Fragmentation of a novel marine peptide, plicatamide, involves an unusual gas-phase intramolecular rearrangement

High-throughput approaches towards the definitive identification of pharmaceutical drug metabolites. 2. An example of how unexpected dissociation behaviour could preclude correct assignment of sites of metabolism

S-oxidation is a common metabolic route for sulfur-containing compounds. Whilst investigating the dissociation of a series of chemically synthesised model S-oxide metabolites, two unexpected losses of 62 m/z units were observed in the collision-induced dissociation (CID) product ion spectrum of protonated 3-dimethylaminomethyl-4-(4-methanesulfinyl-3-methylphenoxy)benzenesulfonamide. A single loss was initially assigned using the low-resolution product ion spectrum, acquired by electrospray ionisation quadrupole ion trap mass spectrometry (ESI-QIT-MS), as methanethial, S-oxide via a charge-remote, four-centred rearrangement. This assignment was consistent with well-documented hydrogen rearrangements in the literature. Further, the loss was not observed for the parent compound. Thus, it was inferred that the site of metabolism was involved in the dissociation and the attractive nature of the four-centred rearrangement meant that the loss of methanethial, S-oxide was a logical assignment. However, deuterium-labelling experiments and accurate mass measurements, performed using electrospray ionisation Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS), showed that the nominal loss of 62 m/z units occurs via two distinct dissociation pathways. Neither of these losses was of methanethial, S-oxide as initially hypothesised from the low-resolution product ion spectrum of the protonated molecule. Mechanisms consistent with the experimental findings are postulated. An MS(3) spectrum of the fully exchanged, deuterated species supported the proposed mechanisms by suggesting that 3-dimethylaminomethyl-4-(4-methanesulfinyl-3-methylphenoxy)benzenesulfonamide has multiple sites of protonation in the gas phase. The planar structures of the posited product ions are likely to provide the driving force for the rearrangements. The relevance of the observations with regards to pharmaceutical drug metabolite identification is discussed.

Cyclization reaction of peptide fragment ions during multistage collisionally activated decomposition: an inducement to lose internal amino-acid residues

During characterization of some peptides (linear precursors of the cyclic peptides showing potential to be anticancer drugs) in an ion trap, it was noted that many internal amino acid residues could be lost from singly charged b ions. The phenomenon was not obvious at the first stage of collisionally activated decomposition (CAD), but was apparent at multiple stages of CAD. The unique fragmentation consisting of multiple steps is induced by a cyclization reaction of b ions, the mechanism of which has been probed by experiments of N-acetylation, MS(n), rearranged-ion design, and activation-time adjustment. The fragmentation of synthetic cyclic peptides demonstrates that a cyclic peptide intermediate (CPI) formed by b ion cyclization exhibits the same fragmentation pattern as a protonated cyclic peptide. Although no rules for the cyclization reaction were discerned in the experiments of peptide modification, the fragmentations of a number of b ions indicate that the "Pro and Asn/Gln effects" can influence ring openings of CPIs. In addition, large-scale losses of internal residues from different positions of a-type ions have been observed when pure helium was used as collision gas. The fragmentation is initiated by a cyclization reaction forming an a-type ion CPI. This CPI with a fixed-charge structure cannot be influenced by the "Pro effect", causing a selective ring opening at the amide bond Pro-Xxx rather than Xxx-Pro. With the knowledge of the unique fragmentations leading to internal residue losses, the misidentification of fragments and sequences of peptides may be avoided.

Characterization of electrospray ionization mass spectrometry for N-diisopropyloxyphosphoryl dipeptide methyl esters

A systematic study of the fragmentation pattern of N-diisopropyloxyphosphoryl (DIPP) dipeptide methyl esters in an electrospray ionization (ESI) tandem mass spectrometry (MS/MS) was presented. A combination of accurate mass measurement and tandem mass spectrometry had been used to characterize the major fragment ions observed in the ESI mass spectrum. It was found that the alkali metal ions acted as a fixed charge site and expelled the DIPP group after transferring a proton to the amide nitrogen. For all the N-phosphoryl dipeptide methyl esters, under the activation of a metal ion, the rearrangement product ion at m/z 163 was observed and confirmed to be the sodium adduct of phosphoric acid mono-isopropyl esters (PAIE), via a specific five-membered penta-co-ordinated phosphorus intermediate. However, no rearrangement ion was observed when a beta-amino acid was at the N-terminal. This could be used to develop a novel method for differentiating isomeric compounds when either alpha- or beta-amino acid are at the N-terminus of peptides. From the [M+Na]+ ESI-MS/MS spectra of N-phosphoryl dipeptide methyl esters (DIPP Xaa1 Xaa2 OMe), the peaks corresponding to the [M+Na Xaa1 C3H6]+ were observed and explained. The [M+Na]+ ESI-MS/MS spectra of N-phosphoryl dipeptide methyl esters with Phe located in the C-terminal, such as DIPPValPheOMe, DIPPLeuPheOMe, DIPPIlePheOMe, DIPPAlaPheOMe and DIPPPhePheOMe, had characteristic fragmentation. Two unusual gas-phase intramolecular rearrangement mechanisms were first proposed for this fragmentation. These rearrangements were not observed in dipeptide methyl ester analogs which did not contain the DIPP at the N-terminal, suggesting that this moiety was critical for the rearrangement.

