An Aggregation Accelerable Pathway of Triplet Excitation Transfer Newly Identified in the LHCII Complexes from Spinach and <i>Bryopsis corticulans</i>

Oxygenic photosynthetic organisms employ multiple photoprotection mechanisms. The major light-harvesting complex of photosystem II of <i>Bryopsis corticulans</i> (<i>B</i>-LHCII) and that of spinach (<i>S</i>-LHCII) are structurally analogous but differ in their pigment compositions. We have attempted to compare, by evaluating the rate of chlorophyll (Chl)-to-carotenoid (Car) triplet excitation transfer (TET), the photoprotection of <i>B</i>- and <i>S</i>-LHCII in light-harvesting and energy-quenched states and observed a fast and a slow TET pathway for the LHCIIs irrespective of the functional states. The fast one in a sub-nanosecond time scale is attributed to the TET from Chl <i>a</i>612 (<i>a</i>603) to L1-Car (L2-Car), whereas the slow one in ∼10 ns is assigned to the TET from Chl <i>a</i>613 to L1-Car. Ongoing from the light-harvesting to the quenched state, the slow TET is accelerated from (14.0 ns)^-1 to (4.7 ns)^-1 for <i>S</i>-LHCII and from (25.0 ns)^-1 to (17.0 ns)^-1 for <i>B</i>-LHCII, becoming dominant for photoprotection at the L1 site. Thus, the TET enhancement and energy-quenching reactivity constitute the synergistic photoprotection of the LHCIIs.