Switchable self-assembly of Prussian blue analogs nano-tiles triggered by salt stimulus.

Prussian blue analogs (PBAs) are materials well known for their bulk physical and (electro)-chemical properties, with an outstanding selectivity for caesium in ion exchange processes. Crystalline nano-tiles, made of copper based PBA, are produced and dispersed in water by tuning their electrostatic interactions. The shape and size of the nano-crystals are determined by combining scattering, microscopic and spectral techniques. We show here that Cu-PBA nano-tiles form planar superstructures by an edge to edge self-assembly process controlled by specific cation effect and ionic strength. Sedimentation and (re-)dispersion of the nano-tiles are found to be fully reversible. This switchable anisotropic self-assembly triggered by salt stimulus makes PBA nanocrystals potentially interesting for applications.

Influence of additives on the structure of surfactant-free microemulsions.

We study the addition of electrolytes to surfactant-free microemulsions in the domain where polydisperse pre-Ouzo aggregates are present. As in previous studies, the microemulsion is the ternary system water/ethanol/1-octanol, where ethanol acts as co-solvent. Addition of electrolytes modifies the static X-ray and neutron scattering, and dynamic light scattering patterns, as well as the position of the miscibility gap, where spontaneous emulsification occurs upon dilution with water. All observations can be rationalized considering that electrolytes are either "salting out" the ethanol, which is the main component of the interface stabilizing the aggregates, or producing charge separation via the antagonistic ion effect discovered by Onuki et al. Amphiphilic electrolytes, such as sodium dodecylsulfate or sodium dietheylhexylphosphate, induce a gradual transition towards monodisperse ionic micelles with their characteristic broad scattering "peak". In these micelles the ethanol plays then the role of a cosurfactant. Dynamic light scattering can only be understood by combination of fluctuations of aggregate concentration due to the vicinity of a critical point and in-out fluctuations of ethanol.

Structure of a liquid/liquid interface during solvent extraction combining X-ray and neutron reflectivity measurements.

We have resolved the molecular structure of a bulk oil/water interface that contains amphiphilic ligand molecules using a combination of X-ray and neutron reflectivity measurements for the first time. This new capability can greatly impact future work in the field of ion separation by phase transfer, i.e. liquid/liquid extraction.

Self-assembly of condensable "bola-amphiphiles" in water/tetraethoxysilane mixtures for the elaboration of mesostructured hybrid materials.

The self-assembly of condensable amphiphile molecules in water is an attractive approach for the synthesis of mesostructured hybrid materials. In this article, we focus on aminoundecyltriethoxysilane (AUT), a condensable "bola-amphiphile", i.e., an amphiphilic molecule possessing two polar heads on both sides of an aliphatic chain. In the present case, one side is a condensable triethoxysilane, and the other side is an amino group. We report on the self-assembly of AUT in mixtures of water and tetraethoxysilane (TEOS). In situ small-angle X-ray scattering (SAXS) measurements allowed us to follow the evolution of the structure from the liquid state up to the solid material formed upon catalytic polycondensation. Depending on the medium composition, hexagonal or lamellar structures can be observed in the final material. These observations allowed us to propose a model for the self-assembly of AUT in water/TEOS mixtures that we were able to validate by simulations of the SAXS profiles. By taking advantage of the modularity of such a system, it proves possible to prepare in a simple way various structured hybrid materials possessing a high number of available organic functions without using sacrificial surfactant molecules.

Second harmonic generation monitoring of nitric acid extraction by a monoamide at the water-dodecane interface.

The interface dynamic properties of a monoamide extractant with potential for application to the front end of the nuclear cycle and to waste treatment are examined by second harmonic generation. The results are compared with bulk nitric ion titration and surface pressure measurements. SH static studies show the extractant reaching the interface and accurately match the IFT measurements. The main feature of the SH dynamic studies is a chaotic fluctuation period, strongly related to intense extraction. Fluctuations are a signature of the interface behaviour during the extraction process. Vertical development of the interface, often called protrusion, remains the most probable origin of the measured fluctuation. Additionally, interfacial measurements show a non-monotonic lag time during extraction, probably related to cooperative effects not observed in the bulk at the working concentration. Such mutual behaviour could be a supplementary prerequisite for the ion transfer across this liquid-liquid interface.

