Influence of Electronic Asymmetry on the Spectroscopic and Photodynamic Properties of the Primary Electron Donor in the Photosynthetic Reaction Center

Photoinduced charge transfer in Zn(II) and Au(III)-ligated symmetric and asymmetric bacteriochlorin dyads: A computational study

The excited-state properties and photoinduced charge-transfer (CT) kinetics in a series of symmetrical and asymmetrical Zn- and Au-ligated meso-meso-connected bacteriochlorin (BChl) complexes are studied computationally. BChl derivatives, which are excellent near-IR absorbing chromophores, are found to play a central role in bacterial photosynthetic reaction centers but are rarely used in artificial solar energy harvesting systems. The optical properties of chemically linked BChl complexes can be tuned by varying the linking group and involving different ligated metal ions. We investigate charge transfer in BChl dyads that are either directly linked or through a phenylene ring (1,4-phenylene) and which are ligating Zn or Au ions. The directly linked dyads with a nearly perpendicular arrangement of the BChl units bear markedly different properties than phenylene linked dyads. In addition, we find that the dielectric dependence of the intramolecular CT rate is very strong in neutral Zn-ligated dyads, whereas cationic Au-ligated dyads show negligible dielectric dependence of the CT rate. Rate constants of the photo induced CT process are calculated at the semiclassical Marcus level and are compared to fully quantum mechanical Fermi's golden rule based values. The rates are calculated using a screened range separated hybrid functional that offers a consistent framework for addressing environment polarization. We study solvated systems in two solvents of a low and a high scalar dielectric constant.

Excitonic Interactions in Bacteriochlorin Homo-Dyads Enable Charge Transfer: A New Approach to the Artificial Photosynthetic Special Pair

Excitonically coupled bacteriochlorin (BC) dimers constitute a primary electron donor (special pair) in bacterial photosynthesis and absorbing units in light-harvesting antenna. However, the exact nature of the excited state of these dyads is still not fully understood. Here, we report a detailed spectroscopic and computational investigation of a series of symmetrical bacteriochlorin dimers, where the bacteriochlorins are connected either directly or by a phenylene bridge of variable length. The excited state of these dyads is quenched in high-dielectric solvents, which we attribute to photoinduced charge transfer. The mixing of charge transfer with the excitonic state causes accelerated (within 41 ps) decay of the excited state for the directly linked dyad, which is reduced by orders of magnitude with each additional phenyl ring separating the bacteriochlorins. These results highlight the origins of the excited-state dynamics in symmetric BC dyads and provide a new model for studying the primary processes in photosynthesis and for the development of artificial, biomimetic systems for solar energy conversion.

Ultrafast Photoinduced Symmetry-Breaking Charge Separation and Electron Sharing in Perylenediimide Molecular Triangles

We report on a visible-light-absorbing chiral molecular triangle composed of three covalently linked 1,6,7,12-tetra(phenoxy)perylene-3,4:9,10-bis(dicarboximide) (PDI) units. The rigid triangular architecture reduces the electronic coupling between the PDIs, so ultrafast symmetry-breaking charge separation is kinetically favored over intramolecular excimer formation, as revealed by femtosecond transient absorption spectroscopy. Photoexcitation of the PDI triangle dissolved in CH2Cl2 gives PDI(+•)-PDI(-•) in τCS = 12.0 ± 0.2 ps. Fast subsequent intramolecular electron/hole hopping can equilibrate the six possible energetically degenerate ion-pair states, as suggested by electron paramagnetic resonance/electron-nuclear double resonance spectroscopy, which shows that one-electron reduction of the PDI triangle results in complete electron sharing among the three PDIs. Charge recombination of PDI(+•)-PDI(-•) to the ground state occurs in τCR = 1.12 ± 0.01 ns with no evidence of triplet excited state formation.

