Observation of the energy-level structure of the low-light adapted B800 LH4 complex by single-molecule spectroscopy

Low-light adapted B800 light-harvesting complex 4 (LH4) from Rhodopseudomonas palustris is a complex in which the arrangement of the bacteriochloropyll a pigments is very different from the well-known B800-850 LH2 complex. For bulk samples, the main spectroscopic feature in the near-infrared is the occurrence of a single absorption band at 802 nm. Single-molecule spectroscopy can resolve the narrow bands that are associated with the exciton states of the individual complexes. The low temperature (1.2 K) fluorescence excitation spectra of individual LH4 complexes are very heterogeneous and display unique features. It is shown that an exciton model can adequately reproduce the polarization behavior of the complex, the experimental distributions of the number of observed peaks per complex, and the widths of the absorption bands. The results indicate that the excited states are mainly localized on one or a few subunits of the complex and provide further evidence supporting the recently proposed structure model.

Circular dichroism of carotenoids in bacterial light-harvesting complexes: experiments and modeling

In this work we investigate the origin and characteristics of the circular dichroism (CD) spectrum of rhodopin glucoside and lycopene in the light-harvesting 2 complex of Rhodopseudomonas acidophila and Rhodospirillum molischianum, respectively. We successfully model their absorption and CD spectra based on the high-resolution structures. We assume that these spectra originate from seven interacting transition dipole moments: the first corresponds to the 0-0 transition of the carotenoid, whereas the remaining six represent higher vibronic components of the S2 state. From the absorption spectra we get an estimate of the Franck-Condon factors of these transitions. Furthermore, we investigate the broadening mechanisms that lead to the final shape of the spectra and get an insight into the interaction energy between carotenoids. Finally, we examine the consequences of rotations of the carotenoid transition dipole moment and of deformations in the light-harvesting 2 complex rings. Comparison of the modeled carotenoid spectra with modeled spectra of the bacteriochlorophyll QY region leads to a refinement of the modeling procedure and an improvement of all calculated results. We therefore propose that the combined carotenoid and bacteriochlorophyll CD can be used as an accurate reflection of the overall structure of the light-harvesting complexes.

Orientation and pore-forming mechanism of a scorpion pore-forming peptide bound to magnetically oriented lipid bilayers

The orientation and pore-forming mechanisms of pandinin 2 (pin2), an antimicrobial peptide isolated from venom of the African scorpion Pandinus imperator, bound to magnetically oriented lipid bilayers were examined by 31P and 13C solid-state, and 15N liquid-state NMR spectroscopy. 31P NMR measurements at various temperatures, under neutral and acidic conditions, showed that membrane lysis occurred only under acidic conditions, and at temperatures below the liquid crystal-gel phase transition of the lipid bilayers, after incubation for two days in the magnet. Differential scanning calorimetry measurements showed that pin2 induced negative curvature strain in lipid bilayers. The 13C chemical shift values of synthetic pin2 labeled at Gly3, Gly8, Leu12, Phe17, or Ser18 under static or slow magic-angle spinning conditions, indicate that pin2 penetrates the membrane with its average helical axis perpendicular to the membrane surface. Furthermore, amide H-D exchange experiments of 15N-Ala4, Gly8, and Ala9 triply-labeled pin2 suggest that this peptide forms oligomers and confirms that the N-terminal region creates membrane pores.

Conformational differences between native and recombinant horseradish peroxidase revealed by tritium planigraphy

Significant conformational differences between native and recombinant horseradish peroxidase have been shown by tritium planigraphy, which includes a method of thermal activation of tritium followed by amino acid analysis of the protein preparation. Comparison of radioactivity distribution among the amino acid residues with the theoretical (calculated) accessibility shows that the recombinant enzyme is characterized by high hydrophobicity and compactness of folding. The protective role of oligosaccharides in native enzyme has been confirmed. An unexpected result of the study is a finding on high accessibility of a catalytic histidine residue in solution. An effect of low dose (3 Gy) of irradiation on the accessibility of amino acid residues has been unequivocally demonstrated. The data can be interpreted as swelling of the compact folding and increase in the surface hydrophilicity of the recombinant enzyme. In the case of native enzyme, irradiation does not cause remarkable changes in the accessibility of amino acid residues indicating the possible extensive radical modification of the native enzyme in the life-course of the cell. The catalytic histidine is an exception. It becomes inaccessible after the enzyme irradiation, while its accessibility in the recombinant enzyme increases. An additional observation of a 5-fold decrease in the rate constant towards hydrogen peroxide points to the destructive effect of irradiation on the hydrogen bond network in the distal domain of the native enzyme molecule and partial collapse of the active site pocket.

