1,368 research outputs found

    Recombination dynamics in bacterial photosynthetic reaction centers

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    The time dependence of magnetic field effects on light absorption by triplet-state and radical ions in quinone-depleted reaction centers of Rhodopseudomonas sphaeroides strain R-26 has been investigated. Measurements on the time scale of the hyperfine interaction in the radical pair [(BChl)2+. ...BPh-.)] provided kinetic data characterizing the recombination process. The results have been interpreted in terms of a recently proposed model that assumes an intermediate electron acceptor (close site) between the bacteriochlorophyll "special pair" (BChl)2 and the bacteriopheophytin BPh (distant site). Recombination is assumed to proceed through this intermediate acceptor. The experiments led to effective recombination rates for the singlet and triplet channel: k(Seff) = 3.9 . 107 s-1 and k(Teff) = 7.4 . 10(8) s-1. These correspond to recombination rates ks = 1 . 10(1) s-1 and kT = 7.1 . 10(11) s-1 in the close configuration. The upper bound of the effective spin dephasing rate k2eff approximately equal to 1 . 10(9) s-1 is identical with the rate of the electron hopping between the distant site of zero spin exchange interaction and the close site of large interaction. Interpretation of data for the case of direct recombination yields the recombination rates, spin dephasing rate, and exchange interaction in a straightforward way

    Determination of QA-content in bacterial reaction centers: an indispensable requirement for quantifying B-branch charge separation

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    AbstractWe have been able to determine the occupancy of the quinone site at the A-branch (QA) of a reaction center preparation with an accuracy of 2%. This is achieved by accumulating the P+Q−A state after multiple actinic excitation and monitoring the extent of the 30 ms ground state bleaching. This bleaching is corrected for deviations from complete saturation due to competing charge separation to the B-branch. On the other hand, knowledge of the QA content is indispensable for determining the yield of B-branch charge separation from nanosecond transients associated with the recombination of P+H−B, which have to be corrected for the nanosecond signal originating from P+H−A of RCs having lost QA

    NO-dependent CaMKII activation during β-adrenergic stimulation of cardiac muscle

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    Aims During β-adrenergic receptor (β-AR) stimulation, phosphorylation of cardiomyocyte ryanodine receptors by protein kinases may contribute to an increased diastolic Ca2+ spark frequency. Regardless of prompt activation of protein kinase A during β-AR stimulation, this appears to rely more on activation of Ca2+/calmodulin-dependent protein kinase II (CaMKII), by a not yet identified signalling pathway. The goal of the present study was to identify and characterize the mechanisms which lead to CaMKII activation and elevated Ca2+ spark frequencies during β-AR stimulation in single cardiomyocytes in diastolic conditions. Methods and results Confocal imaging revealed that β-AR stimulation increases endogenous NO production in cardiomyocytes, resulting in NO-dependent activation of CaMKII and a subsequent increase in diastolic Ca2+ spark frequency. These changes of spark frequency could be mimicked by exposure to the NO donor GSNO and were sensitive to the CaMKII inhibitors KN-93 and AIP. In vitro, CaMKII became nitrosated and its activity remained increased independent of Ca2+ in the presence of GSNO, as assessed with biochemical assays. Conclusions β-AR stimulation of cardiomyocytes may activate CaMKII by a novel direct pathway involving NO, without requiring Ca2+ transients. This crosstalk between two established signalling pathways may contribute to arrhythmogenic diastolic Ca2+ release and Ca2+ waves during adrenergic stress, particularly in combination with cardiac diseases. In addition, NO-dependent activation of CaMKII is likely to have repercussions in many cellular signalling systems and cell type


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    Reaction centers from Rhodobacter sphaeroides have been modified by treatment with sodium borohydride similar to the original procedure [Ditson et al., Biochim. Biophys. Acta 766, 623 (1984)], and investigated spectroscopically and by gel electrophoresis. (1) Low temperature (1.2 K) absorption, fluorescence, absorption- and fluorescence-detected ODMR, and microwave-induced singlet-triplet absorption difference spectra (MIA) suggest that the treatment produces a spectroscopically homogeneous preparation with one of the ‘additional’ bacteriochlorophylls being removed. The modification does not alter the zero field splitting parameters of the primary donor triplet (TP870). (2) From the circular dichroism and Raman resonance spectra in the1500–1800 cm-1 region, the removed pigment is assigned to BchlM, e.g. the "extra" Bchl on the "inactive" M-branch. (3) A strong coupling among all pigment molecules is deduced from the circular dichroism spectra, because pronounced band-shifts and/or intensity changes occur in the spectral components assigned to all pigments. This is supported by distinct differences among the MIA spectra of untreated and modified reaction centers, as well as by Raman resonance. (4) The modification is accompanied by partial proteolytic cleavage of the M-subunit. The preparation is thus spectroscopically homogeneous, but biochemically heterogenous

    Time-resolved measurements of fluorescence from reaction centres of Rhodopseudomonas viridis and the effect of menaquinone reduction

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    AbstractThe kinetics of the fluorescence emitted by the ‘special pair’ of bacteriochlorophyll b molecules in reaction centres from Rhodopseudomonas viridis was recorded in the near infrared, with a time resolution of 1 ns. In nonreduced reaction centres two decay components were resolved with lifetimes of <0.5 and 2.5 ns. Upon reduction of the menaquinone electron acceptor three decay components were detected with lifetimes of < 0.5, 2.5 and 15ns

    Factors influencing the seeking of medical attention with cancer of the colon

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    Thesis (M.S.)--Boston UniversityThis study was designed to determine whether education, marital status, sex, age and religion are factors influencing the time lapse between onset of symptoms and the seeking of medical attention with cancer of the colon