Stacking Interactions and Flexibility of Human Telomeric Multimers.

G-quadruplexes (G4s) are helical four-stranded structures forming from guanine-rich nucleic acid sequences, which are thought to play a role in cancer development and malignant transformation. Most current studies focus on G4 monomers, yet under suitable and biologically relevant conditions, G4s undergo multimerization. Here, we investigate the stacking interactions and structural features of telomeric G4 multimers by means of a novel low-resolution structural approach that combines small-angle X-ray scattering (SAXS) with extremely coarse-grained (ECG) simulations. The degree of multimerization and the strength of the stacking interaction are quantitatively determined in G4 self-assembled multimers. We show that self-assembly induces a significant polydispersity of the G4 multimers with an exponential distribution of contour lengths, consistent with a step-growth polymerization. On increasing DNA concentration, the strength of the stacking interaction between G4 monomers increases, as well as the average number of units in the aggregates. We utilized the same approach to explore the conformational flexibility of a model single-stranded long telomeric sequence. Our findings indicate that its G4 units frequently adopt a beads-on-a-string configuration. We also observe that the interaction between G4 units can be significantly affected by complexation with benchmark ligands. The proposed methodology, which identifies the determinants that govern the formation and structural flexibility of G4 multimers, may be an affordable tool aiding in the selection and design of drugs that target G4s under physiological conditions.

Self-Assembly of Rhamnolipid Bioamphiphiles: Understanding the Structure-Property Relationship Using Small-Angle X-ray Scattering.

The structure-property relationship of rhamnolipids, RLs, well-known microbial bioamphiphiles (biosurfactants), is explored in detail by coupling cryogenic transmission electron microscopy (cryo-TEM) and both ex situ and in situ small-angle X-ray scattering (SAXS). The self-assembly of three RLs with reasoned variation of their molecular structure (RhaC10, RhaC10C10, and RhaRhaC10C10) and a rhamnose-free C10C10 fatty acid is studied in water as a function of pH. It is found that RhaC10 and RhaRhaC10C10 form micelles in a broad pH range and RhaC10C10 undergoes a micelle-to-vesicle transition from basic to acid pH occurring at pH 6.5. Modeling coupled to fitting SAXS data allows a good estimation of the hydrophobic core radius (or length), the hydrophilic shell thickness, the aggregation number, and the surface area per RL. The essentially micellar morphology found for RhaC10 and RhaRhaC10C10 and the micelle-to-vesicle transition found for RhaC10C10 are reasonably well explained by employing the packing parameter (PP) model, provided a good estimation of the surface area per RL. On the contrary, the PP model fails to explain the lamellar phase found for the protonated RhaRhaC10C10 at acidic pH. The lamellar phase can only be explained by values of the surface area per RL being counterintuitively small for a di-rhamnose group and folding of the C10C10 chain. These structural features are only possible for a change in the conformation of the di-rhamnose group between the alkaline and acidic pH.

BioSAXS at European Synchrotron Radiation Facility - Extremely Brilliant Source: BM29 with an upgraded source, detector, robot, sample environment, data collection and analysis software.

As part of its Extremely Brilliant Source (EBS) upgrade project, the ESRF's BM29 BioSAXS beamline was subject to a significant upgrade and refurbishment. In addition to the replacement of the beamline's original bending magnet source by a two-pole wiggler, leading to an increase in brilliance by a factor of 60, the sample environment of the beamline was almost completely refurbished: a vacuum-compatible Pilatus3 X 2M with a sensitive area of 253.7 mm × 288 mm and frame rates up to 250 Hz was installed, increasing the active area available and thus the q-scaling of scattering images taken; the sample changer was replaced with an upgraded version, allowing more space for customizable sample environments and the installation of two new sample exposure units; the software associated with the beamline was also renewed. In addition, the layout and functionality of the BSXCuBE3 (BioSAXS Customized Beamline Environment) data acquisition software was redesigned, providing an intuitive `user first' approach for inexperienced users, while at the same time maintaining more powerful options for experienced users and beamline staff. Additional features of BSXCuBE3 are queuing of samples; either consecutive sample changer and/or SEC-SAXS (size-exclusion chromatography small-angle X-ray scattering) experiments, including column equilibration were also implemented. Automatic data processing and analysis are now managed via Dahu, an online server with upstream data reduction, data scaling and azimuthal integration built around PyFAI (Python Fast Azimuthal Integration), and data analysis performed using the open source FreeSAS. The results of this automated data analysis pipeline are displayed in ISPyB/ExiSAXS. The upgraded BM29 has been in operation since the post-EBS restart in September 2020, and here a full description of its new hardware and software characteristics together with examples of data obtained are provided.

Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual ß-sheet-like domains.

Low-molecular weight gelators (LMWGs) are small molecules (Mw < ∼1 kDa), which form self-assembled fibrillar network (SAFiN) hydrogels in water. A great majority of SAFiN gels are described by an entangled network of self-assembled fibers, in analogy to a polymer in a good solvent. Here, fibrillation of a biobased glycolipid bolaamphiphile is triggered by Ca2+ or Ag+ ions which are added to its diluted micellar phase. The resulting SAFiN, which forms a hydrogel above 0.5 wt%, has a "nano-fishnet" structure, characterized by a fibrous network of both entangled fibers and ß-sheet-like rafts, generally observed for silk fibroin, actin hydrogels or mineral imogolite nanotubes, but generally not known for SAFiN. This work focuses on the strength of the SAFIN gels, their fast recovery after applying a mechanical stimulus (strain) and their unusual resistance to temperature, studied by coupling rheology to small angle X-ray scattering (rheo-SAXS) using synchrotron radiation. The Ca2+-based hydrogel maintains its properties up to 55 °C, while the Ag+-based gel shows a constant elastic modulus up to 70 °C, without the appearance of any gel-to-sol transition temperature. Furthermore, the glycolipid is obtained by fermentation from natural resources (glucose and rapeseed oil), thus showing that naturally engineered compounds can have unprecedented properties, when compared to the wide range of chemically derived amphiphiles.

Ca2+ and Ag+ orient low-molecular weight amphiphile self-assembly into "nano-fishnet" fibrillar hydrogels with unusual ß-sheet-like raft domains.

Low-molecular weight gelators (LMWGs) are small molecules (Mw < ∼1 kDa), which form self-assembled fibrillar network (SAFiN) hydrogels in water when triggered by an external stimulus. A great majority of SAFiN gels involve an entangled network of self-assembled fibers, in analogy to a polymer in a good solvent. In some rare cases, a combination of attractive van der Waals and repulsive electrostatic forces drives the formation of bundles with a suprafibrillar hexagonal order. In this work, an unexpected micelle-to-fiber transition is triggered by Ca2+ or Ag+ ions added to a micellar solution of a novel glycolipid surfactant, whereas salt-induced fibrillation is not common for surfactants. The resulting SAFiN, which forms a hydrogel above 0.5 wt%, has a "nano-fishnet" structure, characterized by a fibrous network of both entangled fibers and ß-sheet-like rafts, generally observed for silk fibroin, actin hydrogels or mineral imogolite nanotubes, but not known for SAFiNs. The ß-sheet-like raft domains are characterized by a combination of cryo-TEM and SAXS and seem to contribute to the stability of glycolipid gels. Furthermore, glycolipid is obtained by fermentation from natural resources (glucose, rapeseed oil), thus showing that naturally engineered compounds can have unprecedented properties, when compared to the wide range of chemically derived amphiphiles.

Chameleonic amphiphile: The unique multiple self-assembly properties of a natural glycolipid in excess of water.

Chameleons are stunning reptiles which change colour according to the surrounding environment. In astrophysics, chameleons are particles whose mass varies in the surrounding matter. Here, we show the chameleonic self-assembly behavior of a low molecular weight (LMW) amphiphile, a broad class of molecules widely studied for several decades. Their ability to self-assemble in water make them both fascinating and useful compounds for a number of applications. Under thermodynamic conditions, their thermotropic and lyotropic phase behavior is generally predicted in relation to their molecular shape, as seen for classical head-tail molecules like surfactants or phospholipids. However, many exceptions do exist, either when amphiphiles have unconventional shapes, e.g., bolaform or gemini, or when they contain functional groups which undergo specific interactions such as H-bonding or π-π stacking. In excess water, surfactants form micelles, phospholipids form vesicles or lamellar phases, and functional amphiphiles often form micelles or fibers. Here, we show the multiphase behavior, much richer and more unpredictable than what it is known for most amphiphiles, of a biobased glycolipid produced by the yeast S. bombicola ΔugtB1. In excess water and within a narrow pH range around neutrality, this compound assembles into micelles, uni- and multilamellar vesicles, lamellae and fibers, simply as a function of changing pH, temperature and counterions. This rich phase behavior is not only interesting in itself, it also generates a number of diverse biocompatible and biodegradable soft self-assembled materials like hydrogels, complex coacervates and drug carriers.

