X-ray absorption spectroscopy on layered photosystem II membrane particles suggests manganese-centered oxidation of the oxygen-evolving complex for the S0-S1, S1-S2, and S2-S3 transitions of the water oxidation cycle

By application of microsecond light flashes the oxygen-evolving complex (OEC) was driven through its functional cycle, the S-state cycle. The S-state population distribution obtained by the application of n flashes (n = 0. 6) was determined by analysis of EPR spectra; Mn K-edge X-ray absorption spectra were collected. Taking into consideration the likely statistical error in the data and the variability stemming from the use of three different approaches for the determination of edge positions, we obtained an upshift of the edge position by 0.8-1.5, 0.5-0.9, and 0.6-1.3 eV for the S0-S1, S1-S2, and S2-S3 transitions, respectively, and a downshift by 2.3-3.1 eV for the S3-S0 transition. These results are highly suggestive of Mn oxidation state changes for all four S-state transitions. In the S0-state spectrum, a clearly resolved shoulder in the X-ray spectrum around 6555 eV points toward the presence of Mn(II). We propose that photosynthetic oxygen evolution involves cycling of the photosystem II manganese complex through four distinct oxidation states of this tetranuclear complex: Mn(II)-Mn(III)-Mn(IV)2 in the S0-state, Mn(III)2-Mn(IV)2 in the S1-state, Mn(III)1-Mn(IV)3 in the S2-state, and Mn(IV)4 in the S3-state.     

Large conductance voltage- and calcium-dependent K+ channel, a distinct member of voltage-dependent ion channels with seven N-terminal transmembrane segments (S0-S6), an extracellular N terminus, and an intracellular (S9-S10) C terminus

Large conductance voltage- and Ca2+-dependent K+ (MaxiK) channels show sequence similarities to voltage-gated ion channels. They have a homologous S1-S6 region, but are unique at the N and C termini. At the C terminus, MaxiK channels have four additional hydrophobic regions (S7-S10) of unknown topology. At the N terminus, we have recently proposed a new model where MaxiK channels have an additional transmembrane region (S0) that confers beta subunit regulation. Using transient expression of epitope tagged MaxiK channels, in vitro translation, functional, and "in vivo" reconstitution assays, we now show that MaxiK channels have seven transmembrane segments (S0-S6) at the N terminus and a S1-S6 region that folds in a similar way as in voltage-gated ion channels. Further, our results indicate that hydrophobic segments S9-S10 in the C terminus are cytoplasmic and unequivocally demonstrate that S0 forms an additional transmembrane segment leading to an exoplasmic N terminus.     

The Torsion-Inversion-Bending Energy Levels in the S1(n, pi*) Electronic State of Acetaldehyde

The membrane permeabilisation properties of six linear natural 18-residue peptaibols, termed trichorzins PA, have been assessed on liposomes and on mollicutes (trivial name, mycoplasmas), a class of parasitic bacteria characterized by a small genome, the lack of a cell wall, a minute cell size, and the incorporation in their plasma membrane of exogenously supplied cholesterol. The trichorzins PA used in this study (PA II, PA IV-VI, PA VIII, and PA IX) differ between them by amino acid or amino alcohol substitutions at positions 4, 7, and 18, and form slightly amphipathic alpha-helices. They proved bactericidal for mollicutes belonging to the genera Acholeplasma, Mycoplasma, and Spiroplasma, with minimal inhibitory concentrations (3.12相似文献