Photo-induced formation of radical anion peptides. Electron photodetachment dissociation experiments.
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Angiotensins↗
Biomedical subjects
Philippe Dugourd
Publications and source records attributed to Philippe Dugourd.
Spectroscopy of isolated, mass-selected tryptophan-Ag3 complexes: a model for photoabsorption enhancement in nanoparticle-biomolecule hybrid systems.
We wish to show that gas-phase studies of small metal cluster-biomolecule complexes provide fundamental insights into the mechanism leading to enhanced optical absorption in nanoparticle-biomolecular hybrid systems. Here we present, for the first time, a joint experimental and theoretical study of photoabsorption and photofragmentation of the silver trimer-tryptophan cation complex ([Trp.Ag3]+). We demonstrate that binding the metal cluster to a biomolecule leads to a remarkably high optical absorption as compared to the bare tryptophan or the [Trp.Ag]+ complex. As calculations show this arises due to coupling between the excitations in the metallic subunit with a charge transfer excitation to the tryptophan molecule.
Models, Biological↗
Electron photodetachment dissociation of DNA polyanions in a quadrupole ion trap mass spectrometer.
We hereby explore the effects of irradiating DNA polyanions stored in a quadrupole ion trap mass spectrometer with an optical parametric oscillator laser between 250 and 285 nm. We studied DNA 6-20-mer single strands and 12-base pair double strands. In all cases, laser irradiation causes electron detachment from the multiply charged DNA anions. Electron photodetachment efficiency directly depends on the number of guanines in the strand, and maximum efficiency is observed between 260 and 275 nm. Subsequent collision-induced dissociation (CID) of the radical anions produced by electron photodetachment results in extensive fragmentation. In addition to neutral losses, a large number of fragments from the w, d, a*, and z* ion series are obtained, contrasting with the w and (a-base) ion series observed in regular CID. The major advantage of this technique, coined electron photodetachment dissociation (EPD) is the absence of internal fragments, combined with good sequence coverage. EPD is therefore a highly promising approach for de novo sequencing of oligonucleotides. EPD of nucleic acids is also expected to give specific radical-induced strand cleavages, with conservation of other fragile bonds, including noncovalent bonds. In effect, preliminary results on a DNA hairpin and on double strands suggest that EPD could also be used to probe intra- and intermolecular interactions in nucleic acids.
DNA↗
Optical properties of gas-phase tryptophan-silver cations: charge transfer from the indole ring to the silver atom.
We present a joint experimental and theoretical investigation of the electronic excitation spectra of the tryptophan-silver complex. The photodissociation spectrum of gas-phase [Trp-Ag]+ was measured from 215 to 330 nm using a quadrupole ion trap coupled to an optical parametric-oscillator laser. The calculated time-dependent density functional theory (TD-DFT) absorption spectra for different prototypes of structures are presented. Low-energy transitions that are experimentally observed are only calculated for the charge-solvation (CS) structures. These transitions are a signature of the metal-pi interaction in [Trp-Ag]+. The recorded spectrum is compared to a Boltzmann average of the absorption spectrum obtained from direct molecular dynamics (MD) simulations involving simultaneous transitions to excited states based on semiempirical configuration interaction (CI) calculations. The results demonstrate that charge transfer can be photoinduced from the indole ring to the silver atom.
Cations, Monovalent↗
UV photodissociation of phospho-seryl-containing peptides: laser stabilization of the phospho-seryl bond with multistage mass spectrometry.
Protonated precursor ions of phosphorylated peptides containing a tyrosyl residue have been subjected to UV laser-induced dissociation (LID) at a wavelength of 220 nm and to collision-induced dissociation (CID) in an ion trap. As expected, neutral loss of the phosphate group is one of the predominant fragmentation channels during CID together with H2O elimination. In contrast, LID leads mainly to the homolytic cleavage of the tyrosyl side chain and a restrained loss of the phosphate group. Interestingly, the intensity of the dephosphorylated fragment ion is greatly minimized when CID is carried out next on the radical precursor ion of the singly and doubly charged species.
Amino Acid Sequence↗
Entropic stabilization of isolated beta-sheets.