New data analysis for BioSAXS at the ESRF.

The second phase of the ESRF upgrade program did not only provide a new storage ring (Extremely Brilliant Source, EBS) but also allowed several beamlines to be refurbished. The BioSAXS beamline (located on port BM29) was upgraded with a new wiggler source and a larger detector. All analysis software has been rewritten to cope with the increased data flux and continues to provide beamline users with reduced and pre-processed data in real time. This article describes FreeSAS, an open-source collection of various small-angle scattering analysis algorithms needed to reduce and analyze BioSAXS data, and Dahu, the tool used to interface data analysis with beamline control. It further presents the data-processing pipelines for the different data acquisitions modes of the beamline, using either a sample changer for individual homogeneous samples or an inline size-exclusion chromatography setup.

Ambiguities in and completeness of SAS data analysis of membrane proteins: the case of the sensory rhodopsin II-transducer complex.

Membrane proteins (MPs) play vital roles in the function of cells and are also major drug targets. Structural information on proteins is vital for understanding their mechanism of function and is critical for the development of drugs. However, obtaining high-resolution structures of membrane proteins, in particular, under native conditions is still a great challenge. In such cases, the low-resolution methods small-angle X-ray and neutron scattering (SAXS and SANS) might provide valuable structural information. However, in some cases small-angle scattering (SAS) provides ambiguous ab initio structural information if complementary measurements are not performed and/or a priori information on the protein is not taken into account. Understanding the nature of the limitations may help to overcome these problems. One of the main problems of SAS data analysis of solubilized membrane proteins is the contribution of the detergent belt surrounding the MP. Here, a comprehensive analysis of how the detergent belt contributes to the SAS data of a membrane-protein complex of sensory rhodopsin II with its cognate transducer from Natronomonas pharaonis (NpSRII-NpHtrII) was performed. The influence of the polydispersity of NpSRII-NpHtrII oligomerization is the second problem that is addressed here. It is shown that inhomogeneity in the scattering length density of the detergent belt surrounding a membrane part of the complex and oligomerization polydispersity significantly impacts on SAXS and SANS profiles, and therefore on 3D ab initio structures. It is described how both problems can be taken into account to improve the quality of SAS data treatment. Since SAS data for MPs are usually obtained from solubilized proteins, and their detergent belt and, to a certain extent, oligomerization polydispersity are sufficiently common phenomena, the approaches proposed in this work might be used in SAS studies of different MPs.

Cellulose Nanocrystal-Fibrin Nanocomposite Hydrogels Promoting Myotube Formation.

Cellulose nanocrystals (CNCs) have been widely studied as fillers to form reinforced nanocomposites with a wide range of applications, including the biomedical field. Here, we evaluated the possibility to combine them with fibrinogen and obtain fibrin hydrogels with improved mechanical stability as potential cellular scaffolds. In diluted conditions at a neutral pH, it was evidenced that fibrinogen could adsorb on CNCs in a two-step process, favoring their alignment under flow. Composite hydrogels could be prepared from concentrated fibrinogen solutions and nanocrystals in amounts up to 0.3 wt %. CNCs induced a significant modification of the initial fibrin fibrillogenesis and final fibrin network structure, and storage moduli of all nanocomposites were larger than those of pure fibrin hydrogels. Moreover, optimal conditions were found that promoted muscle cell differentiation and formation of long myotubes. These results provide original insights into the interactions of CNCs with proteins with key physiological functions and offer new perspectives for the design of injectable fibrin-based formulations.

How to Analyze and Present SAS Data for Publication.

