Changes in the Energy Transfer Pathways within Photosystem II Antenna Induced by Xanthophyll Cycle Activity

Energy transfer pathways between photosystem II (PSII) antenna complexes in intact thylakoid membranes have been studied using low-temperature (77 K) excitation fluorescence spectroscopy. The focus of this study was to see whether de-epoxidation of violaxanthin into zeaxanthin causes any alterations in the energetic couplings between the core antenna complexes CP43 and CP47 and the peripheral light-harvesting antenna (LHCII). It was discovered that the appearance of zeaxanthin caused characteristic alterations in the PSI! excitation fluorescence spectra in the Soret-band region. While in the dark violaxanthin was found to be largely uncoupled from any emitting chlorophylls, its intensive de-epoxidation resulted in the appearance of two additional bands at 509 and 492 nm. The former was attributed to weak coupling of zeaxanthin to emitters in the CP43 and LHCII complexes and the latter to efficient coupling of violaxanthin of the CP29 complex to emitters in the CP43, CP47, and LHCII complexes. The role of CP29-bound violaxanthin is discussed in light of both its efficient energetic coupling and strong physical binding to this complex. The finding that zeaxanthin is energetically coupled to chlorophyll a emitters of the PSI! antenna is discussed with respect to its suggested role as a quencher involved in photoprotective energy dissipation, or non-photochemical quenching (NPQ), in the photosynthetic membrane.