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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 laserdesorbed, 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 defined amino acid are found to block charge transfer. The model qualitatively accounts for the order of the rates of the processes involved.

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Electron solvation dynamics in photoexcited anion clusters of I-(D2O)n=4-6 and I-(H2O)4-6 were probed by using femtosecond photoelectron spectroscopy (FPES). An ultrafast pump pulse excited the anion to the cluster analog of the charge-transfer-to-solvent state seen for I- in aqueous solution. Evolution of this state was monitored by time-resolved photoelectron spectroscopy using an ultrafast probe pulse. The excited n = 4 clusters showed simple population decay, but in the n = 5 and 6 clusters the solvent molecules rearranged to stabilize and localize the excess electron, showing characteristics associated with electron solvation dynamics in bulk water. Comparison of the FPES of I-(D2O)n with I-(H2O)n indicates more rapid solvation in the H2O clusters.