Small Angle X-Ray Scattering of the Thylakoid Membranes of Rhodopseudomonas spheroides in Aqueous Suspensions

The diffraction patterns of particles which have the shape of hollow spheres, i.e. vesicles, can be satisfactorily analyzed by means of a new formula of Weick (1974). This formula is used for the small angle X-ray scattering analysis of aqueous suspensions of thylakoids of Rhodopseudomonas spheroides. Some essential results are: (a) The membrane has a rather asymmetric structure with one layer of low electron density at its inner side and two layers of high electron density near the outer surface of the thylakoids. (b) The distance of the electron density maxima of the latter two layers is 45 ± 5 Å. (c) Between the two maxima is a region of an electron density nearly equal to that of water. (d) The sequence of the peaks is - + 0 + with increasing radius. The peaks extend over an interval of 120 ± 10 Å. (e) The thylakoids are strikingly of the same size. Their diameters, if defined by the outmost layer, vary statistically by about 4% and have an average value of approximately 640 Å.     

Direct Determination of the Structure of Barium Stearate Multilayers by X-Ray Diffraction

Diffraction of X-rays is recorded from barium stearate multilayer systems with from 2 to 60 double layers or unit cells. The generalized Patterson function P′(x) is calculated by an integral Fourier transform of observed intensity data from a specimen containing only two unit cells. The Patterson function P₀(x) of a single unit cell is determined from P′(x) and the electron density distribution of a bimolecular leaflet is obtained by a deconvolution procedure of P₀(x) after Hosemann and Bagchi. The electron density distribution is also calculated independently by a conventional Fourier synthesis with an experimentally established set of phases. The results of the two methods are consistent and fit a physical model of the bimolecular leaflet. A direct analysis, therefore, can be performed if diffraction is observed from multilayer systems with a small number of unit cells.     

Characterization of the Decaheme c-Type Cytochrome OmcA in Solution and on Hematite Surfaces by Small Angle X-Ray Scattering and Neutron Reflectometry

The outer membrane protein OmcA is an 85 kDa decaheme c-type cytochrome located on the surface of the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1. It is assumed to mediate shuttling of electrons to extracellular acceptors that include solid metal oxides such as hematite (α-Fe₂O₃). No information is yet available concerning OmcA structure in physiologically relevant conditions such as aqueous environments. We purified OmcA and characterized its solution structure by small angle x-ray scattering (SAXS), and its interaction at the hematite-water interface by neutron reflectometry. SAXS showed that OmcA is a monomer that adopts a flat ellipsoidal shape with an overall dimension of 34 × 90 × 65 ų. To our knowledge, we obtained the first direct evidence that OmcA undergoes a redox state-dependent conformational change in solution whereby reduction decreases the overall length of OmcA by ∼7 Å (the maximum dimension was 96 Å for oxidized OmcA, and 89 Å for NADH and dithionite-reduced OmcA). OmcA was also found to physically interact with electron shuttle molecules such as flavin mononucleotide, resulting in the formation of high-molecular-weight assemblies. Neutron reflectometry showed that OmcA forms a well-defined monomolecular layer on hematite surfaces, where it assumes an orientation that maximizes its contact area with the mineral surface. These novel insights into the molecular structure of OmcA in solution, and its interaction with insoluble hematite and small organic ligands, demonstrate the fundamental structural bases underlying OmcA's role in mediating redox processes.     

Solution Structure of an Intramembrane Aspartyl Protease via Small Angle Neutron Scattering

Intramembrane aspartyl proteases (IAPs) comprise one of four families of integral membrane proteases that hydrolyze substrates within the hydrophobic lipid bilayer. IAPs include signal peptide peptidase, which processes remnant signal peptides from nascent polypeptides in the endoplasmic reticulum, and presenilin, the catalytic component of the γ-secretase complex that processes Notch and amyloid precursor protein. Despite their broad biomedical reach, basic structure-function relationships of IAPs remain active areas of research. Characterization of membrane-bound proteins is notoriously challenging due to their inherently hydrophobic character. For IAPs, oligomerization state in solution is one outstanding question, with previous proposals for monomer, dimer, tetramer, and octamer. Here we used small angle neutron scattering (SANS) to characterize n-dodecyl-β-D-maltopyranoside (DDM) detergent solutions containing and absent a microbial IAP ortholog. A unique feature of SANS is the ability to modulate the solvent composition to mask all but the enzyme of interest. The signal from the IAP was enhanced by deuteration and, uniquely, scattering from DDM and buffers were matched by the use of both tail-deuterated DDM and D₂O. The radius of gyration calculated for IAP and the corresponding ab initio consensus model are consistent with a monomer. The model is slightly smaller than the crystallographic IAP monomer, suggesting a more compact protein in solution compared with the crystal lattice. Our study provides direct insight into the oligomeric state of purified IAP in surfactant solution, and demonstrates the utility of fully contrast-matching the detergent in SANS to characterize other intramembrane proteases and their membrane-bound substrates.     

