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Dominic Schnitzeler - One of the best experts on this subject based on the ideXlab platform.

  • Finding a complex Polarized Signal in wide-band radio data
    Monthly Notices of the Royal Astronomical Society, 2017
    Co-Authors: Dominic Schnitzeler
    Abstract:

    We present a new algorithm for fitting and classifying Polarized radio sources, which is based on the QU fitting method introduced by O'Sullivan et al. and on our analysis of pulsars. Then we test this algorithm using Monte Carlo simulations of observations in the 16 cm band of the Australia Telescope Compact Array (1.3-3.1 GHz), to quantify how often the algorithm identifies the correct source model, how certain it is of this identification, and how the parameters of the injected and fitted models compare. In our analysis we consider the Akaike and Bayesian Information Criteria, and model averaging. For the observing setup we simulated, the Bayesian Information Criterion, without model averaging, is the best way for identifying the correct model and for estimating its parameters. Sources can only be identified correctly if their parameters lie inside a 'Goldilocks region': strong depolarization makes it impossible to detect sources that emit over a wide range in RM, whereas sources that emit over a narrow range in RM cannot be told apart from simpler sources or sources that emit at only one RM. We identify when emission at similar RMs is 'resolved', and quantify this in a way similar to the Rayleigh criterion in optics. Also, we identify pitfalls in RM synthesis that are avoided by QU fitting. Finally, we show how channel weights can be tweaked to produce apodized RM spectra, that observing time requirements in RM synthesis and QU fitting are the same, and we analyse when to stop RMClean.

  • Finding a faint Polarized Signal in wide-band radio data
    Monthly Notices of the Royal Astronomical Society, 2016
    Co-Authors: Dominic Schnitzeler, Kejia Lee
    Abstract:

    We develop two algorithms, based on maximum likelihood (ML) inference, for estimating the parameters of Polarized radio sources which emit at a single rotation measure (RM), e.g., pulsars. These algorithms incorporate the flux density spectrum of the source, either a power law or a scaled version of the Stokes I spectrum, and a variation in sensitivity across the observing band. We quantify the detection significance and measurement uncertainties in the fitted parameters, and we derive weighted versions of the RM synthesis algorithm which, under certain conditions, maximize the likelihood. We use Monte Carlo simulations to compare injected and recovered source parameters for a range of Signal-to-noise ratios, investigate the quality of standard methods for estimating measurement uncertainties, and search for statistical biases. These simulations consider one frequency band each for the Australia Telescope Compact Array (ATCA), the Square Kilometre Array (SKA), and the Low Frequency Array (LOFAR). We find that results obtained for one frequency band cannot be easily generalized, and that methods which were developed in the past for correcting bias in individual frequency channels do not apply to wide-band data sets. The standard method for estimating the measurement uncertainty in RM is not accurate for sources with non-zero spectral indices. Furthermore, dividing Stokes Q and U by Stokes I to correct for spectral index effects, in combination with RM synthesis, does not maximize the likelihood.

Hirozo Oh-oka - One of the best experts on this subject based on the ideXlab platform.

  • Light-Induced Electron Spin-Polarized (ESP) EPR Signal of the P800+ Menaquinone– Radical Pair State in Oriented Membranes of Heliobacterium modesticaldum: Role/Location of Menaquinone in the Homodimeric Type I Reaction Center
    The journal of physical chemistry. B, 2018
    Co-Authors: Toru Kondo, Masami Kobayashi, Shigeru Itoh, Masahiro Matsuoka, Chihiro Azai, Hirozo Oh-oka
    Abstract:

    Function/location of menaquinone (MQ) was studied in the photosynthetic reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs. The spin-Polarized electron paramagnetic resonance Signals of light-induced radical pair species, which are made of oxidized electron donor bacteriochlorophyll g (P800+) and reduced menaquinone (MQ–) or iron–sulfur cluster (FX–), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-Polarized Signal of P800+FX– radical pair state varied little with respect to the direction of the external magnetic field. It suggested a dominant contribution of the spin evolution on the precursor primary radical pair P800+A0– state with the larger isotropic magnetic exchange interaction J than the anisotropic dipole interaction D. The pure P800+MQ– Signal was simulated by subtracting the effects of spin evolution during the electron-transfer process. It w...

