Multi-photon ionization and fragmentation of uracil: neutral excited-state ring opening and hydration effects.

Multi-photon ionization (MPI) of the RNA base uracil has been studied in the wavelength range 220-270 nm, coinciding with excitation to the S2(ππ*) state. A fragment ion at m/z = 84 was produced by 2-photon absorption at wavelengths ≤232 nm and assigned to C3H4N2O(+) following CO abstraction. This ion has not been observed in alternative dissociative ionization processes (notably electron impact) and its threshold is close to recent calculations of the minimum activation energy for a ring opening conical intersection to a σ(n-π)π* closed shell state. Moreover, the predicted ring opening transition leaves a CO group at one end of the isomer, apparently vulnerable to abstraction. An MPI mass spectrum of uracil-water clusters is presented for the first time and compared with an equivalent dry measurement. Hydration enhances certain fragment ion pathways (particularly C3H3NO(+)) but represses C3H4N2O(+) production. This indicates that hydrogen bonding to water stabilizes uracil with respect to neutral excited-state ring opening.