The site-directed mutation I(L177)H in Rhodobacter sphaeroides reaction center affects coordination of P(A) and B(B) bacteriochlorophylls

To explore the influence of the I(L177)H single mutation on the properties of the nearest bacteriochlorophylls (BChls), three reaction centers (RCs) bearing double mutations were constructed in the photosynthetic purple bacterium Rhodobacter sphaeroides, and their properties and pigment content were compared with those of the correspondent single mutant RCs. Each pair of the mutations comprised the amino acid substitution I(L177)H and another mutation altering histidine ligand of BChl P(A) or BChl B(B). Contrary to expectations, the double mutation I(L177)H+H(L173)L does not bring about a heterodimer RC but causes a 46nm blue shift of the long-wavelength P absorbance band. The histidine L177 or a water molecule were suggested as putative ligands for P(A) in the RC I(L177)H+H(L173)L although this would imply a reorientation of the His backbone and additional rearrangements in the primary donor environment or even a repositioning of the BChl dimer. The crystal structure of the mutant I(L177)H reaction center determined to a resolution of 2.9Å shows changes at the interface region between the BChl P(A) and the monomeric BChl B(B). Spectral and pigment analysis provided evidence for β-coordination of the BChl B(B) in the double mutant RC I(L177)H+H(M182)L and for its hexacoordination in the mutant reaction center I(L177)H. Computer modeling suggests involvement of two water molecules in the β-coordination of the BChl B(B). Possible structural consequences of the L177 mutation affecting the coordination of the two BChls P(A) and B(B) are discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.

Quantum nonequilibrium approach for fast electron transport in open systems: photosynthetic reaction centers

Creation of electrons or excitons by external fields in a system with initially statistically independent unrelaxed vibrational modes leads to an initial condition term. The contribution of this term in the time convolution generalized master-equation approach is studied in the second order of the perturbation theory in path-integral formalism. The developed approach, applied for the analysis of dynamics in the photosynthetic reaction center, exhibits the key role of the initial condition terms at the primary stage of electron transfer.

Structural and spectropotentiometric analysis of Blastochloris viridis heterodimer mutant reaction center

Heterodimer mutant reaction centers (RCs) of Blastochloris viridis were crystallized using microfluidic technology. In this mutant, a leucine residue replaced the histidine residue which had acted as a fifth ligand to the bacteriochlorophyll (BChl) of the primary electron donor dimer M site (HisM200). With the loss of the histidine-coordinated Mg, one bacteriochlorophyll of the special pair was converted into a bacteriopheophytin (BPhe), and the primary donor became a heterodimer supermolecule. The crystals had dimensions 400 x 100 x 100 microm, belonged to space group P4(3)2(1)2, and were isomorphous to the ones reported earlier for the wild type (WT) strain. The structure was solved to a 2.5 A resolution limit. Electron-density maps confirmed the replacement of the histidine residue and the absence of Mg. Structural changes in the heterodimer mutant RC relative to the WT included the absence of the water molecule that is typically positioned between the M side of the primary donor and the accessory BChl, a slight shift in the position of amino acids surrounding the site of the mutation, and the rotation of the M194 phenylalanine. The cytochrome subunit was anchored similarly as in the WT and had no detectable changes in its overall position. The highly conserved tyrosine L162, located between the primary donor and the highest potential heme C(380), revealed only a minor deviation of its hydroxyl group. Concomitantly to modification of the BChl molecule, the redox potential of the heterodimer primary donor increased relative to that of the WT organism (772 mV vs. 517 mV). The availability of this heterodimer mutant and its crystal structure provides opportunities for investigating changes in light-induced electron transfer that reflect differences in redox cascades.

Perturbation of the structure of P680 and the charge distribution on its radical cation in isolated reaction center complexes of photosystem II as revealed by fourier transform infrared spectroscopy

The structure and the electronic properties of P680 and its radical cation in photosystem II (PSII) were studied by means of Fourier transform infrared spectroscopy (FTIR). Light-induced P680+/P680 FTIR difference spectra in the mid- and near-IR regions were measured using PSII membranes from spinach, core complexes from Thermosynechococcus elongatus, and reaction center (RC) complexes (D1-D2-Cytb559) from spinach. The spectral features of the former two preparations were very similar, indicating that the structures of P680 and its radical cation are virtually identical between membranes and cores and between plants and cyanobacteria. In sharp contrast, the spectrum of the RC complexes exhibited significantly different features. A positive doublet at approximately 1724 and approximately 1710 cm-1 due to the 131-keto C=O stretches of P680+ in the membrane and core preparations were changed to a prominent single peak at 1712 cm-1 in the RC complexes. This observation was interpreted to indicate that a positive charge on P680+ was extensively delocalized over the chlorophyll dimer in RC, whereas it was mostly localized on one chlorophyll molecule (70-80%) in intact P680. The significant change in the electronic structure of P680+ in RC was supported by a dramatic change in the characteristics of a broad intervalence band in the near-IR region and relatively large shifts of chlorin ring bands. It is proposed that the extensive charge delocalization in P680+ mainly causes the decrease in the redox potential of P680+/P680 in isolated RC complexes. This potential decrease explains the well-known phenomenon that YZ is not oxidized by P680+ in RC complexes.

