Simulations of Excitonic Couplings in Spinach Light-Harvesting Complex II Using a More Realistic Model in the Presence of Photosystem II Core.

In this study, we employed a method for evaluating dynamic exciton couplings by using atomic transition density moments and molecular dynamics (MD) coordinates. We then applied this computational approach to investigate protein-regulated exciton couplings in spinach light-harvesting complex II (LHCII). Our findings, based on a more realistic all-atom computational model in the presence of the photosystem II (PSII) core and minor antennas, reveal distinct coupling strength patterns for the monomers in LHCII. The probability distributions of excitonic couplings in isolated LHCII always exhibit a single peak, whereas those in the LHCII-PSII complex display a bimodal nature. Such reductions in the excitonic coupling strengths of the main pigments, especially for some chlorophyll-lutein pairs, suggest that the peripheral proteins could modulate energy transfer processes in LHCII-PSII. A closer examination of the key structural parameters reveals that the angles between helices A and B and the distance between CLA612 and LUT620 increase in LHCII-PSII, which are closely associated with the reductions in excitonic couplings.