  • Light-Induced Electron Spin-Polarized (ESP) EPR Signal of the P800+ Menaquinone– Radical Pair State in Oriented Membranes of Heliobacterium modesticaldum: Role/Location of Menaquinone in the Homodimeric Type I Reaction Center
    2018
    Co-Authors: Toru Kondo, Masami Kobayashi, Shigeru Itoh, Masahiro Matsuoka, Chihiro Azai, Hirozo Oh-oka
    Abstract:

    Function/location of menaquinone (MQ) was studied in the photosynthetic reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs. The spin-Polarized electron paramagnetic resonance Signals of light-induced radical pair species, which are made of oxidized electron donor bacteriochlorophyll g (P800+) and reduced menaquinone (MQ–) or iron–sulfur cluster (FX–), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-Polarized Signal of P800+FX– radical pair state varied little with respect to the direction of the external magnetic field. It suggested a dominant contribution of the spin evolution on the precursor primary radical pair P800+A0– state with the larger isotropic magnetic exchange interaction J than the anisotropic dipole interaction D. The pure P800+MQ– Signal was simulated by subtracting the effects of spin evolution during the electron-transfer process. It was concluded that the J value of the P800+MQ– radical pair is negative with an amplitude almost comparable to |D|. It is in contrast to a positive and small J value of the P700+PhyQ– state in photosystem I (PS I). The results indicate similar but somewhat different locations/binding sites of quinones between hRC and PS I

  • An electron spin-Polarized Signal of the P800+A1(Q)- state in the homodimeric reaction center core complex of Heliobacterium modesticaldum.
    Biochemistry, 2008
    Co-Authors: Ryo Miyamoto, Toru Kondo, Shigeru Itoh, Hiroyuki Mino, Hirozo Oh-oka
    Abstract:

    The function of menaquinone as electron acceptor A 1 was identified by EPR in the purified type 1 homodimeric reaction center core complex (RC core) of an anoxygenic photosynthetic bacterium, Heliobacterium modesticaldum. After illumination of the RC core at 210 K in the absence and presence of dithionite, we detected the radical of a special pair of bacteriochlorophyll g molecules (P800 (+)) at g = 2.0033 and a quinone-type radical at g = 2.0062, respectively, at 14 K. Flash excitation of the dark-frozen RC core at 14 K induced two types of transient EPR Signals, i.e., the P800 (+) radical that decayed with a time constant of 3.7 ms and a much faster decay component that showed the electron spin polarization (ESP) pattern of E/A (E, emission; A, absorption). The latter one was assigned to the P800 (+)F X (-) radical pair state. A new ESP Signal that had an apparent A/E/A/E pattern extended to the lower-magnetic-field side was transiently induced by the flash excitation in the RC core that was preilluminated at 210 K in the presence of ascorbate and subsequently cooled to 14 K in the light. The 210 K preillumination of the RC core in the presence of dithionite led to accumulation of the dark stable semiquinone-type Signal at g = 2.0062 and increased the intensity of the light-induced P800 triplet Signal. Flash excitation at 14 K induced the smaller A/E/A/E-type Signal that had the greater contribution of the lower-magnetic-field envelope. This ESP Signal could thus be ascribed to the P800 (+)A 1 (-) radical pair. The kinetics and spectral shape of this ESP Signal suggest that menaquinone serves as secondary electron acceptor A 1 with the molecular orientation of its ring being somewhat different from that of phylloquinone in photosystem I.

Toru Kondo - One of the best experts on this subject based on the ideXlab platform.

  • Light-Induced Electron Spin-Polarized (ESP) EPR Signal of the P800+ Menaquinone– Radical Pair State in Oriented Membranes of Heliobacterium modesticaldum: Role/Location of Menaquinone in the Homodimeric Type I Reaction Center
    The journal of physical chemistry. B, 2018
    Co-Authors: Toru Kondo, Masami Kobayashi, Shigeru Itoh, Masahiro Matsuoka, Chihiro Azai, Hirozo Oh-oka
    Abstract:

    Function/location of menaquinone (MQ) was studied in the photosynthetic reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs. The spin-Polarized electron paramagnetic resonance Signals of light-induced radical pair species, which are made of oxidized electron donor bacteriochlorophyll g (P800+) and reduced menaquinone (MQ–) or iron–sulfur cluster (FX–), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-Polarized Signal of P800+FX– radical pair state varied little with respect to the direction of the external magnetic field. It suggested a dominant contribution of the spin evolution on the precursor primary radical pair P800+A0– state with the larger isotropic magnetic exchange interaction J than the anisotropic dipole interaction D. The pure P800+MQ– Signal was simulated by subtracting the effects of spin evolution during the electron-transfer process. It w...

