Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers

Boxer, Steven G.; Goldstein, Richard A.; Lockhart, David J.; Middendorf, Thomas R.; Takiff, Larry

doi:10.1021/j100363a004

Cited by 127 publications

(73 citation statements)

References 8 publications

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“…A measurement of {A for the lowest electronic transition of the RC, the functional electron-donor state, provides information on the direction of charge displacement associated with this optical transition, and a measurement of {A is both very accurate and free from ambiguities concerning the local field correction (assumingfis a scalar) (12).1 As discussed in detail elsewhere (17), for a C, symmetric dimer such as P, a substantial value of IAAAI is possible; however, the direction of charge displacement must lie along the C2 axis. Because the C2 axis is approximately perpendicular to the transition dipole moment for P (29,30), the observation that CA is rotated to 380 implies that the electronic symmetry of 'P is broken by some feature in its environment (17).…”

Section: Discussionmentioning

confidence: 99%

“…Because the C2 axis is approximately perpendicular to the transition dipole moment for P (29,30), the observation that CA is rotated to 380 implies that the electronic symmetry of 'P is broken by some feature in its environment (17). The angle can be related to the directions ofthe dipoles ofCT states that mix with the lowest energy transition.…”

Section: Discussionmentioning

confidence: 99%

“…The effect of an electric field on the absorption spectra of RCs from several different species has proven to be a useful approach for probing the dipolar character of spectroscopic transitions (11)(12)(13)(14)(15)(16)(17). The absorption Stark effect depends on the square of the magnitude of the change in dipole moment between the ground and excited states, INLAI.…”

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confidence: 99%

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Stark spectroscopy of the Rhodobacter sphaeroides reaction center heterodimer mutant.

Hammes

Mazzola

Boxer

et al. 1990

Proc. Natl. Acad. Sci. U.S.A.

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The effect of an electric field has been measured on the absorption spectrum (Stark effect) of the heterodimer mutant (M)H202L of Rhodobaeter sphaeroides reaction centers, where the primary electron donor consists of one bacteriochlorophyll a and one bacteriopheophytin a. The electronic absorption spectrum of the heterodimer mutant from 820-950 nm is relatively featureless in a poly(vinyl alcohol) film, but it exhibits some structure in a glycerol/water -ass at 77 K. A feature is seen in the Stark effect spectrum of the heterodimer at 77 K centered at 927 and 936 nm in poly(vinyl alcohol) and a glycerol/water glass, respectively. This feature has approximately the same shape and width as the Stark effect for the primary electron donor of the wild type, which consists of a pair of bacteriochlorophyll a molecules. The angle M4 between the transition moment at the frequency of absorption and the difference dipole AIA is 36± + in the wild ty and 32 ± 2 for that feature in the heterodimer. A range of values for IAAI = (13-17)/f Debye units (where f is the local field correction) is obtained for the 936-nm feature in glycerol/water, depending on analysis method. This feature is interpreted as arising from a transition to the lower exciton state of the heterodimer, which is more strongly mixed with a low-lying charge transfer transition than in the wild type.With the recent availability of genes and deletion strains for the photosynthetic bacteria Rhodobacter capsulatus (1, 2) and Rhodobacter sphaeroides (3, 4), amino acid residues in the reaction center (RC) can be manipulated by site-directed mutagenesis to investigate specific aspects of proteinprosthetic group interactions. In R. capsulatus RCs, replacement of the histidine ligand of the M-side bacteriochlorophyll a (BChla) molecule in the special pair primary-electron donor (denoted P) with a noncoordinating side chain leads to loss of the central Mg atom, converting it to a bacteriopheophytin a (BPheoa). This RC, the primary electron donor of which consists of one BChla (DL) and one BPheoa (DM), has been called the heterodimer mutant; the symbol D is used for P in this dimer (2, 5). In contrast to P. in which the lowest singlet electronic absorption band is relatively narrow (full width at half-maximum =500 cm-'), the absorption spectrum of D appears rather featureless and extends from =800-1000 nm. The quantum yield for the initial charge separation step in the heterodimer mutant is =50% of that in wild-type RCs; the remaining 50o of excited RCs decay rapidly to the ground state (5). The mechanism of charge separation in the heterodimer mutant has been suggested (5) to be the following:where HL is the BPheoa electron acceptor. [The notation *D is used to denote the excited state formed upon excitation at 870 nm under conditions ofthe experiments described in refs. 5 and 6. Internal charge transfer (CT) states of D are written as DL+DM-because BPheoa is "-300 meV easier to reduce than BChla (5-7). The hole on D+ may localize on the BChla (DL) half of the ...

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

mentioning

confidence: 99%

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Stark spectroscopy of the Rhodobacter sphaeroides reaction center heterodimer mutant.

Hammes

Mazzola

Boxer

et al. 1990

Proc. Natl. Acad. Sci. U.S.A.

