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1.
Photosynthetic reaction centers (RCs) fromRhodobacter sphaeroides capture solar energy by electron transfer from primary donor D to quinone acceptor QB through the active A branch of electron acceptors. The light-induced electron paramagnetic resonance (EPR) spectrum from native RCs that had Fe2+ replaced by Zn2+ was investigated at cryogenic temperature (80 K, 35 GHz). In addition to the light-induced signal due to the formation of D+.Q A ?. observed previously, a small fraction (ca. 5%) of the signal displayed very different characteristics. The signal was absent in RCs in which the QB was displaced by the inhibitor stigmatellin. Its decay time (τ=6 s) was the same as observed for D+.Q B ?. in mutant RCs lacking QA, which is significantly slower than for D+.Q A ?. (τ=30 ms). Its EPR spectrum was identical to that of D+.Q B ?. . The quantum efficiency for forming the major component of the signal was the same as that found for mutant RCs lacking QA (?=0.2%) and was temperature independent. These results are explained by direct photochemical reduction of QB via B branch electron transfer in a small fraction of native RCs. 相似文献
2.
Influence of heme periphery upon electronic structúre and Mössbauer spectra parameters of hemoglobin
Quantum-chemical analysis of electronic structure peculiarities and mossbauer spectra parameters was performed for penta-coordinated
complex of ferro-protoporphyrin with imidazole (Fe(+2)PPIm). Peripheral substitutes (−CH3, −C2H3, −C2H4COOH) introduced in the porphyrin macrocycle simulated real chemical structure of protoporphyrin (PP) in heme group of Hb.
Calculations displayed that doubly occupied molecular orbitals (MO) of the peripheral substitutes (−CH=CH2 and −CH2−CH2−COOH) always appeared near the occupat ion border. The orientation of vinyl fragment have the essential influence upon Fe5 dorbital populations and quadrupole splittingAEQ for5B1 and5B2 terms. The values of isomer shift were insensitive to that modification of fragment orientation. 相似文献
3.
The concept of introducing an additional, stable paramagnetic species into photosynthetic reaction centres to increase the information content of their spin polarized transient EPR spectra is investigated theoretically. The light-induced electron transfer in such systems generates a series of coupled three-spin states consisting of sequential photoinduced radical pairs coupled to the stable spin which acts as an “observer”. The spin polarized transient EPR spectra are investigated using the coupled three-spin system P+I−Q− A in pre-reduced bacterial reaction centres as a specific example which has been studied experimentally. The evolution of the spin system and the spin polarized EPR spectra of P+I−Q− A and Q− A following recombination of the radical pair (P = primary donor, I = primary acceptor, QA = quinone acceptor) are calculated numerically by solving the equations of motion for the density matrix. The net polarization of the observer spin is also calculated analytically by perturbation theory for the case of a single, short-lived, charge-separated state. The result bears a close resemblance to the chemically induced nuclear polarization (CIDNP) generated in photolysis reactions in which a nuclear spin plays the role of the observer interacting with the radical pair intermediates. However, because the Zeeman frequencies of the three electron spins involved are usually quite similar, the polarization of the electron observer spin in strong magnetic fields can reflect features of the CIDNP effect in both, high and low magnetic fields. The dependence of the quinone spin polarization on the exchange couplings in the three-spin system is investigated by numerical simulations, and it is shown that the observed emissive polarization pattern is compatible with either sign, positive or negative, for a range of exchange couplings, JPI, in the primary pair. The microwave frequency and orientation dependence of the spectra are discussed as two of several possible criteria for determining the sign of JPI. 相似文献
4.
We find theoretically that competition between ∼K
f
q
4 and ∼Qq
2 terms in the Fourier-transformed conformational energy of a single-lipid chain, in combination with interchain entropic repulsion
in the hydrophobic part of the lipid (bi)layer, may cause a crossover on the bilayer pressure-area isotherm P(A)∼(A−A
0)−α. The crossover manifests itself in the transition from α = 5/3 to α = 3. Our microscopic model represents a single-lipid
molecule as a worm-like chain with a finite irreducible cross-section area A
0, a flexural rigidity K
f
, and a stretching modulus Q in a parabolic potential with the self-consistent curvature B(A) formed by entropic interactions between hydrocarbon chains in the lipid layer. The crossover area A* obeys the relation Q/√K
f
B(A*) ≈ 2. We predict a peculiar possibility of deducing the effective elastic moduli K
f
and Q of an individual hydrocarbon chain from the analysis of the isotherm with such a crossover. Also calculated is the crossover-related
behavior of the area compressibility modulus K
A
, the equilibrium area per lipid A
t
, and the chain order parameter S(θ).
