X-ray structure analysis of a membrane protein complex. Electron density map at 3 A resolution and a model of the chromophores of the photosynthetic reaction center from Rhodopseudomonas viridis

Exploiting powder X-ray diffraction for direct structure determination in structural biology: the P2X4 receptor trafficking motif YEQGL

We report the crystal structure of the 5-residue peptide acetyl-YEQGL-amide, determined directly from powder X-ray diffraction data recorded on a conventional laboratory X-ray powder diffractometer. The YEQGL motif has a known biological role, as a trafficking motif in the C-terminus of mammalian P2X4 receptors. Comparison of the crystal structure of acetyl-YEQGL-amide determined here and that of a complex formed with the μ2 subunit of the clathrin adaptor protein complex AP2 reported previously, reveals differences in conformational properties, although there are nevertheless similarities concerning aspects of the hydrogen-bonding arrangement and the hydrophobic environment of the leucine sidechain. Our results demonstrate the potential for exploiting modern powder X-ray diffraction methodology to achieve complete structure determination of materials of biological interest that do not crystallize as single crystals of suitable size and quality for single-crystal X-ray diffraction.

What's in a drop? Correlating observations and outcomes to guide macromolecular crystallization experiments

Observations of crystallization experiments are classified as specific outcomes and integrated through a phase diagram to visualize solubility and thereby direct subsequent experiments. Specific examples are taken from our high-throughput crystallization laboratory which provided a broad scope of data from 20 million crystallization experiments on 12,500 different biological macromolecules. The methods and rationale are broadly and generally applicable in any crystallization laboratory. Through a combination of incomplete factorial sampling of crystallization cocktails, standard outcome classifications, visualization of outcomes as they relate chemically and application of a simple phase diagram approach we demonstrate how to logically design subsequent crystallization experiments.

Structural basis of cyanobacterial photosystem II Inhibition by the herbicide terbutryn

Herbicides that target photosystem II (PSII) compete with the native electron acceptor plastoquinone for binding at the Q(B) site in the D1 subunit and thus block the electron transfer from Q(A) to Q(B). Here, we present the first crystal structure of PSII with a bound herbicide at a resolution of 3.2 Å. The crystallized PSII core complexes were isolated from the thermophilic cyanobacterium Thermosynechococcus elongatus. The used herbicide terbutryn is found to bind via at least two hydrogen bonds to the Q(B) site similar to photosynthetic reaction centers in anoxygenic purple bacteria. Herbicide binding to PSII is also discussed regarding the influence on the redox potential of Q(A), which is known to affect photoinhibition. We further identified a second and novel chloride position close to the water-oxidizing complex and in the vicinity of the chloride ion reported earlier (Guskov, A., Kern, J., Gabdulkhakov, A., Broser, M., Zouni, A., and Saenger, W. (2009) Nat. Struct. Mol. Biol. 16, 334-342). This discovery is discussed in the context of proton transfer to the lumen.

Mechanically, magnetically, and "rotationally aligned" membrane proteins in phospholipid bilayers give equivalent angular constraints for NMR structure determination

The native environment for membrane proteins is the highly asymmetric phospholipid bilayer, and this has a large effect on both their structure and dynamics. Reproducing this environment in samples suitable for spectroscopic and diffraction experiments is a key issue, and flexibility in sample preparation is essential to accommodate the diverse size, shape, and other physical properties of membrane proteins. In most cases, to ensure that the biological activities are maintained, this means reconstituting the proteins in fully hydrated planar phospholipid bilayers. The asymmetric character of protein-containing bilayers means that it is possible to prepare either oriented or unoriented (powder) samples. Here we demonstrate the equivalence of mechanical, magnetic, and what we refer to as "rotational alignment" of membrane proteins in phospholipid bilayer samples for solid-state NMR spectroscopy. The trans-membrane domain of virus protein "u" (Vpu) from human immunodeficiency virus (HIV-1) and the full-length membrane-bound form of fd bacteriophage coat protein in phospholipid bilayers are used as examples. The equivalence of structural constraints from oriented and unoriented (powder) samples of membrane proteins is based on two concepts: (1) their alignment is defined by the direction of the bilayer normal relative to the magnetic field and (2) they undergo rapid rotational diffusion about the same bilayer normal in liquid crystalline membranes. The measurement of angular constraints relative to a common external axis system defined by the bilayer normal for all sites in the protein is an essential element of oriented sample (OS) solid-state NMR.

