Designing Mixed Detergent Micelles for Uniform Neutron Contrast

Micelle-forming detergents provide an amphipathic environment that mimics lipid bilayers and are important tools used to solubilize and stabilize membrane proteins in solution for in vitro structural investigations. Small-angle neutron scattering (SANS) at the neutron contrast match point of detergent molecules allows observing the signal from membrane proteins unobstructed by contributions from the detergent. However, we show that even for a perfectly average-contrast matched detergent there arises significant core–shell scattering from the contrast difference between aliphatic detergent tails and hydrophilic head groups. This residual signal interferes with interpreting structural data of membrane proteins. This complication is often made worse by the presence of excess empty (protein-free) micelles. We present an approach for the rational design of mixed micelles containing a deuterated detergent analog, which eliminates neutron contrast between core and shell and allows the micelle scattering to be fully contrast-matched to unambiguously resolve membrane protein structure using solution SANS

The Internal Organization of Mycobacterial Partition Assembly: Does the DNA Wrap a Protein Core?

<div><p>Before cell division in many bacteria, the ParBs spread on a large segment of DNA encompassing the origin-proximal <em>parS</em> site(s) to form the partition assembly that participates in chromosome segregation. Little is known about the structural organization of chromosomal partition assembly. We report solution X-ray and neutron scattering data characterizing the size parameters and internal organization of a nucleoprotein assembly formed by the mycobacterial chromosomal ParB and a 120-meric DNA containing a <em>parS</em>-encompassing region from the mycobacterial genome. The cross-sectional radii of gyration and linear mass density describing the rod-like ParB-DNA assembly were determined from solution scattering. A “DNA outside, protein inside” mode of partition assembly organization consistent with the neutron scattering hydrogen/deuterium contrast variation data is discussed. In this organization, the high scattering DNA is positioned towards the outer region of the partition assembly. The new results presented here provide a basis for understanding how ParBs organize the <em>parS</em>-proximal chromosome, thus setting the stage for further interactions with the DNA condensins, the origin tethering factors and the ParA.</p> </div

Figure 2

<p><b>Hypothetical three-dimensional organizations of the partition assembly.</b> Two topologically alternative scenarios can be proposed: (a) a “DNA inside, protein outside” model or (b) a “DNA outside, protein inside” model. The DNA is shown as black line, proteins are shown as grey beads. These models can be described as composite cylinders for low-resolution solution scattering experiments. Anticipated shapes at the protein and the DNA match-point for both models (as narrower or hollow cylinders) are shown as cartoons (case I–IV). The handedness and scale are arbitrary. We note that solution scattering cannot distinguish between the left- and the right-handed senses.</p

The Stuhrmann plot (R_g²<i>versus</i> Δρ

<p>^−<b>1</b><b>, Δρ in 10¹⁰ cm</b>^−<b>2</b><b>, R_g in Å) obtained from the SANS dataset of the tbParB-D120 assembly.</b> The real space radii of gyration derived from the corresponding pair distribution functions were used for the calculation of Stuhrmann plot. A straight-line fitted to the data (R² = 0.98) is shown in black.</p

The modified Guinier plot (ln(q.I(q)) <i>versus</i> q², I in cm

<p>^−<b>1</b><b>, q in Å</b>^−<b>1</b><b>) of the “73.5% D₂O” dataset.</b> The R_XS and mass/length of the protein segment were obtained from the slope and the intercept of this plot.</p

Figure 1

<p><b>The size of tbParB-D120 assembly derived from the SAXS data.</b> (a) Intensity (I) in arbitrary unit <i>versus</i> momentum transfer q in Å⁻¹ are plotted at 3 different concentrations. The Guinier plot (lnI(q) <i>versus</i> q², q in Å⁻¹; ) is shown in the inset. These and other graphs presented in this work are prepared using Excel® (Microsoft® corporation). A linear trend-line fitted to the data points is shown in each case. (b) The modified Guinier plot for rod-shaped particle (lnI(q)q <i>versus</i> q², q in Å⁻¹, ) is shown. (c) The pair-distribution function P(r) <i>versus</i> pair-wise distance r in Å. The pair functions shown here and in the <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0052690#pone-0052690-g005" target="_blank">figure 5</a> were calculated with the following boundary conditions: P(r = 0) = 0 and P(r ≥ D_max) = 0. (d) The cross-sectional pair-distribution function (P_XS(r)) <i>versus</i> pair-wise distance r in Å.</p