  • Light-Induced Electron Spin-Polarized (ESP) EPR Signal of the P800+ Menaquinone– Radical Pair State in Oriented Membranes of Heliobacterium modesticaldum: Role/Location of Menaquinone in the Homodimeric Type I Reaction Center
    2018
    Co-Authors: Toru Kondo, Masami Kobayashi, Shigeru Itoh, Masahiro Matsuoka, Chihiro Azai, Hirozo Oh-oka
    Abstract:

    Function/location of menaquinone (MQ) was studied in the photosynthetic reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs. The spin-Polarized electron paramagnetic resonance Signals of light-induced radical pair species, which are made of oxidized electron donor bacteriochlorophyll g (P800+) and reduced menaquinone (MQ–) or iron–sulfur cluster (FX–), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-Polarized Signal of P800+FX– radical pair state varied little with respect to the direction of the external magnetic field. It suggested a dominant contribution of the spin evolution on the precursor primary radical pair P800+A0– state with the larger isotropic magnetic exchange interaction J than the anisotropic dipole interaction D. The pure P800+MQ– Signal was simulated by subtracting the effects of spin evolution during the electron-transfer process. It was concluded that the J value of the P800+MQ– radical pair is negative with an amplitude almost comparable to |D|. It is in contrast to a positive and small J value of the P700+PhyQ– state in photosystem I (PS I). The results indicate similar but somewhat different locations/binding sites of quinones between hRC and PS I

  • An electron spin-Polarized Signal of the P800+A1(Q)- state in the homodimeric reaction center core complex of Heliobacterium modesticaldum.
    Biochemistry, 2008
    Co-Authors: Ryo Miyamoto, Toru Kondo, Shigeru Itoh, Hiroyuki Mino, Hirozo Oh-oka
    Abstract:

    The function of menaquinone as electron acceptor A 1 was identified by EPR in the purified type 1 homodimeric reaction center core complex (RC core) of an anoxygenic photosynthetic bacterium, Heliobacterium modesticaldum. After illumination of the RC core at 210 K in the absence and presence of dithionite, we detected the radical of a special pair of bacteriochlorophyll g molecules (P800 (+)) at g = 2.0033 and a quinone-type radical at g = 2.0062, respectively, at 14 K. Flash excitation of the dark-frozen RC core at 14 K induced two types of transient EPR Signals, i.e., the P800 (+) radical that decayed with a time constant of 3.7 ms and a much faster decay component that showed the electron spin polarization (ESP) pattern of E/A (E, emission; A, absorption). The latter one was assigned to the P800 (+)F X (-) radical pair state. A new ESP Signal that had an apparent A/E/A/E pattern extended to the lower-magnetic-field side was transiently induced by the flash excitation in the RC core that was preilluminated at 210 K in the presence of ascorbate and subsequently cooled to 14 K in the light. The 210 K preillumination of the RC core in the presence of dithionite led to accumulation of the dark stable semiquinone-type Signal at g = 2.0062 and increased the intensity of the light-induced P800 triplet Signal. Flash excitation at 14 K induced the smaller A/E/A/E-type Signal that had the greater contribution of the lower-magnetic-field envelope. This ESP Signal could thus be ascribed to the P800 (+)A 1 (-) radical pair. The kinetics and spectral shape of this ESP Signal suggest that menaquinone serves as secondary electron acceptor A 1 with the molecular orientation of its ring being somewhat different from that of phylloquinone in photosystem I.

