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

Time-resolved FTIR difference spectroscopy for the study of photosystem I with high potential naphthoquinones incorporated into the A1 binding site

Time-resolved step-scan Fourier transform infrared difference spectroscopy has been used to study cyanobacterial photosystem I photosynthetic reaction centers from Synechocystis sp. PCC 6803 (S6803) with four high-potential, 1,4-naphthoquinones incorporated into the A₁ binding site. The high-potential naphthoquinones are 2-chloro-, 2-bromo-, 2,3-dichloro- and 2,3-dibromo-1,4-naphthoquinone. "Foreign minus native" double difference spectra (DDS) were constructed by subtracting difference spectra for native photosystem I (with phylloquinone in the A₁ binding site) from corresponding spectra obtained using photosystem I with the different quinones incorporated. To help assess and assign bands in the difference and double difference spectra, density functional theory based vibrational frequency calculations for the different quinones in solvent, or in the presence of a single asymmetric H- bond to either a water molecule or a peptide backbone NH group, were undertaken. Calculated and experimental spectra agree best for the peptide backbone asymmetrically H- bonded system. By comparing multiple sets of double difference spectra, several new bands for the native quinone (phylloquinone) are identified. By comparing calculated and experimental spectra we conclude that the mono-substituted halogenated NQs can occupy the binding site in either of two different orientations, with the chlorine or bromine atom being either ortho or meta to the H- bonded CO group.

X-ray structure of the Rhodobacter sphaeroides reaction center with an M197 Phe→His substitution clarifies the properties of the mutant complex

The first steps of the global process of photosynthesis take place in specialized membrane pigment-protein complexes called photosynthetic reaction centers (RCs). The RC of the photosynthetic purple bacterium Rhodobacter sphaeroides, a relatively simple analog of the more complexly organized photosystem II in plants, algae and cyanobacteria, serves as a convenient model for studying pigment-protein interactions that affect photochemical processes. In bacterial RCs the bacteriochlorophyll (BChl) dimer P serves as the primary electron donor, and its redox potential is a critical factor in the efficient functioning of the RC. It has previously been shown that the replacement of Phe M197 by His strongly affects the oxidation potential of P (E m P/P+), increasing its value by 125 mV, as well as increasing the thermal stability of RC and its stability in response to external pressure. The crystal structures of F(M197)H RC at high resolution obtained using various techniques presented in this report clarify the optical and electrochemical properties of the primary electron donor and the increased resistance of the mutant complex to denaturation. The electron-density maps are consistent with the donation of a hydrogen bond from the imidazole group of His M197 to the C2-acetyl carbonyl group of BChl PB. The formation of this hydrogen bond leads to a considerable out-of-plane rotation of the acetyl carbonyl group and results in a 1.2 Å shift of the O atom of this group relative to the wild-type structure. Besides, the distance between BChl PA and PB in the area of pyrrole ring I was found to be increased by up to 0.17 Å. These structural changes are discussed in association with the spectral properties of BChl dimer P. The electron-density maps strongly suggest that the imidazole group of His M197 accepts another hydrogen bond from the nearest water molecule, which in turn appears to form two more hydrogen bonds to Asn M195 and Asp L155. As a result of the F(M197)H mutation, BChl PB finds itself connected to the extensive hydrogen-bonding network that pre-existed in wild-type RC. Dissimilarities in the two hydrogen-bonding networks near the M197 and L168 sites may account for the different changes of the E m P/P+ in F(M197)H and H(L168)F RCs. The involvement of His M197 in the hydrogen-bonding network also appears to be related to stabilization of the F(M197)H RC structure. Analysis of the experimental data presented here and of the data available in the literature points to the fact that the hydrogen-bonding networks in the vicinity of BChl dimer P may play an important role in fine-tuning the redox properties of the primary electron donor.

Genetically introduced hydrogen bond interactions reveal an asymmetric charge distribution on the radical cation of the special-pair chlorophyll P680

The special-pair chlorophyll (Chl) P680 in photosystem II has an extremely high redox potential (E_m ) to enable water oxidation in photosynthesis. Significant positive-charge localization on one of the Chl constituents, P_D1 or P_D2, in P680⁺ has been proposed to contribute to this high E_m To identify the Chl molecule on which the charge is mainly localized, we genetically introduced a hydrogen bond to the 13¹-keto C=O group of P_D1 and P_D2 by changing the nearby D1-Val-157 and D2-Val-156 residues to His, respectively. Successful hydrogen bond formation at P_D1 and P_D2 in the obtained D1-V157H and D2-V156H mutants, respectively, was monitored by detecting 13¹-keto C=O vibrations in Fourier transfer infrared (FTIR) difference spectra upon oxidation of P680 and the symmetrically located redox-active tyrosines Y_Z and Y_D, and they were simulated by quantum-chemical calculations. Analysis of the P680⁺/P680 FTIR difference spectra of D1-V157H and D2-V156H showed that upon P680⁺ formation, the 13¹-keto C=O frequency upshifts by a much larger extent in P_D1 (23 cm^-1) than in P_D2 (<9 cm^-1). In addition, thermoluminescence measurements revealed that the D1-V157H mutation increased the E_m of P680 to a larger extent than did the D2-V156H mutation. These results, together with the previous results for the mutants of the His ligands of P_D1 and P_D2, lead to a definite conclusion that a charge is mainly localized to P_D1 in P680<sup/>.