Quantized Diameters in Self-assembled Cyundrical Aggregates of Chlorophylls

Neutron small-angle scattering measurements of several different chlorophylls hydrated in deuterated octane-toluene mixtures show that long, hollow cylinders of aggregated chlorophyll are formed. Clear secondary maxima are present in the scattering, and the cylinder diameters are well determined, but depend on the type of chlorophyll. Chlorophyll-a and Bacteriochlorophyll-a were particularly studied, and several samples of each have been measured. Other chlorophylls have also been studied. The results provide strong evidence that chlorophyll cylinders are only certain sizes, with diameters very nearly in the ratio of small integers. Thus, the cylinder diameters appear to be quantized. Neutron scattering results that further test this quantization property are presented here, together with a proposal for the stereochemical features of chlorophyll aggregation which account for the diameter quantization.

Supramolecular tetrad of subphthalocyanine-triphenylamine-zinc porphyrin coordinated to fullerene as an "antenna-reaction-center" mimic: formation of a long-lived charge-separated state in nonpolar solvent

We report here the formation of a long-lived charge-separated state of a self-assembled donor-acceptor tetrad, formed by axial coordination of a fulleropyrrolidine appended with an imidazole coordinating ligand (C(60)Im) to the zinc center of a subphthalocyanine-triphenylamine-zinc porphyrin (SubPc-TPA-ZnP), as a charge-stabilizing antenna reaction center mimic in toluene. The subphthalocyanine and triphenylamine entities, with their high-energy singlet states, act as an energy-transferring antenna unit to produce a singlet zinc porphyrin. The formation constant for the self-assembled tetrad was determined to be 1.0 x 10(4) M(-1), suggesting a moderately stable complex formation. The geometric and electronic structures of the covalently linked SubPc-TPA-ZnP triad and self-assembled SubPc-TPA-ZnP:C(60)Im tetrad were examined by using an ab initio B3LYP/6-31G method. The majority of the highest occupied frontier molecular orbital was found over the ZnP and TPA entities, whereas the lowest unoccupied molecular orbital was located over the fullerene entity, suggesting the formation of the radical-ion pair (SubPc-TPA-ZnP(*+):C(60)Im(*-)). The redox measurements revealed that the energy level of the radical-ion pair in toluene is located lower than that of the singlet and triplet states of the zinc porphyrin and fullerene entities. The femtosecond transient absorption measurements revealed fast charge separation from the singlet porphyrin to the coordinated C(60)Im with a lifetime of 1.1 ns. Interestingly, slow charge recombination (1.6 x 10(5) s(-1)) and the long lifetime of the charge-separated state (6.6 micros) were obtained in toluene by utilizing the nanosecond transient measurements.

Excited-state vibrational coherence in methanol solution of Zn(II) tetrakis(N-methylpyridyl)porphyrin: charge-dependent intermolecular mode frequencies and implications for electron-transfer dynamics in photosynthetic reaction centers

The nature of the intermolecular vibrational modes between the redox-active chromophores and the protein medium in the photosynthetic reaction center is central to an understanding of the structural origin of the quantum efficiency of the light-driven charge-separation reactions that result in storage of solar energy. In recent work on this issue, we have characterized the low-frequency vibrational coherence of Zn(II) meso-tetrakis(N-methylpyridyl)porphyrin (ZnTMPyP) and compared it to that from bacteriochlorophyll a in polar solution and in the small light-harvesting subunits B820 and B777. The charge-transfer character of ZnTMPyP's π* excited states afford us the opportunity to characterize how the intermolecular vibrational modes and potential with the surrounding medium are affected by the charge on the porphyrin macrocycle. The excited-state vibrational coherence observed with Q-band (S(1) state) excitation at 625 nm of ZnTMPyP in methanol solution contains dominant contributions from a pair of rapidly damped (effective damping time γ < 400 fs) components that are assigned to the hindered translational and librational porphyrin-solvent intermolecular modes. The 256 cm(-1) mean frequency of the intermolecular modes is significantly higher than that observed previously in the ground state, 79 cm(-1), with Soret-band excitation at 420 nm [Dillman et al., J. Phys. Chem. B. 2009, 113, 6127-6139]. The increased mode frequency arises from the activation of the ion-dipole and ion-induced-dipole terms in the intermolecular potential. In the ground state, the π-electron density of ZnTMPyP is mostly confined to the region of the porphyrin macrocycle. In the excited state, the π-electron density is extensively delocalized from the porphyrin out to two of the peripheral N-methylpyridyl rings, each of which carries a single formal charge. The charge-dependent terms contribute to a significant stabilization of the equilibrium geometry of the porphyrin-solvent complex in the excited state. In the photosynthetic reaction center, these terms will play an important role in trapping the charged products of the forward, charge-separation reactions, and the location of the bacteriopheophytin acceptor in a nonpolar region of the structure enhances the rate of the secondary charge-separation reaction.