The conformation of myosin head domains in rigor muscle determined by X-ray interference.

In the absence of adenosine triphosphate, the head domains of myosin cross-bridges in muscle bind to actin filaments in a rigor conformation that is expected to mimic that following the working stroke during active contraction. We used x-ray interference between the two head arrays in opposite halves of each myosin filament to determine the rigor head conformation in single fibers from frog skeletal muscle. During isometric contraction (force T(0)), the interference effect splits the M3 x-ray reflection from the axial repeat of the heads into two peaks with relative intensity (higher angle/lower angle peak) 0.76. In demembranated fibers in rigor at low force (<0.05 T(0)), the relative intensity was 4.0, showing that the center of mass of the heads had moved 4.5 nm closer to the midpoint of the myosin filament. When rigor fibers were stretched, increasing the force to 0.55 T(0), the heads' center of mass moved back by 1.1-1.6 nm. These motions can be explained by tilting of the light chain domain of the head so that the mean angle between the Cys(707)-Lys(843) vector and the filament axis increases by approximately 36 degrees between isometric contraction and low-force rigor, and decreases by 7-10 degrees when the rigor fiber is stretched to 0.55 T(0).

Characterization of porous structure through the dynamical properties of ions confined in sulfonated polyimide ionomers films.

The structure of sulfonated PolyImide (sPI) ionomer membrane has been investigated via the transport properties of ions confined inside. Transport coefficients of N(CH(3))(4)(+) and Na(+) ions have been determined by several techniques in order to get a range of time/space scale as wide as possible: a method using radiotracers, conductivity, pulsed field gradient NMR and NMR quadrupolar relaxation rates determination. For N(CH(3))(4)(+), the self-diffusion has been measured in the direction of membrane plan (parallel) and in the perpendicular direction (transverse), whereas for Na(+) only transverse self-diffusion has been measured. The conductivity of both ions has been measured in the transverse direction. The results show a anisotropic and multiscale structure with a separation phase between hydrophilic and hydrophobic domains that is not well-defined.

Nonhomogeneous textures and banded flow in a soft cubic phase under shear.

We report evidence for two distinct strain-induced orientation transitions in a lyotropic bcc cubic crystal submitted to increasing shear rates. The crystal is built up from copolymer spherical micelles in a selective solvent. The distribution of orientations is characterized by x-ray diffraction: in the two oriented states, the dense <111> rows align along the flow, but they differ from each other in the orientations of the dense layers with respect to the shear plane. These orientation transitions have clear rheological signatures in the form of two stress plateaus each associated with the coexistence of two states of orientation. We compare this behavior with the well documented shear-induced orientation transition in wormlike micelles.

Biophysics and synchrotron radiation where the marriage fails. X-ray damage of lipid membranes and mesophases.

The call for brighter synchrotron X-radiation sources for use in structural biology research is barely audible as we enter the new millennium. Our brightest sources are already creating havoc when used at design specifications because of radiation damage. The time is long overdue to take stock of where we are and where we wish to go with regards to using existing sources and to designing new ones. The problem of radiation damage is particularly severe in studies involving kinetics and mechanism where cryotechniques are not always viable. Accordingly, we need to understand the very nature of radiation damage and to devise means for minimizing it. This is the thrust of the current study as applied to lipid membranes and mesophases. Here, we report on two very different types of radiation damage. One involves a dramatic phase transformation and the other a disordering of lamellar stacking. How beam energy and dose/rate affect damage is also discussed. The work highlights the nature of the damage process and the need for additional studies if we are to make most efficient use of an important resource, synchrotron radiation.

Interference fine structure and sarcomere length dependence of the axial x-ray pattern from active single muscle fibers.