Pyridylporphyrin metallacycles with a slipped cofacial geometry: spectroscopic, X-ray, and photophysical characterization

Treatment of the octahedral Ru(II) complex [trans,cis,cis-RuCl(2)(DMSO-O)(2)(CO)(2)] with an equimolar amount of 5,10-bis(3'-pyridyl)-15,20-diphenylporphyrin (3'-cis-DPyP) yielded, upon selective replacement of the DMSO ligands, the neutral 2 + 2 metallacycle 2. NMR spectroscopy provided unambiguous evidence that only one highly symmetrical species, in which the two chromophores are held in a slipped cofacial arrangement by the external Ru(II) metal fragments, exists in solution. The unprecedented geometry of 2, and of the fully zincated analogue 2a, were confirmed in the solid state by X-ray structural investigations. The spatial arrangement of the two parallel chromophores in 2, with an interplanar distance of 4.18 A and a lateral offset (center-to-center distance) of 9.82 A, is reminiscent of those of the special pair of bacteriophylls in the reaction centers and of adjacent B850 units in the LH2 light-harvesting antenna systems of photosynthetic bacteria. For comparison, the X-ray structure of the corresponding metallacycle with 4'-cis-DPyP, 1a, is also reported. In 1a, the two porphyrins have an almost perfect coplanar arrangement. The semi-zincated metallacycles 1b and 2b, in which only one of the two chromophores bears an inner zinc atom, were prepared from 1 and 2, respectively, and isolated in pure form. Detailed photophysical investigations of the above porphyrin assemblies were performed. In particular, very fast photoinduced intercomponent energy transfer processes from the zinc porphyrin to the free-base unit were detected in the semi-metalated derivatives 1b and 2b (time constants: 14 and 12 ps, respectively).

Energy, Charge, and Spin Transport in Molecules and Self-Assembled Nanostructures Inspired by Photosynthesis

Electron transfer in biological molecules provides both insight and inspiration for developing chemical systems having similar functionality. Photosynthesis is an example of an integrated system in which light harvesting, photoinduced charge separation, and catalysis combine to carry out two thermodynamically demanding processes, the oxidation of water and the reduction of carbon dioxide. The development of artificial photosynthetic systems for solar energy conversion requires a fundamental understanding of electron-transfer reactions between organic molecules. Since these reactions most often involve single-electron transfers, the spin dynamics of photogenerated radical ion pairs provide important information on how the rates and efficiencies of these reactions depend on molecular structure. Given this knowledge, the design and synthesis of large integrated structures to carry out artificial photosynthesis is moving forward. An important approach to achieving this goal is the development of small, functional building blocks, having a minimum number of covalent bonds, which also have the appropriate molecular recognition sites to facilitate self-assembly into a complete, functional artificial photosynthetic system.

Low-frequency resonance Raman studies of the H(M202)G cavity mutant of bacterial photosynthetic reaction centers