Dissecting Proton Delocalization in an Enzyme's Hydrogen Bond Network with Unnatural Amino Acids

Extended hydrogen bond networks are a common structural motif of enzymes. A recent analysis proposed quantum delocalization of protons as a feature present in the hydrogen bond network spanning a triad of tyrosines (Y(16), Y(32), and Y(57)) in the active site of ketosteroid isomerase (KSI), contributing to its unusual acidity and large isotope shift. In this study, we utilized amber suppression to substitute each tyrosine residue with 3-chlorotyrosine to test the delocalization model and the proton affinity balance in the triad. X-ray crystal structures of each variant demonstrated that the structure, notably the O-O distances within the triad, was unaffected by 3-chlorotyrosine substitutions. The changes in the cluster's acidity and the acidity's isotope dependence in these variants were assessed via UV-vis spectroscopy and the proton sharing pattern among individual residues with (13)C nuclear magnetic resonance. Our data show pKa detuning at each triad residue alters the proton delocalization behavior in the H-bond network. The extra stabilization energy necessary for the unusual acidity mainly comes from the strong interactions between Y(57) and Y(16). This is further enabled by Y(32), which maintains the right geometry and matched proton affinity in the triad. This study provides a rich picture of the energetics of the hydrogen bond network in enzymes for further model refinement.

The L(M196)H mutation in Rhodobacter sphaeroides reaction center results in new electrostatic interactions

New histidine residue was introduced in M196 position in the reaction center of Rhodobacter sphaeroides in order to alter polarity of the BChl dimer's protein environment and to study how it affects properties and structure of the primary electron donor P. It was shown that in the absorption spectrum of the mutant RC the 6 nm red shift of the Q Y P band was observed together with considerable decrease of its amplitude. The mid-point potential of P/P (+) in the mutant RC was increased by +65 (±15) mV as compared to the E m P/P (+) value in the wild-type RC suggesting that the mutation resulted in new pigment-protein interactions. Crystal structure of RC L(M196)H determined at 2.4 Å resolution implies that BChl Р В and introduced histidine-M196 organize new electrostatic contact that may be specified either as π-π staking or as hydrogen-π interaction. Besides, the structure of the mutants RC shows that His-M196 apparently became involved in hydrogen bond network existing in BChl Р В vicinity that may favor stability of the mutant RC.

Vibrational techniques applied to photosynthesis: Resonance Raman and fluorescence line-narrowing

Resonance Raman spectroscopy may yield precise information on the conformation of, and the interactions assumed by, the chromophores involved in the first steps of the photosynthetic process. Selectivity is achieved via resonance with the absorption transition of the chromophore of interest. Fluorescence line-narrowing spectroscopy is a complementary technique, in that it provides the same level of information (structure, conformation, interactions), but in this case for the emitting pigment(s) only (whether isolated or in an ensemble of interacting chromophores). The selectivity provided by these vibrational techniques allows for the analysis of pigment molecules not only when they are isolated in solvents, but also when embedded in soluble or membrane proteins and even, as shown recently, in vivo. They can be used, for instance, to relate the electronic properties of these pigment molecules to their structure and/or the physical properties of their environment. These techniques are even able to follow subtle changes in chromophore conformation associated with regulatory processes. After a short introduction to the physical principles that govern resonance Raman and fluorescence line-narrowing spectroscopies, the information content of the vibrational spectra of chlorophyll and carotenoid molecules is described in this article, together with the experiments which helped in determining which structural parameter(s) each vibrational band is sensitive to. A selection of applications is then presented, in order to illustrate how these techniques have been used in the field of photosynthesis, and what type of information has been obtained. This article is part of a Special Issue entitled: Vibrational spectroscopies and bioenergetic systems.

Expression, purification, crystallization and preliminary X-ray structure analysis of wild-type and L(M196)H-mutant Rhodobacter sphaeroides reaction centres

The electron and proton transport mediated by protein-bound cofactors in photosynthesis have been investigated by various methods in order to determine the energetics, the dynamics and the pathway of this process. In purple bacteria, primary photosynthetic charge separation and the build-up of a proton gradient across the periplasmic membrane are catalyzed by the photosynthetic reaction centre (RC). Here, the purification, crystallization and preliminary X-ray analysis of wild-type and L(M196)H-mutant RCs of Rhodobacter sphaeroides are presented, enabling study of the influence of the protein environment of the primary electron donor on the spectral properties and photochemical activity of the RC.

Kinetics and energetics of electron transfer in reaction centers of the photosynthetic bacterium Roseiflexus castenholzii

The kinetics and thermodynamics of the photochemical reactions of the purified reaction center (RC)-cytochrome (Cyt) complex from the chlorosome-lacking, filamentous anoxygenic phototroph, Roseiflexus castenholzii are presented. The RC consists of L- and M-polypeptides containing three bacteriochlorophyll (BChl), three bacteriopheophytin (BPh) and two quinones (Q(A) and Q(B)), and the Cyt is a tetraheme subunit. Two of the BChls form a dimer P that is the primary electron donor. At 285K, the lifetimes of the excited singlet state, P*, and the charge-separated state P(+)H(A)(-) (where H(A) is the photoactive BPh) were found to be 3.2±0.3 ps and 200±20 ps, respectively. Overall charge separation P*→→ P(+)Q(A)(-) occurred with ≥90% yield at 285K. At 77K, the P* lifetime was somewhat shorter and the P(+)H(A)(-) lifetime was essentially unchanged. Poteniometric titrations gave a P(865)/P(865)(+) midpoint potential of +390mV vs. SHE. For the tetraheme Cyt two distinct midpoint potentials of +85 and +265mV were measured, likely reflecting a pair of low-potential hemes and a pair of high-potential hemes, respectively. The time course of electron transfer from reduced Cyt to P(+) suggests an arrangement where the highest potential heme is not located immediately adjacent to P. Comparisons of these and other properties of isolated Roseiflexus castenholzii RCs to those from its close relative Chloroflexus aurantiacus and to RCs from the purple bacteria are made.

