Tagging of protonated Ala-Tyr and Tyr-Ala by crown ether prevents direct hydrogen loss and proton mobility after photoexcitation: importance for gas-phase absorption spectra, dissociation lifetimes, and channels.

Photodissociation of protonated Tyr, Ala-Tyr, Tyr-Ala (Ala = alanine, Tyr = tyrosine), and their complexes with 18-crown-6-ether (CE) was performed in an electrostatic ion storage ring using a tunable laser system. While the three bare ions all absorb strongly at 222 nm, absorption at higher wavelengths was barely visible from sampling the neutrals formed in delayed dissociation. A band at 270 nm was introduced, however, as a consequence of CE attachment to the bare ions. To understand the difference between bare ions and complexes, electronically excited states are considered: The initially reached pipi* state on phenol couples with the dissociative pisigma* state on ammonium, which leads to direct hydrogen loss. Cold radical cations are formed that at high wavelengths do not have enough energy for further dissociation. Excitation within the 222-nm band on the other hand leads to delayed dissociation of stored radical cations that is monitored in the present setup. The pisigma* state moves out of the spectral region upon CE attachment, and instead statistical dissociation is sampled on the microsecond to millisecond time scale at all wavelengths. Our data demonstrate the strength of using supramolecular complexes for action spectroscopy experiments to prevent erroneous spectra as a result of undesired dissociation (H loss) from electronically excited states. The gas-phase absorption spectra firmly establish the perturbations of the phenol electronic structure by a water solvent: The 270-nm band red shifts by approximately 5 nm, whereas the 222-nm band changes by approximately 3 nm. Both transitions occur in the phenol group. These results may be useful for protein dynamics experiments that rely on electronic excitations. Product ion mass spectra of [Tyr + H]+, [Ala-Tyr + H]+, [Tyr-Ala + H]+, [Ala-Tyr + H]+(CE), and [Tyr-Ala + H]+(CE) significantly depend on the excitation wavelength from 210 to 310 nm and on whether the ionizing proton is mobile or not.