic resonance (NMR) in solution, the more reducing ones yielding the higher ratios. Also in some R(1)S-SR(2) disulfides, the ratio of the intensities of the RSH + H(+) and RS(+) ions of each participating thiol shows a linear relationship with the Nernst equation potential difference of the corresponding redox pairs. This behavior allows us to measure the redox potentials of some disulfide/thiol pairs by using different thiol-reducing probes of known oxidoreductive potential as reference. To assist understanding of the fission mechanism of the disulfide bond, the fragments tentatively identified as 'sulfenium' were themselves fragmented; accurate mass measurement of the resulting second-generation fragments demonstrated a loss of thioformaldehyde, thus supporting the assigned structure of this elusive intermediate of the oxidative stress pathway. Understanding this fragmentation process allows us to employ this technique with larger molecules to measure by mass spectrometry the micro-redox properties of different disulfide bonds in peptides with catalytic and signaling biological activity.

Tandem mass spectrometry analysis of synthetic opioid peptide analogs

Five synthetic opioid peptides that were designed to have specific opioid receptor-binding properties were studied by low energy collision-induced dissociation (CID) tandem mass spectrometry (MS/MS). The MS/MS data are required for the analysis of those peptides in ovine plasma in a study to determine the placental transfer of the peptide to the fetus. The synthetic enkephalin-related peptides were: Tyr-D-Arg-Phe-Lys-NH2, (DALDA), N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH, (ICI 174,864), Tyr-D-Thr-Gly-Phe-Leu-Thr, (DTLET), Tyr-D-Pen-Gly-Phe-D-Pen-OH, (DPDPE), and D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2, (CTAP). Liquid secondary ion mass spectrometry (LSIMS) was used for sample desorption-ionization, and a hybrid (E1BE2qQ) tandem mass spectrometer was used to collect the product-ion spectra. A protonated molecule ion, [M + H]+, was observed for each peptide. Amino acid sequence-determining fragment ion were produced by CID and collected by MS/MS for the three linear peptides, and also for the two disulfide-bond-containing peptides in their unreduced and dithiothreitol (DTT)-reduced forms. The detection level for the [M + H]+ ion of DTLET was ca. 3 pmol; and the stabilities of the CTAP and ICI analogs in plasma were studied.

Charge-remote fragmentation in a disulfide-containing peptide, [Pen]-enkephalin, under fast atom bombardment collisionally activated dissociation conditions

Fast atom bombardment collisionally activated dissociation tandem mass spectrometry (FAB CAD MS/MS) of a disulfide-containing peptide, [2-D-penicillamine, 5-D-penicillamine]- enkephalin ([Pen]-enkephalin), is described. Unlike those of most other disulfide-containing peptides investigated, CAD of the native, unreduced protonated molecule of [Pen]-enkephalin yields a relatively large number of fragment ions. Most of the peaks in the CAD spectrum represent fragmentations of the peptide backbone with 'unsymmetric' cleavage of the disulfide bond with charge retention on the N-terminus; the fragment ions generally do not contain a sulfur atom or a disulfide group. The CAD spectrum of the N-terminal ethyl-triphenylphosphonium derivative, on the other hand, shows few fragment ions and is dominated by the single peak at m/z 523. This dominant ion also results from 'unsymmetric' cleavage and corresponds to the analogous protonated species (m/z 235) in the spectrum of the underivatized peptide. The chemical method of charge localization by triphenylphosphonium derivative formation at one end of the peptide is shown to be useful for investigating fragmentation mechanisms in FAB CAD MS/MS. Comparison of the CAD spectra of derivatized and underivatized [Pen]-enkephalin suggests that charge-remote fragmentation plays a significant role in the high-energy dissociation of this disulfide-bonded peptide.

An expanded nomenclature scheme for labeling peptide fragmentations and its use with 'AMASS', a computer program for generating all possible fragment ion structures from known precursors

A nomenclature scheme for exhaustively labeling peptide fragment ions is proposed. The scheme is based on IUPAC nomenclature and the previously proposed Roepstorff nomenclature scheme used to label fragment ions in linear peptides. The descriptor used is specifically defined in order to increase the number of peptide and side chain linkages to which the nomenclature scheme can be applied compared with the Roepstorff scheme. The proposed descriptor can be used unambiguously to assign all possible fragments from linear, cyclized, branched, extended (i.e. beta-amino acids) and retro inverso peptides. A significant advantage of the proposed scheme is its simple interface with the currently accepted Roepstorff scheme. This nomenclature scheme is able to label all theoretical fragments generated by the computer program 'AMASS'. AMASS is proposed as a means of systematically calculating the mass of all possible fragment ions from known precursor structures. The program can help determine whether a peptide fragment was derived from an internal sequence fragment or a combination of side chain and backbone cleavages. The program AMASS and the proposed nomenclature scheme are used to illustrate a procedure for identifying fragment ions in the metastable product ion spectrum of somatostatin-14. We envisage that this procedure will be useful for identifying fragment ions which are characteristic of particular structural arrangements in dicyclic and polycyclic peptides.