Characterization of fluorescent chlorophyll charge-transfer states as intermediates in the excited state quenching of light-harvesting complex II

Light-harvesting complex II (LHCII) is the major antenna complex in higher plants and green algae. It has been suggested that a major part of the excited state energy dissipation in the so-called non-photochemical quenching (NPQ) is located in this antenna complex. We have performed an ultrafast kinetics study of the low-energy fluorescent states related to quenching in LHCII in both aggregated and the crystalline form. In both sample types the chlorophyll (Chl) excited states of LHCII are strongly quenched in a similar fashion. Quenching is accompanied by the appearance of new far-red (FR) fluorescence bands from energetically low-lying Chl excited states. The kinetics of quenching, its temperature dependence down to 4 K, and the properties of the FR-emitting states are very similar both in LHCII aggregates and in the crystal. No such FR-emitting states are found in unquenched trimeric LHCII. We conclude that these states represent weakly emitting Chl-Chl charge-transfer (CT) states, whose formation is part of the quenching process. Quantum chemical calculations of the lowest energy exciton and CT states, explicitly including the coupling to the specific protein environment, provide detailed insight into the chemical nature of the CT states and the mechanism of CT quenching. The experimental data combined with the results of the calculations strongly suggest that the quenching mechanism consists of a sequence of two proton-coupled electron transfer steps involving the three quenching center Chls 610/611/612. The FR-emitting CT states are reaction intermediates in this sequence. The polarity-controlled internal reprotonation of the E175/K179 aa pair is suggested as the switch controlling quenching. A unified model is proposed that is able to explain all known conditions of quenching or non-quenching of LHCII, depending on the environment without invoking any major conformational changes of the protein.