Magic Angle Spinning (MAS) NMR: a new tool to study the spatial and electronic structure of photosynthetic complexes

In the last two decades, Magic Angle Spinning (MAS) NMR has created its own niche in studies involving photosynthetic membrane protein complexes, owing to its ability to provide structural and functional information at atomic resolution of membrane proteins when in the membrane, in the natural environment. The light-harvesting two (LH2) transmembrane complex from Rhodopseudomonas acidophila is used to illustrate the procedure of the technique applicable in photosynthesis research. One- and two-dimensional solid-state NMR experiments involving (13)C- and (15)N-labeled LH2 complexes allow to make a sequence-specific assignment of NMR signals, which forms the basis for resolving structural details and the assessment of charge transfer, electronic delocalization effects, and functional strain in the ground state.

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.

Increasing efficiency of photoelectronic conversion by encapsulation of photosynthetic reaction center proteins in arrayed carbon nanotube electrode

The construction of efficient light energy converting (photovoltaic and photoelectronic) devices is a current and great challenge in science and technology and one that will have important economic consequences. Here we show that the efficiency of these devices can be improved by the utilization of a new type of nano-organized material having photosynthetic reaction center proteins encapsulated inside carbon nanotube arrayed electrodes. In this work, a generically engineered bacterial photosynthetic reaction center protein with specifically synthesized organic molecular linkers were encapsulated inside carbon nanotubes and bound to the inner tube walls in unidirectional orientation. The results show that the photosynthetic proteins encapsulated inside carbon nanotubes are photochemically active and exhibit considerable improvement in the rate of electron transfer and the photocurrent density compared to the material constructed from the same components in traditional lamella configuration.

Redox characteristics of a de novo quinone protein

The electrochemistry of 2,6-dimethylbenzoquinone (DMBQ) has been characterized for three different systems: DMBQ freely solvated in aqueous buffer; DMBQ bound to a neutral, blocked cysteine (N-acetyl-L-cysteine methyl ester) and the resulting DMBQ-bCys compound solvated in aqueous buffer; and DMBQ bound to a small model protein denoted alpha(3)C. The goal of this study is to detect and characterize differences in the redox properties of the protein-ligated DMBQ relative to the solvated quinones. The alpha(3)C protein used here is a tryptophan-32 to cysteine-32 variant of the structurally defined alpha(3)W de novo protein (Dai et al. J. Am. Chem. Soc. 2002, 124, 10952-10953). The properties of alpha(3)C were recently described (Hay et al. Biochemistry 2005, 44, 11891-11902). DMBQ was covalently bound to bCys and alpha(3)C through a sulfur substitution reaction with the cysteine thiol. In contrast to the solvated DMBQ and DMBQ-bCys compounds, diffusion controlled electrochemistry of DMBQ-alpha(3)C showed well-behaved and fully reversible n = 2 oxidation/reduction with a peak separation of approximately 30 mV between pH 5 and 9. DMBQ-alpha(3)C could also be immobilized on a gold electrode modified with a self-assembled monolayer of 3-mercaptopropionoic acid, allowing the measurement, by cyclic voltammetry, of an apparent rate of electron transfer of 22 s(-1). The (cysteine) sulfur substitution significantly lowers one of the hydroquinone pKA's from 10.4 in DMBQ to 6.8 in DMBQ-bCys. This pKA is slightly elevated in DMBQ-alpha(3)C to 7.0 and the E1/2 at pH 7.0 is raised by 110 mV from +190 mV in DMBQ-bCys to +297 mV in DMBQ-alpha(3)C.

Charge Delocalization in the Special-Pair Radical Cation of Mutant Reaction Centers of Rhodobacter sphaeroides from Stark Spectra and Nonadiabatic Spectral Simulations