Structural Insights into Yeast DNA Polymerase δ by Small Angle X-ray Scattering

DNA polymerase δ (Polδ) is a multisubunit polymerase that plays an indispensable role in replication from yeast to humans. Polδ from Saccharomyces cerevisiae is composed of three subunits: Pol3, Pol31, and Pol32. Despite the elucidation of the structures and models of the individual subunits (or portions, thereof), the nature of their assembly remains unclear. We present here a small-angle X-ray scattering analysis of a yeast Polδ complex (Polδ_T) composed of Pol3, Pol31, and Pol32N (amino acids 1-103 of Pol32). From the small angle X-ray scattering global parameters and reconstructed envelopes, we show that Polδ_T adopts an elongated conformation with a radius of gyration (R_g) of ∼ 52 Å and a maximal dimension of ∼ 190 Å. We also propose an orientation for the accessory Pol31-Pol32N subunits relative to the Pol3 catalytic core that best agrees with the experimental scattering profile. The analysis also points to significant conformational variability that may allow Polδ to better coordinate its action with other proteins at the replication fork.     

Acetate in mixotrophic growth medium affects photosystem II in Chlamydomonas reinhardtii and protects against photoinhibition

Chlamydomonas reinhardtii is a photoautotrophic green alga, which can be grown mixotrophically in acetate-supplemented media (Tris–acetate–phosphate). We show that acetate has a direct effect on photosystem II (PSII). As a consequence, Tris–acetate–phosphate-grown mixotrophic C. reinhardtii cultures are less susceptible to photoinhibition than photoautotrophic cultures when subjected to high light. Spin-trapping electron paramagnetic resonance spectroscopy showed that thylakoids from mixotrophic C. reinhardtii produced less ¹O₂ than those from photoautotrophic cultures. The same was observed in vivo by measuring DanePy oxalate fluorescence quenching. Photoinhibition can be induced by the production of ¹O₂ originating from charge recombination events in photosystem II, which are governed by the midpoint potentials (E_m) of the quinone electron acceptors. Thermoluminescence indicated that the E_m of the primary quinone acceptor (Q_A/Q_A⁻) of mixotrophic cells was stabilised while the E_m of the secondary quinone acceptor (Q_B/Q_B⁻) was destabilised, therefore favouring direct non-radiative charge recombination events that do not lead to ¹O₂ production. Acetate treatment of photosystem II-enriched membrane fragments from spinach led to the same thermoluminescence shifts as observed in C. reinhardtii, showing that acetate exhibits a direct effect on photosystem II independent from the metabolic state of a cell. A change in the environment of the non-heme iron of acetate-treated photosystem II particles was detected by low temperature electron paramagnetic resonance spectroscopy. We hypothesise that acetate replaces the bicarbonate associated to the non-heme iron and changes the environment of Q_A and Q_B affecting photosystem II charge recombination events and photoinhibition.     

Oxidation-reduction properties of the electron acceptors of Photosystem II. II. Redox titration at various pH values of the flash-induced formation of C550 in Chlamydomonas Photosystem II particles lacking the secondary quinone electron acceptor

Redox titrations of the flash-induced formation of C550 (a linear indicator of Q^?) were performed between pH 5.9 and 8.3 in Chlamydomonas Photosystem II particles lacking the secondary electron acceptor, B. One-third of the reaction centers show a pH-dependent midpoint potential (E_m,7.5) = ? 30 mV) for redox couple $\text{Q}\text{Q}{}^{\mathrm{?}}$, which varies by ?60 mV/pH unit. Two-thirds of the centers show a pH-independent midpoint potential (E_mm = + 10 mV) for this couple. The elevated pH-independent E_m suggests that in the latter centers the environment of Q has been modified such as to stabilize the semiquinone anion, Q^?. The midpoint potentials of the centers having a pH-dependent E_m are within 20 mV of those observed in chloroplasts having a secondary electron acceptor. It appears therefore that the secondary electron acceptor exerts little influence on the E_m of $\text{Q}\text{Q}{}^{\mathrm{?}}$. An EPR signal at g 1.82 has recently been attributed to a semiquinone-iron complex which comprises Q^?. The similar redox behavior reported here for C550 and reported by others (Evans, M.C.W., Nugent, J.H.A., Tilling, L.A. and Atkinson, Y.E. (1982) FEBS Lett. 145, 176–178) for the g 1.82 signal in similar Photosystem II particles confirm the assignment of this EPR signal to Q^?. At below ?200 mV, illumination of the Photosystem II particles produces an accumulation of reduced pheophytin (Ph^?). At ?420 mV Ph^? appears with a quantum yield of 0.006–0.01 which in this material implies a lifetime of 30–100 ns for the radical pair P-680⁺Ph^?.     

