Photostability and solvation: photodynamics of microsolvated zwitterionic glycine

Photoinduced processes of the zwitterionic glycine solvated with one and two water molecules were investigated using both static (CASSCF, MRCI and EOM-CCSD methods) and excited state non-adiabatic molecular dynamics (full multiple spawning method) approaches. Reaction pathways were identified and the respective yields and timescales determined. Excitation to the two lowest states, corresponding to the onset of the glycine absorption spectrum, was considered and the conformational dependence of the photochemical behavior was explored. The main processes on the femtosecond timescale are C–N dissociation producing the ammonia molecule, dissociation of hydrogen atom from the amino group and regeneration of the non-ionized glycine molecule via hydrogen transfer. It is found that the photochemical reaction yields strongly depend on the cluster conformation. A significant fraction of the electronic population is trapped in the nπ* minimum and the system deactivates only on longer timescales. Photostability of aminoacids can be rationalized in terms of the mechanisms described above. However, to actually reproduce glycine photostability, it is necessary to proceed beyond the cluster models. It is shown that the QM/MM approach within a simple electrostatic embedding scheme represents a reliable approach reproducing the main features of glycine photochemistry.