Stark and absorption spectra for the hole-transfer band of the bacteriochlorophyll special pair in the wild-type and L131LH, M160LH, and L131LH/M160LH mutants of the bacterial reaction center of Rhodobacter sphaeroides are presented, along with extensive analyses based on nonadiabatic spectral simulations. Dramatic changes in the Stark spectra are induced by the mutations, changes that are readily interpreted in terms of the redox-energy asymmetry and degree of charge localization in the special-pair radical cation. The effect of mutagenesis on key properties such as the electronic coupling within the special pair and the reorganization energy associated with intervalence hole transfer are determined for the first time. Results for the L131LH and M160LH/L131LH mutants indicate that these species can be considered as influencing the special pair primarily through modulation of the redox asymmetry, as is usually conceptualized, but M160LH is shown to develop a wide range of effects that can be interpreted in terms of significant mutation-induced structural changes in and around the special pair. The nonadiabatic spectra simulations are performed using both a simple two-state 1-mode and an extensive four-state 70-mode model, which includes the descriptions of additional electronic states and explicitly treats the major vibrational modes involved. Excellent agreement between the two simulation approaches is obtained. The simple model is shown to reproduce key features of the Stark effect of the main intervalence transition, while the extensive model quantitatively reproduces most features of the observed spectra for both the electronic and the phase-phonon regions, thus giving a more comprehensive description of the effect of the mutations on the properties of the special-pair radical cation. These results for a series of closely related mixed-valence complexes show that the Stark spectra provide a sensitive indicator for the properties of the mixed-valence complexes and should serve as an instructive example on the application of nonadiabatic simulations to the study of mixed-valence complexes in general as well as other chemical systems akin to the photosynthetic special pair.

Structural Role of (Bacterio)chlorophyll Ligated in the Energetically Unfavorable β-Position

Chlorophyll is attached to apoprotein in diastereotopically distinct ways, by beta- and alpha-ligation. Both the beta- and alpha-ligated chlorophylls of photosystem I are shown to have ample contacts to apoprotein within their proteinaceous binding sites, in particular, at C-13 of the isocyclic ring. The H-bonding patterns for the C-13(1) oxo groups, however, are clearly distinct for the beta-ligated and alpha-ligated chlorophylls. The beta-ligated chlorophylls frequently employ their C-13(1) oxo in H-bonds to neighboring helices and subunits. In contrast, the C-13(1) oxo of alpha-ligated chlorophylls are significantly less involved in H-bonding interactions, particularly to neighboring helices. Remarkably, in the peripheral antenna, light harvesting complex (LH2) from Rhodobacter sphaeroides, a single mutation in the alpha-subunit, introduced to eliminate H-bonding to the beta-bacteriochlorophyll-B850, which is ligated in the "beta-position," results in significant thermal destabilization of the LH2 in the membrane. In addition, in comparison with wild type LH2, the expression level of the LH2 lacking this H-bond is significantly reduced. These findings show that H-bonding to the C-13(1) keto group ofbeta-ligated (bacterio)-chlorophyll is a key structural motif and significantly contributes to the stability of bacteriochlorophyll proteins in the native membrane. Our analysis of photosystem I and II suggests that this hitherto unrecognized motif involving H-bonding to beta-ligated chlorophylls may be equally critical for the stable assembly of the inner core antenna of these multicomponent chlorophyll proteins.

Strong Effects of an Individual Water Molecule on the Rate of Light-driven Charge Separation in the Rhodobacter sphaeroides Reaction Center

The role of a water molecule (water A) located between the primary electron donor (P) and first electron acceptor bacteriochlorophyll (B(A)) in the purple bacterial reaction center was investigated by mutation of glycine M203 to leucine (GM203L). The x-ray crystal structure of the GM203L reaction center shows that the new leucine residue packs in such a way that water A is sterically excluded from the complex, but the structure of the protein-cofactor system around the mutation site is largely undisturbed. The results of absorbance and resonance Raman spectroscopy were consistent with either the removal of a hydrogen bond interaction between water A and the keto carbonyl group of B(A) or a change in the local electrostatic environment of this carbonyl group. Similarities in the spectroscopic properties and x-ray crystal structures of reaction centers with leucine and aspartic acid mutations at the M203 position suggested that the effects of a glycine to aspartic acid substitution at the M203 position can also be explained by steric exclusion of water A. In the GM203L mutant, loss of water A was accompanied by an approximately 8-fold slowing of the rate of decay of the primary donor excited state, indicating that the presence of water A is important for optimization of the rate of primary electron transfer. Possible functions of this water molecule are discussed, including a switching role in which the redox potential of the B(A) acceptor is rapidly modulated in response to oxidation of the primary electron donor.

Effect of protein orientation on electron transfer between photosynthetic reaction centers and carbon electrodes

A new type of monolayer of photosynthetic reaction centers (RC) with the primary donor facing the carbon electrode has been constructed using a new bifunctional linker and genetically engineered protein. Comparison of protein in two different orientations with linkers binding to the opposite sides of the protein demonstrates the possibility of utilizing the constructed surfaces as photoelectronic devices. The results show improvement of the electron transfer efficiency when RC is bound with the primary donor (P) facing the electrode (P-side). In either protein orientation, electron transfer within the protein is unidirectional and when applying a voltage RC operates as a photorectifier. Electron transfer between the protein and carbon electrodes in the constructed devices is most likely occurring by tunneling.

