Recombination reactions of the primary radical pair in photosystem II (PS II) have been studied in the nanosecond to millisecond time scales by flash absorption spectroscopy. Samples in which the first quinone acceptor (QA) was in the semiquinone form (QA-) or in the doubly reduced state (presumably QAH2) were used. The redox state of QA and the long-lived triplet state of the primary electron donor chlorophyll (3P680) were monitored by EPR. The following results were obtained at cryogenic temperatures (around 20 K). (1) the primary radical pair, P680+Pheo-, is formed with a high yield irrespective of the redox state of QA. (2) The decay of the primary pair is faster with QA- than with QAH2 and could be described biexponentially with t1/2 approximately 20 ns (approximately 65%)/150 ns (approximately 35%) and t1/2 approximately 60 ns (approximately 35%)/250 ns (approximately 65%), respectively. The different kinetics may be due to electrostatic and/or magnetic effects of QA- on charge recombination or due to conformational changes caused by the double reduction treatment. (3) The yield of the triplet state 3P680 was high both with QA- and QAH2. (4) The triplet decay was much faster with QA- [t1/2 approximately 2 microseconds (approximately 50%)/20 microseconds (approximately 50%)] than with QAH2 [t1/2 approximately 1 ms (approximately 65%)/3 ms (approximately 35%)]. The short lifetime of the triplet with QA- explains why it was not detected earlier. The mechanism of triplet quenching in the presence of QA- is not understood; however it may represent a protective process in PS II. (5) Almost identical data were obtained for PS II-enriched membranes from spinach and PS II core preparations from Synechococcus. Room temperature optical studies were performed on the Synechococcus preparation. In samples containing sodium dithionite to form QA- in the dark, EPR controls showed that multiple excitation flashes given at room temperature led to a decrease of the QA-Fe2+ signal, indicating double reduction of QA. During the first few flashes, QA- was still present in the large majority of the centers. In this case, the yield of the primary pair at room temperature was around 50%, and its decay could be described monoexponentially with t1/2 approximately 8 ns (a slightly better fit was obtained with two exponentials: t1/2 approximately 4 ns (approximately 80%)/25 ns (approximately 20%).(ABSTRACT TRUNCATED AT 400 WORDS)

Absorbance difference spectra of the transient states in photosystem II (PS II) have been examined in the Qv absorption region between 660 and 700 nm. The P680+Pheo-/P680Pheo, 3P680/P680, and P680+QA-/P680QA spectra were measured in O2-evolving PS II core complexes from Synechococcus and PS II-enriched membrane fragments from spinach. The low-temperature absorbance difference spectra vary only slightly between both PS II preparations. The 3P680/P680 spectrum is characterized by a bleaching at 685 nm at 25 K and indicates weak exciton coupling with neighboring pigment(s). We conclude that P680 absorbs at 685 nm in more intact PS II preparations at cryogenic temperature. The difference spectra of the radical pairs are strongly temperature dependent. At low temperature the P680+QA-/P680QA- spectrum exhibits the strongest bleaching at 675 nm whereas the P680+Phe-/P680Pheo spectra show two distinct bleaching bands at 674 and 684 nm. It is suggested that an electrochronic red shift resulting in a bleaching at 675 nm and an absorbance increase at about 682 nm dominates the spectral features of the charge-separated states. On the basis of the present results and those in the literature, we conclude that the interactions between the pigments and especially the organization of the primary donor must be quite different in PS II compared to bacterial reaction centers, although the basic structural arrangement of the pigments might be similar. Spectral data obtained with samples in the presence of singly and doubly reduced QA indicate that the primary photochemistry in PS II is not strongly influenced by the redox state of QA at low temperature and confirm the results of the accompanying paper [Van Mieghem, F. J. E., Brettel, K., Hillmann, B., Kamlowski, A., Rutherford, A. W., & Schlodder, E. (1995) Biochemistry 34, 4798-4813]. The spectra of the primary radical pair and the reaction center triplet obtained with more intact PS II preparations differ widely from those of D1/D2/cyt b-559 complexes. In the latter sample, where 3P680 formation results in a bleaching at 680 nm, the P680+Pheo-/P680Pheo spectrum shows only one broad bleaching band at about 680 nm, and the main bleaching due to photoaccumulation of Pheo- at 77 K appears at 682 nm instead of 685 nm in PS II core complexes. This indicates that the removal of the core antenna which is accompanied by the loss of QA causes also structural changes of the reaction center.

The charge separated state P 700•+ A 1•-(P 700 ) primary electron donor, A 1 ) phylloquinone electron acceptor) in photosystem I of oxygenic photosynthesis has been investigated by EPR spectroscopy in frozen solution and single crystals. The transient EPR spectra of P 700•+ A 1 •recorded in frozen solution of fully deuterated samples at X-, Q-, and W-band frequencies are shown to contain sufficient information to yield the orientation of the g-tensors of both A 1•and P 700•+ with respect to the axis connecting both spins. So far incomplete information on the orientation of A 1•relative to the membrane plane has been complemented by data from time-resolved EPR on single crystals measured at Q-band. The phylloquinone headgroup orientation evaluated from the EPR data in the charge-separated state P 700•+ A 1 •is compared with the presently available X-ray structural model. The g-tensor of P 700 •+ has also been determined from cw-EPR experiments at W-band on single crystals, independent of the orientational data of the P 700 •+ g-tensor from the time-resolved EPR experiments.The direction of the principal axes of g(P 700•+ ) differ from the molecular axes system of the chlorophylls comprising P 700 as found previously in the case of P 865•+ in bacterial reaction centers. The implications of the complete structural model from the A 1 •and P 700•+ molecular magnetic interaction tensors in the active charge separated state P 700•+ A 1 •in PS I are discussed.

The light-induced, charge-separated state in single crystals of Photosystem I (PSI) from the cyanobacterium Synechococcus elongatus is investigated with transient, direct-detection EPR spectroscopy. The orientation of the phylloquinone head group of A1 within the PSI reaction center is determined from the orientation dependence of the spin-polarized X-band EPR spectrum of the radical pair made up of the primary donor, P700, and the acceptor, A1. From the angular dependence of the overall spin-polarization pattern an upper limit ∠(c, z d) ≤ 30° is evaluated for the angle between the crystallographic c-axis, collinear with the membrane normal, and the dipolar axis, z d, connecting the electron spin density centers of and . A partially resolved hyperfine coupling (hfc) is assigned to the hfc tensor of the 2-methyl group of A1. Its A ∥ principal axis encloses an angle of β = 35°−55° with c. Simulations of the rotation patterns support a lower limit for the angle ∠(c, z d) ≥ 25° with a larger error than for the upper limit. z d is confirmed to be parallel to both the g xx principal axis of g( ) and to the CO carbonyl bonds within ±5°. With respect to rotation around the gxx axis, the angle between the (c, z d) plane and the quinone plane of the head group can only be specified within an upper limit of 60°. Together with independent knowledge about the location of A1 within the PSI reaction center, a nearly complete structural model for the head group of the functional A1 cofactor is achieved.