In vivo, in vitro, and calculated vibrational spectra of plastoquinone andthe plastosemiquinone anion radical

Plastoquinone (PQ-9) is active as an electron/proton transfer component in photosynthetic membranes. For example, in the photosynthetic complex, photo system II (PSII), PQ-9 acts as Q(A), a one-electron acceptor, and as Q(B), a two electron, two proton accepting species. Light-minus-dark difference F ourier transform infrared (FT-IR) spectroscopy is a technique with which me chanistic information can be obtained concerning PSII. Here, we present com bined experimental and computational studies designed to identify the vibra tional contributions of the electron acceptor, Q(A), in its oxidized and on e-electron reduced states to the difference FT-TR spectrum. Infrared spectr a of decyl-PQ and PQ-9 were obtained; the difference infrared spectra assoc iated with the formation of the corresponding anion radicals were also gene rated in ethanol solutions. Vibrational mode assignments were made based on hybrid Hartree-Fock/density functional (HF/DF) B3LYP calculations with a 6 -31G(d) basis set. Calculations were performed for hydrogen bonded models o f PQ-1 and its radical anion. In addition, a methionine-tolerant strain of the cyanobacterium, Synechocystis sp. PCC 6803, was used to deuterate PQ-9 in PSII. The macrocycle and phytol tail of chlorophyll were not labeled by this procedure. Mass spectral data may be consistent with pal-tial 13(3) me thoxy labeling of chlorophyll, Lack of phytol labeling implies that caroten oids were unlabeled. Difference FT-IR spectra were then obtained by illumin ation at 80 K, resulting in the one-electron reduction of Q(A). When spectr a were obtained of PSII preparations, in which 39% of PQ was H-2(3) labeled and 48% was H-2(6) labeled, isotope-induced shifts were observed. Comparis on of these data to vibrational spectra obtained in vitro and to mode frequ encies and intensities from B3LYP/6-31G(d) calculations provides the basis for vibrational mode assignments.