The heart of oxygenic photosynthesis is photosystem II (PSII), a multisubunit protein complex that uses solar energy to drive the splitting of water and production of molecular oxygen. The effectiveness of the photochemical reaction center of PSII depends on the efficient transfer of excitation energy from the surrounding antenna chlorophylls. A kinetic model for PSII, based on the x-ray crystal structure coordinates of 37 antenna and reaction center pigment molecules, allows us to map the major energy transfer routes from the antenna chlorophylls to the reaction center chromophores. The model shows that energy transfer to the reaction center is slow compared with the rate of primary electron transport and depends on a few bridging chlorophyll molecules. This unexpected energetic isolation of the reaction center in PSII is similar to that found in the bacterial photosystem, conflicts with the established view of the photophysics of PSII, and may be a functional requirement for primary photochemistry in photosynthesis. In addition, the model predicts a value for the intrinsic photochemical rate constant that is 4 times that found in bacterial reaction centers.A s the site of water splitting and oxygen production, photosystem II (PSII) is essential for oxygenic photosynthesis. This multisubunit protein complex consists of at least 17 polypeptides and catalyzes the oxidation of water and the reduction of plastoquinone (1). The PSII holocomplex of higher plants and green algae contains 200-300 chlorophyll (Chl) molecules and various carotenoids that are noncovalently bound to a variety of PSII polypeptides (2). The minimal functionally active PSII complex contains the reaction center (RC) polypeptides (D1, D2, cytochrome b 559 ), the Chla core antenna polypeptides (CP43 and CP47), and the polypeptides of the oxygen-evolving complex. The total number of Chls in this PSII core complex is less than 40 per RC (3). Light energy absorbed by any PSII Chl generates an excited state, which is ultimately transferred to the primary electron donor in photosystem II, the RC photoactive pigment P680. Within the excited-state lifetime, primary charge separation [formation of P680 ϩ and pheophytin Ϫ (Pheo Ϫ )] and charge stabilization (reduction of the primary quinone electron acceptor, Q A , by Pheo Ϫ ) occur with greater than 90% efficiency.The kinetics of excited-state decay in PSII are highly dependent on the redox state of the RC (4). Accordingly, these kinetics contain valuable information about rates and mechanisms of excited-state energy transfer, primary charge separation, and stabilization reactions in the RC complex. Unfortunately, it is very difficult to measure directly these rates by using timeresolved spectroscopic techniques because of the complications of excited-state transfer processes that precede the electron transfer steps. In addition, transient absorption measurements in the Q y band of Chl are complicated by spectral congestion and also by competing absorption, bleaching, and stimulated emission. Although t...

The protein composition, steady state and time-resolved fluorescence emission spectra were studied in solubilized and aggregated LHCII complexes, that were prepared according to two different isolation protocols: (1) by fractionation of cation-depleted thylakoid membranes using the non-ionic detergent Triton X-100 according to the procedure of Burke et al. [(1978) Arch. Biochem. Biophys. 187, 252-263] or (2) by solubilization with N-beta-dodecyl maltoside (beta-DM) of photosystem II (PSII) membrane fragments in the presence of cations [Irrgang et al. (1988) Eur. J. Biochem. 178, 207-217]. Based on the analysis of the decay-associated emission spectra measured at 10 and 80 K five long-wavelength chlorophyll species were identified in aggregated LHCII complexes. These five forms are characterized by emission maxima at 681.5, 683, 687, 695, or 702 nm. All of these forms were found in both types of LHCII preparations but the relative amounts and temperature dependency of these species were markedly different in the aggregated LHCII complexes isolated by the two procedures. It was found that these differences cannot be simply explained by effects due to using a less mild detergent as beta-DM or by an ionic influence of Ca2+. Biochemical analysis of the protein composition showed that beta-DM type LHCII consists of all the chlorophyll (Chl)binding proteins belonging to the antenna system of PSII except the CP29 type II gene product (CP29). In contrast, the Triton X-100-solubilized LHCII is highly depleted in CP26 (CP 29 type I gene product) and is contaminated by a variety of unidentified polypeptides. It is proposed that the aggregates of LHCII prepared using Triton X-100 acquire specific spectral and kinetic features due to interaction between the bulk of LHCII subunits and minor protein(s).