Development of classical models for the description of QM properties in photoinduced processes

Carotenoids are naturally occurring pigments present in photosynthetic organisms, such as plants, bacteria and algae. Specifically, neoxanthin (NEX), violaxanthin (VIO), lutein (LUT), zeaxanthin (ZEA), rhodopsin glucoside (RGT); spirilloxanthin (SPI) and peridinin (PID) are found in Light-Harvesting (LH) complexes where they play different roles in photosynthesis acting as: (1) antennae enhancing the light absorption; (2) photoprotectors, quenching harmful oxidant molecules created in high-light conditions and (3) structure stabilizers. A common methodology to investigate the mechanisms of action of carotenoids in these complexes is the combination of QM methods and classical molecular dynamics based on molecular mechanics force-fields, to simulate the system at in vivo conditions. Unfortunately, for an accurate prediction of the structure and their temperature-dependent fluctuations, it is not sufficient to describe carotenoids with a general force-field but it is necessary to develop specialised force-field parameters. This work presents new force-fields for the different carotenoids, all based on Density Functional Theory (DFT) calculations. In order to validate the quality of the new models, structural and electronic properties, and excitation energy profiles were computed and compared with other available force-fields. As an application, we performed and analysed MD simulations of CP29 and LHCII complexes describing the carotenoids with the new developed models.