Modeling the NMR signatures associated with the functional conformational switch in the major light-harvesting antenna of photosystem II in higher plants

Abstract

The major photosystem II antenna complex, LHCII, possesses an intrinsic conformational switch linked to the formation of a photoprotective, excitation-quenching state. Recent solid state NMR experiments revealed that aggregation-induced quenching in ¹³C-enriched LHCII from C. reinhardtii is associated with changes to the chemical shifts of three specific ¹³C atoms in the Chla conjugated macrocycle. We performed DFT-based NMR calculations on the strongly-quenched crystal structure of LHCII (taken from spinach). We demonstrate that specific Chla–xanthophyll interactions in the quenched structure lead to changes in the Chla¹³C chemical shifts that are qualitatively similar to those observed by solid state NMR. We propose that these NMR changes are due to modulations in Chla–xanthophyll associations that occur due to a quenching-associated functional conformation change in the lutein and neoxanthin domains of LHCII. The combination of solid-state NMR and theoretical modeling is therefore a powerful tool for assessing functional conformational switching in the photosystem II antenna.