Atomic structure and structural stability of Sc75Fe25 nanoglasses.

Nanoglasses are solids consisting of nanometer-sized glassy regions connected by interfaces having a reduced density. We studied the structure of Sc(75)Fe(25) nanoglasses by electron microscopy, positron annihilation spectroscopy, and small-/wide-angle X-ray scattering. The positron annihilation spectroscopy measurements showed that the as-prepared nanoglasses consisted of 65 vol% glassy and 35 vol% interfacial regions. By applying temperature annealing to the nanoglasses and measuring in situ small-angle X-ray scattering, we observed that the width of the interfacial regions increased exponentially as a function of the annealing temperature. A quantitative fit to the small-angle X-ray scattering data using a Debye-Bueche random phase model gave a correlation length that is related to the sizes of the interfacial regions in the nanoglass. The correlation length was found to increase exponentially from 1.3 to 1.7 nm when the sample temperature was increased from 25 to 230 °C. Using simple approximations, we correlate this to an increase in the width of interfacial regions from 0.8 to 1.2 nm, while the volume fraction of interfacial regions increased from 31 to 44%. Using micro-compression measurements, we investigated the deformation behavior of ribbon glass and the corresponding nanoglass. While the nanoglass exhibited a remarkable plasticity even in the annealed state owing to the glass-glass interfaces, the corresponding ribbon glass was brittle. As this difference seems not limited to Sc(75)Fe(25) glasses, the reported result suggest that nanoglasses open the way to glasses with high ductility resulting from the nanometer sized microstructure.

Electrical resistivity and wave character of free electrons in amorphous and nanoglass Sc75Fe25.

The residual electrical resistivity of metallic amorphous alloys, ρ 0, is typically in the range 50 µΩ cm < ρ 0 < 310 µΩ cm corresponding to a mean free path of conduction electrons of order a few interatomic distances. In crystalline metals with low defect levels such as Cu however, the residual electrical resistivity is about ρ 0 ≈ 1.54 × 10-2 µΩ cm, leading to extensive progression of free electrons through the crystalline material, of typically up to 4 × 106 nm. The relatively 'high' values for the electrical resistivity of distorted Sc75Fe25 alloys are discussed here within the framework of the wave character of electrons. The present investigation of amorphous and nanoglass Sc75Fe25 over the temperature range 1.9-320 K, focuses on clarification of the temperature dependence of the resistivity, ρ(T). These alloys systems, which show a range of behaviours for temperature dependent resistivity-including temperature independent residual resistivity, as well as positive and negative polarities for the slope dρ(T)/dT-are examined in detail.

Thermoluminescence and flash-induced oxygen yield in herbicide resistant mutants of the D1 protein in Synechococcus PCC7942.

Several strains of Synechococcus PCC7942 carrying point mutations in the gene psbA were studied by thermoluminescence and polarographic measurement of flash-induced oxygen yield. The following results were obtained: (a) Replacement of Ser-264 in D1 by Ala (mutant Di1) or Gly (mutant G264) resulting in DCMU and atrazine resistance leads to a downshift of the thermoluminescence (TL) B-band peak temperature from 40 degrees C in wild-type thylakoids to about 30 degrees C. In dark adapted samples of both mutants the TL and oxygen yield pattern induced by a train of single turnover flashes were strongly damped indicative of a high miss factor. (b) In contrast to Ser-264 mutants, replacement of Phe-255 in D1 by Tyr (mutant Tyr5) induced strong resistance to atrazine but not to DCMU and did not affect the peak termperature of the B-band and the flash-induced TL and oxygen yield patterns. In this respect mutant Tyr5 resembles the wild type. (c) No significant differences have been found between strains with single site mutations in psbAI and normal psbAII/psbAIII genes, and strains with same mutations in psbAI but additional deletion of psbAII and psbAIII. Obviously in strains were psbAI is present, PS II complexes containing gene products of psbAII and psbAIII are not assembled in detectable amounts. (d) Strains with double mutations at positions 264 and 255 display a downshift of the B-band peak temperature. Their oscillatory patterns of B-band intensity and oxygen yield are highly damped. This behaviour is similar to strains D1 and G264 which are modified at position 264 only. We extend reports on additivity of mutation effects on herbicide binding to binding of QB. (e) Mutations at the QB site not only influence the binding of QB and herbicides but also change the thermoluminescence quantum yield and the lifetimes of the redox states S2 and S3 of the water oxidase. This finding might indicate long ranging effects on Photosystem II exerted by structural modifications of the QB site. From these data we conclude that Ser-264 is essential for binding of atrazine, DCMU and QB, whereas Phe-255 is involved in atrazine binding and its substitution by Tyr does not markedly affect QB or DCMU binding in Synechococcus PCC7942.