Phylogenetic analyses of the core antenna domain: investigating the origin of photosystem I

Phototrophy, the conversion of light to biochemical energy, occurs throughout the Bacteria and plants, however, debate continues over how different phototrophic mechanisms and the bacteria that contain them are related. There are two types of phototrophic mechanisms in the Bacteria: reaction center type 1 (RC1) has core and core antenna domains that are parts of a single polypeptide, whereas reaction center type 2 (RC2) is composed of short core proteins without antenna domains. In cyanobacteria, RC2 is associated with separate core antenna proteins that are homologous to the core antenna domains of RC1. We reconstructed evolutionary relationships among phototrophic mechanisms based on a phylogeny of core antenna domains/proteins. Core antenna domains of 46 polypeptides were aligned, including the RC1 core proteins of heliobacteria, green sulfur bacteria, and photosystem I (PSI) of cyanobacteria and plastids, plus core antenna proteins of photosystem II (PSII) from cyanobacteria and plastids. Maximum likelihood, parsimony, and neighbor joining methods all supported a single phylogeny in which PSII core antenna proteins (PsbC, PsbB) arose within the cyanobacteria from duplications of the RC1-associated core antenna domains and accessory antenna proteins (IsiA, PcbA, PcbC) arose from duplications of PsbB. The data indicate an evolutionary history of RC1 in which an initially homodimeric reaction center was vertically transmitted to green sulfur bacteria, heliobacteria, and an ancestor of cyanobacteria. A heterodimeric RC1 (=PSI) then arose within the cyanobacterial lineage. In this scenario, the current diversity of core antenna domains/proteins is explained without a need to invoke horizontal transfer.

Protein engineering of cytochrome b562 for quinone binding and light-induced electron transfer

The central photochemical reaction in photosystem II of green algae and plants and the reaction center of some photosynthetic bacteria involves a one-electron transfer from a light-activated chlorin complex to a bound quinone molecule. Through protein engineering, we have been able to modify a protein to mimic this reaction. A unique quinone-binding site was engineered into the Escherichia coli cytochrome b(562) by introducing a cysteine within the hydrophobic interior of the protein. Various quinones, such as p-benzoquinone and 2,3-dimethoxy-5-methyl-1,4-benzoquinone, were then covalently attached to the protein through a cysteine sulfur addition reaction to the quinone ring. The cysteine placement was designed to bind the quinone approximately 10 A from the edge of the bound porphyrin. Fluorescence measurements confirmed that the bound hydroquinone is incorporated toward the protein's hydrophobic interior and is partially solvent-shielded. The bound quinones remain redox-active and can be oxidized and rereduced in a two-electron process at neutral pH. The semiquinone can be generated at high pH by a one-electron reduction, and the midpoint potential of this can be adjusted by approximately 500 mV by binding different quinones to the protein. The heme-binding site of the modified cytochrome was then reconstituted with the chlorophyll analogue zinc chlorin e(6). By using EPR and fast optical techniques, we show that, in the various chlorin-protein-quinone complexes, light-induced electron transfer can occur from the chlorin to the bound oxidized quinone but not the hydroquinone, with electron transfer rates in the order of 10(8) s(-1).

A Unified Description of the Electrochemical, Charge Distribution, and Spectroscopic Properties of the Special-Pair Radical Cation in Bacterial Photosynthesis

We apply our four-state 70-vibration vibronic-coupling model for the properties of the photosynthetic special-pair radical cation to: (1) interpret the observed correlations between the midpoint potential and the distribution of spin density between the two bacteriochlorophylls for 30 mutants of Rhodobacter sphaeroides, (2) interpret the observed average intervalence hole-transfer absorption energies as a function of spin density for six mutants, and (3) simulate the recently obtained intervalence electroabsorption Stark spectrum of the wild-type reaction center. While three new parameters describing the location of the sites of mutation with respect to the special pair are required to describe the midpoint-potential data, a priori predictions are made for the transition energies and the Stark spectrum. In general, excellent predictions are made of the observed quantities, with deviations being typically of the order of twice the experimental uncertainties. A unified description of many chemical and spectroscopic properties of the bacterial reaction center is thus provided. Central to the analysis is the assumption that the perturbations made to the reaction center, either via mutations of protein residues or by application of an external electric field, act only to independently modify the oxidation potentials of the two halves of the special pair and hence the redox asymmetry E0. While this appears to be a good approximation, clear evidence is presented that effects of mutation can be more extensive than what is allowed for. A thorough set of analytical equations describing the observed properties is obtained using the Born-Oppenheimer adiabatic approximation. These equations are generally appropriate for intervalence charge-transfer problems and include, for the first time, full treatment of both symmetric and antisymmetric vibrational motions. The limits of validity of the adiabatic approach to the full nonadiabatic problem are obtained.

Hydrogen bonding in a model bacteriochlorophyll-binding site drives assembly of light harvesting complex

In this study, the contribution of intramembrane hydrogen bonding at the interface between polypeptide and cofactor is explored in the native lipid environment by use of model bacteriochlorophyll proteins. In the peripheral antenna complex, LH2, large portions of the transmembrane helices, which make up the dimeric bacteriochlorophyll-binding site, are replaced by simplified, alternating alanine-leucine stretches. Replacement of either one of the two helices with the helices containing the model sequence at a time results in the assembly of complexes with nearly native light harvesting properties. In contrast, replacement of both helices results in the loss of antenna complexes from the membrane. The assembly of such doubly modified complexes is restored by a single intramembrane serine residue at position -4 relative to the liganding histidine of the alpha-subunit. In situ analysis of the spectral properties in a series of site-directed mutants reveals a critical dependence of the model complex assembly on the side chain of the residue at this position in the helix. A hydrogen bond between the hydroxy group of the serine and the 13(1) keto group of one of the central bacteriochlorophylls of the complexes is identified by Raman spectroscopy in the model antenna complex containing one of the alanine-leucine helices. The additional OH group of the serine residue, which participates in hydrogen bonding, increases the thermal stability of the model complexes in the native membrane. Intramembrane hydrogen bonding is thus shown to be a key factor for the binding of bacteriochlorophyll and assembly of this model cofactor-polypeptide site.