Axial x-ray diffraction patterns from single intact fibers of frog skeletal muscle were recorded by using a highly collimated x-ray beam at the European Synchrotron Radiation Facility. During isometric contraction at sarcomere lengths 2.2-3.2 microm, the M3 x-ray reflection, associated with the repeat of myosin heads along the filaments, was resolved into two peaks. The total M3 intensity decreased linearly with increasing sarcomere length and was directly proportional to the degree of overlap between myosin and actin filaments, showing that it comes from myosin heads in the overlap region. The separation between the M3 peaks was smaller at longer sarcomere length and was quantitatively explained by x-ray interference between myosin heads in the two overlap regions of each sarcomere. The relative intensity of the M3 peaks was independent of sarcomere length, showing that the axial periodicities of the nonoverlap and overlap regions of the myosin filament have the same value, 14.57 nm, during active contraction. In resting fibers the periodicity is 14.34 nm, so muscle activation produces a change in myosin filament structure in the nonoverlap as well as the overlap part of the filament. The results establish x-ray interferometry as a new tool for studying the motions of myosin heads during muscle contraction with unprecedented spatial resolution.

Changes in conformation of myosin heads during the development of isometric contraction and rapid shortening in single frog muscle fibres.

1. Two-dimensional X-ray diffraction patterns were recorded at the European Synchrotron Radiation Facility from central segments of intact single muscle fibres of Rana temporaria with 5 ms time resolution during the development of isometric contraction. Shortening at ca 0.8 times the maximum velocity was also imposed at the isometric tetanus plateau. 2. The first myosin-based layer line (ML1) and the second myosin-based meridional reflection (M2), which are both strong in resting muscle, were completely abolished at the plateau of the isometric tetanus. The third myosin-based meridional reflection (M3), arising from the axial repeat of the myosin heads along the filaments, remained intense but its spacing changed from 14.34 to 14.56 nm. The intensity change of the M3 reflection, IM3, could be explained as the sum of two components, I14.34 and I14.56, arising from myosin head conformations characteristic of rest and isometric contraction, respectively. 3. The amplitudes (A) of the X-ray reflections, which are proportional to the fraction of myosin heads in each conformation, changed with half-times that were similar to that of isometric force development, which was 33.5 +/- 2. 0 ms (mean +/- s.d., 224 tetani from three fibres, 4 C), measured from the end of the latent period. We conclude that the myosin head conformation changes synchronously with force development, at least within the 5 ms time resolution of these measurements. 4. The changes in the X-ray reflections during rapid shortening have two temporal components. The rapid decrease in intensity of the 14.56 nm reflection at the start of shortening is likely to be due to tilting of myosin heads attached to actin. The slower changes in the other reflections were consistent with a return to the resting conformation of the myosin heads that was about 60 % complete after shortening of 70 nm per half-sarcomere.

Myosin head movements during isometric contraction studied by X-ray diffraction of single frog muscle fibres.

Time resolved X-ray diffraction experiments in single muscle fibres of the frog at 2.15 microns sarcomere length and 4 degrees C were performed at ID2 (SAXS), ESRF, Grenoble (France) to investigate the structural aspects of cross-bridge action during the development of the isometric tetanic tension (T0). Changes in the low angle myosin-based reflections were measured with 5 ms time resolution by signal averaging data collected with a 10 m camera length and a 2D gas-filled detector. Upon activation the intensity of the first order myosin layer line reflection, I(M1), and the intensity of the second order meridional reflection, I(M2), reduced practically to zero with a half-time which leads the tension rise by 15-20 ms. The complex changes of the intensity of the third order myosin meridional reflection, I(M3), and the increase of its axial spacing from 14.34 nm (at rest) to 14.57 nm (at T0) could be analysed by assuming that they were the result of the combination of the time dependent modulation in intensity of two closely spaced periodicities, one at 14.34 nm, characteristics of the myosin molecule at rest and the other at 14.57 nm, assumed by the myosin as a consequence of the activation and force production. I(14.34) drops monotonically in advance to isometric tension development with a half-time similar to that of I(M1) and I(M2), while I(14.57) rises from zero to a maximum in parallel with tension.

How coenzyme B12 radicals are generated: the crystal structure of methylmalonyl-coenzyme A mutase at 2 A resolution.