  • an electron spin Polarized Signal of the p800 a1 q state in the homodimeric reaction center core complex of heliobacterium modesticaldum
    Biochemistry, 2008
    Co-Authors: Ryo Miyamoto, Shigeru Itoh, Toru Kondo, Hiroyuki Mino, Hirozo Ohoka
    Abstract:

    The function of menaquinone as electron acceptor A 1 was identified by EPR in the purified type 1 homodimeric reaction center core complex (RC core) of an anoxygenic photosynthetic bacterium, Heliobacterium modesticaldum. After illumination of the RC core at 210 K in the absence and presence of dithionite, we detected the radical of a special pair of bacteriochlorophyll g molecules (P800 (+)) at g = 2.0033 and a quinone-type radical at g = 2.0062, respectively, at 14 K. Flash excitation of the dark-frozen RC core at 14 K induced two types of transient EPR Signals, i.e., the P800 (+) radical that decayed with a time constant of 3.7 ms and a much faster decay component that showed the electron spin polarization (ESP) pattern of E/A (E, emission; A, absorption). The latter one was assigned to the P800 (+)F X (-) radical pair state. A new ESP Signal that had an apparent A/E/A/E pattern extended to the lower-magnetic-field side was transiently induced by the flash excitation in the RC core that was preilluminated at 210 K in the presence of ascorbate and subsequently cooled to 14 K in the light. The 210 K preillumination of the RC core in the presence of dithionite led to accumulation of the dark stable semiquinone-type Signal at g = 2.0062 and increased the intensity of the light-induced P800 triplet Signal. Flash excitation at 14 K induced the smaller A/E/A/E-type Signal that had the greater contribution of the lower-magnetic-field envelope. This ESP Signal could thus be ascribed to the P800 (+)A 1 (-) radical pair. The kinetics and spectral shape of this ESP Signal suggest that menaquinone serves as secondary electron acceptor A 1 with the molecular orientation of its ring being somewhat different from that of phylloquinone in photosystem I.

Malik Chami - One of the best experts on this subject based on the ideXlab platform.

  • influence of polarimetric satellite data measured in the visible region on aerosol detection and on the performance of atmospheric correction procedure over open ocean waters
    Optics Express, 2011
    Co-Authors: Tristan Harmel, Malik Chami
    Abstract:

    An original atmospheric correction algorithm, so-called multi-directionality and POLarization-based Atmospheric Correction (POLAC), is described. This algorithm is based on the characteristics of the multidirectional and polarimetric data of the satellite PARASOL (CNES). POLAC algorithm is used to assess the influence of the polarimetric information in the visible bands on the retrieval of the aerosol properties and the water-leaving radiance over open ocean waters. This study points out that the use of the Polarized Signal significantly improves the aerosol type determination. The use of the Polarized information at one visible wavelength only, namely 490 nm, allows providing estimates of the Angstrom exponent of aerosol optical depth with an uncertainty lower than 4%. Based on PARASOL observations, it is shown that the detection of the fine aerosols is improved when exploiting polarization data. The atmospheric component of the satellite Signal is then better modeled, thus improving de facto the water-leaving radiance estimation.

  • invariance of Polarized reflectance measured at the top of atmosphere by parasol satellite instrument in the visible range with marine constituents in open ocean waters
    Optics Express, 2008
    Co-Authors: Tristan Harmel, Malik Chami
    Abstract:

    The influence of oceanic constituents on the Polarized reflectance measured at the top of atmosphere (TOA) over open ocean waters in one visible band is investigated. First, radiative transfer modelling is used to quantify the effects of biomass concentration on the TOA Polarized Signal for a wide range of observation geometries. The results showed that the TOA Polarized reflectance remains insensitive to variations in the chlorophyll a concentration whatever the geometrical conditions in oligotrophic and mesotrophic waters, which represent about 90% of the global ocean. The invariance of the Polarized Signal with water content is explained by the prevailing influence of both atmospheric effects and skylight reflections at the sea surface on the polarization state of the radiation reaching the top of atmosphere level. The simulations also revealed that multidirectional and Polarized TOA reflectances obtained in the visible spectrum are powerful tools for the discrimination between the aerosol optical properties. In the second part of the paper, the theoretical results are rigorously validated using original multiangle and Polarized measurements acquired by PARASOL satellite sensor, which is used for the first time for ocean color purposes. First, a statistical analysis of the geometrical features of PARASOL instrument showed that the property of invariance of the TOA Polarized reflectance is technically verified for more than 85% of viewed targets, and thus, indicating the feasibility of separating between the atmospheric and oceanic parameters from space remotely sensed Polarized data. Second, PARASOL measurements acquired at regional and global scales nicely corroborated the simulations. This study also highlighted that the radiometric performance of the Polarized visible wavelength of PARASOL satellite sensor can be used either for the aerosol detection or for atmospheric correction algorithms over open ocean waters regardless of the biomass concentration.