The text was submitted by the authors in English. 相似文献
5.
The chemical composition of ultrafine amorphous Fe−B powders prepared by a chemical reduction depends on the mixed molar ratio
of KBH4 to Fe ions. We propose the following reaction processes for the formation of ultrafine Fe−B powders: (1) 4Fe2++2BH4−+6OH−→2Fe2B+6H2O+H2 and (2) 4Fe2++2BH4−+7OH−→2Fe3B+Fe+BO2+5H2O+5/2H2. 相似文献
6.
Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) techniques were used to investigate the electronic structure of the primary (Q A ?? ) and secondary (Q B ?? ) ubiquinone electron acceptors in reaction centers (RCs) of the photosynthetic bacteriumRhodobacter sphaeroides. To reduce the EPR linewidth, the high-spin Fe2+ present in native RCs was replaced by diamagnetic Zn2+. Experiments were performed both on frozen solutions and single crystals at microwave frequencies of 9, 35 and 94 GHz. Differences in the EPR/ENDOR spectra were observed for Q A ?? and Q B ?? , which are attributed to different environments of the quinones in the RC. The differences exhibited themselves in: (i) the g-tensors, (ii) the17O and13C hyperfine coupling (hfc) constants of the quinones labeled at the carbonyl group, (iii) the1H-hfcs of the quinone ring and (iv) the exchangeable protons hydrogen bonded to the carbonyl oxygens. From these results and from H/D exchange experiments, the following conclusions were drawn: both Q A ?? and Q B ?? have at least two hydrogen bonds of different strengths to the carbonyl oxygens. The hydrogen bonds for Q A ?? are stronger and more asymmetric than for Q B ?? . For Q A ?? the stronger bond (to O4) was assigned to His(M219) and the weaker (to O1) to Ala(M260). For Q B ?? the stronger bond (to O4) was assigned to His(L190), with several weaker bonds (to O1) to Ser(L223), Ile(224) and Gly(L225). From the temperature dependence of the hfcs of the exchangeable protons some dynamic properties of the RC were deduced. Hfcs with more distant nitrogens were observed by electron spin echo envelope modulation (ESEEM). For Q A ?? they were assigned to Nδ of His(M219) and to the peptide backbone nitrogen of Ala(M260) and for Q B ?? to Nδ of His(L190). These interactions indicate the extent of the electron wave function, which is important for the understanding of the electron transfer mechanism. Based on the magnetic resonance results, the function of the quinone acceptors in the reaction center is discussed. 相似文献
7.
V. S. Pavlovich 《Journal of Applied Spectroscopy》2006,73(3):328-339
A detailed derivation is presented for relations making it possible to describe the effect of temperature on the halfwidth
of the P960 and P870 absorption bands and also on the electron transfer (ET) rate at reaction centers (RCs) of the purple
bacteria Rps. viridis and Rb. sphaeroides. Primary electron transfer is considered as a resonant nonradiative transition between
P* and P+B
L
−
states (where P is a special pair, BL is an additional bacteriochlorophyll in the L branch of the reaction center). It has been shown that the vibrational hα mode with frequency 130–150 cm−1 controls primary electron transfer. It has been found that the matrix element of the electronic transition between the states
P* and P+B
L
−
is equal to 12.7 ± 0.9 and 12.0 ± 1.2 cm−1 for Rps. viridis and Rb. sphaeroides respectively. The mechanism is discussed for electron transport from P* and BL and then to bacteriopheophytin HL.
__________
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 294–303, May–June, 2006. 相似文献
8.