Low-frequency (90-435 cm(-1)) NIR-excitation (875-900 nm) resonance Raman (RR) studies are reported for the H(M202)G cavity mutant of bacterial photosynthetic reaction centers (RCs) from Rb. sphaeroides that was first described by Goldsmith et al. [(1996) Biochemistry 35: 2421-2428]. In this mutant, the His residue that axially ligates the Mg ion of the M-side bacteriochlorophyll (BChl) of the special pair primary donor (P) is replaced by a non-ligating Gly residue. Regardless, the Mg ion of P(M) in the H(M202)G RCs remains pentacoordinates and is presumably ligated by a water molecule, although this axial ligand has not been definitively identified. The low-frequency RR studies of the H(M202)G RCs are accompanied by studies of RCs exchanged with D(2)O and incubated with imidazole (Im). The RR studies of the cavity mutant RCs reveal the following: (1) The structure of P(M) in the H(M202)G RCs is different from that of the wild-type, consistent with an altered BChl core. (2) A water ligand for P(M) in the H(M202)G RCs is generally consistent with the low-frequency RR spectra. The Mg-OH(2) stretching vibration is tentatively assigned to a band at 318 cm(-1), a frequency higher than that of the Mg-His stretch of the native pigment ( approximately approximately 235 cm(-1)). (3) The BChl core structure of P(M) in the cavity mutant is rendered similar (but not identical) to that of the wild-type when the adventitious water axial ligand is replaced by Im. (4) Exchange with D(2)O results in more global structural changes, likely involving the protein, which in turn affect the structure of the BChls in P. (5) Assignment of the low-frequency vibrational spectrum of P is generally more complex than originally suggested.

Probing the Contribution of Electronic Coupling to the Directionality of Electron Transfer in Photosynthetic Reaction Centers

Subpicosecond transient absorption studies are reported for a set of Rhodobacter (R.) capsulatus bacterial photosynthetic reaction centers (RCs) designed to probe the origins of the unidirectionality of charge separation via one of two electron transport chains in the native pigment-protein complex. All of the RCs have been engineered to contain a heterodimeric primary electron donor (D) consisting of a bacteriochlorophyll (BChl) and a bacteriopheophytin (BPh). The BPh component of the M heterodimer (Mhd) or L heterodimer (Lhd) is introduced by substituting a Leu for His M200 or His L173, respectively. Previous work on primary charge separation in heterodimer mutants has not included the Lhd RC from R. capsulatus, which we report for the first time. The Lhd and Mhd RCs are used as controls against which we assess RCs that combine the heterodimer mutations with a second mutation (His substituted for Leu at M212) that results in replacement of the native L-side BPh acceptor with a BChl (beta). The transient absorption spectra reveal clear evidence for charge separation to the normally inactive M-side BPh acceptor (H(M)) in Lhd-beta RCs to form D+H(M)- with a yield of approximately 6%. This state also forms in Mhd-beta RCs but with about one-quarter the yield. In both RCs, deactivation to the ground state is the predominant pathway of D decay, as it is in the Mhd and Lhd single mutants. Analysis of the results indicates an upper limit ofV2L/V2m < or = 4 for the contribution of the electronic coupling elements to the relative rates of electron transfer to the L versus M sides of the wild-type RC. In comparison to the L/M rate ratio (kL/kM) approximately 30 for wild-type RCs, our findings indicate that electronic factors contribute approximately 35% at most to directionality with the other 65% deriving from energetic considerations, which includes differences in free energies, reorganization energies, and contributions of one- and two-step mechanisms on the two sides of the RC.

Density Functional Theory Calculations on the Dielectric Constant Dependence of the Oxidation Potential of Chlorophyll: Implication for the High Potential of P680 in Photosystem II†

The primary donor chlorophyll (Chl) of photosystem II (PSII), P680, has an extremely high oxidation redox potential (E(ox)) of approximately 1.2 V, which is essential for photosynthetic water oxidation. The mechanism for achieving a high potential such as that of P680 has been one of the central questions in photosynthesis research. Here, we have examined the dielectric constant (epsilon) dependence of the E(ox) of monomer Chl using density functional theory calculations with the polarizable continuum model. The calculated E(ox) of a model Chl compound exhibited a sharp increase with a decrease in epsilon in the relatively low epsilon region (epsilon < 5). In contrast, in the higher-epsilon region, E(ox) was rather insensitive to epsilon and converged to a constant value at very high epsilon values. This tendency in the high-epsilon region explains the experimental E(ox) values of isolated Chl a that have been observed in a relatively narrow range of 0.74-0.93 V. The E(ox) of Chl in an ideal hydrophobic protein was estimated to be approximately 1.4 V at an epsilon value of 2. This value indicates that Chl in a hydrophobic environment originally has a high E(ox) that is sufficient for oxidizing water (E(ox) = 0.88 V at pH 6). On the basis of the reported X-ray crystallographic structures, the protein environment of P680 in PSII was estimated to be more hydrophobic than that of the primary donors in bacterial reaction centers. It is therefore suggested that the low-dielectric environment around P680 is one of the major factors in its very high E(ox), and thus, introducing nonpolar amino acids into the binding pocket of P680 was an important process in the evolution of PSII.