Differential extraction and structural specificity of specialized ubiquinone molecules in secondary electron transfer in chromatophores from Rhodopseudomonas sphaeroides, Ga

In chromatophores from the facultative photosynthetic bacterium, Rhodopseudomonas sphaeroides, Ga, the function of ubiquinone-10 (UQ-10) at two specialized binding sites (Q_B and Q_Z) has been determined by kinetic criteria. These were the rate of rereduction of flash-oxidized [BChl]₂⁺ through the back reaction, or the binary pattern of cytochrome b₅₆₁ (for the Q_b site), and the rapid rate of rereduction of flash-oxidized cytochrome c, or the relative amplitude of the antimycin-sensitive Phase III ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{~ 1.5}\text{ms}$) of the carotenoid spectral shift induced by a single turnover flash at E_h ~ 100 mV (for the Q_Z site). The phenomenon associated with the two binding sites behaved differently on extraction of UQ from lyophilized chromatophores using isooctane. By this selective extraction procedure it has been possible to show that UQ-10 molecules are required at different concentrations in the membrane for specific redox events in secondary electron transfer. The reduction of cytochrome b occurs in particles which no longer show the phenomena associated with Q_Z, but still possess a large proportion of Q_b, while rapid rereduction of flash-oxidized cytochrome c requires an additional complement of UQ-10 (Q_Z). Extracted particles lacking Q_Z and a large amount of Q_B have been reconstituted with different UQ homologs (UQ-1, UQ-3, and UQ-10). Specific redox events have been studied in reconstituted particles. All UQ homologs act as secondary acceptors from the reaction center; UQ-3 and UQ-10, but not UQ-1, are also able to reconstitute the function of Q_Z as electron donor to cytochrome c. Only UQ-10, however, is able to restore normal rates of the overall cyclic electron transfer induced by a train of flashes, and maximal rates of the light-induced ATP synthesis. The results are interpreted in terms of Q-cycle mechanisms in which quinone and quinol at both the Q_Z and Q_b sites are in rapid equilibrium with the quinone pool.     

Monomeric Amyloid Beta Peptide in Hexafluoroisopropanol Detected by Small Angle Neutron Scattering

Small proteins like amyloid beta (Aβ) monomers are related to neurodegenerative disorders by aggregation to insoluble fibrils. Small angle neutron scattering (SANS) is a nondestructive method to observe the aggregation process in solution. We show that SANS is able to resolve monomers of small molecular weight like Aβ for aggregation studies. We examine Aβ monomers after prolonged storing in d-hexafluoroisopropanol (dHFIP) by using SANS and dynamic light scattering (DLS). We determined the radius of gyration from SANS as 1.0±0.1 nm for Aβ_1–40 and 1.6±0.1 nm for Aβ_1–42 in agreement with 3D NMR structures in similar solvents suggesting a solvent surface layer with 5% increased density. After initial dissolution in dHFIP Aβ aggregates sediment with a major component of pure monomers showing a hydrodynamic radius of 1.8±0.3 nm for Aβ_1–40 and 3.2±0.4 nm for Aβ_1–42 including a surface layer of dHFIP solvent molecules.     

Electrogenic reduction of the secondary quinone acceptor in chromatophores of Rhodospirillum rubrum: Rapid kinetics measurements

Electron transfer Q_A → Q_B has been reconstituted with added Q-10 in Rhodospirillum rubrum chromatophores associated with a phospholipid-impregnated collodion film. Rapid kinetics measurements of laser flash-induced ΔΨ generated in the chromatophores show that whereas electron transfer from Q_a⁻ to Q_B upon the first flash is not electrogenic in dark-adapted chromatophores, reduction of Q⁻_B to Q_bh₂ induced by the second flash gives rise to an electrogenic phase with τ = 250 μs at pH 7.5 which contributes about 10% to the total ΔΨ generated upon the flash. The electrogenic phase is ascribed to vectorial protonation of Q²⁻_B.     