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“…In particular, Stark spectroscopy shows that a substantial separation of charge occurs upon photoexcitation of P, and the change in dipole moment between the ground and excited state, ∆µ, is substantially larger for P than for a monomeric BChl (2). Significantly, the measured angle A between ∆µ and the transition dipole moment of P demonstrates that ∆µ is not parallel to the C 2 symmetry axis.…”

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confidence: 98%

Excited-State Electronic Asymmetry of the Special Pair in Photosynthetic Reaction Center Mutants: Absorption and Stark Spectroscopy

1999

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The electronic absorption line shape and Stark spectrum of the lowest energy Q y transition of the special pair in bacterial reaction centers contain a wealth of information on mixing with charge transfer states and electronic asymmetry. Both vary greatly in mutants that perturb the chemical composition of the special pair, such as the heterodimer mutants, and in mutants that alter interactions between the special pair and the surrounding reaction center protein, such as those that add or remove hydrogen bonds. The conventional and higher-order Stark spectra of a series of mutants are presented with the aim of developing a systematic description of the electronic structure of the excited state of the special pair that initiates photosynthetic charge separation. The mutants L168HF, M197FH, L131LH and L131LH/M160LH/ M197FH are known to have different hydrogen-bonding patterns to the special pair; however, they exhibit Stark effects that are very similar to wild type. By contrast, the addition of a hydrogen bond to the M-side keto carbonyl group of the special pair in M160LH greatly affects both the absorption and Stark spectra. The heterodimer special pairs, L173HL and M202HL, exhibit much larger Stark effects than wild type, with the greatest effect in the M-side mutant. Double mutants that combine the M-side heterodimer and a hydrogen-bond addition to the L-side of the special pair decrease the magnitude of the Stark effect. These results suggest that the electronic asymmetry of the dimer can be perturbed either by the formation of a heterodimer or by adding or deleting a hydrogen bond to a keto carbonyl group. From the pattern observed, it is concluded that the charge transfer state P L + P M -has a larger influence on the excited state of the dimer in wild type than the P L -P M + charge transfer state. Furthermore, asymmetry can be varied continuously, from extreme cases in which the heterodimer and hydrogen-bond effects work together, to cases in which hydrogen bonding offsets the effects of the heterodimer, to cases in which the homodimer is perturbed by hydrogen bonds. This leads to a unified model for understanding the effects of perturbations on the electronic symmetry of the special pair, and this can be connected with perturbations on the properties of many other systems such as donor-acceptor-substituted polyenes.Photoexcitation of a strongly interacting pair of bacteriochlorophyll (BChl) 1 molecules called the special pair or P in bacterial photosynthetic reaction centers (RCs) initiates a series of light-driven charge separation reactions. The singlet excited state of P, 1 P, transfers an electron within a few picoseconds to H L , a bacteriopheophytin (BPhe) molecule that is the initial electron acceptor (1). As shown in the upper part of Figure 1, there are two similar sets of chromophores related by a local C 2 axis of symmetry that can serve as electron acceptors, yet only the chromophores on the L side (the right side as illustrated, sometimes denoted the A branch) participate in the i...

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“…Three nonlinear optical parameters can be determined from Stark spectroscopy [28,29]; (1) the transition dipole-moment polarizability and its hyperpolarizability (D factor), (2) the change of polarizability upon photoexcitation (∆α), and (3) the change of dipole-moment upon photoexcitation (∆µ). Stark spectroscopy has been extensively applied to the studies of RCs since the early 1980s [27,[30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] (for excellent reviews by Boxer et al, see [46,47]). The most remarkable characteristic of the nonlinear optical properties of the RC can be found in its large ∆µ value of P. This is initially assumed to be caused by mixing with a charge-transfer (CT) state [37][38][39].…”

Section: Local Electrostatic Field Induced By the Carotenoid Bound Tomentioning

confidence: 99%

Structures and functions of carotenoids bound to reaction centers from purple photosynthetic bacteria

Hashimoto

Fujii

Yanagi

et al. 2006

Pure and Applied Chemistry

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The photoprotective function of 15,15'-cis-carotenoids bound to the photosynthetic reaction centers (RCs) of purple bacteria has been studied using carotenoids reconstituted into carotenoidless RCs from Rhodobacter sphaeroides strain R26.1. The triplet-energy level of the carotenoid has been proposed to affect the quenching of the triplet state of special-pair bacteriochlorophyll (P). This was investigated using microsecond flash photolysis to detect the carotenoid triplets as a function of the number of conjugated double bonds, n. The carotenoid triplet signals were extracted by using singular-value decomposition (SVD) of the huge matrices data, and were confirmed for those having n = 8 to 11. This interpretation assumes that the reconstituted carotenoids occupy the same binding site in the RC. We have been able to confirm this assumption using X-ray crystallography to determine the structures of carotenoidless, wild-type carotenoid-containing, and 3,4-dihydro-spheroidene-reconstituted RCs. The X-ray study also emphasized the importance of the methoxy group of the carotenoids for binding to the RCs. Electroabsorption (Stark) spectroscopy was used to investigate the effect of the carotenoid on the electrostatic field around P. This electrostatic field changed by 10 % in the presence of the carotenoid.

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Excited states, electron-transfer reactions, and intermediates in bacterial photosynthetic reaction centers

Cited by 127 publications

References 8 publications

Stark spectroscopy of the Rhodobacter sphaeroides reaction center heterodimer mutant.

Stark spectroscopy of the Rhodobacter sphaeroides reaction center heterodimer mutant.

Excited-State Electronic Asymmetry of the Special Pair in Photosynthetic Reaction Center Mutants: Absorption and Stark Spectroscopy

Structures and functions of carotenoids bound to reaction centers from purple photosynthetic bacteria

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