Ultrafast Photoinduced Charge Separation Resulting from Self-assembly of a Green Perylene-based Dye into π-Stacked Arrays

Condensation of 3,4,5-tris(n-dodecyloxy)aniline with the green chromophore 1,7-bis(N-pyrrolidinyl)perylene-3,4;9,10-tetracarboxylic dianhydride yields N,N'-bis(3,4,5-tris(n-dodecyloxy)phenyl)-1,7-bis(N-pyrrolidinyl)perylene-3,4;9,10-bis(dicarboximide), 5PDI-TAP, which absorbs light strongly from 550 to 750 nm. 5PDI-TAP dissolves readily in methylcyclohexane (MCH), resulting in self-assembly into H-aggregates. Small-angle X-ray scattering data obtained on 10(-4) M solutions of 5PDI-TAP in MCH show that the aggregates are pi-stacked monodisperse pentamers. Femtosecond transient absorption spectroscopy on solutions of (5PDI-TAP)5 in MCH shows evidence of charge separation occurring with tau < or = 150 fs between adjacent stacked members of 5PDI-TAP within the pentamer followed by charge recombination with tau = 860 ps. Transmission electron microscopy of 5PDI-TAP films cast from solution show isolated bundles of columnar aggregates. (5PDI-TAP)n is a potentially useful material for organic photovoltaics because efficient photoinduced charge generation is an intrinsic property of the assembly.

Combining Light-Harvesting and Charge Separation in a Self-Assembled Artificial Photosynthetic System Based on Perylenediimide Chromophores

Self-assembly of robust perylenediimide chromophores is used to produce an artificial light-harvesting antenna structure that in turn induces self-assembly of a functional special pair that undergoes ultrafast, quantitative charge separation. The structure consists of four 1,7-(3',5'-di-tert-butylphenoxy)perylene-3,4:9,10-perylene-3,4:9,10-bis(carboximide) (PDI) molecules attached to a single 1,7-bis(pyrrolidin-1-yl)perylene-3,4:9,10-perylene-3,4:9,10-bis(carboximide) (5PDI) core, which self-assembles to form (5PDI-PDI4)2 in toluene. The system is characterized using both structural methods (NMR, SAXS, mass spectroscopy, and GPC) and photophysical methods (UV-vis, time-resolved fluorescence, and femtosecond transient absorption spectroscopy). Energy transfer from (PDI)2 to (5PDI)2 occurs with tau = 21 ps, followed by excited-state symmetry breaking of 1*(5PDI)2 to produce 5PDI*+-5PDI*- quantitatively with tau = 7 ps. The ion pair recombines with tau = 420 ps. Electron transfer occurs only in the dimeric system and does not occur in the disassembled monomer, thus mimicking both antenna and special pair function in photosynthesis.

Excited-state symmetry breaking in cofacial and linear dimers of a green perylenediimide chlorophyll analogue leading to ultrafast charge separation

Photoexcitation of chromophoric dimers constrained to a symmetric pi-stacked geometry by their molecular structure usually produces excimers independent of solvent polarity, while dimers with edge-to-edge perpendicular pi systems undergo excited-state symmetry breaking in highly polar solvents leading to intradimer charge separation. We present direct evidence for symmetry breaking in the lowest excited singlet state of a symmetric cofacial dimer of 1,7-bis(pyrrolidin-1'-yl)-perylene-3,4:9,10-bis(dicarboximide) (5PDI) in the low polarity solvent toluene to produce a radical ion pair quantitatively. This dimer, cof-5PDI2, was synthesized by attaching two 5PDI chromophores via imide groups to a xanthene spacer. For comparison, a linear symmetric dimer, lin-5PDI2, was prepared in which the 5PDI chromophores are linked end-to-end via a N-N single bond between their imides. The edge-to-edge pi systems of the 5PDI chromophores within lin-5PDI2 are perpendicular to one another. Ground-state absorption spectra of both 5PDI dimers show exciton coupling, which is consistent with the orientation of the 5PDI chromophores relative to one another. Ultrafast transient absorption spectroscopy following excitation of the dimers with 700 nm, 100 fs laser pulses shows that quantitative intradimer electron transfer occurs in cof-5PDI2 in toluene with tau = 0.17 ps followed by charge recombination to the ground state with tau = 222 ps. Similar measurements on lin-5PDI2 reveal that photoinduced electron transfer does not occur in toluene, but occurs in more polar solvents such as 2-methyltetrahydrofuran, wherein tau = 55 ps for charge separation and tau = 99 ps for charge recombination. Excited-state symmetry breaking in 5PDI dimers provides new routes to biomimetic charge separation and storage assemblies that can be more easily prepared and modified than those based on multiple tetrapyrrole macrocycles.

