The nature of carotenoid S* state and its role in the nonphotochemical quenching of plants

Light-harvesting complexes (LHCs) of plants serve as antennas to collect light and transfer the absorbed energy to reaction centers, but also regulate energy transport by dissipating the excitation energy of chlorophylls. This process, known as nonphotochemical quenching (NPQ), seems to be activated by conformational changes within the LHCs, but the identity of the quenching species remains elusive. Recent spectroscopic measurements suggest the so-called S* dark state of the bound carotenoids as the quencher of chlorophyll excitation. Here, we employ excited state nonadiabatic dynamics simulations to investigate lutein embedded in different conformations of a minor LHC of plants, CP29. We reveal that different conformations of the complex differentially stabilize the minor s-trans conformer of lutein with respect to the dominant s-cis one. Our findings also demonstrate that it is the s-trans conformer that presents the spectroscopic signatures of the S* state and rationalize how it is able to accept energy from the closest chlorophyll, providing thus a relationship between LHC conformation and quenching.