State- and time-resolved observation of ultrafast intermolecular proton transfer in hydrated biomolecules

Proton transfer underpins number of chemical and biochemical processes, yet its sub-100 fs dynamics have rarely been captured in real time. Here, we report direct and time-resolved observation of ionizing radiation-induced proton transfer in a heteroaromatic hydrate: the pyrrole-water complex. Both the electron-impact and strong-field laser experiments create a locally and doubly charged pyrrole unit (C₄H₅N²⁺), which immediately (within 60 fs) donates a proton to the adjacent H₂O, generating deprotonated C₄H₄N⁺ and hydronium H₃O⁺ cations that subsequently undergo Coulomb explosion. The electron-impact experiments directly revealed initial states and provided dynamical insights through fragment ions and electron coincidence momentum imaging. The strong-field femtosecond laser experiments tracked the ultrafast dynamics of proton transfer; complementary ab initio calculations unraveled the dynamical details. The 50-60 fs proton transfer qualifies as one of the fastest acid-base reactions observed to date. This study offers a novel perspective on radiation-induced proton transfer in hydrated biomolecules.