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Chloroplasts are central to the provision of energy for green plants. Their photosynthetic membrane consists of two major complexes converting sunlight: photosystem I (PSI) and photosystem II (PSII). The energy flow toward both photosystems is regulated by light-harvesting complex II (LHCII), which after phosphorylation can move from PSII to PSI in the so-called state 1 to state 2 transition and can move back to PSII after dephosphorylation. To investigate the changes of PSI and PSII during state transitions, we studied the structures and frequencies of all major membrane complexes from Arabidopsis thaliana chloroplasts at conditions favoring either state 1 or state 2. We solubilized thylakoid membranes with digitonin and analyzed the complete set of complexes immediately after solubilization by electron microscopy and image analysis. Classification indicated the presence of a PSI-LHCII supercomplex consisting of one PSI-LHCI complex and one LHCII trimer, which was more abundant in state 2 conditions. The presence of LHCII was confirmed by excitation spectra of the PSI emission of membranes in state 1 or state 2. The PSI-LHCII complex could be averaged with a resolution of 16 A, showing that LHCII has a specific binding site at the PSI-A, -H, -L, and -K subunits.

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Density functional calculations of the total energy have been used to determine minimum energy structures for YH 3 . Small, symmetry lowering displacements of the hydrogen atoms lead to a structure with an energy which is lower than that of any other structure considered so far and the opening of a large band gap sufficient to explain the recently observed metal-insulator transition in the YH x system. [ S0031-9007(97) There has recently been a breakthrough in the preparation and study of the physical properties of rare-earth hydrides [1]. By depositing thin films (ϳ500 nm) of rare-earth metals on a transparent substrate and coating them with an optically thin protective layer of palladium through which hydrogen could diffuse, it was possible to observe a metal-insulator transition as a function of the H concentration optically as well as in electrical transport measurements. The transition could be cycled repeatedly without any apparent deterioration of the sample.For 2 , x , 3 Huiberts et al.[1] interpreted their extensive optical and electrical measurements on YH x in terms of a cubic dihydride b phase and of a hexagonal trihydride g phase. In the cubic YH 2 phase the two tetrahedral sites in the fcc Y lattice are (nominally) occupied and the octahedral sites unoccupied. This phase is quite well characterized from measurements on bulk samples and is metallic with a conductivity which is about a factor of 5 higher than that of yttrium metal. There have been detailed experimental studies of the optical properties [2] which could be interpreted in terms of conventional, selfconsistently calculated one-electron band structures [3]. For x . 2 the octahedral sites are believed to become randomly occupied until the trihydride phase is nucleated when x exceeds 2.09 [4]. The properties of the hexagonal YH 3 phase were not well known until now because bulk samples disintegrate when x approaches three. To explain their measurements, Huiberts et al. required the hexagonal YH 3 phase to have a band gap of 1.8 eV and, for substoichiometric trihydrides YH 32d , shallow donor states were assumed to be formed about 0.4 eV below the conduction band edge. This picture was supported by Hall measurements, a negative temperature coefficient of the electrical resistivity ͑dr͞dT͒, and the dependence of the resistivity on d [1,5]. The metal-insulator transition in YH x occurs on going from the cubic b phase to the hexagonal g phase.A number of electronic structure calculations have been performed for YH 3 . In the most recent work [6,7] based on the HoD 3 structure derived from neutron scattering experiments [8][9][10], it was concluded that YH 3 should be metallic which is in disagreement with Huiberts' interpretation of his experiments. To reconcile these contradictory results, strong correlation effects have been invoked [1,7,11]. In this Letter we use Car-Parrinello calculations to determine minimum energy structures for hexagonal YH 3 [12]. We show that although YH 3 is metallic in the high symmetry HoD 3 structure, there exists...