Photoinhibition affects the non-heme iron center in photosystem II.

Effects on the PS II acceptor side caused by exposure to strong white light (180 W/m2) of PS II membrane fragments (spinach) at pH 6.5 and 0 degrees C were analyzed by measuring low temperature EPR signals and flash-induced transient changes of the fluorescence quantum yield. The following results were obtained: (a) the extent of the light induced g = 1.9 EPR signal as a measure of photochemical Fe2+QA- formation declines with progressing photoinhibition. The half-life of this effect is independent of the absence or presence of an exogenous electron acceptor during the photoinhibitory treatment; (b) in samples photoinhibited in the absence of an electron acceptor and subsequently incubated with K3[Fe(CN)6] in the dark, the extent of the g = 8 EPR signal (reflecting the oxidized Fe3+ form of the endogenous non-heme iron center) and of the flash-induced change of the fluorescence yield (as a measure of fast electron transfer from QA- to Fe3+ after the first flash; [see (1992) Photosynth. Res. 31, 113-126] exhibits the same dependence on photoinhibition time as the g = 1.9 EPR signal; (c) in samples photoinhibited in the presence of an exogenous electron acceptor, the signals reflecting Fe(3+)-formation and fast electron transfer from QA- to Fe3+ decline faster than the g = 1.9 EPR signal. These results provide for the first time direct evidence that the endogenous non-heme iron center located between QA and QB is susceptible to modifications by light stress. The implications of this finding will be discussed.

Effects of photoinhibition on the PS II acceptor side including the endogenous high spin Fe(2+) in thylakoids, PS II-membrane fragments and PS II core complexes.

Effects of photoinhibition at 0 °C on the PS II acceptor side have been analyzed by comparative studies in isolated thylakoids, PS II membrane fragments and PS II core complexes from spinach under conditions where degradation of polypeptide(s) D1(D2) is highly retarded. The following results were obtained by measurements of the transient fluorescence quantum and oxygen yield, respectively, induced by a train of short flashes in dark-adapted samples: (a) in the control the decay of the fluorescence quantum yield is very rapid after the first flash, if the dark incubation was performed in the presence of 300 µM K3[Fe(CN)6]; whereas, a characteristic binary oscillation was observed in the presence of 100 µM phenyl-p-benzoquinone with a very fast relaxation after the even flashes (2nd, 4th. . . ) of the sequence; (b) illumination of the samples in the presence of K3[Fe(CN)6] for only 5 min with white light (180 W m(-2)) largely eliminates the very fast fluorescence decay after the first flash due to QA (-) reoxidation by preoxidized endogenous non-heme Fe(3+), while a smaller effect arises on the relaxation kinetics of the fluorescence transients induced by the subsequent flashes; (c) the extent of the normalized variable fluorescence due to the second (and subsequent) flash(es) declines in all sample types with a biphasic time dependence at longer illumination. The decay times of the fast (6-9 min) and the slow degradation component (60-75 min) are practically independent of the absence or presence of K3[Fe(CN)6] and of anaerobic and aerobic conditions during the photo-inhibitory treatment, while the relative extent of the fast decay component is higher under anaerobic conditions. (d) The relaxation kinetics of the variable fluorescence induced by the second (and subsequent) flash(es) become retarded due to photoinhibition, and (e) the oscillation pattern of the oxygen yield caused by a flash train is not drastically changed due to photoinhibition.Based on these findings, it is concluded that photoinhibition modifies the reaction pattern of the PS II acceptor side prior to protein degradation. The endogenous high spin Fe(2+) located between QA and QB is shown to become highly susceptible to modification by photoinhibition in the presence of K3[Fe(CN)6] (and other exogenous acceptors), while the rate constant of QA (-) reoxidation by QB(QB (-)) and other acceptors (except the special reaction via Fe(3+)) is markedly less affected by a short photoinhibition. The equilibrium constant between QA (-) and QB(QB (-)) is not drastically changed as reflected by the damping parameters of the oscillation pattern of oxygen evolution.