Modeling the bacterial photosynthetic reaction center. 4. The structural, electrochemical, and hydrogen-bonding properties of 22 mutants of Rhodobacter sphaeroides

Site-directed mutagenesis has been employed by a number of groups to produce mutants of bacterial photosynthetic reaction centers, with the aim of tuning their operation by modifying hydrogen-bond patterns in the close vicinity of the "special pair" of bacteriochlorophylls P identical with P(L)P(M). Direct X-ray structural measurements of the consequences of mutation are rare. Attention has mostly focused on effects on properties such as carbonyl stretching frequencies and midpoint potentials to infer indirectly the induced structural modifications. In this work, the structures of 22 mutants of Rhodobacter sphaeroides have been calculated using a mixed quantum-mechanical molecular-mechanical method by modifying the known structure of the wild type. We determine (i) the orientation of the 2a-acetyl groups in the wild type, FY(M197), and FH(M197) series mutants of the neutral and oxidized reaction center, (ii) the structure of the FY(M197) mutant and possible water penetration near the special pair, (iii) that significant protein chain distortions are required to assemble some M160 series mutants (LS(M160), LN(M160), LQ(M160), and LH(M160) are considered), (iv) that there is competition for hydrogen-bonding between the 9-keto and 10a-ester groups for the introduced histidine in LH(L131) mutants, (v) that the observed midpoint potential of P for HL(M202) heterodimer mutants, including one involving also LH(M160), can be correlated with the change of electrostatic potential experienced at P(L), (vi) that hydrogen-bond cleavage may sometimes be induced by oxidation of the special pair, (vii) that the OH group of tyrosine M210 points away from P(M), and (viii) that competitive hydrogen-bonding effects determine the change in properties of NL(L166) and NH(L166) mutants. A new technique is introduced for the determination of ionization energies at the Koopmans level from QM/MM calculations, and protein-induced Stark effects on vibrational frequencies are considered.

Noise breaking the twofold symmetry of photosynthetic reaction centers: electron transfer

In this work we present a stochastic model to elucidate the unidirectionality of the primary charge separation process in the bacterial reaction centers where two symmetric ways of electron transfer (ET), starting from the common electron donor, are possible. We have used a model of three sites/molecules with ET beginning at site 1 with the option to proceed to site 2 or site 3. If the direct ET between sites 2 and 3 is not allowed and electron cannot escape from the system then it is shown that the different stochastic fluctuations in the energy of sites and the interaction between sites on these two ways are sufficient to cause the transient asymmetric electron distribution at site 2 and 3 during relaxation to the steady state. This means that overall asymmetric ET can be caused by the transient asymmetric electron distribution if there is a possibility for an electron to escape from the three-site system. To explore this possibility we have introduced a sink into the model at the end of each of the sites 2 and 3. The dependence of the asymmetry in electron transfer on the value of the sink parameter, introduced through an additional imaginary diagonal matrix element of the Hamiltonian, was investigated. Results show indeed that the unidirectionality of the electron transfer generated in the system of three molecules depends strongly on the sink parameter value.

