WHAT IS THE BIOPHYSICAL MECHANISM OF 1CHL* DE-EXCITATION?
The pH- and xanthophyll-dependent conformational change and the PsbS protein are necessary for qE, but the actual biophysical mechanism of 1Chl* de-excitation is still unknown. The central and long-standing question is whether the involvement of xanthophylls is direct or indirect. The xanthophylls may act indirectly as allosteric regulators of the LHCs that cause a switch from light harvesting to energy dissipation (qE; Horton et al., 2000). In this case the conformational change must somehow facilitate1Chl* de-excitation, which may occur by internal conversion of Chl itself to the ground state, releasing excitation energy as heat. Isolated, detergent-solubilized LHCs can exhibit pH-dependent Chl fluorescence quenching, which is inhibited by violaxanthin and promoted by zeaxanthin (Ruban et al., 1997). On the other hand, xanthophylls may directly de-excite1Chl*. This is theoretically possible according to recent spectroscopic experiments, which showed that isolated xanthophyll cycle pi
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