The role of far-red fluorescing chlorophylls in the quenching of LHCII

In photosynthetic organisms, the major light-harvesting antenna protein LHCII performs the dual function of capturing excitation energy and protecting the photosystem from over-excitation via non-photochemical quenching (NPQ). We investigated the role of far-red-fluorescing chlorophylls (emission > 700 nm) in quenching LHCII by comparing isolated trimeric LHCII (denoted F680) with aggregated LHCII prepared in a glycerol-rich medium (denoted F730). Time-resolved fluorescence and ultrafast transient-absorption spectroscopy, including an intensity-cycling scheme to separate annihilation-free dynamics, were used. We find that the red-emissive chlorophyll states are populated in < 100 ps and display properties consistent with states having a low transition-dipole moment. Despite the significant shortening of the chlorophyll-a excited-state lifetimes in the F730 sample (hundreds of picoseconds vs. several nanoseconds in F680), no long-lived excited-state species attributable to the far-red emitters were detected in the transient-absorption data. These findings suggest that the far-red-fluorescing chlorophylls are not the direct quenchers but rather markers of the quenched LHCII configuration and highlight the necessity to better define aggregate size and internal organization for elucidating the molecular mechanism of quenching.