Electron-Capture and -Transfer Dissociation of Peptides Tagged with Tunable Fixed-Charge Groups: Structures and Dissociation Energetics

Abstract

Pyridiniummethylcarbonyl moieties that were previously designed on the basis of electronic structure analysis are now utilized as fixed-charge tags with tunable electronic properties to be used for N-terminal peptide derivatization and sequencing by electron-transfer dissociation. Dipeptides AK and KA were derivatized at the peptide N-terminus with 4-dimethylaminopyridinium-N-acetyl (DMAP-ac) and pyridinium-N-acetyl (pyrid-ac) tags of increasing intrinsic recombination energies. Upon the capture of a free electron or electron transfer from fluoranthene anions, (DMAP-ac-AK+H) 2+ , (DMAP-ac-KA+H) 2+ , (pyrid-ac-AK+H) 2+ and (pyridac-KA+H) 2+ ions, as well as underivatized (AK+2H) 2+ , completely dissociated. The fixed-charge tags steered the dissociation upon electron transfer to form abundant backbone N-C α bond cleavages, whereas the underivatized peptide mainly underwent H-atom and side-chain losses. Precursor ion structures for the tagged peptides were analyzed by an exhaustive conformational search combined with B3LYP/6-31+G(d,p) geometry optimization and single-point energy calculations in order to select the global energy minima. Structures, relative energies, transition states, ion-molecule complexes, and dissociation products were identified for several chargereduced species from the tagged peptides. The electronic properties of the charge tags and their interactions with the peptide moieties are discussed. Electrospray ionization and electrontransfer dissociation of larger peptides are illustrated with a DMAP-tagged pentapeptide.

Chemical Derivatization of Peptide Carboxyl Groups for Highly Efficient Electron Transfer Dissociation

“…We speculate that the pyridine functional group sequesters the transferred electron and produces uninformative side-chain fragmentation products rather than backbone cleavages, as has been observed for ETD and ECD of various peptides having heterocyclic aromatic groups [8, 55–57]. Interestingly, Turecek has found that a pyridinium group, attached to a peptide’s N-terminus, served as a good functional group for improving ETD [58, 59]. While similar in structure, the pyridinium differs from the pyridine employed here in that the nitrogen atom resides on the opposite side of the aromatic ring and it is quaternary, rather than tertiary.…”

Principles of electron capture and transfer dissociation mass spectrometry applied to peptide and protein structure analysis

“…Generally, the peptides are protonated, although use of metal cations or synthetic charge tags can provide additional charge sites by chemical modifications. [11][12][13] An additional increase in peptide charge, or supercharging, is achieved by modifying solvent composition and ion source parameters, as discussed elsewhere. 14 For simplicity, here only protonation as a means of peptide or protein charging is considered.…”

“…(1) Identification of the precursor ion and charge-reduced species. Since the m/z value of the precursor ion is known, use the formula q = n À 1, where q is the charge state and n is the number of isotopic peaks per 1 m/z unit, to determine the charge state of the precursor ion (since 12 C and 13 C isotopes differ by B1 Da, Table 1). Knowing the m/z value and the charge state of the precursor ion, it is possible to derive the m/z value of the charge-reduced species.…”

Tunable Charge Tags for Electron-Based Methods of Peptide Sequencing: Design and Applications

“…The N,N-dimethylaminopyridyl-1-methylcarboxamido (DMAP) group has recently been shown to substantially improve ETD sequence coverage of small peptides [52]. We therefore focused on studying a broader series of penta-, nona-, and decapeptides tagged with DMAP at the Ntermini.…”