The New York Consortium on Membrane Protein Structure (NYCOMPS): a high-throughput platform for structural genomics of integral membrane proteins

The New York Consortium on Membrane Protein Structure (NYCOMPS) was formed to accelerate the acquisition of structural information on membrane proteins by applying a structural genomics approach. NYCOMPS comprises a bioinformatics group, a centralized facility operating a high-throughput cloning and screening pipeline, a set of associated wet labs that perform high-level protein production and structure determination by x-ray crystallography and NMR, and a set of investigators focused on methods development. In the first three years of operation, the NYCOMPS pipeline has so far produced and screened 7,250 expression constructs for 8,045 target proteins. Approximately 600 of these verified targets were scaled up to levels required for structural studies, so far yielding 24 membrane protein crystals. Here we describe the overall structure of NYCOMPS and provide details on the high-throughput pipeline.

Protein crystallization using microfluidic technologies based on valves, droplets, and SlipChip

To obtain protein crystals, researchers must search for conditions in multidimensional chemical space. Empirically, thousands of crystallization experiments are carried out to screen various precipitants at multiple concentrations. Microfluidics can manipulate fluids on a nanoliter scale, and it affects crystallization twofold. First, it miniaturizes the experiments that can currently be done on a larger scale and enables crystallization of proteins that are available only in small amounts. Second, it offers unique experimental approaches that are difficult or impossible to implement on a larger scale. Ongoing development of microfluidic techniques and their integration with protein production, characterization, and in situ diffraction promises to accelerate the progress of structural biology.

Dielectric asymmetry in the photosynthetic reaction center

Although the three-dimensional structure of the bacterial photosynthetic reaction center (RC) reveals a high level of structural symmetry, with two nearly equivalent potential electron transfer pathways, the RC is functionally asymmetric: Electron transfer occurs along only one of the two possible pathways. In order to determine the origins of this symmetry breaking, the internal electric field present in the RC when charge is separated onto structurally characterized sites was probed by using absorption band shifts of the chromophores within the RC. The sensitivity of each probe chromophore to an electric field was calibrated by measuring the Stark effect spectrum, the change in absorption due to an externally applied electric field. A quantitative comparison of the observed absorption band shifts and those predicted from vacuum electrostatics gives information on the effective dielectric constant of the protein complex. These results reveal a significant asymmetry in the effective dielectric strength of the protein complex along the two potential electron transfer pathways, with a substantially higher dielectric strength along the functional pathway. This dielectric asymmetry could be a dominant factor in determining the functional asymmetry of electron transfer in the RC.

The Photosynthetic Reaction Center from the Purple Bacterium Rhodopseudomonas viridis

The history and methods of membrane protein crystallization are described. The solution of the structure of the photosynthetic reaction center from the bacterium Rhodopseudomonas viridis is described, and the structure of this membrane protein complex is correlated with its function as a light-driven electron pump across the photosynthetic membrane. Conclusions about the structure of the photosystem II reaction center from plants are drawn, and aspects of membrane protein structure are discussed.

Stark effect spectroscopy of Rhodobacter sphaeroides and Rhodopseudomonas viridis reaction centers

The nature of the initially excited state of the primary electron donor or special pair has been investigated by Stark effect spectroscopy for reaction centers from the photosynthetic bacteria Rhodopseudomonas viridis and Rhodobacter sphaeroides at 77 K. The data provide values for the magnitude of the difference in permanent dipole moment between the ground and excited state, [unk]Deltamu[unk], and the angle [unk] between Deltamu and the transition dipole moment for the electronic transition. [unk]Deltamu[unk] and [unk] for the lowest-energy singlet electronic transition associated with the special pair primary electron donor were found to be very similar for the two species. [unk]Deltamu[unk] for this transition is substantially larger than for the Q(y) transitions of the monomeric pigments in the reaction center or for pure monomeric bacteriochlorophylls, for which Stark data are also reported. We conclude that the excited state of the special pair has substantial charge-transfer character, and we suggest that charge separation in bacterial photosynthesis is initiated immediately upon photoexcitation of the special pair. Data for Rhodobacter sphaeroides between 340 and 1340 nm are presented and discussed in the context of the detection of charge-transfer states by Stark effect spectroscopy.

