The Genuine Article Number: RK018
Weinkauf R., Schanen P., Yang D., Sonkara S., Schlag EW .
ELEMENTARY PROCESSES IN PEPTIDES - ELECTRON MOBILITY AND
DISSOCIATIONS IN PEPTIDE CATIONS IN THE GAS PHASE
Journal of Physical Chemistry. 99(28):11255-11265, 1995 Jul 13.
Pulsed laser desorption. Ionization mass-spectrometry. Resonant 2-photon
ionization. Supersonic molecular-beams. Charge-transfer. Exciplex formation.
Bichromophoric molecules. Solvent polarity. Jet. Spectroscopy.
Neutral peptides of natural amino acids of the type (X)(n)-Y (n = 1,2,3) are
prepared in the gas phase by laser desorption and supersonic cooling. Local
ionization is performed by resonant laser excitation in aromatic amino acids
(Y) located at the C-terminal end. In a one-color experiment, subsequent UV
photofragmentation of the cation is shown to directly reflect the prior charge
migration in these large molecules. Peptides are engineered, which show
either fragment ions originating from the chromophore or from the opposite
N-terminal side (X). The results show that by changing local ionization
energies and thus absolute positions of ionic dissociation energies, one has
complete control over different paths of chemical reactivity. The length
dependence of the process shows, that charge mobility seems to be not the
bottleneck for dissociation pathways at high internal energies: charge
transfer over more than 10 sigma-bonds is shown. When we apply a local
picture and estimate local ionization potentials, we find, for the peptides used
here, that after localized ionization, positive charge is statically localized at
the initial prepared site. In a two-color experiment (UV + VIS) we observe
indications that in the photoexcited tripeptide cation Leu-Leu-Tyr charge
transfer can occur at internal energies of about 2.2 eV, an energy at which no
dissociation occurs. We interpret the process in terms of direct
photoexcitation into a charge transfer (CT) band or by a photoexcitation to a
localized state followed by nonradiative relaxation to a CT state. For the
charge-transfer process we propose a through bond HOMO electron transfer
(hole transfer) as the relevant mechanim. Consequences of our findings for
charge migration and fragmentation processes in peptides are discussed.
Physical chemistry/chemical physics.
Current Contents/Physical, Chemical & Earth Sciences
Reprint available from:
INST PHYSICAL CHEMISTRY & ELECTROCHEMISTRY I