Lipid Membrane Regulation of Chlorophyll Triplet Excitation Quenching in Photosynthetic Light-Harvesting Complex LHCII

Carotenoid (Car) in photosynthesis plays an essential role in photoprotection by quenching chlorophyll triplet excitation ( 3 Chl * ) via a Chl-to-Car triplet energy transfer (TET). However, mechanistic studies on the Car triplet photoprotection (CTP) have been complicated by the involvement of the O 2 quenching and the complexity of the thylakoid membrane. To clarify the interplay of the TET with the effects of O 2 and lipid–protein interaction, we prepared nanodiscoidal lipid–protein assemblies of the light-harvesting complexes of photosystem II (LHCII) of Bryopsis corticulans and spinach and examined their triplet excitation dynamics in a broad temporal regime of 1–10 5 ns. For both kinds of LHCII complexes, besides the well-known ultrafast TET at both L1 and L2 sites, an O 2 insensitive, slow TET reaction at the L1 (but not L2) site proceeding with a time constant of 11–25 ns was verified. Lipid membranes can substantially accelerate the slow TET reaction and prolong the 3 Car * lifetime. On the other hand, both kinds of LHCII complexes bear a minor fraction of Car-unquenchable 3 Chl *, i.e., 2.2% and 4.6% (1.5% and 2.2%) for Bry. corticulans (spinach) LHCII complexes in lipid and aqueous phases, respectively, which can be fully quenched by O 2 . In addition, the lipid membrane promotes the O 2 accessibility of LHCII proteins and, compared to L2, the L1 site is (30–130)% more permissive to O 2 access. The heterogeneous 3 Chl * deactivation pathways and CTP potency of Bry. corticulans and spinach are compared.