Temperature-dependent electric deflection measurements have been performed for a series of unsolvated alanine-based peptides (Ac-WA(n)-NH(2), where Ac = acetyl, W = tryptophan, A = alanine, and n = 3, 5, 10, 13, and 15). The measurements are interpreted using Monte Carlo simulations performed with a parallel tempering algorithm. Despite alanine's high helix propensity in solution, the results suggest that unsolvated Ac-WA(n)-NH(2) peptides with n > 10 adopt beta-sheet conformations at room temperature. Previous studies have shown that protonated alanine-based peptides adopt helical or globular conformations in the gas phase, depending on the location of the charge. Thus, the charge more than anything else controls the structure.
Alanine↗
Photodissociation spectroscopy of trapped protonated tryptophan.
We have coupled a quadrupole ion trap with a frequency doubled optical parametric oscillator laser. The photodissociation spectrum of the protonated tryptophan ion from 215 to 320 nm is reported. The yields of fragmentation on each mass channel as a function of the laser wavelength were obtained. We also report experiments involving multiple stages of laser induced dissociation and discuss possible structures for the fragmentation products.
Gas Chromatography-Mass Spectrometry↗
Specific photodissociation of peptides with multi-stage mass spectrometry.
We report collision-induced dissociation (CID) and laser-induced dissociation (LID) performed at different wavelengths between 220 and 280 nm of the peptides leucine-enkephalin (protonated) and gramicidin A (sodiated). Hydrogen-atom losses and side-chain cleavages were observed in LID experiments. These losses depend on the laser wavelength and lead to the formation of radical ions. The fragmentations of these radicals, which are not observed in CID experiments, were investigated in multi-stage mass spectrometry experiments.
Enkephalin, Leucine↗
Application of molecular beam deflection time-of-flight mass spectrometry to peptide analysis.
The application of molecular beam deflection time-of-flight mass spectrometry (MBD-TOFMS) to peptide identification is described. The technique permits a simultaneous measurement of molecular mass and electric dipole susceptibility. The mass and susceptibility are not strongly correlated, and the results can be presented as a two-dimensional map. The susceptibility provides a useful way to disperse isobaric and isomeric peptides, and at least for small peptides, the susceptibility is significantly different for different amino acid sequences. Results for peptides in the mass range 1000-2300 Da show that the mass and susceptibility lead to a higher identification score than mass spectra alone.
Algorithms↗
Direct probing of zwitterion formation in unsolvated peptides.
Molecular beam electric deflection measurements have been used to determine electric susceptibilities for small unsolvated alanine-based peptides. The electric susceptibility provides information about the charge distribution within the peptide and can be used to distinguish between zwitterionic and canonical forms. Measured electric susceptibilities for WAn peptides (n = 1-5) are similar to those for capped Ac-WAn-NH2 peptides (which cannot form zwitterions). Susceptibilities calculated using a simulated tempering-based approach are substantially larger for the zwitterionic form than for the canonical form. The measured susceptibilities are in good agreement with those calculated for the canonical form. For the larger peptides, the lowest potential energy structure found in the simulations is hairpin-like, while the lowest free energy structure found at room temperature is extended. The zwitterionic form is constrained by intramolecular interactions which make it entropically unfavorable.
Alanine↗
Electric susceptibility of unsolvated glycine-based peptides.
The DC electric susceptibilities of unsolvated glycine-based peptides, WGn (W = tryptophan and G = glycine) with n = 1-5, have been measured by deflection of a molecular beam in an electric field. These are the first electric deflection measurements performed on peptides. At 300 K the susceptibilities are in the range of 200-400 A(3). By far the largest contribution to the susceptibilities is from the permanent dipole moment of the peptides. The results indicate that the peptides do not have rigid conformations with fixed dipoles. Instead the dipole is averaged as the peptides explore their energy landscape. For a given WGn peptide, all molecules have almost the same average dipole, which suggests that they all explore a similar energy landscape on the microsecond time scale of the measurement. The measured susceptibilities are in good overall agreement with values calculated from the average dipole moment deduced from Monte Carlo simulations.
Electrochemistry↗