Measurement of the extent of electron transfer to the bacteriopheophytin in the M-subunit in reaction centers of Rhodopseudomonas viridis

Abstract

We have measured the extent of flash-induced electron transfer from the bacteriochlorophyll dimer, P, to the bacteriopheophytin in the M-subunit, HM, in reaction centers of Rhodopseudomonas viridis. This has been done by measuring the transient states produced by excitation of reaction centers trapped in the PHL (-)HM state at 90 K. Under these conditions the normal forward electron transfer to the bacteriopheophytin in the L-subunit, HL, is blocked and the yield of transient P(+)HM (-) can be estimated with respect to the lifetime of P(*). Under these conditions flash induced absorbance decreases of the bacteriochlorophyll dimer 990 nm band suggest that a transient P(+) state is formed with a quantum yield of 0.09±0.06 compared to that formed during normal photochemistry. These transient measurements provide an upper limited on the yield of a transient P(+) HM (-) state. An estimate of 0.09 as the yield of the P(+) HM (-) state is consistent with all current observations. This estimate and the lifetime of P(*) suggest that the electron transfer rate from P(*) to HM, kM, is about 5 × 10(9) sec(-1) (τM = 200ps). These measurements suggest that the a branching ratio kL/kM is on the order of 200. The large value of the branching ratio is remarkable in view of the structural symmetry of the reaction center. This measurement should be useful for electron transfer calculations based upon the reaction center structure.

B-Side Electron Transfer To Form P+HB- in Reaction Centers from the F(L181)Y/Y(M208)F Mutant of Rhodobacter capsulatus

“…The H A and H B anion absorption bands are known to be very broad (40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) nm full width at half-maximum, as in the spectra for the wild-type RC in Figure 3) with peaks in the difference spectra near ∼665 and ∼645 nm, respectively. With ∼10% of P* decaying directly to the ground state, and 90% the expected combined yield of P + H A -and P + H B -, we should find an integrated 620-700 nm absorption that is reduced in magnitude by only ∼10% compared to wild type (assuming the anions of H A and H B have equal oscillator strength).…”

Wavelength and intensity dependent primary photochemistry of isolated Photosystem II reaction centers at 5°C

“…Insight into the mechanism of primary charge separation in bacterial reaction centers has been gained from pre-reduction of the bacteriopheophytin primary electron acceptor with chemical reductants, such as sodium dithionite [45]. A similar strategy can be employed to reduce Pheo a in the PSII RC prior to flash excitation [2,18,46].…”

Low-Temperature Femtosecond-Resolution Transient Absorption Spectroscopy of Large-Scale Symmetry Mutants of Bacterial Reaction Centers

“…A number of steady state and nanosecond resolution absorbance measurements have been performed in which reaction centers with both the primary quinone (Q A ) 1 and the A side bacteriopheophytin (H A ) reduced have been shown to form the anion of the B side bacteriopheophytin (H B ) with a limited quantum yield (approximately 8%-9%; Tiede et al, 1987;Kellogg et al, 1989). Comparing this yield to the overall decay rate of the initial electron donor excited singlet state (P*) under the same conditions (20 ps with reduced Q A and H A ; Holten et al, 1978) a branching ratio of 200:1 in favor of the A side was determined (Kellogg et al, 1989). Calculations of relative electron transfer rates along the two paths based on the crystal structure of Rhodopseudomonas (Rp.)…”