Structural, dynamic, and energetic aspects of long-range electron transfer in photosynthetic reaction centers

Intramolecular electron transfer within proteins plays an essential role in biological energy transduction. Electron donor and acceptor cofactors are bound in the protein matrix at specific locations, and protein-cofactor interactions as well as protein conformational changes can markedly influence the electron transfer rates. To assess these effects, we have investigated charge recombination from the primary quinone acceptor to the special pair bacteriochlorophyll dimer in wild-type reaction centers of Rhodobacter sphaeroides and four mutants with widely modified free energy gaps. After light-induced charge separation, the recombination kinetics were measured in the light- and dark-adapted forms of the protein from 10 to 300 K. The data were analyzed by using the spin-boson model, which allowed us to self-consistently determine the electronic coupling energy, the distribution of energy gaps, the spectral density of phonons, and the reorganization energy. The analysis revealed slow changes of the energy gap after charge separation. Interesting correlations of the control parameters governing electron transfer were found and related to structural and dynamic properties of the protein.

LZnX complexes of tripodal ligands with intramolecular RN–H hydrogen bonding groups: structural implications of a hydrogen bonding cavity, and of X/R in the hydrogen bonding geometry/strength

Tripodal ligands N(CH2Py)3-n(CH2Py-6-NHR)n(R=H, n=1-3 L1-3, n=0 tpa; R=CH2tBu, n=1-3 L'1-3) are used to investigate the effect of different hydrogen bonding microenvironments on structural features of their LZnX complexes (X=Cl-, NO3-, OH-). The X-ray structures of [(L2)Zn(Cl)](BPh4)2.0.5(H2O.CH3CN), [(L3)Zn(Cl)](BPh4)3.CH3CN, [(L'1)Zn(Cl)](BPh4) 1', [(L'2)Zn(Cl)](BPh4)2'.CH3OH, and [(L'3)Zn(Cl)](BPh4)3' have been determined and exhibit trigonal bipyramidal geometries with intramolecular (internal) N-HCl-Zn hydrogen bonds. The structure of [(L'2)Zn(ONO2)]NO3 4'.H2O with two internal N-HO-Zn hydrogen bonds has also been determined. The axial Zn-Cl distance lengthens from 2.275 A in [(tpa)Zn(Cl)](BPh4) to 2.280-2.347 A in 1-3, 1'-3'. Notably, the average Zn-N(py) distance is also progressively lengthened from 2.069 A in [(tpa)Zn(Cl)](BPh4) to 2.159 and 2.182 A in the triply hydrogen bonding cavity of 3 and 3', respectively. Lengthening of the Zn-Cl and Zn-N(py) bonds is accompanied by a progressive shortening of the trans Zn-N bond from 2.271 A in [(tpa)Zn(Cl)](BPh4) to 2.115 A in 3 (2.113 A in 3'). As a result of the triply hydrogen bonding microenvironment the Zn-Cl and Zn-N(py) distances of 3 are at the upper end of the range observed for axial Zn-Cl bonds, whereas the axial Zn-N distance is one of shortest among N4 ligands that induce a trigonal bipyramidal geometry. Despite the rigidity of these tripodal ligands, the geometry of the intramolecular RN-HX-Zn hydrogen bonds (X=Cl-, OH-, NO3-) is strongly dependent on the nature of X, however, on average, similar for R=H, CH2tBu.

Relative importance of hydrogen bonding and coordinating groups in modulating the zinc–water acidity

The presence of second-sphere -NH(2) groups in the proximity of a zinc(ii)-bound water molecule enhances its acidity by ca. 2 pK(a) units.

Identification of intramembrane hydrogen bonding between 131 keto group of bacteriochlorophyll and serine residue α27 in the LH2 light-harvesting complex

Intramembrane hydrogen bonding and its effect on the structural integrity of purple bacterial light-harvesting complex 2, LH2, have been assessed in the native membrane environment. A novel hydrogen bond has been identified by Raman resonance spectroscopy between a serine residue of the membrane-spanning region of LH2 alpha-subunit, and the C-13(1) keto carbonyl of bacteriochlorophyll (BChl) B850 bound to the beta-subunit. Replacement of the serine by alanine disrupts this strong hydrogen bond, but this neither alters the strongly red-shifted absorption nor the structural arrangement of the BChls, as judged from circular dichroism. It also decreases only slightly the thermal stability of the mutated LH2 in the native membrane environment. The possibility is discussed that weak H-bonding between the C-13(1) keto carbonyl and a methyl hydrogen of the alanine replacing serine(-4) or the imidazole group of the nearby histidine maintains structural integrity in this very stable bacterial light-harvesting complex. A more widespread occurrence of H-bonding to C-13(1) not only in BChl, but also in chlorophyll proteins, is indicated by a theoretical analysis of chlorophyll/polypeptide contacts at <3.5 A in the high-resolution structure of Photosystem I. Nearly half of the 96 chlorophylls have aa residues suitable as hydrogen bond donors to their keto groups.

