Photodissociation of protonated tryptophan and alteration of dissociation pathways by complexation with crown ether

The behavior of protonated tryptophan (TrpH+) and its complex with 18-crown-6-ether (CE) after photoexcitation has been explored based on measurements of dissociation lifetimes, fragmentation channels, and absorption spectra using an electrostatic ion storage ring. A recent implementation of pulsed power supplies for the ring elements with microsecond response times allows us to identify the daughter ion fragment masses and to disentangle fragmentation that occurs from excited states immediately after photoexcitation from that occurring on a longer time scale of several microseconds to milliseconds. We find that attachment of crown ether significantly alters the dissociation channels since it renders the πσ*(NH3) state inaccessible and hence prevents the N-H bond breakage which is an important fragmentation channel of TrpH+. As a result, on a long time scale (>10 μs), photoexcited TrpH+(CE) decays exponentially whereas TrpH+ displays a power-law decay. The only ions remaining in the latter case are Trp+· radical cations with a broad internal energy distribution caused by the departing hydrogen. Large changes in the fragment branching ratios as functions of excitation wavelength between 210 and 290 nm were found for both TrpH+ and TrpH+(CE).