Electronic structure of the oxidized primary electron donor of the HL (M202) and HL (L173) heterodimer mutants of the photosynthetic bacterium Rhodobacter sphaeroides: ENDOR on single crystals of reaction centers

Dehydration affects the electronic structure of the primary electron donor in bacterial photosynthetic reaction centers: evidence from visible-NIR and light-induced difference FTIR spectroscopy

The photosynthetic reaction center (RC) is a membrane pigment-protein complex that catalyzes the initial charge separation reactions of photosynthesis. Following photoexcitation, the RC undergoes conformational relaxations which stabilize the charge-separated state. Dehydration of the complex inhibits its conformational dynamics, providing a useful tool to gain insights into the relaxational processes. We analyzed the effects of dehydration on the electronic structure of the primary electron donor P, as probed by visible-NIR and light-induced FTIR difference spectroscopy, in RC films equilibrated at different relative humidities r. Previous FTIR and ENDOR spectroscopic studies revealed that P, an excitonically coupled dimer of bacteriochlorophylls, can be switched between two conformations, P866 and P850, which differ in the extent of delocalization of the unpaired electron between the two bacteriochlorophyll moieties (PL and PM) of the photo-oxidized radical P(+). We found that dehydration (at r = 11%) shifts the optical Qy band of P from 866 to 850-845 nm, a large part of the effect occurring already at r = 76%. Such a dehydration weakens light-induced difference FTIR marker bands, which probe the delocalization of charge distribution within the P(+) dimer (the electronic band of P(+) at 2700 cm(-1), and the associated phase-phonon vibrational modes at around 1300, 1480, and 1550 cm(-1)). From the analysis of the P(+) keto C[double bond, length as m-dash]O bands at 1703 and 1713-15 cm(-1), we inferred that dehydration induces a stronger localization of the unpaired electron on PL(+). The observed charge redistribution is discussed in relation to the dielectric relaxation of the photoexcited RC on a long (10(2) s) time scale.

George Feher: a pioneer in reaction center research

Our understanding of photosynthesis has been greatly advanced by the elucidation of the structure and function of the reaction center (RC), the membrane protein responsible for the initial light-induced charge separation in photosynthetic bacteria and green plants. Although today we know a great deal about the details of the primary processes in photosynthesis, little was known in the early days. George Feher made pioneering contributions to photosynthesis research in characterizing RCs from photosynthetic bacteria following the ground-breaking work of Lou Duysens and Rod Clayton (see articles in this issue by van Gorkom and Wraight). The work in his laboratory at the University of California, San Diego, started in the late 1960s and continued for over 30 years. He isolated a pure RC protein and used magnetic resonance spectroscopy to study the primary reactants. Following this pioneering work, Feher studied the detailed structure of the RC and the basic electron and proton transfer functions that it performs using a wide variety of biophysical and biochemical techniques. These studies, together with work from many other researchers, have led to our present detailed understanding of these proteins and their function in photosynthesis. The present article is a brief historical account of his pioneering contributions to photosynthesis research. A more detailed description of his work can be found in an earlier biographical paper (Feher in Photosynth Res 55:1-40, 1998a).

High-field EPR

Among the numerous spectroscopic techniques utilized in photosynthesis research, high-field/high-frequency EPR and its pulse extensions ESE, ENDOR, ESEEM, and PELDOR play an important role in the endeavor to understand, on the basis of structure and dynamics data, dominant factors that control specificity and efficiency of light-induced electron- and proton-transfer processes in primary photosynthesis. Short-lived transient intermediates of the photocycle can be characterized by high-field EPR techniques, and detailed structural information can be obtained even from disordered sample preparations. The chapter describes how multifrequency high-field EPR methodology, in conjunction with mutation strategies for site-specific isotope or spin labeling and with the support of modern quantum-chemical computation methods for data interpretation, is capable of providing new insights into the photosynthetic transfer processes. The information obtained is complementary to that of protein crystallography, solid-state NMR and laser spectroscopy.

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.

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.

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 3: interpretation of effects of site-directed mutagenesis on the special-pair midpoint potential

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

Heterodimeric versus homodimeric structure of the primary electron donor in Rhodobacter sphaeroides reaction centers genetically modified at position M202

Using light-induced Fourier-transform infrared (FTIR) difference spectroscopy of the photo-oxidation of the primary donor (P) in chromatophores from Rhodobacter sphaeroides, we examined a series of site-directed mutants with His M202 changed to Gly, Ser, Cys, Asn or Glu in order to assess the ability of these side chains to ligate the Mg atom of one of the two bacteriochlorophylls (BChl) constituting P. In the P+QA-/PQA FTIR difference spectra of the mutants HG(M202), HS(M202), HC(M202) and HN(M202), the presence of a specific electronic transition at approximately 2650-2750 cm-1 as well as of associated vibrational (phase-phonon) bands at approximately 1560, 1480 and 1290 cm-1 demonstrate that these mutants contain a BChl/BChl homodimer like that in native reaction centers with the charge on P+ shared between the two coupled BChl. In contrast, the absence of all of these bands in HE(M202) shows that this mutant contains a BChl/bacteriopheophytin heterodimer with the charge localized on the single BChl, as previously determined for the mutant HL(M202). Furthermore, the spectra of the heterodimers HE(M202) and HL(M202) are very similar in the 4000-1200 cm-1 IR range. Perturbations of the 10a-ester and 9-keto carbonyl modes for both the P and P+ states are observed in the homodimer mutants reflecting slight variations in the conformation and/or in position of P. These perturbations are likely to be due to a repositioning of the dimer in the new protein cavity generated by the mutation.

NEW EPR METHODS FOR INVESTIGATING PHOTOPROCESSES WITH PARAMAGNETIC INTERMEDIATES

▪ Abstract  Some of the significant advances in time-resolved multifrequency electron paramagnetic resonance (EPR) methods are reviewed, with the explicit focus on studies of light-driven processes and photoreactions in real time. Prominent examples are excited state electron transfer reactions with transient charge-separated radical pairs playing a central role. Paramagnetic intermediates and products are key functional states; thus EPR is the method of choice for their characterization. Photogenerated spin polarization and coherences as process-inherent features add the practical advantage of compensation in the trade-off between sensitivity and time resolution. Additionally, they provide detailed structural and dynamic information on the photoreactive system. Significance and specificity of the results achieved for charge separation in photosynthetic reaction centers and donor-acceptor model complexes indicate highly promising perspectives in photochemical research.

Advanced EPR spectroscopy on electron transfer processes in photosynthesis and biomimetic model systems

This review focuses on the recent advances in EPR spectroscopy as they are applied both to photoinduced electron transfer in the photosynthetic apparatus and to biomimetic systems. The review deals with time-resolved direct-detection cw and pulsed EPR and ENDOR methods, both at conventional bands [X-(9.5 GHz), K-(24 GHz), and Q-(35 GHz)(] and at high frequency bands (W-band, 95 GHz, and even higher frequency bands). EPR studies on photosynthetic and model systems in their doublet, triplet and radical pair states are surveyed, including their static and dynamic properties. APplications of time-resolved EPR in studying photoinduced electron and energy transfer in isotropic and anisotropic environments, and the concepts of electron spin polarization and magnetic field effects in photochemical reactions are also reviewed.