Energetics and mechanisms of high efficiency of charge separation and electron transfer processes in Rhodobacter sphaeroides reaction centers

Effects of environmental changes due to D(2)O/H(2)O substitution and cryosolvent addition on the energetics of the special pair and the rate constants of forward and back electron transfer reactions in the picosecond-nanosecond time domain have been studied in isolated reaction centers (RC) of the anaxogenic purple bacterium Rhodobacter sphaeroides. The following results were obtained: (i). replacement of H(2)O by D(2)O or addition of either 70% (v/v) glycerol or 35% (v/v) DMSO do not influence the absorption spectra; (ii). in marked contrast to this invariance of absorption, the maxima of fluorescence spectra are red shifted relative to control by 3.5, 6.8 and 14.5 nm for RCs suspended in glycerol, D(2)O or DMSO, respectively; (iii). D(2)O/H(2)O substitution or DMSO addition give rise to an increase of the time constants of charge separation (tau(e)), and Q(A)(-) formation (tau(Q)) by a factors of 2.5-3.1 and 1.7-2.5, respectively; (iv). addition of 70% glycerol is virtually without effect on the values of tau(e) and tau(Q); (v). the midpoint potential E(m) of P/P(+) is shifted by about 30 and 45 mV towards higher values by addition of 70% glycerol and 35% DMSO, respectively, but remains invariant to D(2)O/H(2)O exchange; and (vi). an additional fast component with tau(1)=0.5-0.8 ns in the kinetics of charge recombination P(+)H(A)(-)-->P*(P)H(A) emerges in RC suspensions modified either by D(2)O/H(2)O substitution or by DMSO treatment. The results have been analysed with special emphasis on the role of deformations of hydrogen bonds for the solvation mechanism of nonequilibrium states of cofactors. Reorientation of hydrogen bonds provides the major contribution of the very fast environmental response to excitation of the special pair P. The Gibbs standard free energy gap between 1P* and P(+)B(A)(-) due to solvation is estimated to be approximately 70, 59 and 48 meV for control, D(2)O- and DMSO-treated RC samples, respectively.

Tuning of the redox potential of the primary electron donor in reaction centres of purple bacteria: effects of amino acid polarity and position

Mutation of residues His L168 and Phe M197 in the reaction centre from Rhodobacter sphaeroides has an unusually strong effect on the mid-point redox potential (E(m)) of the pair of bacteriochlorophylls that form the primary donor of electrons, tuning E(m) over a range of nearly 250 mV. This effect is correlated to the accompanying change in the permanent dipole of the L168 or M197 residue, suggesting it is mediated by changes in charge-dipole interactions. Comparisons with mutations made at a variety of other positions show that this correlation is particular to this residue pair, perhaps reflecting their proximity to the ring I regions of the dimer bacteriochlorophylls that form the overlap region between these molecules.

Differentiating the orientations of photosynthetic reaction centers on Au electrodes linked by different bifunctional reagents

The photosynthetic reaction center (RC) composite film was fabricated by self-assembled monolayers (SAMs) on the Au electrode with two different bifunctional reagents, 4-aminothiophenol (ATP) and 2-mercaptoethylamine (MEA), respectively. The square wave voltametry (SWV), bulk electrolysis and photocurrent test were employed for characterizing the composite film. The dramatic different electrochemical characteristics were observed for the two types of films, which strongly suggested an orientational difference for RC arising from the structural difference between the two bifunctional reagents. For RC-MEA film, three redox peaks which implying electron transfer (ET) between the primary donor (P) and the bacteriopheophytin (Bphe) were observed. While for RC-ATP film, two redox peaks implying ET between the nonheme iron and the primary quinone (Q(A)) were observed. The ET behavior driven by electric field also supported the result that the RC could be linked to the electrode at different sites. The site-specific immobilization approach reported here supplies a method to differentiate the protein orientation.

Tuning of the optical and electrochemical properties of the primary donor bacteriochlorophylls in the reaction centre from Rhodobacter sphaeroides: spectroscopy and structure

A series of mutations have been introduced at residue 168 of the L-subunit of the reaction centre from Rhodobacter sphaeroides. In the wild-type reaction centre, residue His L168 donates a strong hydrogen bond to the acetyl carbonyl group of one of the pair of bacteriochlorophylls (BChl) that constitutes the primary donor of electrons. Mutation of His L168 to Phe or Leu causes a large decrease in the mid-point redox potential of the primary electron donor, consistent with removal of this strong hydrogen bond. Mutations to Lys, Asp and Arg cause smaller decreases in redox potential, indicative of the presence of weak hydrogen bond and/or an electrostatic effect of the polar residue. A spectroscopic analysis of the mutant complexes suggests that replacement of the wild-type His residue causes a decrease in the strength of the coupling between the two primary donor bacteriochlorophylls. The X-ray crystal structure of the mutant in which His L168 has been replaced by Phe (HL168F) was determined to a resolution of 2.5 A, and the structural model of the HL168F mutant was compared with that of the wild-type complex. The mutation causes a shift in the position of the primary donor bacteriochlorophyll that is adjacent to residue L168, and also affects the conformation of the acetyl carbonyl group of this bacteriochlorophyll. This conformational change constitutes an approximately 27 degrees through-plane rotation, rather than the large into-plane rotation that has been widely discussed in the context of the HL168F mutation. The possible structural basis of the altered spectroscopic properties of the HL168F mutant reaction centre is discussed, as is the relevance of the X-ray crystal structure of the HL168F mutant to the possible structures of the remaining mutant complexes.

