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.

Laser photodissociation at 355 nm (near-UVPD) of phosphopeptide cation-radicals provides information on the position of the radical center and the phosphorylated serine residue. Cationradicals of the z type are produced by electron transfer dissociation of peptide dications from pSAAAR, ApSAAR, AApSAR, and AAApSR and found to absorb light at 355 nm to undergo photodissociation. Near-UVPD can induce backbone dissociation of the CO-NH bond next to the radical center on its N-terminal side concurrent with only minor radical scrambling and loss of the post-translational modification. The pSer residue effectively blocks backbone cleavage when the radical site is in its closest vicinity. Near-UVPD of loss-of-ammonia fragment ions produced by ETD results in backbone dissociations that distinguish isomeric ions differing in the site of ammonia loss. The experimental data are interpreted with the help of ab initio and density functional theory calculations of ion structures, transition states, and excitation energies.

Abstract. Electron transfer to gas-phase peptide ions with diazirine-containing amino acid residue photoleucine (L*) triggers diazirine ring reduction followed by cascades of residue-specific radical reactions. Upon electron transfer, substantial fractions of (GL*GGR +2H) +• cation-radicals undergo elimination of [NH O-labeled peptide ions and found to specifically involve the amide oxygen of the N-terminal residue. The structures, energies, and electronic states of the peptide radical species were elucidated by a combination of near-UV photodissociation experiments and electron structure calculations combining ab initio and density functional theory methods. Electron transfer reaching the ground electronic states of charge reduced (GL*GGR +2H) +• cation-radicals was found to reduce the diazirine ring. In contrast, backbone N−C α bond dissociations that represent a 60%-75% majority of all dissociations because of electron transfer are predicted to occur from excited electronic states.

Hyaluronic acid is a naturally occurring linear polysaccharide with substantial medical potential. In this work, discrimination of tyramine-based hyaluronan derivatives was accessed by ion mobility-mass spectrometry of deprotonated molecules and nuclear magnetic resonance spectroscopy. As the product ion mass spectra did not allow for direct isomer discrimination in mixture, the reductive labeling of oligosaccharides as well as stable isotope labeling was performed. The ion mobility separation of parent ions together with the characteristic fragmentation for reduced isomers providing unique product ions allowed us to identify isomers present in a mixture and determine their mutual isomeric ratio. The determination used simple recalculation of arrival time distribution areas of unique ions to areas of deprotonated molecules. Mass spectrometry data were confirmed by nuclear magnetic resonance spectroscopy.