Primary electron transfer kinetics in bacterial reaction centers with modified bacteriochlorophylls at the monomeric sites BA,B.

Abstract

The primary electron transfer has been investigated by femtosecond time-resolved absorption spectroscopy in two chemically modified reaction centers (RC) of Rhodobacter sphaeroides, in which the monomeric bacteriochlorophylls BA and BB have both been exchanged by 132-hydroxybacteriochlorophyll a or [3-vinylJ-132-hydroxybac-teriochlorophyll a. The kinetics of the primary electron transfer are not influenced by the 132-hydroxy modification. In RCs containing [3-vinyl]-132-hydroxybacteriochlorophyll a the primary rate is reduced by a factor of 10. HThe three known x-ray structures of bacterial reaction centers (RCs) show basically the same arrangement of the cofactors (see refs. 1-4). They form two branches [A (active) and B (inactive)], which are arranged in an approximate twofold symmetry. Surprisingly, the electron transfer (ET) has been found to proceed exclusively along the A branch (5-7). From the primary donor, P870 or P960, an electron is transferred to the acceptor quinone (QA) The structural arrangement of the chromophores suggests an ET via the two intervening pigments-i.e., the monomeric bacteriochlorophyll (BChl) BA and the bacteriopheophytin (BPhe) HAWhile there is general agreement that the BPhe HA is an intermediate electron acceptor (5-9), the role of the BChl BA located between the BChl dimer P and the BPhe HA is still controversially discussed. First studies with sufficient time resolution and an appropriate excitation wavelength led to the conclusion that the state P+B-(P, primary donor) is not a real intermediate in the ET (5,10,11). However, a direct ET from P* to HA over an edge-to-edge distance of '10 A can be accounted for only by a "superexchange" transfer via a virtual intermediate P+B-(12-15 Kinetic measurements were performed at T 298 K in cuvettes of 1-mm path length under stirring. The sample volume was 0.2-0.3 ml. The transmission was between T 10%o and 50% at A = 860 nm (80-25 ,uM The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Carotenoid triplet state formation in Rhodobacter sphaeroides R-26 reaction centers exchanged with modified bacteriochlorophyll pigments and reconstituted with spheroidene

“…Once again the temperature dependence in the 105 to 150 K range show that these signals are uniformly reduced upon raising the temperature. This is consistent with the observation that an exchange of Phe for BPhe has only little effect on the neighboring pigments in the B-pockets (Scheer et al 1992). Indeed, one would not expect any significant change in the rate of energy transfer from the primary donor to the carotenoid upon exchange ofPhe a for the native BPhe molecules at sites H A and H B.…”

“…Phe a and [3-vinyl]-132-OH-BChl a were prepared as before (Struck et al 1992). 132-OH-Zn-BChl a was obtained by metalation of the respective bacteriopheophytin with zinc acetate in acetic acid (Fiedor et al 1993, unpublished results), or by transmetalation of the cadmium complex (Hartwich et al 1993).…”

“…Recent work has shown that only the pigments on the so-called Aside of the reaction center participate in the photoinduced electron transfer from the primary donor to the BPhe at site H A (Kirmaier et al 1985, Zinth et al 1985. The precise role of the bridging BChl A in the primary electron transfer photochemistry is the subject of some controversy and intense scrutiny (Finkele 1992).…”

The First Picoseconds in Bacterial Photosynthesis—Ultrafast Electron Transfer for the Efficient Conversion of Light Energy

“…[125,126] As expected from a stepwise reaction model a reduction in primary ET rate with the accompanying significant decrease of the transient P + B A À population and less efficient charge separation was found. Important information was obtained when the secondary acceptor BPhe was replaced by different types of modified BPhe and Phe molecules.…”

Femtosecond Spectroscopy of the Primary Electron Transfer in Photosynthetic Reaction Centers

“…sphaeroides were studied where the bacteriochlorophyll a molecule at the monomeric position BA and Bß were exchanged by [3-vinyl]-13 2 -OH-BChl a molecules [19]. The modification due to the 3-vinyl group is expected to change the redox potential of the BChl and as a consequence the energy of the radical pair State P+BA".…”