Formation of charge separated state P⁺Q⁻_A and triplet state ³P at low temperature in Rhodobacter sphaeroides (R‐26) reaction centers in which bacteriopheophytin a is replaced by plant pheophytin a

Abstract

Low temperature optical and photochemical properties of Rhodobacter sphaeroides (R-26) reaction centers, in which bacteriopheophytin a has been replaced by plant pheophytin a, are reported. Modified reaction centers preserve the ability for photoinduced electron transfer from the primary electron donor P to the primary quinone acceptor QA at 80K. The triplet state ESR signal of modified reaction centers with prereduced QA at 10K shows an electron spin polarization pattern and ZFS parameters analogous to those for the triplet state 3p in non-treated reaction centers. It was found that at low temperature both P+QA and 3p states are formed via a precursor radical pair P+I-in which I is the introduced plant pheophytin molecule. This shows that acceptor systems of bacterial and plant (photosystem I1) reaction centers are mutually replacable in structural and functional aspects.

Formation of a Long-Lived P+BA- State in Plant Pheophytin-Exchanged Reaction Centers of Rhodobacter sphaeroides R26 at Low Temperature

“…At a delay of 1.2 ns (solid line), the 1020 nm band, the net bleaching at 800 nm, and the line-broadening signal at 670 nm have diminished considerably, indicating decay of the P + B Aradical pair. The difference spectrum at 1.2 ns very much resembles the state P + Q Aas measured previously in H A,B -exchanged and H B -only exchanged RCs (22,31), with a bleaching of P, a major band shift of B A and/or B B , and a minor band shift of Phe. The absorption increase at 770 nm is due to H A in H B -only exchanged RCs.…”

Characterization of Pheophytin Ground States in Rhodobacter sphaeroides R26 Photosynthetic Reaction Centers from Multispin Pheophytin Enrichment and 2-D 13C MAS NMR Dipolar Correlation Spectroscopy

“…This shows that almost all Φ B has been replaced. The relative amount of Φ A replacement is more difficult to estimate from the absorption peak at 545 nm, since the reconstituted Pheo a also absorbs at the same wavelength (Shkuropatov et al, 1994). From the ratio of the peaks at 676 nm (Pheo a) and 890 nm (P), we obtain that 88 ( 5% of the total amount of BPheo has been replaced by labeled Pheo a.…”

“…sphaeroides R26 were isolated by treatment of chromatophores with LDAO, followed by purification with DEAE-Sephacel chromatography (Feher & Okamura, 1978). Replacement of the BPheo by labeled Pheo a has been done using a slightly modified method described earlier (Scheer et al, 1992;Shkuropatov et al, 1994). Briefly, 22 batches of 10 mL each containing about 14 µM RC, 50 µM labeled Pheo a, 0.1% LDAO, 1 mM EDTA, 10 mM Tris, pH 8, and 10% acetone were incubated for 90 min at 42 ( 0.5 °C.…”

Chlorophyll a Synthesis in Rhodobacter sphaeroides by Chlorophyll Synthase of Nicotiana tabacum

“…The reaction center (RC) complex of R. sphaeroides has four BChl a P and two bacteriopheophytin a P (Bpheo a P , Mg 2+ -free form of BChl a P ) molecules. When pheophytin a P (Pheo a P , Mg 2+ -free form of Chl a P ) is provided to an isolated RC complex, the Bpheo a P of the RC complex can be easily replaced by Pheo a P in the presence of a mild detergent, such as lauryldimethylamine oxide, with heating at 42 °C [ 31 , 32 , 33 , 34 , 35 ]. The modified RC complex containing Pheo a P illustrated the electron transfer kinetics with a decreased quantum yield for both triplet special pair formation and quinone reduction compared with the native RC [ 34 , 35 ].…”