A multigeneration analysis of cytochrome b(562) redox variants: evolutionary strategies for modulating redox potential revealed using a library approach

The redox potential of cytochromes sets the energy yield possible in metabolism and is also a key determinant of the rate at which redox reactions proceed. Here, the heme protein, cytochrome b(562), is used to study the in vitro evolution of redox potential within a library of variants containing the same structural archetype, the four-helix bundle. Multisite variations in the active site of cytochrome b(562) were introduced. A library of variants containing random mutations in place of R98 and R106 was created, and the redox potentials of a statistical sampling of this library were measured. This procedure was carried out for both the low- and high-potential variants of a previously studied F61X/F65X, first-generation library [Springs, S. L., Bass, S. E., and McLendon, G. L. (2000) Biochemistry 39, 6075]. The second-generation library reported here has a range of redox potentials which is greater than 40% (160 mV) of the known accessible potential among cytochromes with identical axial ligands (but different folds) and exceeds the range exhibited phylogenetically by the cytochrome c' family which internally maintains the same axial ligation and fold. A statistical analysis of the libraries examined reveals that the redox potential of WT cyt b(562) is found at the high-potential extremum of the distribution, indicating that this protein apparently evolved to differentially stabilize the reduced protein. The 2.7 A crystal structure of F61I/F65Y/R106L (low-potential variant of the second-generation library) was solved and is compared to the wild-type structure and the 2.2 A resolution structure of the F61I/F65Y variant (low-potential variant of the first-generation library). The structures indicate that charge-dipole effects are responsible for shifting the redox equilibrium toward the oxidized state in both the F61I/F65Y and F61I/F65Y/R106L variants. Specifically, a new protein dipole is introduced into the heme microenvironment as a result of the F65Y mutation, two new internal water molecules (one in hydrogen-bonding distance of Y65) are found, and in the case of F61I/F65Y/R106L (DeltaE(m) = 158 mV vs NHE), increased solvent exposure of the heme as a result of the R106L substitution is identified.

Electron spin echo envelope modulation spectroscopy in photosystem I

The applications of electron spin echo envelope modulation (ESEEM) spectroscopy to study paramagnetic centers in photosystem I (PSI) are reviewed with special attention to the novel spectroscopic techniques applied and the structural information obtained. We briefly summarize the physical principles and experimental techniques of ESEEM, the spectral shapes and the methods for their analysis. In PSI, ESEEM spectroscopy has been used to the study of the cation radical form of the primary electron donor chlorophyll species, P(700)(+), and the phyllosemiquinone anion radical, A(1)(-), that acts as a low-potential electron carrier. For P(700)(+), ESEEM has contributed to a debate concerning whether the cation is localized on a one or two chlorophyll molecules. This debate is treated in detail and relevant data from other methods, particularly electron nuclear double resonance (ENDOR), are also discussed. It is concluded that the ESEEM and ENDOR data can be explained in terms of five distinct nitrogen couplings, four from the tetrapyrrole ring and a fifth from an axial ligand. Thus the ENDOR and ESEEM data can be fully accounted for based on the spin density being localized on a single chlorophyll molecule. This does not eliminate the possibility that some of the unpaired spin is shared with the other chlorophyll of P(700)(+); so far, however, no unambiguous evidence has been obtained from these electron paramagnetic resonance methods. The ESEEM of the phyllosemiquinone radical A(1)(-) provided the first evidence for a tryptophan molecule pi-stacked over the semiquinone and for a weaker interaction from an additional nitrogen nucleus. Recent site-directed mutagenesis studies verified the presence of the tryptophan close to A(1), while the recent crystal structure showed that the tryptophan was indeed pi-stacked and that a weak potential H-bond from an amide backbone to one of the (semi)quinone carbonyls is probably the origin of the to the second nitrogen coupling seen in the ESEEM. ESEEM has already played an important role in the structural characterization on PSI and since it specifically probes the radical forms of the chromophores and their protein environment, the information obtained is complimentary to the crystallography. ESEEM then will continue to provide structural information that is often unavailable using other methods.

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.

Effect of D2O and cryosolvents on the redox properties of bacteriochlorophyll dimer and electron transfer processes in Rhodobacter sphaeroides reaction centers

Effects of environmental changes on the reaction pattern of excitation energy trapping and transformation into the "stable" radical pair P+Q(A)-, have been analyzed in isolated reaction centers of the anoxygenic purple bacterium Rhodobacter sphaeroides. The following results were obtained: (a) replacement of exchangeable protons by deuterons significantly retarded the electron transfer steps of primary charge separation, leading to the radical pair P+I- and of the subsequent reoxidation of I- by the quinone acceptor Q(A) but has virtually no effect on the midpoint potential of P/P+ that was found to be 430+/-20 mV; (b) addition of 70% (v/v) glycerol causes a shift of Em by about 30 mV towards higher values whereas the kinetics of the electron transfer reactions remain almost unaffected; (c) in the presence of the cryoprotectant DMSO, a combined effect arises, i.e. a retardation of the electron transfer kinetics comparable to that induced by H/D exchange and simultaneously an upshift of the Em value to 475+/-20 mV, resembling the action of glycerol. These results are discussed within the framework of effects on the midpoint potential due to the dielectric constant of the medium and changes of the charge distribution in the vicinity of the redox groups and the influence of relaxation processes on electron transfer reactions.