R. Yu. Abdulsabirov I. I. Antonova S. L. Korableva N. M. Nizamutdinov V. G. Stepanov N. M. Khasanova 《Physics of the Solid State》1997,39(3):423-425
The orientational dependences of the EPR spectra of Fe3+-doped LiCaAlF6 single crystals (space group P31c, Z=2), grown at the Laboratory of Magnetic Radio Spectroscopy at Kazan’ State University, have been investigated in detail.
The spectrum is described by a trigonal spin Hamiltonian with the following parameters: B
20=40.072×10−4 cm−1, B
40=−5.799×10−4 cm−1, B
43=−4.281×10−4 cm−1, A
s=24.33±1, A
p=6.13±1, g
∥=g
⊥=2.00217±0.0003. A theoretical calculation of the hyperfine structure parameters shows that they are described quite well
when allowance is made for the overlapping of the wave functions of the paramagnetic center and the ligands (F−).
Fiz. Tverd. Tela (St. Petersburg) 39, 488–490 (March 1997) 相似文献
9.
B. J. van Rossum W. B. S. van Liemt P. Gast J. Lugtenburg H. J. M. de Groot 《Applied magnetic resonance》2007,31(1-2):145-158
The photosynthetic reaction center (RC) of the photosynthetic bacteriumRhodobacter sphaeroides R26 contains two quinones, QA and QB. Solid-state heteronuclear (1H?13C) dipolar correlation spectroscopy has been used to study the binding of the quinones in the ground state for RCs reconstituted with l-13C ubiquinone-10. Lee-Goldburg cross-polarization buildup curves are recorded to determine distancesr CH between the l-13C carbon labels and the protons involved in the polarization transfer. The l-13C of both QA and QB have intermolecular correlations with protons that resonate downfield, in the region of hydrogen-bonding protons. The distances between the carbon labels and the correlated protons are short, 0.21±0.01 nm. Hence the nuclear magnetic resonance provides evidence for strong hydrogen-bonding interactions at the l-C=O of both QA and QB for RCs in the ground state. The environment of the l-13C of the QB is structurally heterogeneous compared to that of the QA. The data can be reconciled with a strong H-bonding interaction of the l-C=O of QA with Ala M260 NH, and with complex hydrogen bonding involving NH of Ile-L224 and of Gly-L225, and possibly the Ser-L223 hydroxyl group of the l-C=O of the QB, in the proximal site. 相似文献
10.
The primary processes of bacterial photosynthesis take place in a membrane-bound protein called the reaction center (RC). The process involves light-induced electron transfer from a primary electron donor to a sequence of electron acceptors. The terminal acceptors, operating sequentially, are two ubiquinones, QA and QB, that couple magnetically to a high-spin (S = 2) Fe2+ forming an Fe2+-Q? complex. We have used a variety of techniques to investigate the electronic as well as some features of the spatial structure of the Fe2+-Q? complex in RCs fromRb. sphaeroides. These include: (a) static magnetization measurements in the temperature range of 0.7 to 190 K and magnetic fields up to 8 kG, (b) EPR spectroscopy at helium temperatures at 1.2, 9, and 35 GHz, (c) Mössbauer spectroscopy and (d) extended X-ray fine structure (EXAFS) determinations. The results of these measurements showed that Fe2+ resides in an asymmetric ligand field environment forming 6 ligands with a combination of oxygens and nitrogens. From the EXAFS results the distances of the Fe2+ to the first and third coordination shells were determined. These spatial features were subsequently corroborated by the X-ray structure of the RC, which showed the environment of the Fe to be a distorted octahedron, the base plane of which is formed by three N?’s of histidines and one carbonyl oxygen; the apex is formed by a fourth N? and a second carbonyl oxygen. The distances from the Fe2+ to the first and third shell were in good agreement with the values obtained from EXAFS. The most detailed information of the electronic structure of the Fe2+-Q? complex was obtained from the EPR spectra using the spin Hamiltonian formalism. The following spin-Hamiltonian parameters were obtained: for the crystalline field parameters,D = 7.60 K,E/D = 0.25; for the electronicg-values, gFe, x = 2.16, gFe, y = 2.27, gFe, z = 2.04 and for the antiferromagnetic exchange interaction,J x = ?0.13 K,J y = ?0.58 K,J z = ?0.58 K. The use of the spin Hamiltonian was validated by comparing the results obtained from it with those obtained from an exact numerical solution of the 25 lowest energy levels of the orbital Hamiltonian. Possible roles that Fe2+ may play in the function of the RC are discussed. They include a structural role in which Fe2+ liganded to four histidine nitrogens imparts a rigid structure to a four α-helix bundle and a role in the electron transfer kinetics, most notably from the intermediate acceptor I? (bacteriopheophytin) to QA. Replacement of Fe2+ by diamagnetic Zn2+ retained all observed native characteristics of the RC. 相似文献
11.