  • radiative transfer model for the computation of radiance and polarization in an ocean atmosphere system polarization properties of suspended matter for remote sensing
    Applied Optics, 2001
    Co-Authors: Malik Chami, Richard P Santer, Eric Dilligeard
    Abstract:

    A radiative transfer code termed OSOA for the ocean–atmosphere system that is able to predict the total and the Polarized Signals has been developed. The successive-orders-of-scattering method is used. The air–water interface is modeled as a planar mirror. Four components grouped by their optical properties, pure seawater, phytoplankton, nonchlorophyllose matter, and yellow substances, are included in the water column. Models are validated through comparisons with standard models. The numerical accuracy of the method is better than 2%; high computational efficiency is maintained. The model is used to study the influence of polarization on the detection of suspended matter. Polarizing properties of hydrosols are discussed: phytoplankton cells exhibit weak polarization and small inorganic particles, which are strong backscatterers, contribute appreciably to the Polarized Signal. Therefore the use of the Polarized Signal to extract the sediment signature promises good results. Also, Polarized radiance could improve characterization of aerosols when open ocean waters are treated.

Shigeru Itoh - One of the best experts on this subject based on the ideXlab platform.

  • Light-Induced Electron Spin-Polarized (ESP) EPR Signal of the P800+ Menaquinone– Radical Pair State in Oriented Membranes of Heliobacterium modesticaldum: Role/Location of Menaquinone in the Homodimeric Type I Reaction Center
    The journal of physical chemistry. B, 2018
    Co-Authors: Toru Kondo, Masami Kobayashi, Shigeru Itoh, Masahiro Matsuoka, Chihiro Azai, Hirozo Oh-oka
    Abstract:

    Function/location of menaquinone (MQ) was studied in the photosynthetic reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs. The spin-Polarized electron paramagnetic resonance Signals of light-induced radical pair species, which are made of oxidized electron donor bacteriochlorophyll g (P800+) and reduced menaquinone (MQ–) or iron–sulfur cluster (FX–), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-Polarized Signal of P800+FX– radical pair state varied little with respect to the direction of the external magnetic field. It suggested a dominant contribution of the spin evolution on the precursor primary radical pair P800+A0– state with the larger isotropic magnetic exchange interaction J than the anisotropic dipole interaction D. The pure P800+MQ– Signal was simulated by subtracting the effects of spin evolution during the electron-transfer process. It w...

  • Light-Induced Electron Spin-Polarized (ESP) EPR Signal of the P800+ Menaquinone– Radical Pair State in Oriented Membranes of Heliobacterium modesticaldum: Role/Location of Menaquinone in the Homodimeric Type I Reaction Center
    2018
    Co-Authors: Toru Kondo, Masami Kobayashi, Shigeru Itoh, Masahiro Matsuoka, Chihiro Azai, Hirozo Oh-oka
    Abstract:

    Function/location of menaquinone (MQ) was studied in the photosynthetic reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs. The spin-Polarized electron paramagnetic resonance Signals of light-induced radical pair species, which are made of oxidized electron donor bacteriochlorophyll g (P800+) and reduced menaquinone (MQ–) or iron–sulfur cluster (FX–), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-Polarized Signal of P800+FX– radical pair state varied little with respect to the direction of the external magnetic field. It suggested a dominant contribution of the spin evolution on the precursor primary radical pair P800+A0– state with the larger isotropic magnetic exchange interaction J than the anisotropic dipole interaction D. The pure P800+MQ– Signal was simulated by subtracting the effects of spin evolution during the electron-transfer process. It was concluded that the J value of the P800+MQ– radical pair is negative with an amplitude almost comparable to |D|. It is in contrast to a positive and small J value of the P700+PhyQ– state in photosystem I (PS I). The results indicate similar but somewhat different locations/binding sites of quinones between hRC and PS I

