Mechanisms of UV photodissociation of small protonated peptides.

Photofragmentation of protonated dipeptides by 263 nm photons is investigated with an experimental technique based on the detection in coincidence of the ionic and neutral fragments. With this method, it is possible to determine whether the fragmentation takes place in one or several steps. The timing of these steps can also be evaluated. The interpretation of the various fragmentation pathways is tentatively developed along the same line as that previously proposed for tryptophan. The fragmentation can be explained by two types of mechanisms: internal conversions and direct fragmentations triggered by the migration of the photoactive electron on positive charged sites or on oxygen sites.

Mechanisms of photoinduced Calpha[Single Bond]Cbeta bond breakage in protonated aromatic amino acids.

Photoexcitation of protonated aromatic amino acids leads to C(alpha)[Single Bond]C(beta) bond breakage among other channels. There are two pathways for the C(alpha)[Single Bond]C(beta) bond breakage, one is a slow process (microseconds) that occurs after hydrogen loss from the electronically excited ion, whereas the other is a fast process (nanoseconds). In this paper, a comparative study of the fragmentation of four molecules shows that the presence of the carboxylic acid group is necessary for this fast fragmentation channel to occur. We suggest a mechanism based on light-induced electron transfer from the aromatic ring to the carboxylic acid, followed by a fast internal proton transfer from the ammonium group to the negatively charged carboxylic acid group. The ion formed is a biradical since the aromatic ring is ionized and the carbon of the COOH group has an unpaired electron. Breakage of the weak C(alpha)[Single Bond]C(beta) bond gives two even-electron fragments and is expected to quickly occur. The present experimental results together with the ab initio calculations support the interpretation previously proposed.