Femtosecond transient absorption spectroscopy in the range of 500-1040 nm was used to study electron transfer at 5 K in reaction centers of Rhodobacter sphaeroides R26 in which the... Show moreFemtosecond transient absorption spectroscopy in the range of 500-1040 nm was used to study electron transfer at 5 K in reaction centers of Rhodobacter sphaeroides R26 in which the bacteriopheophytins (BPhe) were replaced by plant pheophytin a (Phe). Primary charge separation took place with a time constant of 1.6 ps, similar to that found in native RCs. Spectral changes around 1020 nm indicated the formation of reduced bacteriochlorophyll (BChl) with the same time constant, and its subsequent decay in 620 ps. This observation identifies the accessory BChl as the primary electron acceptor. No evidence was found for electron transfer to Phe, indicating that electron transfer from B-A(-) occurs directly to the quinone (Q(A)) through superexchange. The results are explained by a model in which the free energy level of P(+)Phe(-) lies above that of P+BA-, which itself is below P*. Assuming that the pigment exchange does not affect the energy levels of P* and P+BA-, our results strongly support a two-step model for primary electron transfer in the native bacterial RC, with no, or very little, admixture of superexchange. Show less
The quantum yield of the formation of the charge-separated state P(+)Q(A)(-) in reaction centers (RCs) of Rhodobacter sphaeroides R-26, in which the bacteriopheophytins in both the active (A) and... Show moreThe quantum yield of the formation of the charge-separated state P(+)Q(A)(-) in reaction centers (RCs) of Rhodobacter sphaeroides R-26, in which the bacteriopheophytins in both the active (A) and the inactive (B) branch are replaced by pheophytin (Pheo) a (Phi(A,B)-exchanged RCs), shows a positive temperature dependence: it is 38 +/- 5% between 10 and 60 K, increases with temperature to 72 +/- 5% at 200 K and shows a minor additional increase above this temperature. The temperature dependence of the quantum yield of P(+)Q(A)(-) formation in Phi(A,B)-exchanged RCs is modelled in the framework of a reaction scheme with the energy level of P(+)Pheo(A)(-) placed above P+BA- (Shkuropatov, A. Ya. and Shuvalov, V.A. (1993) FEES Lett. 322, 168-172), by the introduction of direct electron transfer from B-A(-) to Q(A), assisted by a superexchange-mechanism via P(+)Pheo(A)(-). The observed triples formation Phi(A,B)-exchanged RCs with pre-reduced Q(A) at cryogenic temperatures (quantum yield less than or equal to 12%) is attributed to a residual fraction of RCs in which only Phi(B) was exchanged for Pheo a. The lack of triplet formation in pre-reduced Phi(A,B)-exchanged RCs is consistent with our kinetic model, since this predicts that at low temperatures the state P(+)Pheo(A)(-) is not populated. (C) 1997 Elsevier Science B.V. Show less
Bacteriopheophytin (BPheo) a of reaction centers (RCs) of Rhodobacter sphaeroides R-26 has been exchanged with pheophytin (Pheo) a. By varying the incubation temperature of the pigment exchange... Show moreBacteriopheophytin (BPheo) a of reaction centers (RCs) of Rhodobacter sphaeroides R-26 has been exchanged with pheophytin (Pheo) a. By varying the incubation temperature of the pigment exchange procedure two types of RCs were obtained, with either only the BPheo in the B-chain (BPheo(B)) or both BPheo(B) and BPheo(A) replaced by Pheo a. For the two RC types absorption and CD spectra at 6 K as well as P(+)Q(A)(-) difference spectra at 10 K are compared with those of native RCs. The most pronounced differences are observed in the Q(Y) and Q(X) regions of the (B)Pheos. The P(+)Q(A)(-) decay halftime is for RCs with Pheo a in both chains 10-15 ms longer than for native RCs and RCs that still have a BPheo in the A-chain,at all temperatures between 10 and 290 K. At low temperatures all three RC types showed biphasic P(+)Q(A)(-) recombination. Show less