Reverse electron flow in chloroplasts

Energy dependent reverse electron flow reactions in isolated thylakoids provide a unique tool to study, in the dark, the coupling between the ATP synthase, proton transport and the electron transfer system. Appropriate experimental conditions have been established to follow experimentally the following reactions:

1. ATP driven proton uptake into the inner-thylakoid space, which requires preactivation of the ATP synthase.
2. ATP driven reverse electron transport, which involves proton transport as an intermediate, and results in the reduction of Q_A by an externally added electron donor.
3. ATP driven luminescence, which requires the presence of an oxidized partner on the water side of photosystem II, and involves electron transport from Q_B to Q_A.
4. ΔpH driven reverse electron flow, which does not require the participation of the ATP synthase, and uses reduced intermediates between the two photosystems as electron donors for the reduction of Q_A.
5. ΔpH driven luminescence which again uses reduced intermdiates between the two photosystems as electron donors for Q_A reduction, and requires the presence of an oxidized partner on the water side of photosystem II.

Several of these reactions have been shown to occur in intact chloroplasts and may provide an important regulatory mechanism in vivo.     

Photosynthetic response of clusterbean chloroplasts to UV-B radiation: Energy imbalance and loss in redox homeostasis between QA and QB of photosystem II

The effects of ultraviolet-B (UV-B: 280-320 nm) radiation on the photosynthetic pigments, primary photochemical reactions of thylakoids and the rate of carbon assimilation (P_n) in the cotyledons of clusterbean (Cyamopsis tetragonoloba) seedlings have been examined. The radiation induces an imbalance between the energy absorbed through the photophysical process of photosystem (PS) II and the energy consumed for carbon assimilation. Decline in the primary photochemistry of PS II induced by UV-B in the background of relatively stable P_n, has been implicated in the creation of the energy imbalance_. The radiation induced damage of PS II hinders the flow of electron from Q_A to Q_B resulting in a loss in the redox homeostasis between the Q_A to Q_B leading to an accumulation of Q_A⁻. The accumulation of Q_A⁻ generates an excitation pressure that diminishes the PS II-mediated O₂ evolution, maximal photochemical potential (F_v/F_m) and PS II quantum yield (Φ_{PS II}). While UV-B radiation inactivates the carotenoid-mediated protective mechanisms, the accumulation of flavonoids seems to have a small role in protecting the photosynthetic apparatus from UV-B onslaught. The failure of protective mechanisms makes PS II further vulnerable to the radiation and facilitates the accumulation of malondialdehyde (MDA) indicating the involvement of reactive oxygen species (ROS) metabolism in UV-B-induced damage of photosynthetic apparatus of clusterbean cotyledons.     

Possible roles of two quinone molecules in direct and indirect proton pumps of bovine heart NADH-quinone oxidoreductase (complex I)

In many energy transducing systems which couple electron and proton transport, for example, bacterial photosynthetic reaction center, cytochrome bc₁-complex (complex III) and E. coli quinol oxidase (cytochrome bo₃ complex), two protein-associated quinone molecules are known to work together. T. Ohnishi and her collaborators reported that two distinct semiquinone species also play important roles in NADH-ubiquinone oxidoreductase (complex I). They were called SQ_Nf (fast relaxing semiquinone) and SQ_Ns (slow relaxing semiquinone). It was proposed that Q_Nf serves as a “direct” proton carrier in the semiquinone-gated proton pump (Ohnishi and Salerno, FEBS Letters 579 (2005) 4555), while Q_Ns works as a converter between one-electron and two-electron transport processes. This communication presents a revised hypothesis in which Q_Nf plays a role in a “direct” redox-driven proton pump, while Q_Ns triggers an “indirect” conformation-driven proton pump. Q_Nf and Q_Ns together serve as (1e^?/2e^?) converter, for the transfer of reducing equivalent to the Q-pool.     

Structure of Supercritical Fluid Krypton at Small Scattering Angle Using Parallel Molecular Dynamics Simulation

Abstract

We present a parallel algorithm for molecular dynamics involving short-range two- and three-body potentials and the pair-correlation function, g(r). The method is based on a spatial decomposition of the simulation box that takes advantage of a linked-cell list, and allows a load balanced partition of the computations of both the forces and g(r) over the processors. The tests of the program is conducted by evaluating the efficiency for both the thermalization phase and the production phase of the simulation. This method is successfully applied to the calculation of the direct correlation function of fluid krypton at small scattering angle along the T = 297 K supercritical isotherm.     

