Biochemistry 1995,34, zyxwvut 12761 zyxwv - 12767 12761 Secondary Electron Transfer Processes in Membranes of zyxw Heliobacillus mobilist Su Lin, Hung-Cheng Chiou, and Robert E. Blankenship" Department of Chemistry and Biochemistry, Center zyxwvut for the Study of Early Events in Photosynthesis, Arizona State University, Tempe, Arizona 85287-1604 Received January 31, 1995; Revised Manuscript Received July 24, 1995@ ABSTRACT: Picosecond transient absorption difference spectroscopic experiments were performed on membranes of the antennaheaction center complex of Heliobacillus mobilis to study the electron transfer processes. Particular emphasis was placed on the blue spectral region, where the difference spectra of iron- sulfur centers and quinones are significantly different. Spectra were measured at room temperature in the wavelength region from 400 to 470 nm and from 630 to 730 nm. Laser excitation was into the 788 nm zyxwvutsrqp Qy band of the bacteriochlorophyll g of the reaction center complex. Global analysis in both wavelength regions reveals three kinetic components. A 25 ps phase originates from the decay of the excited state of antenna to form the primary charge-separated state P798+Ao-; a 600 ps component is assigned to the electron transfer from the primary electron acceptor Ao to a secondary electron acceptor; a nondecaying component on the time scale measured represents the formation of the secondary charge-separated state. When the secondary electron acceptors were reduced by adding dithionite at pH 11, the 600 ps component disappeared. Only a 25 ps component and a constant were observed in the 630-730 nm region. The 25 ps component is assigned to the excitation decay in the antenna and the formation of P798+Ao-, just as in the nonreduced sample. In the reduced sample, the P798'Ao- state does not decay on the time scale measured. In the 400-470 nm region, the same kinetic behavior was observed. The absorption difference spectra of the primary and the secondary electron acceptor were constructed from different charge-separated states. The Ao- - A0 spectrum resembles the spectrum of the same state from photosystem I, which also contains a Chl a molecule as the primary electron acceptor. The secondary electron acceptor X has an zy X- - X difference spectrum similar to F, in photosystem I from higher plants. The spectra do not give any evidence in favor of a quinone acceptor in heliobacteria, although they do not rule out the possibility that such an acceptor is present. Heliobacteria are a group of newly discovered non-oxygen evolving photosynthetic bacteria (Gest & Favinger, 1983; Beer-Romero & Gest, 1987; Madigan, 1992; Madigan & Ormerod, 1995). These organisms contain a very simple photosynthetic apparatus in which antenna and reaction center are organized as a single pigment-protein complex (Trost & Blankenship, 1989; Van de Meent et al., 1990). The major pigment in heliobacteria is bacteriochlorophyll (BChl)' g (Brockmann & Lipinski, 1983; Michalski et al., 1987). About 35-40 BChl g pigments were found per complex associated with a primary electron donor. The primary electron donor is believed to be formed by a dimer of BChl g and is named P798 after its Q, transition peak position at room temperature (Fuller et al., 1985; Nuijs et al., 1985). The primary electron acceptor A0 is identified as an 8l-hydroxy chlorophyll (Chl) a molecule (Van de Meent et al., 1991). Menaquinones and iron-sulfur centers are also found in the reaction center complex of the This work was supported by Grant MCB 9418415 from the National Science Foundation. This is Publication No. 239 from the Arizona State University Center for the Study of Early Events in Photosynthesis. * To whom correspondence should be addressed. @ Abstract published in Advance ACS Abstracts, September zyxwvut 15, 1995. ' Abbreviations: Hc. mobilis, Heliobacillus mobilis; (B)Chl, (bac- terio)chlorophyll; P700, primary electron donor of photosystem I absorbing at 700 nm; P798, primary electron donor of heliobacteria absorbing at 798 nm; Ao, primary electron acceptor in photosystem I; AI, secondary electron acceptor in photosystem I; Fx, zyxwvut FA&, iron- sulfur centers that act as secondary electron acceptors in photosystem I. heliobacteria (Prince et al., 1985; Brok et al., 1986; Trost & Blankenship, 1989; Hiraishi, 1989; Fischer, 1990; Nitschke et al., 1990; Liebl et al., 1993). However, the understanding of their function in electron transfer in heliobacteria is incomplete. A number of studies have shown that the reaction center of heliobacteria is similar in many ways to those of the green sulfur bacteria and photosystem I from higher plants (Prince et al., 1985; Nitschke et al., 1990; Buttner et al., 1992; Liebl et al., 1993). They all contain iron-sulfur center clusters as one of the early electron acceptors and are therefore known as the Fe-S type of reaction center (Blankenship, 1992). The electron carriers in this type of reaction center have consider- ably lower redox potential than those of purple bacteria, green filamentous bacteria and photosystem I1 in higher plants. Electron transfer processes in the reaction center of photosystem I have been extensively studied [for recent reviews, see Sttif (1992) and Golbeck (1994)l. The initial charge separation to form the P700+Ao- state requires 1-3 ps when the excitation resides on the special pair P700 (Holzwarth et al., 1993; Hastings et al., 1994; Kumazaki et al. 1994). The observed chlorophyll excited state lifetime is longer (25-30 ps), due to the fact that most of the time the excitation resides on one of the antenna pigments instead of the special pair. The primary electron acceptor A0 is thought to be a Chl a molecule (Golbeck & Bryant, 1991). The absorption dif- ference spectrum associated with the reduction of A0 obtained 0006-2960/95/0434-12761$09.00/0 0 1995 American Chemical Society