Significance of low-frequency local fluctuation motions in the transmembrane B and C alpha-helices of bacteriorhodopsin, to facilitate efficient proton uptake from the cytoplasmic surface, as revealed by site-directed solid-state 13C NMR

13C NMR spectra of [1-13C]Val- or -Pro-labeled bacteriorhodopsin (bR) and its single or double mutants, including D85N, were recorded at various pH values to reveal conformation and dynamics changes in the transmembrane alpha-helices, in relation to proton release and uptake between bR and the M-like state caused by modified charged states at Asp85 and the Schiff base (SB). It was found that the D85N mutant acquired local fluctuation motion with a frequency of 10(4) Hz in the transmembrane B alpha-helix, concomitant with deprotonation of SB in the M-like state at pH 10, as manifested from a suppressed 13C NMR signal of the [1-13C]-labeled Val49 residue. Nevertheless, local dynamics at Pro50 neighboring with Val49 turned out to be unchanged, irrespective of the charged state of SB as viewed from the 13C NMR of [1-13C]-labeled Pro50. This means that the transmembrane B alpha-helix is able to acquire the fluctuation motion with a frequency of 10(4) Hz beyond the kink at Pro50 in the cytoplasmic side. Concomitantly, fluctuation motion at the C helix with frequency in the order of 10(4) Hz was found to be prominent, due to deprotonation of SB at pH 10, as viewed from the 13C NMR signal of Pro91. Accordingly, we have proposed here a novel mechanism as to proton uptake and transport based on a dynamic aspect that a transient environmental change from a hydrophobic to hydrophilic nature at Asp96 and SB is responsible for the reduced p Ka value which makes proton uptake efficient, as a result of acquisition of the fluctuation motion at the cytoplasmic side of the transmembrane B and C alpha-helices in the M-like state. Further, it is demonstrated that the presence of a van der Waals contact of Val49 with Lys216 at the SB is essential to trigger this sort of dynamic change, as revealed from the 13C NMR data of the D85N/V49A mutant.

Effect of micellar charge on the conformation and dynamics of melittin

Electrostatic interactions play a crucial role in modulating and stabilizing molecular interactions in membranes and membrane-mimetic systems such as micelles. We have monitored the change in the conformation and dynamics of the cationic hemolytic peptide melittin bound to micelles of various charge types, utilizing fluorescence and circular dichroism (CD) spectroscopy. The sole tryptophan of melittin displays a red-edge excitation shift (REES) of 3-6 nm when bound to anionic, nonionic, and zwitterionic micelles. This suggests that melittin is localized in a restricted environment, probably in the interfacial region of the micelles, and this region offers considerable restriction to the reorientational motion of the solvent dipoles around the excited state tryptophan in melittin. Further, the rotational mobility of melittin is considerably reduced in these micelles and is found to be dependent on the surface charge of micelles. Interestingly, our results show that melittin does not partition into cetyltrimethylammonium bromide (CTAB) micelles owing to electrostatic repulsion between melittin and CTAB micelles, both of which carry a positive charge. In addition, the fluorescence lifetime of melittin is modulated in micelles of different charge types. The lowest mean fluorescence lifetime is observed in the case of melittin bound to anionic sodium dodecyl sulfate (SDS) micelles. CD spectroscopy shows that micelles induce significant helicity to melittin, with maximum helicity being induced in the case of melittin bound to SDS micelles. Fluorescence quenching measurements using the neutral aqueous quencher acrylamide show differential accessibility of melittin in various types of micelles. Taken together, our results show that micellar surface charge can modulate the conformation and dynamics of melittin. These results could be relevant to understanding the role of the surface charge of membranes in the interaction of membrane-active, amphiphilic peptides with membranes.

