Identification of a photochemically inactive pheophytin molecule in the spinach D 1-D 2-cyt b559 complex

Abstract

A photochemically inactive pheophytin (Pheo) a molecule in the spinach D 1-D 2-cyt b 559 complex, which contains approximately 6 chlorophyll (Chl) a molecules and two β-carotene molecules for every two Pheo a molecules, was identified by fluorescence excitation spectroscopy at 77K, and confirmed by absorption, linear dichroism (LD), and magnetic circular dichroism spectra. When the emission was monitored at 740 nm, photochemically active Pheo a was detected as peaks at 418, 513, 543, and 681 nm, as described in previous reports. However, when the emission was monitored at 665 nm, two bands were observed at 414 and 537 nm; these bands were insensitive to photochemical reduction of the primary acceptor and thus appear to be due to photochemically inactive Pheo a. The Qy band of this Pheo a molecule at 77K was estimated to be at 671 ± 1 nm, based on the magnitude of the band-shift in photochemically active Pheo a. Two β-carotene molecules in the complex were also discriminated by fluorescence excitation and LD spectra. These observations were discussed in the light of current understanding of the molecular organization of pigments and the relationship of pigment arrangement to optical properties in the reaction center of photosystem II.