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Highly resolved, simultaneously recorded absorption and CD spectra of active Photosystem II (PSII) samples in the range 440-750 nm is reported and compared. The similarity of Stark data in plants and Synechocystis points to minimal interactions of Pheo(sub D1) with nearby chlorophyll pigments in active Photosystem II (PSII) preparations.

(Carbido)decaruthenium carbonyl clusters were analyzed by electron spectroscopy, magnetic susceptibility, near infrared and electron paramagnetic resonance spectrometric studies. The results revealed that the the metal cluster compound exhibits properties that are characteristic of high-nuclearity carbonyl clusters in terms of having 'molecular' rather than 'mesometallic' character.

The roles of aminosugar and duplex binding in the thiol activation of calicheamicin (CLM) for DNA cleavage are investigated through direct kinetic measurements. The second-order reaction rate of CLM is compared with two of its close structural analogues, which lack CLM's free amine. Results indicate that the aminosugars of both CLM and neocarzinostatin participate in thiol activation and that DNA binding modestly affects reaction rates in CLM and mixed disulfides.

We report and compare highly resolved, simultaneously recorded absorption and CD spectra of active Photosystem II (PSII) samples in the range 440-750 nm. From an appropriately scaled comparison of spinach membrane fragment (BBY) and PSII core spectra, we show that key features of the core spectrum are quantitatively represented in the BBY data. PSII from the cyanobacterium Synechocystis 6803 display spectral features in the Q[SUBy] region of comparable width (50-70 cm[SUP-1] fwhm) to those seen in plant PSII but the energies of the resolved features are distinctly different. A comparison of spectra taken of PSII poised in the S[SUB1]Q[SUBA] and S[SUB2]Q[SUBA] redox states reveals electrochromic shifts largely attributable to the influence of Q[SUBA-] on Pheo[SUBD1]. This allows accurate determinations of the Pheo[SUBD1] Q[SUBy] absorption positions to be at 685.0 nm for spinach cores, 685.8 nm for BBY particles, and 683.0 nm for Synechocystis. These are discussed in terms of earlier reports of the Pheo[SUBD1] energies in PSII. The Q[SUBx] transition of Pheo[SUBD1] undergoes a blue shift upon Q[SUBA] reduction, and we place a lower limit of 80 cm[SUP-1] on this shift in plant material. By comparing the magnitude of the Stark shifts of the Q[SUBx] and Q[SUBy] bands of Pheo[SUBD1], the directions of the transition-induced dipole moment changes, Δμ[SUBx] and Δμ[SUBy], for this functionally important pigment could be determined, assuming normal magnitudes of the Δμ's. Consequently, Δμ[SUBx] and Δμ[SUBy] are determined to be approximately orthogonal to the directions expected for these transitions. Low-fluence illumination experiments at 1.7 K resulted in very efficient formation of Q[SUBA-]. This was accompanied by cyt b[SUB559] oxidation in BBYs and carotenoid oxidation in cores. No chlorophyll oxidation was observed. Our data allow us to estimate the quantum efficiency of PSII at this temperature to be of the order 0.1-1. No Stark shift associated with the S[SUB1]-to-S[SUB2] transition of the Mn cluster is evident in our samples. The similarity of Stark data in plants and Synechocystis points to minimal interactions of Pheo[SUBD1] with nearby chloropyll pigments in active PSII preparations. This appears to be at variance with interpretations of experiments performed with inactive solubilized reaction-center preparations. [ABSTRACT FROM AUTHOR]