The Genuine Article Number: VU825
Weinkauf R., Schanen P., Metsala A., Schlag EW., Burgle M., Kessler H .
HIGHLY EFFICIENT CHARGE TRANSFER IN PEPTIDE CATIONS IN THE GAS
PHASE-THRESHOLD EFFECTS AND MECHANISM
Journal of Physical Chemistry. 100(47):18567-18585, 1996 Nov 21.
Range electron-transfer. Multiphoton dissociation spectra. Molecular ion
spectroscopy. Long-range. Energy-transfer. Supersonic jet. Benzene cation.
Radiationless transitions. Photoelectron-spectra. Exciplex formation.
We present new experimental data demonstrating specific, photoactivated
positive charge migration in isolated peptide radical cations. The effect
exhibits a threshold behavior, which we can directly correlate with energetics
of local electronic states. A new very efficient mechanism for charge transfer
in cations is proposed that involves an extended coulomb state (EC) of
shakeup character. Our investigations are performed on laser-desorbed,
cooled, neutral peptides in the gas phase. Charge localization in the peptide
is achieved by resonant UV two-photon ionization at an aromatic
chromophore. Charge flow in the cations can be activated by absorption of a
first visible (VIS) photon. Presence of charge in the aromatic chromophore is
probed by resonant absorption of a second VIS photon and monitored by
dissociation. While this charge detection is found to work in isolated,
positively charged chromophores or amino acids, it is efficiently quenched in
some peptides. We explain this by photoactivated charge transfer and charge
storage in nonaromatic groups of the peptides. At threshold this process is
found to be strongly dependent on amino acid substitution even far away
from the site of photoactivation. For analysis we first set up a local molecular
orbital model for peptide cations and subsequently obtain a landscape of
local electronic cation states formed by local hole and low-lying extended
coulomb states. Charge transfer is found to be a through-bond mechanism
involving energetically accessible electronic states along the path of charge
flow. Charge transfer between hole states is mediated with very high
efficiency through saturated carbon bridges by extended coulomb states. This
new mechanism seems to be generally applicable to large extended
molecular radical cations. Only barriers of the size of a full length of a certain
amino acid are found to block charge transfer. The model qualitatively
accounts for the order of the rates of the processes involved. [References:
Physical chemistry/chemical physics.
Current Contents/Physical, Chemical & Earth Sciences
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