Modeling the bacterial photosynthetic reaction center 3: interpretation of effects of site-directed mutagenesis on the special-pair midpoint potential

Interpretation of changes in midpoint potential of the "special pair" in bacterial photosynthetic reaction centers caused by site-directed mutagenesis is discussed in terms of a simple tight-binding model which relates them to concomitant variations in spin distribution between the two bacteriochlorophyll molecules of the special pair. Our analysis improves on previous similar ones by Allen and co-workers [Artz, K., Williams, J. C., Allen, J. P., Lendzian, F., Rautter, J., and Lubitz, W. (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 13582; Ivancich, A., Artz, K., Williams, J. C., Allen, J. P., and Mattioli, T. A. (1998) Biochemistry 37, 11812] in that it is both more complete, including electron-phonon coupling, and more accurate. It is applied to analyze data for a series of M160 mutants of Rhodobacter sphaeroides, yielding a value of 0.18+/-0.03 eV for the electronic coupling energy between the highest occupied levels of the two bacteriochlorophylls in the wild-type and a value of the energy offset E(o) between the highest occupied molecular orbitals of the L and M bacteriochlorophylls of 0.14+/-0.03 eV. For a mutant in which the electron hole in the special pair cation is located entirely on the reactive (L) side, a potential of 641+/-30 mV with respect to the normal hydrogen electrode is predicted. This agrees well with the average value ca. 650 mV observed for the heterodimer mutant HL(M202) in which the bacteriochlorophyll on the unreactive M side has been replaced by a bacteriopheophytin, causing extensive charge localization. However, the deduced coupling is found to be very sensitive to small changes in the assumptions used in the model, and various important chemical effects remain to be included.

Relationship between altered structure and photochemistry in mutant reaction centers in which bacteriochlorophyll replaces the photoactive bacteriopheophytin

Qy-excitation resonance Raman (RR) spectra are reported for two mutant reaction centers (RCs) from Rhodobacter capsulatus in which the photoactive bacteriopheophytin (BPhL) is replaced by a bacteriochlorophyll (BChl) molecule, designated beta. The pigment change in both mutants is induced via introduction of a histidine residue near the photoactive cofactor. In one mutant, L(M212)H, the histidine is positioned over the core of the cofactor and serves as an axial ligand to the Mg+2 ion. In the other mutant, F(L121)H/F(L97)V, the histidine is positioned over ring V of the cofactor, which is nominally too distant to permit bonding to the Mg+2 ion. The salient observations are as follows: (1) The beta cofactor in F(L121)H/F(L97)V RCs is a five-coordinate BChl molecule. However, there is no evidence for the formation of a Mg-His bond. This bond is either much weaker than in the L(M212)H RCs or completely absent, the latter implying coordination by an alternative ligand. The different axial ligation for beta in the F(L121)H/F(L97)V versus L(M212)H RCs in turn leads to different conformations of the BChl macrocycles. (2) The C9-keto group of beta in F(L121)H/F(L97)V RCs is free of hydrogen bonding interactions, unlike the L(M212)H RCs in which the C9-keto of beta is hydrogen bonded to Glu L104. The interactions between other peripheral substituents of beta and the protein are also different in the F(L121)H/F(L97)V RCs versus L(M212)H RCs. Accordingly, the position and orientation of beta in the protein is different in the two beta-containing RCs. Nonetheless, previous studies have shown that the primary electron transfer reactions are very similar in the two mutants but differ in significant respects compared to wild-type RCs. Collectively, these observations indicate that changes in the conformation of a photoactive tetrapyrrole macrocycle or its interactions with the protein do not necessarily lead to significantly perturbed photochemistry and do not underlie the altered primary events in beta-type RCs.

Primary charge separation routes in the BChl:BPhe heterodimer reaction centers of Rhodobacter sphaeroides

Energy transfer and the primary charge separation process are studied as a function of excitation wavelength in membrane-bound reaction centers of Rhodobacter sphaeroides in which the excitonically coupled bacteriochlorophyll homodimer is converted to a bacteriochlorophyll-bacteriopheophytin heterodimer, denoted D [Bylina, E. J., and Youvan, D. C. (1988) Proc. Natl. Acad. Sci. U.S. A. 85, 7226]. In the HM202L heterodimer reaction center, excitation of D using 880 nm excitation light results in a 43 ps decay of the excited heterodimer, D. The decay of D results for about 30% in the formation of the charge separated state D+QA- and for about 70% in a decay directly to the ground state. Upon excitation of the monomeric bacteriochlorophylls using 798 nm excitation light, approximately 60% of the excitation energy is transferred downhill to D, forming D. Clear evidence is obtained that the other 40% of the excitations results in the formation of D+QA- via the pathway BA --> BA+HA- --> D+HA- --> D+QA-. In the membrane-bound "reversed" heterodimer reaction center HL173L, the simplest interpretation of the transient absorption spectra following B excitation is that charge separation occurs solely via the slow D-driven route. However, since a bleach at 812 nm is associated with the spectrum of D in the HL173L reaction center, it cannot be excluded that a state including BB is involved in the charge separation process in this complex.

