The photosynthetic apparatus of plants and algae has evolved a process of feedback control of energy input into the photosynthetic reaction centers, revealed in observations of the nonphotochemical quenching of chlorophyll fluorescence (NPQ) 2 (1-3). Exposure to high light intensity leads to closure of a large fraction of photosystem II (PSII) reaction centers and the buildup of the proton gradient across the photosynthetic membrane (⌬pH). The latter prompts the transition of the PSII light harvesting antenna (LHCII) into the photoprotective state (4 -6), where a large proportion of the absorbed energy is rapidly converted into heat, preventing photoinhibitory damage of reaction centers. The molecular mechanism of NPQ is currently under scrupulous multidisciplinary investigation (7-14).Many of these studies have centered on the search for pigments associated with light harvesting complexes that may act as energy traps/quenchers. In recent years certain evidence has accumulated that xanthophylls may play a central role in photoprotective energy dissipation. A carotenoid radical cation, suggested to be zeaxanthin, was found to correlate with the level of NPQ and was proposed to act as a direct quencher of chlorophyll excitation (7, 10). Other studies indicated the involvement of energy transfer to the S 1 state of LHCII-bound lutein 1 (9, 15). Evidence of these putative quenchers occurring in vivo was provided (7, 9), yet little is known about how ⌬pH activates them. ⌬pH is believed to protonate luminal residues on the PSII subunit PsbS and LHC complexes causing certain conformational changes to occur, which are necessary for NPQ (5, 6). These conformational changes are not well understood but can be traced by changes in LHCII-bound pigments detected by absorption and resonance Raman spectroscopy (9,(15)(16)(17)(18)(19)(20).Both the proposed zeaxanthin-and lutein-quenching mechanisms assume very close (van der Waals) contact between the quenching xanthophyll and chlorophyll. Such an interaction would, most likely, affect the energy levels of the pigments involved, which may be required to convert the xanthophyll into an energy quencher. Indeed, a subpopulation of zeaxanthin molecules in PSII was reported to undergo an absorption redshift in the NPQ state (18 -20). Recently, lutein absorption was also found to be decreased in the dissipative state (20), interestingly this loss of molar extinction was enhanced by the presence of zeaxanthin.Whereas many studies have probed the role of xanthophylls, relatively little is known about the state of chlorophylls upon formation of NPQ. Some research suggests that chlorophyll associates may be formed in the NPQ state, which potentially could act as quenchers (21-23). It has been proposed that such associates have red-shifted absorption and fluorescence spectra. Low temperature (77K) fluorescence spectra of quenched LHCII aggregates possess a 20-nm red-shifted band, F700 (22), which was also detected in leaves and thylakoids frozen in the NPQ state (21, 24). The F700 emitting...