Combining UV photodissociation with electron transfer for peptide structure analysis

Abstract

The combination of near-UV photodissociation with electron transfer and collisional activation provides a new tool for structure investigation of isolated peptide ions and reactive intermediates. Two new types of pulse experiments are reported. In the first one called UV/Vis photodissociation-electron transfer dissociation (UVPD-ETD), diazirine-labeled peptide ions are shown to undergo photodissociation in the gas phase to form new covalent bonds, guided by the ion conformation, and the products are analyzed by electron transfer dissociation. In the second experiment, called ETD-UVPD wherein synthetic labels are not necessary, electron transfer forms new cation-peptide radical chromophores that absorb at 355 nm and undergo specific backbone photodissociation reactions. The new method is applied to distinguish isomeric ions produced by ETD of arginine containing peptides.

Near-UV Photodissociation of Tryptic Peptide Cation Radicals. Scope and Effects of Amino Acid Residues and Radical Sites

“…Mass spectra were measured on an LTQ-XL-ETD linear ion trap (LIT) tandem mass spectrometer (ThermoElectron Fisher, San Jose, CA, USA) that has been modified to allow photodissociation studies [19]. Doubly charged peptide ions were generated by electrospray of solutions in 50:50:0.5 methanol-water-acetic acid and mass-selected in the LIT.…”

“…Recently, photodissociation has been combined with another ion activation method, electron transfer dissociation (ETD) [17], to drive nonselective dissociations of primary peptide fragment ions produced by ETD, thus increasing the number of fragment ions to be potentially used in peptide sequencing [18]. Near-UV photodissociation at 355 nm of ETD peptide fragment ions of the z-type has been shown to be a selective tool for determining the ion structure by targeting the radical chromophores generated by electron transfer [19]. Infrared multiphoton dissociation of ETD-produced peptide fragment ions has also been implemented to provide useful information of the structure and conformation of c- [20] and z-type [21] ions.…”

“…These correspond to [ • AAXAK + ], or [z 5 + H] +• , ions according to previous studies of ion structure that established that the ammonia molecule selectively originates from the peptide N-terminus [33,34]. Analogous fragment ions by ETD loss of ammonia from the Arg-C-terminated (AAXAR) series were not considered because they are likely to be mixtures of isomers formed by ammonia elimination from the N-terminus and the Arg side chain [19,34]. Third, [ • XAR + ], or [z 3 + H] +• ions that have the C α -radical defect at the variable X residue were studied for the Arg C-terminated peptides.…”

Efficient Covalent Bond Formation in Gas-Phase Peptide–Peptide Ion Complexes with the Photoleucine Stapler

“…To accomplish photodissociation of trapped ions in the LIT, the LTQ-XL ETD mass spectrometer was modified as reported previously [26,34,35]. Briefly, the chemical ionization (CI) source for the anion production was modified by drilling a 1-mm diameter hole into the insert block to provide a line of sight path to the LIT.…”

“…Formation of covalent bonds in gas-phase ion complexes has been reported for several systems that relied on collision-induced chemical reactions between an anion and a cation in a complex that mimicked chemical methods used in solution [23,24]. In addition, covalent bond formation in gasphase ion complexes with 18-crown-6-ether has been accomplished by collision-induced dissociation of diazomalonate derivatives [25] or photodissociation of a diazirine labeled peptide [26] in which reactive carbenes played the role of reactive intermediates. The singular example whereby photoactivation of a peptide ion led to amide bond formation was severely limited by indiscriminate regioselectivity, use of vacuum ultraviolet light (157 nm), and low yield (<1%) [27].…”