Steady-State Chlorophyll a Fluorescence Transients during Ammonium Assimilation by the N-Limited Green Alga Selenastrum minutum

The assimilation of ammonium by the N-limited green alga Selenastrum minutum results in the suppression of photosynthetic electron flow from H₂O to CO₂ (6, 7, 18). In this study, results are presented which describe the correponding change in steady-state chlorophyll a fluorescence. The addition of ammonium resulted in a transient decline in fluorescence followed by a marked increase. Fluorescence did not return to control levels until the added ammonium had been assimilated. Analysis of the fluorescence transients showed that ammonium assimilation resulted in a rapid increase in nonphotochemical quenching (Q_e) peaking 10 to 15 seconds after ammonium addition. Q_e then decreased dramatically reaching a minimum value approximately 45 seconds following ammonium addition and returned to the control level only after the added ammonium had been assimilated. There were no effects of ammonium addition on photochemical quenching (Q_q) for approximately 10 to 15 seconds at which time both gross O₂ evolution (as measured by mass spectrometry) and Q_q declined. In the presence of d,l-glyceraldehyde or when cells were held at the CO₂ compensation point, the addition of ammonium resulted in a decline in Qe 10 to 15 seconds after addition. The Q_e peak and the Q_q decline were absent. These results imply that the transient increase in Q_e and the subsequent decline in Q_q may be attributed to the decline in Calvin cycle activity during ammonium assimilation. The decline in Q_e is apparently a direct result of ammonium assimilation. The observation that the Q_e peak precedes the Q_q decline would be consistent with the decreases in Calvin cycle carbon flow occurring at the kinase reactions prior to glyceraldehyde-3-phosphate dehydrogenase.     

Singlet and triplet excited carotenoid and antenna bacteriochlorophyll of the photosynthetic purple bacterium Rhodospirillum rubrum as studied by picosecond absorbance difference spectroscopy

Picosecond absorbance difference spectra at a number of delay times after a 35 ps excitation pulse and kinetics of absorbance changes were measured in chromatophores of the photosynthetic purple bacterium Rhodospirillum rubrum after chemical oxidation of the primary electron donor P-875. Kinetics and spectra were measured of the excited singlet states of carotenoid and bacteriochlorophyll a and also of the triplet state of the carotenoid. The excited singlet state of carotenoid, produced by direct excitation at 532 nm, is characterized by a bleaching of the ground state absorption bands in the region 450–490 nm and by an absorbance increase with a maximum near 570 nm. Its lifetime was calculated to be 0.6 ± 0.1 ps in vitro and less than 1 ps in vivo. The triplet state of carotenoid in vivo is formed within 100 ps after direct carotenoid excitation via a pathway that does not involve excited states of bacteriochlorophyll. Singlet excitation of a bacteriochlorophyll a molecule causes the bleaching of its Q_x and Q_y absorption bands, and is probably associated with blue shifts of the Q_y absorption band of about six neighboring bacteriochlorophyll molecules. Upon increasing the excitation density, the average lifetime of the singlet excitations on bacteriochlorophyll decreased from about 350 ps to about 10 ps or less. The results are in quantitative agreement with the known effect of singlet-singlet annihilation upon the fluorescence yield, and furthermore show that no bacteriochlorophyll or carotenoid triplet formation is associated with this annihilation.     

Molecular and quantitative trait variation within and among small fragmented populations of the endangered plant species Psilopeganum sinense

Background and Aims

Natural selection and genetic drift are important evolutionary forces in determining genetic and phenotypic differentiation in plant populations. The extent to which these two distinct evolutionary forces affect locally adaptive quantitative traits has been well studied in common plant and animal species. However, we know less about how quantitative traits respond to selection pressures and drift in endangered species that have small population sizes and fragmented distributions. To address this question, this study assessed the relative strengths of selection and genetic drift in shaping population differentiation of phenotypic traits in Psilopeganum sinense, a naturally rare and recently endangered plant species.

Methods

Population differentiation at five quantitative traits (Q_ST) obtained from a common garden experiment was compared with differentiation at putatively neutral microsatellite markers (F_ST) in seven populations of P. sinense. Q_ST estimates were derived using a Bayesian hierarchical variance component method.

Key Results

Trait-specific Q_ST values were equal to or lower than F_ST. Neutral genetic diversity was not correlated with quantitative genetic variation within the populations of P. sinense.