The QCD factorization theorem for diffractive processes in DIS is used to derive formulae for the leading twist contribution
to the nuclear shadowing of parton distributions in the low thickness limit (due to the coherent projectile (photon) interactions
with two nucleons). Based on the current analyzes of diffraction at HERA we find that the average strength of the interactions
which govern diffraction in the gluon sector at x≤ 10−3, Q
0= 2 GeV is ∼50mb. This is three times larger than in the quark sector and suggests that applicability of DGLAP approximation
requires significantly larger Q
0 in the gluon sector. We use this information on diffraction to estimate the higher order shadowing terms due to the photon
interactions with N≥ 3 nucleons which are important for the scattering of heavy nuclei and to calculate nuclear shadowing and Q
2 dependence of gluon densities. For the heavy nuclei the amount of the gluon shadowing: G
A(x,Q
0
2) /AG
N(x,Q
0
2)|x ≤ 10−3∼ 0.25–0.4 is sensitive to the probability of the small size configurations within wave function of the gluon “partonometer”
at the Q
0 scale. At this scale for A∼ 200 the nonperturbative contribution to the gluon density is reduced by a factor of 4–5 at x≤ 10−3 unmasking PQCD physics in the gluon distribution of heavy nuclei. We point out that the shadowing of this magnitude would
strongly modify the first stage of the heavy ion collisions at the LHC energies, and also would lead to large color opacity
effects in eA collisions at HERA energies. In particular, the leading twist contribution to the cross section of the coherent
J/ψ production off A≥ 12 nuclei at s
−2≥ 70 GeV is strongly reduced as compared to the naive color transparency expectations. The Gribov black body limit for F
2A(x,Q
2) is extended to the case of the gluon distributions in nuclei and shown to be relevant for the HERA kinematics of eA collisions.
Properties of the final states are also briefly discussed.
Received: 12 March 1999 相似文献
12.
Summary Point defects in annealed B2-phase FeAl samples in the range 47–53 at.% Fe were studied using57Fe M?ssbauer spectroscopy. Spectra were analyzed using local environment models according to which point defects in atomic
shells close to probe atoms induce shifts in the nuclear monopole interaction. For well-annealed samples, better results were
obtained assuming only the presence of FeAl antisite and VFe vacancy defects, and not of AlFe antisite defects. Monopole interactions of57Fe probes on the Fe and Al sublattices having no defects in the first two shells were about +0.27 and −0.03 mm s−1, respectively, with respect to Fe in alpha-Fe metal. The shifts induced by FeAl and VFe defects in the first shells of Fe probes on the Fe and Al sublattices were −0.15 and −0.24 mm s−1, respectively, and, in the second shells, +0.06 and +0.011 mm s−1. In addition to structural defects needed to accommodate deviations from stoichiometry, annealed samples were found to contain
several percents of FeAl and VFe defects due to lattice disorder, with greater disorder in Fe-deficient alloys.
Paper presented at the ICAME-95, Rimini, 10–16 September 1995. 相似文献
13.
Magnetic properties, arising from surface exchange and interparticle interactions of the Fe3O4 (magnetite) nanoparticles, were investigated in the temperature range of 5–300 and 120–300 K using vibrating sample magnetometer
technique and electron spin resonance spectroscopy, respectively. The research was based on to figure out the origin of intraparticle
interactions and the change of interparticle interactions in wide size range Fe3O4 nanoparticles. The analyses were done for samples having almost same particle size distributions. The average particle sizes
were changed in between 30 ± 2 and 34 ± 2 nm. The observed magnetization values were demonstrated the mixture of single-domain
size particles, exhibiting both single-domain (SD) and superparamagnetic (SPM) states. The symmetry of resonance curves changed
according to the ratio of SD and SPM-stated particles in mixture under located temperature. The changes of anisotropy up to
domain state were understood by freezing magnetic moment in glycerol matrix from room temperature to 120 K under 5-kG field.