SAS is a powerful technique to investigate oligomeric state and domain organization of macromolecules, e.g. proteins and nucleic acids, under physiological, functional and even time resolved conditions. However, reconstructing three dimensional structures from SAS data is inherently ambiguous, as no information about orientation and phase is available. In addition experimental artifacts such as radiation damage, concentration effects and incorrect background subtraction can hinder the interpretation of even lead to wrong results. In this chapter, explanations on how to analyze data and how to assess and minimize the influence of experimental artifacts on the data. Furthermore, guidelines on how to present the resulting data and models to demonstrate the data supports the conclusion being made and that it is not biased by artifacts, will be given.

Innovative High-Throughput SAXS Methodologies Based on Photonic Lab-on-a-Chip Sensors: Application to Macromolecular Studies.

The relevance of coupling droplet-based Photonic Lab-on-a-Chip (PhLoC) platforms and Small-Angle X-Ray Scattering (SAXS) technique is here highlighted for the performance of high throughput investigations, related to the study of protein macromolecular interactions. With this configuration, minute amounts of sample are required to obtain reliable statistical data. The PhLoC platforms presented in this work are designed to allow and control an effective mixing of precise amounts of proteins, crystallization reagents and buffer in nanoliter volumes, and the subsequent generation of nanodroplets by means of a two-phase flow. Spectrophotometric sensing permits a fine control on droplet generation frequency and stability as well as on concentration conditions, and finally the droplet flow is synchronized to perform synchrotron radiation SAXS measurements in individual droplets (each one acting as an isolated microreactor) to probe protein interactions. With this configuration, droplet physic-chemical conditions can be reproducibly and finely tuned, and monitored without cross-contamination, allowing for the screening of a substantial number of saturation conditions with a small amount of biological material. The setup was tested and validated using lysozyme as a model of study. By means of SAXS experiments, the proteins gyration radius and structure envelope were calculated as a function of protein concentration. The obtained values were found to be in good agreement with previously reported data, but with a dramatic reduction of sample volume requirements compared to studies reported in the literature.

Coupling High Throughput Microfluidics and Small-Angle X-ray Scattering to Study Protein Crystallization from Solution.

In this work, we propose the combination of small-angle X-ray scattering (SAXS) and high throughput, droplet based microfluidics as a powerful tool to investigate macromolecular interactions, directly related to protein solubility. For this purpose, a robust and low cost microfluidic platform was fabricated for achieving the mixing of proteins, crystallization reagents, and buffer in nanoliter volumes and the subsequent generation of nanodroplets by means of a two phase flow. The protein samples are compartmentalized inside droplets, each one acting as an isolated microreactor. Hence their physicochemical conditions (concentration, pH, etc.) can be finely tuned without cross-contamination, allowing the screening of a huge number of saturation conditions with a small amount of biological material. The droplet flow is synchronized with synchrotron radiation SAXS measurements to probe protein interactions while minimizing radiation damage. To this end, the experimental setup was tested with rasburicase (known to be very sensitive to denaturation), proving the structural stability of the protein in the droplets and the absence of radiation damage. Subsequently weak interaction variations as a function of protein saturation was studied for the model protein lysozime. The second virial coefficients (A2) were determined from the X-ray structure factors extrapolated to the origin. A2 obtained values were found to be in good agreement with data previously reported in literature but using only a few milligrams of protein. The experimental results presented here highlight the interest and convenience of using this methodology as a promising and potential candidate for studying protein interactions for the construction of phase diagrams.

Use of dynamic light scattering and small-angle X-ray scattering to characterize new surfactants in solution conditions for membrane-protein crystallization.

The structural and interactive properties of two novel hemifluorinated surfactants, F2H9-ß-M and F4H5-ß-M, the syntheses of which were based on the structure and hydrophobicity of the well known dodecyl-ß-maltoside (DD-ß-M), are described. The shape of their micellar assemblies was characterized by small-angle X-ray scattering and their intermicellar interactions in crystallizing conditions were measured by dynamic light scattering. Such information is essential for surfactant phase-diagram determination and membrane-protein crystallization.

BioSAXS Sample Changer: a robotic sample changer for rapid and reliable high-throughput X-ray solution scattering experiments.