Directed mutations affecting spectroscopic and electron transfer properties of the primary donor in the photosynthetic reaction center

Oligonucleotide-mediated mutagenesis has been used to change the histidine residues that act as axial ligands to the central Mg(2+) ions of the "special pair" bacteriochlorophylls in the reaction center of Rhodobacter capsulatus. Histidine-173 of the L subunit has been replaced with glutamine, while histidine-200 of the M subunit has been replaced with glutamine, leucine, or phenylalanine. When leucine or phenylalanine is introduced at M200, one of the special pair bacteriochlorophylls is converted to bacteriopheophytin, which generates a heterodimer at the special pair binding site. The pigment composition of the reaction center is unaltered when either histidine is replaced with glutamine. All of these mutant reaction centers are photochemically active, although the electron transfer properties of heterodimer-containing reaction centers are altered. These mutations begin to define the structural parameters that determine whether bacteriochlorophyll or bacteriopheophytin will be incorporated into the tetrapyrrole binding sites of the photosynthetic reaction center. Our results demonstrate that the properties of the photosynthetic reaction center can be changed by directed mutagenesis, which makes this complex an excellent model for testing theories of electron transfer in biological systems.

Infrared spectroelectrochemistry of bacteriochlorophylls and bacteriopheophytins: Implications for the binding of the pigments in the reaction center from photosynthetic bacteria

The IR spectra of the bacteriochlorophyll a and b cations and the bacteriopheophytin a and b anions were obtained by using an IR and optically transparent electrochemical cell. Prominent effects of radical formation on the vibrational spectra were found for bands assigned to the ester, keto, and acetyl C=O groups and for vibrations from macrocycle bonds. The (radical-minus-neutral) difference spectra are compared to the light-induced difference spectra of the primary donor photooxidation and the intermediary acceptor photoreduction in the reaction center of photosynthetic bacteria. Light-induced absorbance changes from bacteriochlorophyll a-containing reaction centers bear striking similarities to the electrochemically induced absorbance changes observed upon formation of bacteriochlorophyll a(+)in vitro. Comparison of the radical formation in vitro in a hydrogen-bonding or a nonhydrogen-bonding solvent suggests an ester C=O group hydrogen bonded in the neutral state but free in the cation state. For the keto C=O group, the same comparison indicates one free carbonyl group. The (anion-minus-neutral) difference spectra of bacteriopheophytin a and b exhibit a single band in the ester C=O frequency range. In contrast, two bands are observed in the difference spectra of the intermediary acceptor reduction in the reaction center of Rhodopseudomonas viridis. The higher frequency band exhibits a sensitivity to (1)H-(2)H exchange, which suggests a contribution from a protonated carboxyl group of an amino acid side chain.

Femtosecond spectroscopy of electron transfer in the reaction center of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26: Direct electron transfer from the dimeric bacteriochlorophyll primary donor to the bacteriopheophytin acceptor with a time constant of 2.8 +/- 0.2 psec

The primary light-induced charge separation in reaction centers from Rhodopseudomonas sphaeroides R-26 has been investigated after excitation with laser pulses of 150 fsec duration within the longwave absorption band of the primary donor at 850 nm. An excited state of the primary donor, characterized by a broad absorption spectrum extending over the whole spectral range investigated (545-1240 nm), appeared within 100 fsec and gave rise to stimulated emission in the 870- to 1000-nm region with a 2.8-psec lifetime. The photooxidation of the primary donor, as measured at 1240 nm, and the photoreduction of the bacteriopheophytin acceptor, monitored at 545 nm and 675 nm, have been found to proceed simultaneously with a time constant of 2.8 +/- 0.2 psec. Kinetics of absorbance changes at other probe wavelengths gave no indication that an accessory bacteriochlorophyll is involved as a transient electron acceptor.

Primary electron transfer reactions in modified reaction centers from Rhodopseudomonas sphaeroides

Absorption spectra were measured by means of an optical multichannel analyzer in Rhodopseudomonas sphaeroides R-26 reaction centers (RCs) modified by treatment with NaBH(4) at various times (>/=1 ps) after the onset of a short excitation flash at 880 nm. Most of these RCs (75-95%) have only one "monomeric" bacteriochlorophyll-800 (B(1)) molecule and are as active as the original RCs. The duration of the excitation and measuring pulses was approximately 33 ps. If the center of the excitation pulse preceded the center of the measuring pulse by 36-40 ps, the formation of a state P(E) (early state), which is converted to the state P(F) (P(+) bacteriopheophytin(-)) in 4 +/- 1 ps (1/e time), was observed. Also the kinetics and the spectrum of the stimulated emission (reflecting the kinetics and the emission spectrum of the excited state P(*)) were determined. The difference spectrum of the state P(E) approximately equals the sum of the spectra of the states P(*) ( approximately 65%) and (1)[P(+)B(1) (-)] ( approximately 35%). This indicates that B(1) (-) is an intermediate in the electron transfer from P(*) to bacteriopheophytin, H(1), transferring this electron with a rate constant of (4 x 0.35 ps)(-1) = 7 x 10(11) s(-1).