Influence of beta-radiation sterilisation in properties of new chitosan/soybean protein isolate membranes for guided bone regeneration

Novel chitosan (cts) and soybean protein isolate (SI) blended membranes were prepared. These membranes were produced by solvent casting. Besides combining the advantages of both materials, cts/SI membranes exhibit a biphasic structure that will eventually originate in situ porous formation, through a two-step degradation mechanism. In this particular work the effect of beta-radiation over the properties of these membranes was evaluated. beta-radiation sterilisation was performed at three different doses (25, 50 and 100 kGy) and eventual surface chemical changes were evaluated by Fourier transformed infrared--with attenuated total reflection and contact angle measurements. Moreover, eventual bulk properties changes due to beta-radiation were assessed by means of mechanical tensile tests and water uptake measurements. In general, no substantial changes were detected on the studied properties, with the exception of the surface energy that was found to be slightly increased for higher applied doses.

Effect of external stresses on protein conformation: a computer modelling study

The increasing use of digital technologies such as mobile phones has led to major health concerns about the effects of non-ionizing pulsed radiation exposure. We believe that the health implications of exposure to radiation cannot be fully understood without establishing the molecular mechanisms of biological effects of pulsed microwaves. We aim to establish methods for studying the molecular mechanisms of protein structural and energetic changes occurring due to external stresses related to non-ionizing radiation by using a combination of experimental and theoretical approaches. In this paper, we present the results from our fully atomistic simulation study of chemical and thermal stress response of a prototype protein, insulin. We performed a series of molecular dynamics simulations of insulin in solution under equilibrium conditions, under chemical stress (imitated by reducing the disulfide bonds in the protein molecule), and under short-lived thermal stress (imitated by increasing simulation temperature for up to 2 ns). The resultant protein conformational behaviour was analysed for various properties with the aim of establishing analysis routines for classification of protein unfolding pathways and associated molecular mechanisms.

Circular dichroism spectra of human hemoglobin reveal a reversible structural transition at body temperature

Previously we have shown that human red blood cells (RBCs) undergo a sudden change from blocking to passing through a 1.3+/-0.2-microm micropipette when applying an aspiration pressure of 2.3 kPa at a critical transition temperature (Tc = 36.4+/-0.3 degrees C). Low-shear viscosity measurements suggested that changes in the molecular properties of hemoglobin might be responsible for this effect. To evaluate structural changes in hemoglobin at the critical temperature, we have used circular dichroism (CD) spectroscopy. The thermal denaturation curves of human hemoglobin A (HbA) and hemoglobin S (HbS) upon heating between 25 and 60 degrees C were non-linear and showed accelerated denaturation between 35 and 39 degrees C with a midpoint at 37.2+/-0.6 degrees C. The transition was reversible below 39 degrees C and independent of solution pH (pH 6.8-7.8). It was also independent of the oxygenation state of hemoglobin, since a sample that was extensively deoxygenated with N2 showed a similar transition by CD. These findings suggest that a structural change in hemoglobin may enable the cellular passage phenomenon as well as the temperature-dependent decrease in viscosity of RBC solutions.

Soft X-Ray-Induced Decomposition of Amino Acids: An XPS, Mass Spectrometry, and NEXAFS Study

Decomposition of five amino acids, alanine, serine, cysteine, aspartic acid, and asparagine, under irradiation with soft X rays (magnesium Kalpha X-ray source) in ultra-high vacuum was studied by means of X-ray photoelectron spectrometry (XPS) and mass spectrometry. A comparative analysis of changes in XPS line shapes, stoichiometry and residual gas composition indicates that the molecules decompose by several pathways. Dehydration, decarboxylation, decarbonylation, deamination and desulfurization of pristine molecules accompanied by desorption of H2, H2O, CO2, NH3 and H2S are observed with rates depending on the specific amino acid. NEXAFS spectra of cysteine at the carbon, oxygen and nitrogen K-shell and sulfur L2,3 edges complement the XPS and mass spectrometry data and show that the exposure of the sample to an intense soft X-ray synchrotron beam results in the formation of C-C and C-N double and triple bonds. Qualitatively, the amino acids studied can be arranged in the following ascending order of radiation stability: serine<alanine<aspartic acid<cysteine<asparagine.

