B Side Electron Transfer in a Rhodobacter sphaeroides Reaction Center Mutant in Which the B Side Monomer Bacteriochlorophyll Is Replaced with Bacteriopheophytin:  Low-Temperature Study and Energetics of Charge-Separated States

Abstract

The mutation, HL(M182), in the Rhodobacter sphaeroides reaction center (RC) results in the replacement of the monomer bacteriochlorophyll (BChl) on the inactive side (B side) of the RC with a bacteriopheophytin (BPheo; the new cofactor is referred to as φB). In φB-containing RCs, the first excited state of the primary donor (P*) decays with an accelerated time constant of 2.6 ± 0.1 ps at room temperature as compared to 3.1 ± 0.2 ps in wild-type (WT) RCs. At low temperatures, P* decay is essentially the same in the HL(M182) mutant and WT RCs: 1.4 ± 0.1 ps at 77 K and 1.1 ± 0.1 ps at 9 K. The yield of the charge-separated P+φB - state decreases from 35% at room temperature to 12% at 77 and 9 K. The decreased P+φB - yield is apparently due to the fact that the rate of the charge separation along the A side of the RC at low temperature increases, while the rate along the B side remains essentially unchanged. From measurements of the long-lived fluorescence decay at room temperature, the standard free energy of the P+φB - state is estimated to be about 0.16 ± 0.04 eV below P*. Given a difference between the midpoint potentials of BChl and BPheo of 0.26 ± 0.03 V, the standard free energy of the P+BB - state in WT RC is estimated to be 0.1 ± 0.07 eV above P*.

Formation of a Semiquinone at the QB Site by A- or B-Branch Electron Transfer in the Reaction Center from Rhodobacter sphaeroides

“…Most of these studies have concentrated on formation of the P + H Bstate (18, 20-23, [26][27][28]. Where the possibility of forward electron transfer past P + H B -has been considered, studies have focused on the decay kinetics of the P + H B -state to a presumed P + Q B -state or, as described above, on long-lived (millisecond) absorbance changes associated with P + (19,24,25,(29)(30)(31).…”

“…The structural and energetic basis of this asymmetry has been the subject of intense interest, and in recent years mutagenesis has been used in attempts to affect the balance of transmembrane electron transfer along the two branches (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31). To date, all of these investigations have involved transient absorption studies in the visible or near infrared, carried out in the presence or absence of inhibitors of ubiquinone reduction at the Q B site.…”

Photochemical Conversion of Solar Energy

“…[33,34] The key molecular components are a bacteriochlorophyll "special pair" (P), a bacteriochlorophyll monomer (BC), a bacteriopheophytin (BP), a quinone (Q A ), and a four-heme ctype cytochrome (Cyt). These molecules are held in a fixed geometry by surrounding proteins, so that the twofold axis of P [35] is perpendicular to the membrane, the periplasmic face lies approximately between P and Cyt, and the cytoplasmic face lies at the level of Q A . In the reaction center, excitation of P by absorption of light or, more commonly, by singlet-singlet energy transfer from various antenna systems, is followed by very fast (~3 ps) electron transfer to the BP "primary" acceptor (whether the interposed BC plays the role of mediator in a superexchange mechanism or directly intervenes as an intermediate electron acceptor has been the object of debate).…”

Temperature Dependence of Electron Transfer to the M-Side Bacteriopheophytin in Rhodobacter capsulatus Reaction Centers

“…[58][59][60][61] Calculations and experiments have provided estimates or bracketed ranges for the free energies of the charge-separated states in the wild-type RC: P + B L -0.05-0.1 eV below P*; 29,50,51,[62][63][64][65][66][67][68][69][70] P + H L -∼0.25 eV below P* when relaxed; [71][72][73][74][75] P + B M -0.1-0.2 eV above P* and P + H M -below P* by no more than ∼0.15 eV and probably within 0.1 eV. [50][51][52][76][77][78][79] Systematic efforts to manipulate the free energy differences of the L-and M-branch charge-separated states by site-directed mutagenesis of key amino acids, including those near B M and B L , have led to mutant RCs in which electron transfer to the M branch competes effectively with charge separation to the L branch, yielding P + H M -(reviewed in ref 80). In one such strategy in Rb.…”