Ion-Induced Dipole Interactions and Fragmentation Times: Cα-Cß Chromophore Bond Dissociation Channel.

The fragmentation times corresponding to the loss of the chromophore (Cα-Cß bond dissociation channel) after photoexcitation at 263 nm have been investigated for several small peptides containing tryptophan or tyrosine. For tryptophan-containing peptides, the aromatic chromophore is lost as an ionic fragment (m/z 130), and the fragmentation time increases with the mass of the neutral fragment. In contrast, for tyrosine-containing peptides the aromatic chromophore is always lost as a neutral fragment (mass = 107 amu) and the fragmentation time is found to be fast (<20 ns). These different behaviors are explained by the role of the postfragmentation interaction in the complex formed after the Cα-Cß bond cleavage.

Vacuum ultraviolet photoionization study of gas phase vitamins A and B1 using aerosol thermodesorption and synchrotron radiation.

Gas-phase studies of biomolecules are often difficult to initiate because of the thermolability of these systems. Such studies are nevertheless important to determine fundamental intrinsic properties of the molecules. Here we present the valence shell photoionization of gas-phase vitamins A and B1 close to their ionization threshold. The study was performed by means of an aerosol thermodesorption source coupled to an electron/ion coincidence spectrometer and synchrotron radiation (SOLEIL facility, France). Ion yield curves were recorded for both molecules over a few electronvolt energy range and the threshold photoelectron spectrum was also obtained for vitamin A. Some fundamental properties were extracted for both ions such as adiabatic and the three first vertical ionization energies of retinol (IEad = 6.8 ± 0.2 eV and IEvert = 7.4, 8.3, and 9.2 eV) and dissociation appearance energies for the main fragment ions of vitamin B1. Analysis of the data was supported by ab initio calculations which show a very good agreement with the experimental observations.

Prompt and slow electron-detachment-dissociation/electron-photodetachment-dissociation of a 21-mer peptide.

Electron detachment dissociation (EDD) and electron photodetachment dissociation (EPD) are relatively new dissociation methods that involve electron detachment followed by radical-driven dissociation from multiply deprotonated species. EDD yields prompt dissociation whereas only electron detachment is obtained by EPD; subsequent vibrational activation of the charge-reduced radical anion is required to obtain the product ions. Herein, the fragmentation patterns that were obtained by EDD and by vibrational activation of the charge-reduced radical anions that were produced through EDD or EPD (activated-EDD and activated-EPD) were compared. The observed differences were related to the dissociation kinetics and/or the contribution of electron-induced dissociation (EID). Time-resolved double-resonance experiments were performed to measure the dissociation rate constants of the EDD product ions. Differences in the formation kinetics were revealed between the classical EDD/EPD 'a(⋅)(i)/''x(j) complementary ions and some 'a(⋅)(i)/c(i)/'''z(⋅)(j) product ions, which were produced with slower dissociation rate constants, owing to the presence of specific neighbouring side chains. A new fragmentation pathway is proposed for the formation of the slow-kinetics 'a(⋅)(i) ions.

Determination of peptide topology through time-resolved double-resonance under electron capture dissociation conditions.

Characterizing the conformation of biomolecules by mass spectrometry still represents a challenge. With their knotted structure involving a N-terminal macrolactam ring where the C-terminal tail of the peptide is threaded and sterically trapped, lasso peptides constitute an attractive model for developing methods for characterizing gas-phase conformation, through comparison with their unknotted topoisomers. Here, the kinetics of electron capture dissociation (ECD) of a lasso peptide, capistruin, was investigated by electrospray ionization-Fourier transform ion cyclotron resonance mass spectrometry and compared to that of its branched-cyclic topoisomer, lactam-capistruin. Both peptides produced rather similar ECD spectra but showed different extent of H(•) transfer from c(i)' to z(j)(•) ions. Time-resolved double-resonance experiments under ECD conditions were performed to measure the formation rate constants of typical product ions. Such experiments showed that certain product ions, in particular those related to H(•) transfer, proceeded through long-lived complexes for capistruin, while fast dissociation processes predominated for lactam-capistruin. The formation rate constants of specific ECD product ions enabled a clear differentiation of the lasso and branched-cyclic topoisomers. These results indicate that the formation kinetics of ECD product ions constitute a new way to explore the conformation of biomolecules and distinguish between topoisomers and, more generally, conformers.