Recording of blue light-induced energy and volume changes within the wild-type and mutated phot-LOV1 domain from Chlamydomonas reinhardtii

The time-resolved thermodynamics of the flavin mononucleotide (FMN)-binding LOV1 domain of Chlamydomonas reinhardtii phot (phototropin homolog) was studied by means of laser-induced optoacoustic spectroscopy. In the wild-type protein the early red-shifted intermediate LOV(715) exhibits a small volume contraction, DeltaV(715) = -1.50 ml/mol, with respect to the parent state. LOV(715) decays within few micro s into the covalent FMN-Cys-57 adduct LOV(390), that shows a larger contraction, DeltaV(390) = -8.8 ml/mol, suggesting a loss of entropy and conformational flexibility. The high energy content of LOV(390), E(390) = 180 kJ/mol, ensures the driving force for the completion of the photocycle and points to a strained photoreceptor conformation. In the LOV-C57S mutated protein the photoadduct is not formed and DeltaV(390) is undetected. Large effects on the measured DeltaVs are observed in the photochemically competent R58K and R58K/D31Q mutated proteins, with DeltaV(390) = -2.0 and -1.9 ml/mol, respectively, and DeltaV(715) approximately 0. The D31Q and D31N substitutions exhibit smaller but well-detectable effects. These results show that the photo-induced volume changes involve the protein region comprising Arg-58, which tightly interacts with the FMN phosphate group.

X-ray induced reduction of the crystal of high-molecular-weight cytochrome c revealed by microspectrophotometry

The crystal structures of high-molecular-weight cytochrome c (HMC) from Desulfovibrio vulgaris Hildenborough in the transient and reduced states have been determined at 2.8 A resolution. An absorption spectrum measured with microspectrophotometer indicated that about 86% of the hemes were reduced after 45 min irradiation of the X-ray beam. Further exposure for 90 min did not significantly change the spectrum. These results suggest that HMC in the crystalline state is easily reduced by illumination of the X-ray beam from synchrotron radiation.

RF absorption involving biological macromolecules

The fundamental intramolecular frequency of a globular protein can be obtained from the measurements of acoustic velocities of bulk protein matter. This lowest frequency for common size molecules is shown to be above several hundred GHz. All modes below this frequency would then be intermolecular modes or bulk modes of the molecule and surrounding matter or tissue. The lowest frequency modes of an extended DNA double helix are also shown to be bulk modes because of interaction with water. Only DNA modes, whose frequency is well above 4 GHz, can be intrahelical modes, that is, confined to the helix rather than in the helix plus surroundings. Near 4 GHz, they are heavily damped and, therefore, not able to resonantly absorb. Modes that absorb radio frequency (RF) below this frequency are bulk modes of the supporting matter. Bulk modes rapidly thermalize all absorbed energy. The implication of these findings for the possibility of athermal RF effects is considered. The applicability of these findings for other biological molecules is discussed.

Gamma irradiation of intravenous immunoglobulin

This paper describes gamma irradiation of a biotherapeutic product under conditions (the Clearant Process") that protect proteins and foster inactivation of viruses and other pathogens. The treated product was immunoglobulin paste from cold ethanol fractionation of human plasma, a process intermediate in the production of intravenous immunoglobulin (IGIV). The frozen paste was irradiated on dry ice to 45 kGy, conditions that inactivate > or = 4 log10 of non-enveloped viruses and > or = 6 log10 of enveloped viruses. When IGIV purified from the irradiated paste was characterized, no protein aggregation, fragmentation, oxidation or denaturation was detected and Fab functionality remained intact.

