Modeling of fluorescence quenching by lutein in the plant light-harvesting complex LHCII.

Photoprotective non-photochemical quenching (NPQ) in higher plants is the result of the formation of energy quenching traps in the light-harvesting antenna of photosystem II (PSII). It has been proposed that this quenching trap is a lutein molecule closely associated with the chlorophyll terminal emitter of the major light-harvesting complex LHCII. We have used a combination of time-dependent density functional theory (TD-DFT) and the semiempirical MNDO-CAS-CI method to model the chlorophyll-lutein energy transfer dynamics of the highly quenched crystal structure of LHCII. Our calculations reveal that the incoherent "hopping" of energy from Chla612 to the short-lived, dipole forbidden 2(1)A(g)(-) state of lutein620 accounts for the strong fluorescence quenching observed in these crystals. This adds weight to the argument that the same dissipative pathway is responsible for in vivo NPQ.