Orientation-Dependent Order–Disorder Transition of Block Copolymer Lamellae in Electric Fields

Electric fields have been shown to stabilize the disordered phase of near-critical block copolymer solutions. Here, we use in situ synchrotron small-angle X-ray scattering to examine how the initial orientation of lamellar domains with respect to the external field (φ) affects the shift in the order–disorder transition temperature (<i>T</i>_ODT) of lyotropic solutions of poly­(styrene-<i>b</i>-isoprene) in toluene. We find a downward shift of the transition temperature, which scales with lamellar orientation as Δ<i>T</i>_ODT ∼ cos² φ, in accordance with theory

A Small-Angle Neutron Scattering Study of the Equilibrium Conformation of Polyelectrolytes in Stoichiometric Saloplastic Polyelectrolyte Complexes

Stoichiometric polyelectrolyte complexes, PECs, from fully sulfonated poly­(styrenesulfonate), PSS, as polyanion and poly­(diallyldimethylammonium chloride), PDADMA, as polycation, were prepared by mixing them at optimized polyelectrolyte and NaCl concentrations. The complexes were compacted by ultracentrifugtion and then annealed in NaCl solutions at elevated temperatures to allow the polymers to fully intermix and relax. Small-angle neutron scattering, SANS, with contrast matching, was used to study single polyelectrolyte chain dimensions in PECs made from a mixture of deuterated and protonated PSS chains. Two PSS molecular weights in PECs were investigated at various ionic strengths. SANS curves, form factor fits, and corresponding Kratky plots indicate the Gaussian nature of the polyelectrolyte chains in the complexes regardless of molecular weight. PSS coils were slightly larger than the unperturbed dimension, more so for the higher molecular weight material, which was attributed to an effective stiffening of the chain due to ladderlike interactions between polyelectrolytes

Scattering Neutrons along the Polyelectrolyte Complex/Coacervate Continuum

The coil size of narrow molecular weight distribution deuterated poly­(styrene­sulfonate), PSS, within a polyelectrolyte complex doped with KBr was tracked across the continuum from solid to coacervate to solution using small-angle neutron scattering. While PSS alone in solution exhibited the familiar and pronounced “polyelectrolyte effect” of coil shrinkage with increasing [KBr], the radius of gyration <i>R</i>_g of the PSS in the complex remained surprisingly constant up to 1.4 M KBr, which is close to the transition between complex and coacervate behavior. Thereafter, <i>R</i>_g decreased with increasing KBr, remaining slightly larger than <i>R</i>_g for PSS in KBr alone. Upturns in the scattering at low angle, seen for complexes in lower [KBr], are consistent with porosity, observed macroscopically as whitening of the bulk complexa universal property of polyelectrolyte complexes. Reasons for this porosity, imaged by scanning electron microscopy, are discussed. At high <i>q</i> ranges, a correlation peak between deuterated coils of PSS was observed

Unraveling the Single-Nanometer Thickness of Shells of Vesicle-Templated Polymer Nanocapsules

Vesicle-templated nanocapsules have emerged as a viable platform for diverse applications. Shell thickness is a critical structural parameter of nanocapsules, where the shell plays a crucial role providing mechanical stability and control of permeability. Here we used small-angle neutron scattering (SANS) to determine the thickness of freestanding and surfactant-stabilized nanocapsules. Despite being at the edge of detectability, we were able to show the polymer shell thickness to be typically 1.0 ± 0.1 nm, which places vesicle-templated nanocapsules among the thinnest materials ever created. The extreme thinness of the shells has implications for several areas: mass-transport through nanopores is relatively unimpeded; pore-forming molecules are not limited to those spanning the entire bilayer; the internal volume of the capsules is maximized; and insight has been gained on how polymerization occurs in the confined geometry of a bilayer scaffold, being predominantly located at the phase-separated layer of monomers and cross-linkers between the surfactant leaflets