Effects of 2.45 GHz microwave fields on liposomes entrapping glycoenzyme ascorbate oxidase: Evidence for oligosaccharide side chain involvement

Previous observations reported by our group indicate that 2.45 GHz microwave fields at specific absorption rate (SAR) of 5.6 W/kg reduce the enzyme activity rate of ascorbate oxidase (AO) trapped in liposomes. In this study, we report dose-response studies on these AO containing liposomes irradiated at different SAR values (1.4, 2.8, 4.2, and 5.6 W/kg). No response was observed for SAR below 5.6 W/kg. Liposomes entrapping functional AO in its deglycated form (AO-D) were also used. In this case, no MW related enzyme activity changes were observed, demonstrating a direct involvement of oligosaccharide chains of AO. Furthermore, the catalytic properties of both AO and AO-D were not impaired by MW irradiation, neither in homogeneous solution nor loaded in liposomes, excluding possible changes in the conformation of enzyme as a mechanism. Our results suggest that the oligosaccharide chains of AO are critical to elicit the microwave observed effects on lipid membrane.

Non-thermal effects of electromagnetic fields at mobile phone frequency on the refolding of an intracellular protein: Myoglobin

Non-thermal effects induced by exposure to microwave electromagnetic field (MW-EMF) at 1.95 MHz, a frequency used in mobile communication, have been observed on the refolding kinetics of the heme binding site in an intracellular protein: tuna myoglobin, starting from acidic conditions. We have selected myoglobin because it can be considered a good model to study protein interactions with MW-EMF for its well-known high-resolution crystallographic structure. Myoglobin solutions at pH 3.0 were subjected to 3 h exposure to microwave field (with a specific absorption rate of 51 +/- 1 mW/g); the heme site refolding has been followed by measuring the molecular absorption in the Soret spectral region and the data were fitted to a bi-exponential model. The kinetics of exposed samples appear to be slowered by MW-EMF action. Moreover, the tryptophanyl lifetime distribution of the exposed protein, as deduced by the analysis of the fluorescence emission decay from its single tryptophan, appears sharper if compared to non-exposed protein samples. This observation suggests that the presence of MW-EMF could affect the propensity of protein molecules to populate specific conformational substates among which myoglobin molecules fluctuate at acidic pH. Changes in the structural fluctuation caused by MW perturbation can affect differently the aggregation process that occurs competitively during the protein folding, so representing a potential risk for protein "misfolding." These data suggest that MW-EMF could have also biochemical and, consequently, biological effects on eukaryotic cells that are still under investigation.

Photodynamics of red fluorescent proteins studied by fluorescence correlation spectroscopy

Red fluorescent proteins are important tools in fluorescence-based life science research. Recently, we have introduced eqFP611, a red fluorescent protein with advantageous properties from the sea anemone Entacmaea quadricolor. Here, we have studied the submillisecond light-driven intramolecular dynamics between bright and dark states of eqFP611 and, for comparison, drFP583 (DsRed) by using fluorescence correlation spectroscopy on protein solutions. A three-state model with one dark and two fluorescent states describes the power-dependence of the flickering dynamics of both proteins at different excitation wavelengths. It involves two light-driven conformational transitions. We have also studied the photodynamics of individual (monomeric) eqFP611 molecules immobilized on surfaces. The flickering rates and dark state fractions of eqFP611 bound to polyethylene glycol-covered glass surfaces were identical to those measured in solution, showing that the bound FPs behaved identically. A second, much slower flickering process was observed on the 10-ms timescale. Deposition of eqFP611 molecules on bare glass surfaces yielded bright fluorescence without any detectable flickering and a >10-fold decreased photobleaching yield. These observations underscore the intimate connection between protein motions and photophysical processes in fluorescent proteins.

Energetics of echinomycin binding to DNA

Differential scanning calorimetry and UV thermal denaturation have been used to determine a complete thermodynamic profile for the bis-intercalative interaction of the peptide antibiotic echinomycin with DNA. The new calorimetric data are consistent with all previously published binding data, and afford the most rigorous and direct determination of the binding enthalpy possible. For the association of echinomycin with DNA, we found DeltaG degrees = -7.6 kcal mol(-1), DeltaH = +3.8 kcal mol(-1) and DeltaS = +38.9 cal mol(-1) K(-1) at 20 degrees C. The binding reaction is clearly entropically driven, a hallmark of a process that is predominantly stabilized by hydrophobic interactions, though a deeper analysis of the free energy contributions suggests that direct molecular recognition between echinomycin and DNA, mediated by hydrogen bonding and van der Waals contacts, also plays an important role in stabilizing the complex.

