QM/MM Studies of the Photochemistry of Biochromophores in Solution and in Protein Environments
Organic Molecular Chemistry - Synthesis and Characterisation
Final Report Abstract
The aim of this project was the simulation of the photoinduced nonadiabatic dynamics of urocanic acid (UA), a UV filter found in human skin, in a water sphere and of Orange Carotenoid Protein (OCP), a photoactive protein found in the light-harvesting complex of cyanobacteria. Both of these studies were aimed at being performed with the QM/MM methodology and the ab initio multiple spawning (AIMS) method. To conduct these studies, suitable electronic-structure methods needed to be identified and optimally set up for the description of the chromophores in these molecules. These tests were performed for isolated UA and for isolated b-carotene, the latter as a frequently studied molecule in experiments and a prototype for carotenoids. Numerous tests to employ the FOMO-CASCI and the SA-CASSCF methods for the simulation of UA were unsuccessful. These methods are either unable to yield the correct np*/pp* state order (FOMO-CASCI) or are not stable enough (SA-CASSCF) during dynamics simulations of this molecule. A new DFT-based electronic-structure method, which was recently developed in the Martínez group and yields the correct state order, enabled an AIMS simulation of UA. These simulations show that photoexcited UA deactivates to the electronic ground state on a picosecond time scale and that the quantum yield for E→Z photoisomerization will be found at ~50%. As a precursor project for the later simulation of OCP, the photoinduced dynamics of the bright state/dark state internal-conversion process in b-carotene was simulated. For this, the simple SA4-CASSCF(4,4) electronic-structure setup, which includes only four of the many p orbitals of the molecule in the active space, was found to be viable, describing the crucial dark and bright low-lying singlet excited states correctly and rendering them close in energy for sampled structures around the ground-state minimum. These simulations have shown that the two states start to interact and exchange population within the first few femtoseconds of the simulation and that their interaction is driven by the dynamical motion along C– C and C=C stretch vibrations (which are responsible for changes in the bond-length-alternation motion). Comparisons of our results with experimental data (absorption and fluorescence spectra, time-resolved fluorescence decay) shows good agreement. The identification of the SA4-CASSCF(4,4) electronic-structure setup for b-carotene and the new DFT-based electronic-structure method for UA for conducting successful nonadiabatic dynamics simulations of these molecules will allow us to simulate OCP and UA in a water sphere next by employing a QM/MM setup. Also, the identification of these electronic-structure setups allows for future simulations of the photoinduced dynamics of a range of polyenes and carotenoids and of a broader variety of biochromophores that have low-lying np* excited states.