  • An electron spin-Polarized Signal of the P800+A1(Q)- state in the homodimeric reaction center core complex of Heliobacterium modesticaldum.
    Biochemistry, 2008
    Co-Authors: Ryo Miyamoto, Toru Kondo, Shigeru Itoh, Hiroyuki Mino, Hirozo Oh-oka
    Abstract:

    The function of menaquinone as electron acceptor A 1 was identified by EPR in the purified type 1 homodimeric reaction center core complex (RC core) of an anoxygenic photosynthetic bacterium, Heliobacterium modesticaldum. After illumination of the RC core at 210 K in the absence and presence of dithionite, we detected the radical of a special pair of bacteriochlorophyll g molecules (P800 (+)) at g = 2.0033 and a quinone-type radical at g = 2.0062, respectively, at 14 K. Flash excitation of the dark-frozen RC core at 14 K induced two types of transient EPR Signals, i.e., the P800 (+) radical that decayed with a time constant of 3.7 ms and a much faster decay component that showed the electron spin polarization (ESP) pattern of E/A (E, emission; A, absorption). The latter one was assigned to the P800 (+)F X (-) radical pair state. A new ESP Signal that had an apparent A/E/A/E pattern extended to the lower-magnetic-field side was transiently induced by the flash excitation in the RC core that was preilluminated at 210 K in the presence of ascorbate and subsequently cooled to 14 K in the light. The 210 K preillumination of the RC core in the presence of dithionite led to accumulation of the dark stable semiquinone-type Signal at g = 2.0062 and increased the intensity of the light-induced P800 triplet Signal. Flash excitation at 14 K induced the smaller A/E/A/E-type Signal that had the greater contribution of the lower-magnetic-field envelope. This ESP Signal could thus be ascribed to the P800 (+)A 1 (-) radical pair. The kinetics and spectral shape of this ESP Signal suggest that menaquinone serves as secondary electron acceptor A 1 with the molecular orientation of its ring being somewhat different from that of phylloquinone in photosystem I.

  • an electron spin Polarized Signal of the p800 a1 q state in the homodimeric reaction center core complex of heliobacterium modesticaldum
    Biochemistry, 2008
    Co-Authors: Ryo Miyamoto, Shigeru Itoh, Toru Kondo, Hiroyuki Mino, Hirozo Ohoka
    Abstract:

    The function of menaquinone as electron acceptor A 1 was identified by EPR in the purified type 1 homodimeric reaction center core complex (RC core) of an anoxygenic photosynthetic bacterium, Heliobacterium modesticaldum. After illumination of the RC core at 210 K in the absence and presence of dithionite, we detected the radical of a special pair of bacteriochlorophyll g molecules (P800 (+)) at g = 2.0033 and a quinone-type radical at g = 2.0062, respectively, at 14 K. Flash excitation of the dark-frozen RC core at 14 K induced two types of transient EPR Signals, i.e., the P800 (+) radical that decayed with a time constant of 3.7 ms and a much faster decay component that showed the electron spin polarization (ESP) pattern of E/A (E, emission; A, absorption). The latter one was assigned to the P800 (+)F X (-) radical pair state. A new ESP Signal that had an apparent A/E/A/E pattern extended to the lower-magnetic-field side was transiently induced by the flash excitation in the RC core that was preilluminated at 210 K in the presence of ascorbate and subsequently cooled to 14 K in the light. The 210 K preillumination of the RC core in the presence of dithionite led to accumulation of the dark stable semiquinone-type Signal at g = 2.0062 and increased the intensity of the light-induced P800 triplet Signal. Flash excitation at 14 K induced the smaller A/E/A/E-type Signal that had the greater contribution of the lower-magnetic-field envelope. This ESP Signal could thus be ascribed to the P800 (+)A 1 (-) radical pair. The kinetics and spectral shape of this ESP Signal suggest that menaquinone serves as secondary electron acceptor A 1 with the molecular orientation of its ring being somewhat different from that of phylloquinone in photosystem I.