The shift of H
R values to higher magnetic fields and the more symmetric resonance spectrum proved the effect of anisotropy and interparticle
interactions fields on magnetic behave. In addition, the origin of intra-interaction was exposed from Fe3+ centers and exchange coupling in between Fe2+, Fe3+, and O−, and Fe3+ centers found from g factor (g). 相似文献
14.
L. L. Feezel F. Reiss-Husson I. Agalidis U. H. Smith M. C. Thurnauer J. R. Norris 《Applied magnetic resonance》1990,1(2):255-265
Recently, a general model has been developed to explain electron spin polarized (ESP) electron paramagnetic resonance (EPR) signals found in systems where radical pairs are formed sequentially. The photosynthetic bacterial reaction center (RC) is such a system in which we can experimentally vary parameters (lifetime, structure, and magnetic interactions in the sequentially formed radical pairs) that affect ESP development in order to test this model. In Fe2+-depleted transfer step from intermediate radical pair, P 870 + Q a ? which is produced in an electron transfer step from intermediate radical pair, P 870 + I?. (P 870 + is the oxidized primary donor, a special pair of bacteriochlorophyll molecules, I? is the reduced bacteriopheophytin acceptor, and Q a ? is the reduced primary quinone acceptor.) The lifetime of P 870 + I? can be shortened relative to the lifetime of P 870 + I? in Fe2+-depleted RCs by substitution of Zn2+. We report the first observation of X-band and Q-band ESP EPR signals due to P 870 + Q? from bacterial reaction centers that contain Zn2+. Comparison of these signals to those observed from Fe2+-depleted bacterial reaction centers shows intensity differences and g-factor shifts. The results are discussed in terms of the general sequential radical pair model. 相似文献
15.
J. -M. Gaite V. V. Izotov S. I. Nikitin S. Y. Prosvirnin 《Applied magnetic resonance》2001,20(3):307-315
Two synthetic beryls (Al2Be3Si6O18) of different color (purple and blue-green) were studied with electron paramagnetic resonance (EPR) and optical spectroscopy.
In both crystals, the known spectra of Cu2+ and Fe3+ were observed with the same relative intensity. In the purple sample heated at 700°C in hydrogen atmosphere, two different
kinds of Mn2+ EPR spectra were observed. The main one is pseudoaxial, it arises from ions substituted for Al3+ at position 4c of the structure. The weaker one is more complex, it has orthorhombic symmetry and is characterized by an
unusually large zero-field splitting (B
20 = 741 · 10−4 cm−1) and an isotropic hyperfine constantA = 70 G. This spectrum arises from Mn2+ at position 6f in the structure, normally occupied by Be. From optics, the blue-green color arises from Cu2+, while the purple one is due to Mn3+. 相似文献
16.
Structural factors that provide localization of excited states and determine the properties of primary donor and acceptor of electron in the reaction center of photosystem II (PSII RC) are studied. The results of calculations using stationary and time-dependent density functional theory indicate an important role of protein environments of chlorophylls PA, PB, BA, and BB and pheophytins HA and HB in the area with a radius of no greater than ≤10 Å in the formation of excitonic states of PSII RC. When the neighboring elements are taken into account, the wavelength of long-wavelength Q y transition of chlorophyll molecules is varied by about 10 nm. The effect is less developed for pheophytin molecules (Δλ ? 2 nm). The following elements strongly affect energy of the transition: HisA198 and HisD197 amino-acid residues that serve as ligands of magnesium atoms affect PA and PB, respectively; MetA183 affects PA; MetA172 and MetD198 affect BA; water molecules that are located above the planes of the BA and BB macrocycles form H bonds with carbonyl groups; and phytol chains of PA and PB affect BA, BB, HA, and HB. The analysis of excitonic states, mutual positions of molecular orbitals of electron donors and acceptors, and matrix elements of electron transfer reaction shows that (i) charge separation between BA and HA and PB and BA is possible in the active A branch of cofactors of PSII RC and (ii) electron transfer is blocked at the BB - HB fragment in inactive B branch of PSII RC. 相似文献
17.