Are the conformational dynamics and the ligand binding properties of myoglobin affected by exposure to microwave radiation?

The global uptake of mobile communication emphasizes the question about possible adverse consequences of the exposure to low-level radiofrequency radiation from mobile phones on human health as result of so-called "non-thermal effects". In order to state safety guidelines it seems appropriate to start by excluding, if possible, non-specific effects on structural and dynamic properties of fundamental biomolecules such as proteins. Proteins are flexible polyelectrolytes; thus, they are susceptible, in principle, to the action of electromagnetic fields. In this article, we investigated the effects of microwaves on structural and functional properties of Tunnus tynnus myoglobin at 1.95 GHz, a frequency used by new wireless microwave communication systems. The protein solution was exposed for 2.5 h to 51 mW/g SAR (specific absorption rate) level. Measurements of absorption spectroscopy, circular dichroism and fluorescence emission decay in the frequency domain do not exhibit any influence of the radiation on the native structural state of protein macromolecules.

Away from the edge: SAD phasing from the sulfur anomalous signal measured in-house with chromium radiation

Anomalous scattering with soft X-ray radiation opens new possibilities in phasing for macromolecular crystallography. Anomalous scattering from S atoms collected on an in-house chromium radiation source (lambda = 2.29 A) was used to phase the X-ray diffraction data of thaumatin (22 kDa) and trypsin (24 kDa) crystals. The contribution to the anomalous term, Deltaf" = 1.14 e(-), from sulfur for Cr Kalpha radiation is doubled compared with that for Cu Kalpha radiation, Deltaf" = 0.56 e(-). The direct-methods programs RANTAN or SHELXD successfully found sulfur positions using data sets with resolution limited to 3.5 A. The statistical phasing program SHARP was used to produce the electron-density maps using the sulfur anomalous signal alone at low resolution ( approximately 3.5 A). An interpretable electron-density map for each structure was obtained solely from the phases derived from single-wavelength anomalous dispersion (SAD) data obtained using Cr Kalpha radiation. Much fewer data (that is, lower redundancy) are required for this sulfur SAD phasing procedure compared with the highly redundant data reported in the sulfur SAD phasing procedure with Cu Kalpha radiation. Cr Kalpha radiation can also improve the strength of anomalous scattering of many other intrinsic elements in macromolecules, such as calcium, zinc and phosphorus, because of the increased Deltaf". Furthermore, the anomalous scattering of selenium is increased substantially from 1.14 e(-) with Cu Kalpha radiation to 2.28 e(-) with Cr Kalpha radiation. In order to measure the small Bijvoet differences accurately, several devices were developed for the experiment, including an Osmic Confocal MaxFlux optic optimized for Cr Kalpha radiation, a helium path and a beam stop. In the cases studied here, radiation damage to the samples and reduction of anomalous signal were observed in some long exposure time data sets. Therefore, an adequate data-collection strategy to maximize the completeness in a short scan range was used in subsequent data collections. The results show that the anomalous signal of S atoms can be collected quickly. Since the absorption of solvent and the loop may no longer be negligible with Cr Kalpha radiation, the orientation of the crystal and exposure time were taken into account in order to minimize the effects of radiation damage and absorption. This experimental study shows that using Cr Kalpha radiation from an in-house rotating-anode X-ray generator can provide sufficient phasing power from sulfur anomalous signals to routinely phase protein diffraction data.