Isotope effect on electron transfer in reaction centers from Rhodopseudomonas sphaeroides

Previous ENDOR studies on reaction centers from Rhodopseudomonas sphaeroides have shown the presence of two hydrogen-bonded protons associated with the primary, ubiquinone, acceptor Q(A). These protons exchange with deuterons from solvent (2)H(2)O. The effect of this deuterium substitution on the charge-recombination kinetics (BChl)(2) (+)Q(A) (-) --> (BChl)(2)Q(A) has been studied with a sensitive kinetic difference technique. The electron-transfer rate was found to increase with deuterium exchange up to a maximum Deltak/k of 5.7 +/- 0.3%. The change in rate was found to have an exchange time of 2 hr, which matched the disappearance of the ENDOR lines due to the exchangeable protons. These results indicate that these protons play a role in the vibronic coupling associated with electron transfer. A simple model for the isotope effect on electron transfer predicts a maximum rate increase of 20%, which is consistent with the experimental results.

Stoichiometric model of the photosynthetic unit of Ectothiorhodospira halochloris

A stoichiometric model of the photosynthetic unit of Ectothiorhodospira halochloris has been obtained by means of scanning transmission electron microscope mass determination and mass mapping in conjunction with polyacrylamide gel electrophoresis. One reaction center, consisting of four single polypeptides, including one cytochrome, is surrounded by six identical light-harvesting complexes, each containing three polypeptides with 2:2:2 stoichiometry. This stoichiometric model was incorporated into the three-dimensional structure of the photosynthetic unit as derived from surface relief reconstructions of the two surfaces of shadowed membranes. The reaction center protrudes substantially from both membrane surfaces and has the cytochrome attached to the periplasmic face in a noncentrosymmetric fashion. The reaction center may assume various orientations within the photosynthetic complexes.

Simulation of photochemical hole-burning experiments on photosynthetic reaction centers

An effective Hamiltonian formalism is used to calculate the homogeneous linewidth of long-wavelength absorption in the photosynthetic reaction center. Agreement with the experimental values of approximately 400 cm(-1) for the hole width of the 990-nm band of Rhodopseudomonas viridis is obtained. The anomalously (two orders of magnitude) large width is explained in terms of resonant coupling to charge transfer states. These results support a dynamical model of primary charge separation [Friesner, R. & Wertheimer, R. (1982) Proc. Natl. Acad. Sci USA 79, 2138-2142] in which such resonant coupling was also concluded to be important.

Isolation of a photosystem II reaction center consisting of D-1 and D-2 polypeptides and cytochrome b-559

A photosystem II reaction center complex consisting of D-1 and D-2 polypeptides and cytochrome b-559 was isolated from spinach grana thylakoids, treated with 4% (wt/vol) Triton X-100, by ion-exchange chromatography using DEAE-Toyopearl 650S. The isolated complex appears to contain five chlorophyll a, two pheophytin a, one beta-carotene, and one or two cytochrome b-559 heme(s) (molar ratio) and exhibits a reversible absorbance change attributable to the photochemical accumulation of reduced pheophytin typical for the intermediary electron acceptor of photosystem II reaction center. These results strongly suggest that the site of primary charge separation in photosystem II is located on the heterodimer composed of D-1 and D-2 subunits.

Primary photochemistry of iron-depleted and zinc-reconstituted reaction centers from Rhodopseudomonas sphaeroides

The primary photochemistry of Fe-depleted and Zn-reconstituted reaction centers from Rhodopseudomonas sphaeroides R-26.1 was studied by transient absorption spectroscopy and compared with native, Fe(2+)-containing reaction centers. Excitation of metal-free reaction centers with 30-ps flashes produced the initial charge-separated state P(+)I(-) (P(+)BPh(-), where P is the primary donor and BPh is bacteriopheophytin) with a yield and visible/near-infrared absorption difference spectrum indistinguishable from that observed in native reaction centers. However, the lifetime of P(+)I(-) was found to increase approximately 20-fold to 4.2 +/- 0.3 ns (compared to 205 ps in native reaction centers), and the yield of formation of the subsequent state P(+)Q(A) (-) (Q(A) is the primary quinone acceptor) was reduced to 47 +/- 5% (compared to essentially 100% in native reaction centers). The remaining 53% of the metal-free reaction centers were found to undergo charge recombination during the P(+)I(-) lifetime to yield both the ground state (28 +/- 5%) and the triplet state P(R) (25 +/- 5%). Reconstitution of Fe-depleted reaction centers with Zn(2+) restored the "native" photochemistry. Possible mechanisms responsible for the reduced decay rate of P(+)I(-) in metal-free reaction centers are discussed.