Effects of hydrogen bonds on the redox potential and electronic structure of the bacterial primary electron donor

The primary donor, P, of photosynthetic bacterial reaction centers (RCs) is a dimer of excitonically interacting bacteriochlorophyll (BChl) molecules. The two constituents are named PL and PM to designate their close association with the L- and M-subunits, respectively, of the RC protein. A series of site-directed mutants of RCs from Rhodobacter sphaeroides has been constructed in order to model the effects of hydrogen bonding on the redox midpoint potential and electronic structure of P. The leucine residue at position M160 was genetically replaced with eight other amino acid residues capable of donating a hydrogen bond to the C9 keto carbonyl group of the PM BChl a molecule of P. Fourier transform (FT) (pre)resonance Raman spectroscopy with 1064 nm excitation was used to (i) determine the formation and strengths of hydrogen bonds on this latter keto carbonyl group in the reduced, neutral state (PO), and (ii) determine the degree of localization of the positive charge on one of the two constituent BChl molecules of P in its oxidized, radical cation state (P*+). A correlation was observed between the strength of the hydrogen bond and the increase in PO/P*+ redox midpoint potential. This correlation is less pronounced than that observed for another series of RC mutants where hydrogen bonds to the four pi-conjugated carbonyl groups of P were broken or formed uniquely involving histidinyl residues [Mattioli, T. A., Lin, X., Allen, J. P. and Williams, J. C. (1995) Biochemistry 34, 6142-6152], indicating that histidinyl residues are more effective in raising the PO/P*+ redox midpoint potential via hydrogen bond formation than are other hydrogen bond-forming residues. In addition, an increase in positive charge localization is correlated with the strength of the hydrogen bond and with the PO/P*+ redox midpoint potential. This latter correlation was analyzed using an asymmetric bacteriochlorophyll dimer model based on Hückel-type molecular orbitals in order to obtain estimates of certain energetic parameters of the primary donor. Based on this model, the correlation is extrapolated to the case of complete localization of the positive charge on PL and gives a predicted value for the P/P+ redox midpoint potential similar to that experimentally determined for the Rb. sphaeroides HL(M202) heterodimer. The model yields parameters for the highest occupied molecular orbital energies of the two BChl a constituents of P which are typical for the oxidation potential of isolated BChl a in vitro, suggesting that the protein, as compared to many solvents, does not impart atypical redox properties to the BChl a constituents of P.

Relationship between the oxidation potential and electron spin density of the primary electron donor in reaction centers from Rhodobacter sphaeroides

The primary electron donor in bacterial reaction centers is a dimer of bacteriochlorophyll a molecules, labeled L or M based on their proximity to the symmetry-related protein subunits. The electronic structure of the bacteriochlorophyll dimer was probed by introducing small systematic variations in the bacteriochlorophyll-protein interactions by a series of site-directed mutations that replaced residue Leu M160 with histidine, tyrosine, glutamic acid, glutamine, aspartic acid, asparagine, lysine, and serine. The midpoint potentials for oxidation of the dimer in the mutants showed an almost continuous increase up to approximately 60 mV compared with wild type. The spin density distribution of the unpaired electron in the cation radical state of the dimer was determined by electron-nuclear-nuclear triple resonance spectroscopy in solution. The ratio of the spin density on the L side of the dimer to the M side varied from approximately 2:1 to approximately 5:1 in the mutants compared with approximately 2:1 for wild type. The correlation between the midpoint potential and spin density distribution was described using a simple molecular orbital model, in which the major effect of the mutations is assumed to be a change in the energy of the M half of the dimer, providing estimates for the coupling and energy levels of the orbitals in the dimer. These results demonstrate that the midpoint potential can be fine-tuned by electrostatic interactions with amino acids near the dimer and show that the properties of the electronic structure of a donor or acceptor in a protein complex can be directly related to functional properties such as the oxidation-reduction midpoint potential.