Optimizing data collection for structure determination

The ultimate purpose of diffraction data collection is to produce a data set which will result in the required structural information about the molecule of interest. This usually entails collecting a complete and accurate set of reflection intensities to as high a resolution as possible. In practice, the characteristics of the crystal and properties of the X-ray source can be limiting factors to the data-set quality that can be achieved and a careful strategy has to be used to extract the maximum amount of information from the data within the experimental constraints. In the particular case of data intended for phasing using anomalous dispersion, the synchrotron beamline properties are relevant to determine how many wavelengths (one or more) should be used for the experiment and what the wavelength values should be. This will in turn affect the detailed strategy for data collection, including decisions about the data-collection sequence and how much data to collect at each wavelength. Collection of multiwavelength anomalous dispersion (MAD) data at three different wavelengths can provide very accurate experimental phases. Two-wavelength MAD experiments may offer the best compromise between phase quality and minimizing the effects of radiation damage to the sample. However, MAD experiments are demanding in terms of beamline wavelength range, easy tunability, stability and reproducibility. When the beamline cannot fulfill these demands, single-wavelength experiments may be a better option.

Effects of ELF magnetic field on membrane protein structure of living HeLa cells studied by Fourier transform infrared spectroscopy

The effects of exposure to a 50 Hz magnetic field (maximum of 41.7 to 43.6 mT) on the membrane protein structures of living HeLa cells were studied using attenuated total reflection infrared spectroscopy. One min of such exposure shifted peak absorbance of the amide I band to a smaller wave number, reduced peak absorbance of the amide II band, and increased absorbance at around 1600 cm(-1). These results suggest that exposure to the ELF magnetic field has reversible effects on the N-H inplane bending and C-N stretching vibrations of peptide linkages, and changes the secondary structures of alpha-helix and beta-sheet in cell membrane proteins.

Spectroscopic characterization of a semi-stable, charge-separated state in Cu2+-substituted reaction centers from Rhodobacter sphaeroides

In reaction centers from Rhodobacter sphaeroides exposed to continuous illumination in the presence of an inhibitor of the Q(A)(-) to Q(B) electron transfer, a semi-stable, charge-separated state was formed with halftimes of formation and decay of several minutes. When the non-heme iron was replaced by Cu(2+), the decay of the semi-stable, charge-separated state became much slower than in centers with bound Fe(2+) with about the same rate constant for formation. In Cu(2+)-substituted reaction centers, the semi-stable state was associated with an EPR signal, significantly different from that observed after chemical reduction of the acceptor-side quinone or after illumination at low temperature, but similar to that of an isolated Cu(2+) in the absence of magnetic interaction. The EPR results, obtained with Cu(2+)-substituted reaction centers, suggest that the slow kinetics of formation and decay of the charge-separated, semi-stable state is associated with a structural rearrangement of the acceptor side and the immediate environment of the metal-binding site.

Evidence for spontaneous structural changes in a dark-adapted state of photosystem II

Photosystem II catalyzes photosynthetic water oxidation in plants, green algae, and cyanobacteria. The manganese-containing active site cycles through a series of five oxidation states, S(n), where n refers to the number of oxidizing equivalents stored. In this report, reaction-induced Fourier transform infrared and electron paramagnetic resonance spectra of the S(1)-to-S(2) transition are presented. These data suggest that changes in carboxylate ligation to manganese, changes in secondary structure, and/or changes in polarity occur during dark adaptation in the S(1) state. These spontaneous structural changes are attributed to a S(1)' intermediate, at the same oxidation level as S(1), in the process of photosynthetic water oxidation.

Electron transport dynamics at the quinone acceptor site of bacterial photosynthetic reaction centers as probed using fast temperature changes

Methods of laser-induced temperature jumps and fast freezing were used for testing the rates of thermoinduced conformational transitions of reaction center (RC) complexes in chromatophores and isolated RC preparations of various photosynthesizing purple bacteria. An electron transfer reaction from primary to secondary quinone acceptors was used as a probe of electron transport efficiency. The thermoinduced transition of the acceptor complex to the conformational state facilitating electron transfer to the secondary quinone acceptor was studied. To investigate the dynamics of spontaneous decay of the RC state induced by the thermal pulse, the thermal pulse was applied either before or during photoinduced activation of electron transport reactions in the RC acceptor complex. The maximum effect was observed if the thermal pulse was applied against the background of steady-state photoactivation of the RC. It was shown that neither the characteristic time of the thermoinduced transition within the temperature range 233-253 K nor the characteristic time of spontaneous decay of this state at 253 K exceeded several tens of milliseconds. Independent support of the estimates was obtained from experiments with varied cooling rates of the samples tested.