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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The dissociation of peptide ions has been found to have ultrafast components that in many ways are uniquely different from typical unimolecular kinetics. As such, some peptide reactions provide new channels, which do not conform to statistical models of reaction kinetics. When the dissociation rates are in the 100 fs range, they are in a time scale where statistical methods do not yet apply, although molecules that have not yet dissociated will later in time undergo statistical redistribution of their excess energy, which, however, may not lead to noticeable reactivity within the experimental time frames for large peptides and hence are simply dissipative. This work is meant to reconcile the long time statistical results of Lifshitz et al. (2003) with the work of Schlag et al. (1995/6) that suggests an alternate parallel and much faster time scale for dissociation. It is argued that the two sets of results and interpretations augment one another and in fact open up a most interesting new field of peptide kinetics in addition to the unimolecular behavior, which becomes de facto arrested by the shear size of the molecule being unable to find a transition state on any reasonable time scale.

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