Functional characterisation of a purified homogeneous Photosystem II core complex with high oxygen evolution capacity from spinach.

The functional properties of a purified homogeneous spinach PS II-core complex with high oxygen evolution capacity (Haag et al. 1990a) were investigated in detail by measuring thermoluminescence and oscillation patterns of flash induced oxygen evolution and fluorescence quantum yield changes. The following results were obtained: a) Depending on the illumination conditions the PS II-core complexes exhibit several thermoluminescence bands corresponding to the A band, Q band and Zv band in PS II membrane fragments. The lifetime of the Q band (Tmax=10°C) was determined to be 8s at T=10°C. No B band corresponding to S2QB (-) or S3QB (-) recombination could be detected. b) The flash induced transient fluorescence quantum yield changes exhibit a multiphasi relaxation kinetics shich reflect the reoxidation of Q A (-) . In control samples without exogenous acceptors this process is markedly slower than in PS II membrane fragments. The reaction becomes significantly retarded by addition of 10 µM DCMU. After dark incubation in the presence of K3[Fe(CN)6 c) Excitation of dark-adapted samples with a train of short saturating flashes gives rise to a typical pattern dominated by a high O2 yield due to the third flash and a highly damped period four oscillation. The decay of redox states S2 and S3 are dominated by short life times of 4.3 s and 1.5 s, respectively, at 20°C. The results of the present study reveal that in purified homogeneous PS II-core complexes with high oxygen evolution isolated from higher plants by ß-dodecylmaltoside solubilization the thermodynamic properties and the kinetic parameters of the redox groups leading to electron transfer from water to QA are well preserved. The most obvious phenomenon is a severe modification of the QB binding site. The implications of this finding are discussed.

Deformation-mechanism map for nanocrystalline metals by molecular-dynamics simulation.

Molecular-dynamics simulations have recently been used to elucidate the transition with decreasing grain size from a dislocation-based to a grain-boundary-based deformation mechanism in nanocrystalline f.c.c. metals. This transition in the deformation mechanism results in a maximum yield strength at a grain size (the 'strongest size') that depends strongly on the stacking-fault energy, the elastic properties of the metal, and the magnitude of the applied stress. Here, by exploring the role of the stacking-fault energy in this crossover, we elucidate how the size of the extended dislocations nucleated from the grain boundaries affects the mechanical behaviour. Building on the fundamental physics of deformation as exposed by these simulations, we propose a two-dimensional stress-grain size deformation-mechanism map for the mechanical behaviour of nanocrystalline f.c.c. metals at low temperature. The map captures this transition in both the deformation mechanism and the related mechanical behaviour with decreasing grain size, as well as its dependence on the stacking-fault energy, the elastic properties of the material, and the applied stress level.

Charge-induced reversible strain in a metal.

Dimension changes on the order of 0.1% or above in response to an applied voltage have been reported for many types of materials, including ceramics, polymers, and carbon nanostructures, but not, so far, for metals. We show that reversible strain amplitudes comparable to those of commercial piezoceramics can be induced in metals by introducing a continuous network of nanometer-sized pores with a high surface area and by controlling the surface electronic charge density through an applied potential relative to an electrolyte impregnating the pores.

Modification of the properties of S2 state in photosynthetic O2-evolving center by replacement of chloride with other anions.

Properties of the S2 state formed in photosystem II membranes in which Cl- had been replaced by various anions were investigated by means of thermoluminescence measurements and low temperature EPR spectroscopy. The Br--substituted membranes showed the normal thermoluminescence B-band arising from S2Q-B charge recombination, whereas the SO2-4-, F--, CH3COO--, and NO-3-substituted membranes showed modified B-bands with variously upshifted peak temperatures. The extent of the peak temperature upshift varied in parallel with the extent of inhibition of O2 evolution depending on the anion species. A normal EPR S2 multiline signal was induced in Br--substituted membranes, but its amplitude was reduced to less than 10% in F--, NO-3-, CH3COO--, and SO2-4-substituted membranes, In contrast, the g = 4.1 signal from S2 was markedly enhanced in F-- and NO-3-substituted membranes, not much affected in CH3COO-- and SO2-4-substituted membranes, and decreased to 70% in Br--substituted membranes. Based on these data, the effect of various types of S2 modification on the O2-evolving activity was discussed. It was suggested that anions have an important role in regulating the interaction between the Mn atoms, and thereby adjust the redox properties of the S2 state to enable further transitions beyond S2.