Conclusions

Despite the prevalent empirical evidence for Q_ST > F_ST, the results instead suggest a definitive role of stabilizing selection and drift leading to phenotypic differentiation among small populations. Three traits exhibited a significantly lower Q_ST relative to F_ST, suggesting that populations of P. sinense might have experienced stabilizing selection for the same optimal phenotypes despite large geographical distances between populations and habitat fragmentation. For the other two traits, Q_ST estimates were of the same magnitude as F_ST, indicating that divergence in these traits could have been achieved by genetic drift alone. The lack of correlation between molecular marker and quantitative genetic variation suggests that sophisticated considerations are required for the inference of conservation measures of P. sinense from neutral genetic markers.     

Effect of inhibitors,redox state and isoprenoid chain length on the affinity of ubiquinone for the secondary acceptor binding site in the reaction centers of photosynthetic bacteria

Quinone and inhibitor binding to Rhodopseudomonas sphaeroides (R-26 and GA) reaction centers were studied using spectroscopic methods and by direct adsorption of reaction centers onto anion exchange filters in the presence of ¹⁴C-labelled quinone or inhibitor. These measurements show that as secondary acceptor, Q_B, ubiquinone (UQ) is tightly bound in the semiquinone form and loosely bound in the quinone and quinol forms. The quinol is probably more loosely bound than the quinone. o-Phenanthroline and terbutryn, a triazine inhibitor, compete with UQ and with each other for binding to the reaction center. Inhibition by o-phenanthroline of electron transfer from the primary to the secondary quinone acceptor (Q_A to Q_B) occurs via displacement of UQ from the Q_B binding site. Displacement of UQ by terbutryn is apparently accessory to the inhibition of electron transfer. Terbutryn binding is lowered by reduction of Q_B to Q^?_B but is practically unaffected by reduction of Q_A to Q^?_A in the absence of Q_B. UQ-9 and UQ-10 have a 5- to 6-fold higher binding affinity to the Q_B site than does UQ-1, indicating that the long isoprenoid chain facilitates the binding to the Q_B site.     

Calculation of the energy confinement time and power threshold for L-H transitions in a tokamak with allowance for the parameters of the transport barrier

The ASTRA-ETL code is used to simulate L-H transition scenarios and calculate the energy confinement time and the threshold power of the L-H transition as functions of the averaged electron density 〈n〉, the averaged magnetic field B, the neutral density n _n , and the neutral temperature T _n , as well as the values of T _Se , T _Si , and n _S at the separatrix. It is shown that the linear dependence of the threshold power of the L-H transition on the averaged electron density, Q _L-H∝〈n〉, is associated with an increase in the viscosity of a poloidally rotating plasma due to charge exchange and is governed exclusively by an increase in the neutral density n _n . When the averaged electron density 〈n〉 is low, the threshold power rises because T _Si and T _Se increase. The accuracy of predictions for the power threshold of the L-H transition can be improved if the scaling of Q _L-H versus 〈n〉 and B is derived by processing experimental data from discharges with close parameter values at the separatrix. The hysteresis effect during an L-H-L transition triggered by varying the input power is modeled. The global energy confinement time τ_E is shown to increase linearly with 〈n〉 in the range 〈n〉<3.6×10¹⁹ m^?3 and to saturate at higher electron densities; this behavior is found to be characteristic of the Ohmic, L-, and H-modes. The saturation is associated with the fact that losses via the ion channel (when the transport coefficients are density-independent) dominate over losses via the electron channel. The dependence of τ _E on the input power is determined from the calculated database and is found to be τ _E =0.12Q _L-H ^?0.46 at a fixed averaged electron density 〈n〉. In the simulations of the L-H transition, the energy confinement time τ _E increases by a factor of 2 only if the thermal diffusivity inside the transport barrier is lower than that in the central plasma by a factor of more than 6.     

Low resolution crystal structure of hagfish insulin

Insulin from the Atlantic hagfish, Myxine glutinosa, crystallizes in space group P4₁2₁2 with a monomer in the asymmetric unit. The application of the Rossmann &; Blow (1962) rotation function, utilizing the known 2-zinc pig insulin crystal structure, has established the existence of an insulin dimer containing a crystallographic 2-fold axis. The position of the hagfish insulin molecule in the unit cell has been determined and a set of calculated phases derived. These are compared to phases found from isomorphous replacement studies. A 6 Å resolution electron density map has been calculated which shows the A and B chains are folded in a similar way to pig insulin and that the monomers are similarly organized into dimers.