A MOLECULAR MECHANISM FOR Q(E)-QUENCHING

We discuss energy-dependent fluorescence lowering (q(E)-quenching), and suggest a model to explain the experimental data currently available. The main elements of the model are: (a) the q(E)-quenching reflects a mechanism associated with a component of the light-harvesting antenna rather than the reaction center of photosystem (PS) II - we suggest that it occurs through formation of an efficient quencher in one of the minor chlorophyll protein (CP) complexes; (b) the minor CPs have glutamate residues instead of glutamines at positions shown in light-harvesting complex II (LHCII) to be ligands to chlorophylls near the lumenal interface. We suggest that the quenching reflects a change in ligation of chlorophyll on protonation of these glutamate residues leading to formation of an exciton coupled dimer with a neighboring pigment, in which additional energy levels allow vibrational relaxation of the excited singlet. The model accounts for the dependence on low lumenal pH, the ligand residue changes between LHCII and the minor CPs, the preferential distribution of components of the xanthophyll cycle in the minor CPs, the inhibition of q(E)-quenching by DCCD, and the specific binding of DCCD to the minor CPs.