Functional characterization of mutant strains of the cyanobacterium Synechocystis sp. PCC 6803 lacking short domains within the large, lumen-exposed loop of the chlorophyll protein CP47 in photosystem II.

Several autotrophic mutant strains of Synechocystis sp. PCC 6803 carrying short deletions or a single-site mutation within the large, lumen-exposed loop (loop E) of the chlorophyll a-binding photosystem II core protein, CP47, are analyzed for their functional properties by measuring the flash-induced pattern of thermoluminescence, oxygen yield, and fluorescence quantum yield. A physiological and biochemical characterization of these mutant strains has been given in two previous reports [Eaton-Rye, J.J., & Vermaas, W.F.J. (1991) Plant Mol. Biol. 17, 1165-1177; Haag, E., Eaton-Rye, J.J., Renger, G., & Vermaas, S. F.J. (1993) Biochemistry 32, 4444-4454]. The results of the present study show that deletion of charged and conserved amino acids in a region roughly located between residues 370 and 390 decreases the binding affinity of the extrinsic PS II-O protein to photosystem II. Marked differences with PSII-O deletion mutants are observed with respect to Ca2+ requirement and the flash-induced pattern of oxygen evolution. Under conditions where a sufficient light activation is provided, the psbB mutants assayed in this study reveal normal S-state parameters and lifetimes. The results bear two basic implications: (i) the manganese involved in water oxidation can still be bound in a functionally normal or only slightly distorted manner, and (ii) the binding of the extrinsic PS II-O protein to photosystem II is impaired in mutants carrying a deletion in the domain between residues 370 and 390, but the presence of the PS II-O protein is still of functional relevance for the PS II complex, e.g., for maintenance of a high-affinity binding site for Ca2+ and/or involvement during the process of photoactivation.

Involvement of the CP47 protein in stabilization and photoactivation of a functional water-oxidizing complex in the cyanobacterium Synechocystis sp. PCC 6803.

Oscillation patterns of the oxygen yield per flash induced by a train of single-turnover flashes were measured as a function of dark incubation and different pre-illumination conditions in several autotrophic mutant strains of Synechocystis sp. PCC 6803 carrying short deletions within the large, lumen-exposed hydrophilic region (loop E) of the chlorophyll a-binding photosystem II protein CP47. A physiological and biochemical characterization of these mutant strains has been presented previously [Eaton-Rye, J. J., & Vermaas, W. F. J. (1991) Plant Mol. Biol. 17, 1165-1177; Haag, E., Eaton-Rye, J. J., Renger, G., & Vermaas, W. F. J. (1993) Biochemistry 32, 4444-4454], and some functional properties were described recently [Gleiter, H. M., Haag, E., Shen, J.-R., Eaton-Rye, J. J., Inoue, Y., Vermaas, W. F. J., & Renger, G. (1994) Biochemistry 33, 12063-12071]. The present study shows that in several mutants the water-oxidizing complex (WOC) became inactivated during prolonged dark incubation, whereas the WOC of the wild-type strain remained active. The rate and extent of the inactivation in the mutants depend on the domain of loop E, where 3-8 amino acid residues were deleted. The most pronounced effects are observed in mutants delta(A373-D380) and delta(R384-V392). A competent WOC can be restored from the fully inactivated state by illumination with short saturating flashes. The number of flashes required for this process strongly depends on the site at which a deletion has been introduced into loop E. Again, the most prominent effects were found in mutants delta(A373-D380) and delta(R384-V392). Interestingly, the number of flashes required for activation was reduced by more than an order of magnitude in both mutants by the addition of 10 mM CaCl2 to the cell suspension. On the basis of a model for photoactivation proposed by Tamura and Cheniae (1987) [Biochim. Biophys. Acta 890, 179-194], a scheme is presented for the processes of dark inactivation and photoactivation in these mutants. The results presented here corroborate an important role of the large hydrophilic domain (loop E) of CP47 in a functional and stable WOC.