Role of charge-transfer states in bacterial photosynthesis

Photon echo, photon-echo excitation, and "hole-burning" data recorded in the 800-990 nm region of Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis reaction centers are reported. The primary process in these reaction centers, following excitation, was found to occur in approximately 25 fsec; the long-wavelength band of the primary electron donor (P) was largely homogeneously broadened. In accordance with our previous explanation of hole-burning and photon-echo measurements on Rb. sphaeroides [Meech, S. R., Hoff, A. J. & Wiersma, D. A. (1985) Chem. Phys. Lett. 121, 287-292], we interpret this as resulting from a dephasing of the excitation in P into a background of strongly coupled charge-transfer states. The previously reported picosecond lifetime of the excited P state is assigned to decay of these strongly mixed states. Further, a coupling between P and an adjacent bacteriochlorophyll was observed. The extent of this coupling and the role of charge-transfer states in the functioning of reaction centers is discussed.

Femtosecond spectroscopy of excitation energy transfer and initial charge separation in the reaction center of the photosynthetic bacterium Rhodopseudomonas viridis

Reaction centers from the photosynthetic bacterium Rhodopseudomonas viridis have been excited within the near-infrared absorption bands of the dimeric primary donor (P), of the "accessory" bacteriochlorophylls (B), and of the bacteriopheophytins (H) by using laser pulses of 150-fsec duration. The transfer of excitation energy between H, B, and P occurs in slightly less than 100 fsec and leads to the ultrafast formation of an excited state of P. This state is characterized by a broad absorption spectrum and exhibits stimulated emission. It decays in 2.8 +/- 0.2 psec with the simultaneous oxidation of the primary donor and reduction of the bacteriopheophytin acceptor, which have been monitored at 545, 675, 815, 830, and 1310 nm. Although a transient bleaching relaxing in 400 +/- 100 fsec is specifically observed upon excitation and observation in the 830-nm absorption band, we have found no indication that an accessory bacteriochlorophyll is involved as a resolvable intermediary acceptor in the primary electron transfer process.

Exciton interactions in reaction centers of the photosynthetic bacterium Rhodopseudomonas viridis probed by optical triplet-minus-singlet polarization spectroscopy at 1.2 K monitored through absorbance-detected magnetic resonance

Linear dichroic triplet-minus-singlet [LD-(T - S)] spectra of isolated reaction centers of the photosynthetic bacterium Rhodopseudomonas viridis have been measured at 1.2 K with the linear dichroic absorbance-detected magnetic resonance (LD-ADMR) technique for two mutually perpendicular directions of the preferred axis. The LD-(T - S) spectra have been calibrated with respect to the corresponding (T - S) spectra as a function of applied microwave power and quantitatively interpreted using the formalism of photoselection. The transition moment of the optical transition at 1007 nm makes angles of 72 degrees +/- 5 degrees and 15 degrees +/- 5 degrees with the triplet x and y spin axes, respectively. The experimental spectra have been simulated employing exciton theory and using the atomic coordinates of the resolved crystal structure of the reaction center. The spectral interpretation yields the angles between the transition moments of the various absorption bands of the (T - S) spectra and the triplet axes, and between the moments themselves, with the triplet state of the primary donor (3)P localized on the P-bacteriochlorophyll b in the "active" (L) chain.

Determination of the primary charge separation rate in isolated photosystem II reaction centers with 500-fs time resolution

We have measured directly the rate of formation of the oxidized chlorophyll a electron donor (P680(+)) and the reduced electron acceptor pheophytin a(-) (Pheoa(-)) following excitation of isolated spinach photosystem II reaction centers at 4 degrees C. The reaction-center complex consists of D(1), D(2), and cytochrome b-559 proteins and was prepared by a procedure that stabilizes the protein complex. Transient absorption difference spectra were measured from 440 to 850 nm as a function of time with 500-fs resolution following 610-nm laser excitation. The formation of P680(+)-Pheoa(-) is indicated by the appearance of a band due to P680(+) at 820 nm and corresponding absorbance changes at 505 and 540 nm due to formation of Pheoa(-). The appearance of the 820-nm band is monoexponential with tau = 3.0 +/- 0.6 ps. The time constant for decay of (1*)P680, the lowest excited singlet state of P680, monitored at 650 nm, is tau = 2.6 +/- 0.6 ps and agrees with that of the appearance of P680(+) within experimental error. Treatment of the photosystem II reaction centers with sodium dithionite and methyl viologen followed by exposure to laser excitation, conditions known to result in accumulation of Pheoa(-), results in formation of a transient absorption spectrum due to (1*)P680. We find no evidence for an electron acceptor that precedes the formation of Pheoa(-).

