Modelling Photoinduced Events in Solvated Bio-Cromophores by Hybrid QM/MM Approaches

The aim of the study has been to provide the rationale underlying the photo-induced processes and dynamics that occur in solvated biological systems such as retinal PSB cromophores and nucleotides. For such purpose, QM/MM setups and computational protocols have been developed and validated on the native and 10-methylated PSB retinal chromophores and on the GMP. COBRAMM has been used for the simulations, and scripts allowing QM/MM IRC calculations and conical intersection optimizations have been developed to tackle the QM/MM study of complex systems. It has been disclosed that the 10-methylation in all-trans RPSB retinal triggers a dramatic change in the excited state subpicosecond dynamics because the methyl group in 10-position stabilizes an excited state minimum with a large charge-transfer character and alternated C-C bonds favoring an efficient photoisomerization. Water-solvated GMP using multireference perturbation theory QM/MM techniques has been studied, disclosing the importance of the environment displaying qualitative differences for the ππ*La and ππ*Lb states whose spectra are shifted compared to their gas-phase counterparts. The ππ*La state is considered the main spectroscopic state driving the ultra-fast deactivation processes that characterize GMP during UV-light irradiation. A shallow stationary point towards the end of the ππ* La MEP has been characterized, with two different CIs with the ground state that account for the two fastest decay times experimentally measured. Upon initial Lb absorption, two CIs between the ππ *Lb and La states have also been located. CIs between the nO π* and the ππ *Lb and La states have also been characterized along its relaxation route, with a minimum in the nO π* state expected to vertically emit at ~2.7eV. Both ππ *Lb and nO π* are suggested to contribute to the longest-lived experimental timescale.