Yu. V. Zorenko M. M. Batenchuk V. I. Gorbenko M. V. Pashkovskii I. V. Konstankevich 《Journal of Applied Spectroscopy》1999,66(6):953-959
We consider the factors determining the energy yield of crystallophosphors based on single-crystal films of oxides of the
system Al2−Y2O3−R2O3 (R is the rare-earth ions) that are used as different types of luminescent converters. Special features of producing the
films by the method of liquid-phase epitaxy from Pb−B2O3 and Bi2O3-based fluxes are analyzed, and the advantages of this method for production of efficient luminophors, which are based on
high-melting oxides doped with mercury-like impurities, are shown. It is established that the main factor that bounds the
crystallophosphor luminous yield is the presence of donor-acceptor complexes of the Pb2+−Pb4+ and Fe3+−Fe2+-type that form different channels of dissipation of excitation energy. The means of minimizing the contribution of these
ions to the processes of excitation-energy relaxation are discussed.
Institute of Applied Physics, I. Franko L'vov State University, 49 General Chuprynka Str., L'vov, 290044, Ukraine. Translated
from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 6, pp. 819–823, November–December, 1999. 相似文献
18.
The magnetic properties of the spinel series GexCu1?xFe2O4 (X = 0 to 0.8) have been investigated by means of Mössbauer spectroscopy. Mössbauer spectra for X = 0.0 to 0.6 suggest the existence of two hyperfine fields, one due to the Fe3+ tetrahedral ions (A-sites) and the other due to Fe3+ octahedral ions (B-sites), while for X = 0.8 it shows the superposition of hyperfine field split spectra from A- and B-site ions and a broad central line spectrum. For 0.2 ? X ? 0.4, fast electron exchange among octahedral iron ions occurs as in Fe3O4. The variations of nuclear magnetic fields at the A- and B-sites are explained on the basis of AB and BB supertransferred hyperfine interactions. 相似文献
19.
Ramesh Narayan 《Pramana》1981,17(1):13-23
A simple theory is developed which shows that the regions of stability of the CsCl, NaCl and ZnS structures can be demarcated
in a two-dimensional plot of the radius ratio versus the strength of the van der Waals interaction. There is good agreement
with experiment. The effect of pressure on these structures is explained qualitatively. The increased occurrence of the ZnS
structure and the decreased stability of the CsCl structure in the A2+ B2− crystals compared to the A+B− crystals is also explained. Finally it is shown that the radius ratio and the polarizabilities of the ions are the important
factors that determine the structures of AB2 crystals. 相似文献
20.
The serum albumin is the most abundant protein in blood plasma and the iron is essential for many cellular processes. However,
the interaction between Fe3+ and haem-free serum albumin remains unclear. Here we provide evidence for the fact that haem-free BSA possesses one specific
Fe3+-binding site. The binding of Fe3+ to BSA results in a significant quenching of the Trp fluorescence of BSA. The average apparent dissociation constant value
for the interaction of Fe3+ and BSA is 3.46 × 10−8 ± 3 × 10−10 M at 37 °C and 3.30 × 10−8 ± 5 × 10−10 M at 25 °C, respectively, as determined by fluorescence titration. Addition of 50 μM Fe2+ to 1 μM BSA results in an obvious hysteretic effect on the fluorescence of BSA. The time-dependent fluorescence quenching
of BSA by Fe2+ is not caused by the Fe2+-induced conformational change of BSA, but the oxygen-dependent oxidation of Fe2+ to Fe3+. Fe2+ undergoes an oxygen-dependent oxidation to Fe3+ under aerobic conditions, which is accelerated by the interaction of BSA with Fe3+ and extensively inhibited under anaerobic conditions. The results suggest that BSA may take part in non-transferrin bound
iron transfer. 相似文献