Excitation trapping and primary charge stabilization in Rhodopseudomonas viridis cells, measured electrically with picosecond resolution

The transmembrane primary charge separation in the photosynthetic bacterium Rhodopseudomonas viridis was monitored by electric measurements of the light-gradient type [Trissl, H. W. & Kunze, U. (1985) Biochim. Biophys. Acta 806, 136-144]. Excitation of whole cells with 30-ps laser pulses at either 532 nm or 1064 nm gave rise to a biphasic increase of the photovoltage. The fast phase, contributing about 50% of the total, rose with an exponential time constant </=40 ps and was independent of the redox state of the quinone electron acceptor. It is assigned to the migration of the excitation energy in the antenna and its subsequent trapping by the reaction center, monitored by the ultrafast charge separation between the primary electron donor and the bacteriopheophytin intermediary acceptor. The slower phase (125 +/- 50 ps) only occurred when the quinone was oxidized and disappeared when it was reduced (either chemically or photochemically). It is assigned to the forward electron transfer from the bacteriopheophytin to the quinone. The relative amplitudes of these two electrogenic steps demonstrate that the bacteriopheophytin intermediary acceptor is located halfway between the primary donor and the quinone.

Analysis of optical spectra from single crystals of Rhodopseudomonas viridis reaction centers

Absorption spectra and light-induced absorbance changes of crystals from Rhodopseudomonas viridis reaction centers are recorded. A theoretical analysis of the absorption and circular dichroism spectra is presented, yielding a consistent picture of spectroscopic and structural information.

Effects of nuclear spin polarization on reaction dynamics in photosynthetic bacterial reaction centers

Singlet-triplet mixing in the initial radical-pair state, P[unk]I[unk], of photosynthetic bacterial reaction centers is due to the hyperfine mechanism at low magnetic fields and both the hyperfine and Deltag mechanisms at high magnetic fields (>1 kG). Since the hyperfine field felt by the electron spins in P[unk]I[unk] is dependent upon the nuclear spin state in each radical, the relative probabilities of charge recombination to the triplet state of the primary electron donor, (3)PI, or the ground state, PI, will depend on the nuclear spin configuration. As a result these recombination products will have non-equilibrium distributions of nuclear spin states (nuclear spin polarization). This polarization will persist until the (3)PI state decays. In addition, due to unequal nuclear spin relaxation rates in the diamagnetic PI and paramagnetic (3)PI states, net polarization of the nuclear spins can result, especially in experiments that involve recycling of the system through the radical-pair state. This net polarization can persist for very long times, especially at low temperatures. Nuclear spin polarization can have consequences on any subsequent process that involves re-formation of the radical-pair state.Numerical calculations of the nuclear polarization caused by both of these mechanics are presented, including the effect of such polarization on subsequent yields of (3)PI, (3)PI decay rates, the decay rate of the radical pair, and saturation behavior. The effect of this polarization under certain circumstances can be very dramatic and can explain previously noted discrepancies between experiments and theories that do not include nuclear spin polarization effects. Our analysis suggests new classes of experiments and indicates the need to reinterpret some past experimental results.

Spectroscopic determinants in the reaction center of rhodopseudomonas viridis

Assignments are proposed for the long wavelength absorption bands observed in the reaction center of Rhodopseudomonas viridis. The assignments are based on a theoretical treatment in which quantum mechanical calculations are first carried out on the individual chromophores of the reaction center. The energies and wave functions that are obtained are then introduced into an exciton-type perturbation treatment in which extensive configuration interaction is carried out between the excited states of the four bacteriochlorophylls and two bacteriopheophytins of the reaction center. Calculated values for absorption maxima, transition moments, linear dichroism, and rotational strength are compared with experiments in an attempt to distinguish among different assignments. The calculations alone do not lead to unambiguous assignments; indeed it is difficult to account for the reaction center spectra without introducing assumptions as to the effects of the protein on the energy levels of the individual molecules. Even if these effects are treated as free parameters, the experimental spectra still provide useful constraints that restrict the models that are possible. The major result of this work is that the weak 850-nm absorption band is due, primarily, to the higher energy exciton state of the bacteriochlorophyll special pair. Accounting for the 960-nm absorption band of the low energy exciton state of the special pair requires either that a large spectroscopic effect of the protein be introduced, or possibly, that charge transfer states play a major spectroscopic role. The difference in spectra seen in the formation of oxidized or triplet state reaction centers can be understood in terms of a combination of electrochromic effects and modified exciton interactions.

