Photodissociation of protonated amino acids and peptides in an ion storage ring. Determination of Arrhenius parameters in the high-temperature limit

We have measured the time dependence of the fragmentation of protonated amino acids and peptides upon UV excitation in an electrostatic ion storage ring. After absorption of a 266 nm photon, protonated Trp (TrpH⁺) has a lifetime of 10-20 μs but also a component with a millisecond lifetime is present. The long lifetime may be due to fluorescence, which leads to ions with lower excitation energy, or to the decay of the Trp^+• radical cation formed after prompt hydrogen loss in the laser interaction region. Only one component with a lifetime of about 10 μs was detected for TyrH ⁺. The lifetime of photoexcited PheH⁺ is even shorter with an upper limit of a few microseconds. For the singly protonated tripeptides (LysTrpLysH⁺ and LysTyrLysH⁺), the decay curves are found to consist of a single component that can be reproduced with an assumption of statistical decay after equilibration of the photon energy among all vibrational modes. The rate constant is expressed in the Arrhenius form in terms of the microcanonical temperature, and the decay rate is obtained by integration over the energy distribution, which has a spread corresponding to the canonical energy distribution at room temperature. The resulting deviation from exponential decay makes it possible to determine the decay parameters from a measurement at a single photon wavelength. Activation energies of E_a = 1.24 ± 0.07 and 1.5 ± 0.4 eV were determined for LysTrpLysH ⁺ and LysTyrLysH⁺, respectively, with pre-exponential factors of A_d = 10^11.1±0.5 and 10 ^12.9±2.6 s^-1.