Fragmentation of tunichrome Sp-1 is dominated by an unusual gas-phase intramolecular rearrangement

Tunichrome Sp-1 is a modified pentapeptide from the ascidian Styela plicata, having the structure H-DOPA-DOPA-Gly-Pro-dcDeltaDOPA (where DOPA = 3,4-dihydroxyphenylalanine and dcDeltaDOPA = decarboxy-(E)-alpha,beta-dehydro-DOPA). The tandem mass spectrum of the peptide is dominated by a number of abundant fragment ions that involve a gas-phase rearrangement where the dcDeltaDOPA group becomes covalently attached to the N-terminus. The high degree of rearrangement in Sp-1 compared with a related octapeptide, plicatamide, allowed for detailed multiple mass spectrometric (MS(n)) (up to n = 6) experiments, and hence permitted a detailed assessment of the origin and routes to the formation of the various rearrangement ions. Analyses on both a triple-quadrupole and a quadrupole time-of-flight mass spectrometer were made to ascertain whether the gas-phase rearrangements observed for tunichrome Sp-1 were unique to an ion trap mass spectrometer (i.e. the hypothesis being that perhaps the extended trapping times were required to facilitate this unusual gas-phase rearrangement). Interestingly, analyses on both the triple-quadrupole and quadruple time-of-flight mass spectrometers revealed an identical phenomenon, with the rearrangement fragment ions present at approximately the same abundance as the non-rearranged a-, b- and y-type sequence ions. We suggest that the smaller size of tunichrome Sp-1 compared with plicatamide facilitates the transfer of the dcDeltaDOPA group in this gas-phase rearrangement. This rearrangement was not observed for peptide analogs of tunichrome Sp-1 that did not contain the dcDeltaDOPA at the C-terminus, confirming that the presence of dcDeltaDOPA is critical for the rearrangement.

Plicatamide, an antimicrobial octapeptide from Styela plicata hemocytes

Plicatamide (Phe-Phe-His-Leu-His-Phe-His-dc Delta DOPA), where dc Delta DOPA represents decarboxy-(E)-alpha,beta-dehydro-3,4-dihydroxyphenylalanine, is a potently antimicrobial octapeptide from the blood cells of the solitary tunicate, Styela plicata. Wild type and methicillin-resistant Staphylococcus aureus (MRSA) responded to plicatamide exposure with a massive potassium efflux that began within seconds. Soon thereafter, treated bacteria largely ceased consuming oxygen, and most became nonviable. Native plicatamide also formed cation-selective channels in model lipid bilayers composed of bacterial lipids. Methicillin-resistant S. aureus treated with plicatamide for 5 min contained prominent mesosomes as well as multiple, small dome-shaped surface protrusions that suggested the involvement of osmotic forces in its antimicrobial effects. To ascertain the contribution of the C-terminal dc Delta DOPA residue to antimicrobial activity, we synthesized several analogues of plicatamide that lacked it. One of these peptides, PL-101 (Phe-Phe-His-Leu-His-Phe-His-Tyr-amide), closely resembled native plicatamide in its antimicrobial activity and its ability to induce potassium efflux. Plicatamide was potently hemolytic for human red blood cells but did not lyse ovine erythrocytes. The small size, rapid action, and potent anti-staphylococcal activity of plicatamide and PL-101 make them intriguing subjects for future antimicrobial peptide design.

An unusual rearrangement of Zofenopril, a new ACE inhibitor drug: mass spectrometric and conformational studies

Zofenopril (1) is a new ACE inhibitor, used in therapy for hypertension and post-myocardial infarction. The protonated quasi-molecular ion (m/z 430) of 1, obtained under positive electrospray ionization conditions, loses a benzoic acid molecule (m/z 308), which in turn decomposes via loss of CO (m/z 280) when low-energy collisional-induced dissociation (CID) and in-source experiments are performed. This rearrangement is the main fragmentation process and can be observed both in-source and in the product ion tandem mass spectra, using either an ion trap or a triple quadrupole instrument. Other known diastereoisomers of 1, an impurity with an acetyl in the place of the benzoyl group (2) and an impurity with two propanoyl chains in series (3), give the same rearrangement. On the other hand, the mass spectra of the methyl ester (4) and an impurity with two proline moieties (5) do not show this unusual fragmentation. Time-resolved CID spectra of 1 show that the rearrangement occurs after about 2 ms, a time scale comparable to those of the other non-rearrangement cleavages. These experiments suggest a conformation in the gas phase for 1 in which the benzoyl group is close to the hydroxyl of the carboxylic acid group, from which the rearrangement could readily occur. Since compounds 4 and 5 do not show the same behaviour, the presence of a carboxylic acid in the proline ring seems to play a crucial role in the rearrangement, probably due to an intramolecular hydrogen bond. To confirm this hypothesis, deuterium exchanges in mass spectrometric experiments and a conformational analysis via computational methods were performed.