Kinetic and Energetic Model for the Primary Processes in Photosystem II

A detailed model for the kinetics and energetics of the exciton trapping, charge separation, charge recombination, and charge stabilization processes in photosystem (PS) II is presented. The rate constants describing these processes in open and closed reaction centers (RC) are calculated on the basis of picosecond data (Schatz, G. H., H. Brock, and A. R. Holzwarth. 1987. Proc. Natl. Acad. Sci. USA. 84:8414-8418) obtained for oxygen-evolving PS II particles from Synechococcus sp. with approximately 80 chlorophylls/P(680). The analysis gives the following results. (a) The PS II reaction center donor chlorophyll P(680) constitutes a shallow trap, and charge separation is overall trap limited. (b) The rate constant of charge separation drops by a factor of approximately 6 when going from open (Q-oxidized) to closed (Q-reduced) reaction centers. Thus the redox state of Q controls the yield of radical pair formation and the exciton lifetime in the Chl antenna. (c) The intrinsic rate constant of charge separation in open PS II reaction centers is calculated to be approximately 2.7 ps(-1). (d) In particles with open RC the charge separation step is exergonic with a decrease in standard free energy of approximately 38 meV. (e) In particles with closed RC the radical pair formation is endergonic by approximately 12 meV. We conclude on the basis of these results that the long-lived (nanoseconds) fluorescence generally observed with closed PS II reaction centers is prompt fluorescence and that the amount of primary radical pair formation is decreased significantly upon closing of the RC.

Elastic interactions of photosynthetic reaction center proteins affecting phase transitions and protein distributions

Reaction-center proteins of Rhodopseudomonas Sphaeroides reconstituted into phosphatidylcholine vesicles shift and broaden the fluid-gel transition of the lipid bilayer. The amount of broadening and temperature shift of the transition depend both on protein concentration and on lipid chain length. In particular, the direction of the transition temperature shift is very sensitive to lipid chain length. Electron micrographs show homogeneous protein distribution on the fluid surface whereas the solid phase contains protein aggregates the type depending on chain length. The results can qualitatively be understood in the framework of a mattress model of lipid/protein interactions in membranes.

An overview on chlorophylls and quinones in the photosystem I-type reaction centers

Minor but key chlorophylls (Chls) and quinones in photosystem (PS) I-type reaction centers (RCs) are overviewed in regard to their molecular structures. In the PS I-type RCs, the prime-type chlorophylls, namely, bacteriochlorophyll (BChl) a' in green sulfur bacteria, BChl g' in heliobacteria, Chl a' in Chl a-type PS I, and Chl d' in Chl d-type PS I, function as the special pairs, either as homodimers, (BChl a')(2) and (BChl g')(2) in anoxygenic organisms, or heterodimers, Chl a/a' and Chl d/d' in oxygenic photosynthesis. Conversions of BChl g to Chl a and Chl a to Chl d take place spontaneously under mild condition in vitro. The primary electron acceptors, A (0), are Chl a-derivatives even in anoxygenic PS I-type RCs. The secondary electron acceptors are naphthoquinones, whereas the side chains may have been modified after the birth of cyanobacteria, leading to succession from menaquinone to phylloquinone in oxygenic PS I.

Protein folding in membranes

Separation of cells and organelles by bilayer membranes is a fundamental principle of life. Cellular membranes contain a baffling variety of proteins, which fulfil vital functions as receptors and signal transducers, channels and transporters, motors and anchors. The vast majority of membrane-bound proteins contain bundles of alpha-helical transmembrane domains. Understanding how these proteins adopt their native, biologically active structures in the complex milieu of a membrane is therefore a major challenge in today's life sciences. Here, we review recent progress in the folding, unfolding and refolding of alpha-helical membrane proteins and compare the molecular interactions that stabilise proteins in lipid bilayers. We also provide a critical discussion of a detergent denaturation assay that is increasingly used to determine membrane-protein stability but is not devoid of conceptual difficulties.