On a chlorophyll-caroteinoid coupling in LHCII

Higher plants have evolved elaborate mechanisms to fine-tune their photosynthetic activity for the optimal yield under low light and at the same time they cope with its fluctuations. Proposals in the literature have indicated that a specific chlorophyll-a (Chl-a)/carotenoid (Car) pair within the major light harvesting complex (LHCII) of photosystem II can play the role of a quencher in terms of energy transfer between the Q(y) state of Chl-a to the short-lived dark S-1 state of the carotenoid. For the respective coupling calculation, we employ the TrESP approach (transition charge from electrostatic potential) which needs atomic transition charges as input. In this study, we determine these charges using different quantum chemical approaches as well as compare to existing data. Furthermore, the excitonic couplings for the Chl-a/Car pair are determined along molecular dynamics trajectories and the results are discussed in terms of varying pH and ionic strengths.