Small-angle X-ray scattering (SAXS) of macromolecules in solution is in increasing demand by an ever more diverse research community, both academic and industrial. To better serve user needs, and to allow automated and high-throughput operation, a sample changer (BioSAXS Sample Changer) that is able to perform unattended measurements of up to several hundred samples per day has been developed. The Sample Changer is able to handle and expose sample volumes of down to 5 µl with a measurement/cleaning cycle of under 1 min. The samples are stored in standard 96-well plates and the data are collected in a vacuum-mounted capillary with automated positioning of the solution in the X-ray beam. Fast and efficient capillary cleaning avoids cross-contamination and ensures reproducibility of the measurements. Independent temperature control for the well storage and for the measurement capillary allows the samples to be kept cool while still collecting data at physiological temperatures. The Sample Changer has been installed at three major third-generation synchrotrons: on the BM29 beamline at the European Synchrotron Radiation Facility (ESRF), the P12 beamline at the PETRA-III synchrotron (EMBL@PETRA-III) and the I22/B21 beamlines at Diamond Light Source, with the latter being the first commercial unit supplied by Bruker ASC.

ISPyB for BioSAXS, the gateway to user autonomy in solution scattering experiments.

Logging experiments with the laboratory-information management system ISPyB (Information System for Protein crystallography Beamlines) enhances the automation of small-angle X-ray scattering of biological macromolecules in solution (BioSAXS) experiments. The ISPyB interface provides immediate user-oriented online feedback and enables data cross-checking and downstream analysis. To optimize data quality and completeness, ISPyBB (ISPyB for BioSAXS) makes it simple for users to compare the results from new measurements with previous acquisitions from the same day or earlier experiments in order to maximize the ability to collect all data required in a single synchrotron visit. The graphical user interface (GUI) of ISPyBB has been designed to guide users in the preparation of an experiment. The input of sample information and the ability to outline the experimental aims in advance provides feedback on the number of measurements required, calculation of expected sample volumes and time needed to collect the data: all of this information aids the users to better prepare for their trip to the synchrotron. A prototype version of the ISPyBB database is now available at the European Synchrotron Radiation Facility (ESRF) beamline BM29 and is already greatly appreciated by academic users and industrial clients. It will soon be available at the PETRA III beamline P12 and the Diamond Light Source beamlines I22 and B21.

Cryogenic x-ray diffraction microscopy utilizing high-pressure cryopreservation.

We present cryo x-ray diffraction microscopy of high-pressure-cryofixed bacteria and report high-convergence imaging with multiple image reconstructions. Hydrated D. radiodurans cells were cryofixed at 200 MPa pressure into ∼10-µm-thick water layers and their unstained, hydrated cellular environments were imaged by phasing diffraction patterns, reaching sub-30-nm resolutions with hard x-rays. Comparisons were made with conventional ambient-pressure-cryofixed samples, with respect to both coherent small-angle x-ray scattering and the image reconstruction. The results show a correlation between the level of background ice signal and phasing convergence, suggesting that phasing difficulties with frozen-hydrated specimens may be caused by high-background ice scattering.

Upgraded ESRF BM29 beamline for SAXS on macromolecules in solution.

Small-angle X-ray scattering (SAXS) measurements of proteins in solution are becoming increasingly popular with biochemists and structural biologists owing to the presence of dedicated high-throughput beamlines at synchrotron sources. As part of the ESRF Upgrade program a dedicated instrument for performing SAXS from biological macromolecules in solution (BioSAXS) has been installed at the renovated BM29 location. The optics hutch has been equipped with new optical components of which the two principal elements are a fixed-exit double multilayer monochromator and a 1.1 m-long toroidal mirror. These new dedicated optics give improved beam characteristics (compared with the previous set-up on ID14-3) regarding the energy tunability, flux and focusing at the detector plane leading to reduced parasitic scattering and an extended s-range. User experiments on the beamline have been successfully carried out since June 2012. A description of the new BioSAXS beamline and the set-up characteristics are presented together with examples of obtained data.

Continuous thermal collapse of the intrinsically disordered protein tau is driven by its entropic flexible domain.