BACKGROUND: The enzyme methylmalonyl-coenzyme A (CoA) mutase, an alphabeta heterodimer of 150 kDa, is a member of a class of enzymes that uses coenzyme B12 (adenosylcobalamin) as a cofactor. The enzyme induces the formation of an adenosyl radical from the cofactor. This radical then initiates a free-radical rearrangement of its substrate, succinyl-CoA, to methylmalonyl-CoA. RESULTS: Reported here is the crystal structure at 2 A resolution of methylmalonyl-CoA mutase from Propionibacterium shermanii in complex with coenzyme B12 and with the partial substrate desulpho-CoA (lacking the succinyl group and the sulphur atom of the substrate). The coenzyme is bound by a domain which shares a similar fold to those of flavodoxin and the B12-binding domain of methylcobalamin-dependent methionine synthase. The cobalt atom is coordinated, via a long bond, to a histidine from the protein. The partial substrate is bound along the axis of a (beta/alpha)8 TIM barrel domain. CONCLUSIONS: The histidine-cobalt distance is very long (2.5 A compared with 1.95-2.2 A in free cobalamins), suggesting that the enzyme positions the histidine in order to weaken the metal-carbon bond of the cofactor and favour the formation of the initial radical species. The active site is deeply buried, and the only access to it is through a narrow tunnel along the axis of the TIM barrel domain.

Structure of influenza virus haemagglutinin complexed with a neutralizing antibody.

Haemagglutinin (HA) is the influenza surface glycoprotein that interacts with infectivity-neutralizing antibodies. As a consequence of this immune pressure, it is the variable virus component, which is important in antigenic drift, that results in recurrent epidemics of influenza. We have determined the crystallographic structure of a complex formed between the antigen-binding fragment (Fab) of a neutralizing antibody and the membrane-distal domain ('HA top') of a HA subunit prepared from HA in its membrane-fusion-active conformation. A dramatic change is seen in the structure of the Fab-combining site on complex formation. Our results indicate that neutralization of infectivity by this antibody involves the inhibition of receptor binding, and demonstrate how influenza virus can maintain its conserved receptor-binding site despite the immune selective pressure for change in this region of the molecule; they also contribute to a complete description of the endosomal pH-induced fusion-active HA structure.

The structural basis for seryl-adenylate and Ap4A synthesis by seryl-tRNA synthetase.

BACKGROUND: Seryl-tRNA synthetase is a homodimeric class II aminoacyl-tRNA synthetase that specifically charges cognate tRNAs with serine. In the first step of this two-step reaction, Mg.ATP and serine react to form the activated intermediate, seryl-adenylate. The serine is subsequently transferred to the 3'-end of the tRNA. In common with most other aminoacyl-tRNA synthetases, seryl-tRNA synthetase is capable of synthesizing diadenosine tetraphosphate (Ap4A) from the enzyme-bound adenylate intermediate and a second molecule of ATP. Understanding the structural basis for the substrate specificity and the catalytic mechanism of aminoacyl-tRNA synthetases is of considerable general interest because of the fundamental importance of these enzymes to protein biosynthesis in all living cells. RESULTS: Crystal structures of three complexes of seryl-tRNA synthetase from Thermus thermophilus are described. The first complex is of the enzyme with ATP and Mn2+. The ATP is found in an unusual bent conformation, stabilized by interactions with conserved arginines and three manganese ions. The second complex contains seryl-adenylate in the active site, enzymatically produced in the crystal after soaking with ATP, serine and Mn2+. The third complex is between the enzyme, Ap4A and Mn2+. All three structures exhibit a common Mn2+ site in which the cation is coordinated by two active-site residues in addition to the alpha-phosphate group from the bound ligands. CONCLUSIONS: Superposition of these structures allows a common reaction mechanism for seryl-adenylate and Ap4A formation to be proposed. The bent conformation of the ATP and the position of the serine are consistent with nucleophilic attack of the serine carboxyl group on the alpha-phosphate by an in-line displacement mechanism leading to the release of the inorganic pyrophosphate. A second ATP molecule can bind with its gamma-phosphate group in the same position as the beta-phosphate of the original ATP. This can attack the seryl-adenylate with the formation of Ap4A by an identical in-line mechanism in the reverse direction. The divalent cation is essential for both reactions and may be directly involved in stabilizing the transition state.