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

Fine Tuning of Retinal Photoinduced Decay in Solution

Single methylation at position C₁₀ of the all-trans retinal protonated Schiff base switches its excited-state decay in methanol from a slower picosecond into an ultrafast, protein-like subpicosecond process. QM/MM modeling in conjunction with on-the-fly excited-state dynamics provides fundamental understanding of the fine-tuning mechanics that “catalyzes” the photoinduced decay of solvated retinals. Methylation alters the interplay between the ionic S₁ and covalent S₂ states, reducing the excited-state lifetime by favoring the formation of a S₁ transient fluorescent state with fully inverted bond lengths that accounts for the recorded transient spectroscopy and from which a space-saving conical intersection seam is quickly (<1 ps) reached. Minimal and apparently innocent chemical modifications thus affect the characteristic intramolecular charge-transfer of the S₁ state as well as the interaction with the covalent S₂ excited state, eventually providing the high tunability of retinal photophysics and photochemistry and delivering a new concept for the rational design of retinal-based photoactive molecular devices

Deciphering the photochemical mechanisms describing the UV-induced processes occurring in solvated guanine monophosphate

International audienc

Excited state evolution of DNA stacked adenines resolved at the CASPT2//CASSCF/Amber level: from the bright to the excimer state and back

International audienc

Probing deactivation pathways of DNA nucleobases by two-dimensional electronic spectroscopy: first principles simulations

International audienc

Probing deactivation pathways of DNA nucleobases by two-dimensional electronic spectroscopy: first principles simulations.

The SOS//QM/MM [Rivalta et al., Int. J. Quant. Chem., 2014, 114, 85] method consists of an arsenal of computational tools allowing accurate simulation of one-dimensional (1D) and bi-dimensional (2D) electronic spectra of monomeric and dimeric systems with unprecedented details and accuracy. Prominent features like doubly excited local and excimer states, accessible in multi-photon processes, as well as charge-transfer states arise naturally through the fully quantum-mechanical description of the aggregates. In this contribution the SOS//QM/MM approach is extended to simulate time-resolved 2D spectra that can be used to characterize ultrafast excited state relaxation dynamics with atomistic details. We demonstrate how critical structures on the excited state potential energy surface, obtained through state-of-the-art quantum chemical computations, can be used as snapshots of the excited state relaxation dynamics to generate spectral fingerprints for different de-excitation channels. The approach is based on high-level multi-configurational wavefunction methods combined with non-linear response theory and incorporates the effects of the solvent/environment through hybrid quantum mechanics/molecular mechanics (QM/MM) techniques. Specifically, the protocol makes use of the second-order Perturbation Theory (CASPT2) on top of Complete Active Space Self Consistent Field (CASSCF) strategy to compute the high-lying excited states that can be accessed in different 2D experimental setups. As an example, the photophysics of the stacked adenine-adenine dimer in a double-stranded DNA is modeled through 2D near-ultraviolet (NUV) spectroscopy

Multiple Decay Mechanisms and 2D-UV Spectroscopic Fingerprints of Singlet Excited Solvated Adenine-Uracil Monophosphate

International audienc