The tau protein belongs to the category of Intrinsically Disordered Proteins (IDP), which in their native state lack a folded structure and fluctuate between many conformations. In its physiological state, tau helps nucleating and stabilizing the microtubules' (MTs) surfaces in the axons of the neurons. Tau is mainly composed by two domains: (i) the binding domain that tightly bounds the MT surfaces and (ii) the projection domain that exerts a long-range entropic repulsive force and thus provides the proper spacing between adjacent MTs. Tau is also involved in the genesis and in the development of the Alzheimer disease when it detaches from MT surfaces and aggregates in paired helical filaments. Unfortunately, the molecular mechanisms behind these phenomena are still unclear. Temperature variation, rarely considered in biological studies, is here used to provide structural information on tau correlated to its role as an entropic spacer between adjacent MTs surfaces. In this paper, by means of small-angle X-ray scattering and molecular dynamics simulation, we demonstrate that tau undergoes a counterintuitive collapse phenomenon with increasing temperature. A detailed analysis of our results, performed by the Ensemble Optimization Method, shows that the thermal collapse is coupled to the occurrence of a transient long-range contact between a region encompassing the end of the proline-rich domain P2 and the first part of the repeats domain, and the region of the N-terminal domain entailing residues 80-150. Interestingly these two regions involved in the tau temperature collapse belong to the flexible projection domain that acts as an entropic bristle and regulates the MTs' architecture. Our results show that temperature is an important parameter that influences the dynamics of the tau projection domain, and hence its entropic behavior.

Salt effects in the formation of self-assembled lithocholate helical ribbons and tubes.

The formation of self-assembled nanotubes is usually accounted for by anisotropic elastic properties of membranelike precursors. We present experimental data as evidence of the role played by electrostatics in the formation of self-assembled tubes in alkaline aqueous suspensions of lithocholic acid (LCA). Striking salt effects are characterized by comparing the rheological, dynamical, and scattering properties of systems prepared either in stoichiometric neutralization conditions (SC) of LCA or in a large excess of sodium hydroxide (EOC, experimentally optimized conditions) and finally, in two steps: stoichiometric neutralization followed by an appropriate addition of NaCl (AISC). The SC liquid system is originally made up of loose helical ribbons (previous transmission electron microscopy data), and upon aging they exhibit both intra- and interordering processes. Initially, the helical ribbons are loose and progressively wind around a cylinder (R = 330 Å) with their edges exposed to the solvent. They can be temporarily organized in a centered rectangular two-dimensional lattice (pgg, a = 224 Å, b = 687 Å). Upon further aging, the ribbons wind into more compact helical ribbons (or tubes with helical grooves): their edges are less-exposed and their ordering vanishes. Upon addition of NaCl salt (as in the AISC systems), the specific screening of the intra-aggregate electrostatic repulsions induces the closure of the ribbons into tubes (R(ext) = 260 Å, R(int) = 245 Å as in the EOC systems). Simultaneously with the closure of the ribbons into plain tubes, a drastic enhancement of their interconnectivity through van der Waals attractions develops. Eventually, gels are obtained with networks having hexagonal bundles of tubes.

A spring-loaded release mechanism regulates domain movement and catalysis in phosphoglycerate kinase.

Phosphoglycerate kinase (PGK) is the enzyme responsible for the first ATP-generating step of glycolysis and has been implicated extensively in oncogenesis and its development. Solution small angle x-ray scattering (SAXS) data, in combination with crystal structures of the enzyme in complex with substrate and product analogues, reveal a new conformation for the resting state of the enzyme and demonstrate the role of substrate binding in the preparation of the enzyme for domain closure. Comparison of the x-ray scattering curves of the enzyme in different states with crystal structures has allowed the complete reaction cycle to be resolved both structurally and temporally. The enzyme appears to spend most of its time in a fully open conformation with short periods of closure and catalysis, thereby allowing the rapid diffusion of substrates and products in and out of the binding sites. Analysis of the open apoenzyme structure, defined through deformable elastic network refinement against the SAXS data, suggests that interactions in a mostly buried hydrophobic region may favor the open conformation. This patch is exposed on domain closure, making the open conformation more thermodynamically stable. Ionic interactions act to maintain the closed conformation to allow catalysis. The short time PGK spends in the closed conformation and its strong tendency to rest in an open conformation imply a spring-loaded release mechanism to regulate domain movement, catalysis, and efficient product release.