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We consider a model of the photosystem II (PS II) reaction center in which its spectral properties result from weak ('100 cm-') excitonic interactions between the majority of reaction center chlorins. Such a model is consistent with a structure similar to that of the reaction center of purple bacteria but with a reduced coupling of the chlorophyll special pair. We find that this model is consistent with many experimental studies of PS II. The similarity in magnitude of the exciton coupling and energetic disorder in PS II results in the exciton states being structurally highly heterogeneous.This model suggests that P680, the primary electron donor of PS II, should not be considered a dimer but a multimer of several weakly coupled pigments, including the pheophytin electron acceptor. We thus conclude that even if the reaction center of PS II is structurally similar to that of purple bacteria, its spectroscopy and primary photochemistry may be very different.The primary processes of photosynthesis involve the absorption of solar energy by an array of light-harvesting pigments, typically chlorophyll, embedded in pigment-protein complexes within a lipid membrane. The resulting chlorophyll excited state is rapidly transferred to a primary electron donor species within a photosynthetic reaction center, where the energy is trapped by a sequence of electron transfer reactions (1). The close proximity of the chlorophylls within the pigment-protein complexes gives rise to dipole-dipole coupling between the pigments. This coupling is responsible for Forster energy transfer between the chlorophylls and may also result in energetic shifts and delocalization of the excited states (exciton interactions) (1). Exciton interactions are important for photosynthetic function in, for example, defining the precursor state to the initial charge separation reaction (2) and are also important in many nonbiological supramolecular systems (3). Moreover, as exciton interactions can strongly influence the properties of optical transitions monitored in many studies of photosynthetic complexes, their consideration can be essential in the interpretation of experimental results.In this paper we consider the importance of exciton interactions within the photosystem two (PS II) reaction center. The initial charge separation reaction in PS II results in oxidation of the primary electron donor, a chlorophyll species referred to as P680 (due to a characteristic bleaching observed at 680 nm upon oxidation of this species) and reduction of a pheophytin molecule. This electron transfer reaction is of particular interest as the resulting species P680+ is thought to be the most oxidizing species found in living organisms, with a potential of +1.1 eV (compare with 0.4-0.5 eV for the primary electron donors of other photosynthetic reaction centers). This high oxidizing potential is essential to PS II'sThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in ac...

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Nonphotochemical hole-burned spectra obtained as a function of burn wavelength at 4.2 K are reported for the isolated L H C I1 peripheral antenna complex of photosystem 11. The lowest-energy state of the trimer complex is shown to lie at 680 nm, 4 nm below the most intense Chl a band at 676 nm. The linear electronphonon coupling for the 680-nm state is characterized and used to predict that its fluorescence origin should lie at 681 nm, precisely coincident with the observed origin at 4.2 K. The 680-nm band carries the equivalent absorption strength of about one chlorophyll a molecule per Cs trimer complex, which contains about 27 chlorophyll a molecules. The 680-nm absorption band possesses an inhomogeneous width of -120 cm-I, and its zero-phonon line distribution function is largely uncorrelated with those of the higher-energy states. Zerophonon hole widths are used to determine that the fluorescent 680-nm state dephases in 10 ps at 4.2 K. An interpretation of this dephasing is given in terms of the trimer of subunits structure. Based on the satellite hole structure observed upon hole burning into the 680-nm state, two new states at 674 and 678 nm are identified.The possibility that these three states are excitonically correlated is considered. The observed trend in the zero-phonon hole burning efficiency as a function of burn frequency is qualitatively consistent with the states at energies higher than 680 nm having ultrashort lifetimes at 4.2 K.