12 research outputs found
Comparison of the Thermal Denaturing of Human Serum Albumin in the Presence of Guanidine Hydrochloride and 1‑Butyl-3-methylimidazolium Ionic Liquids
The
interaction of proteins with aqueous solutions of ionic liquids
(ILs) has attracted considerable recent attention owing to the challenges
of finding biocompatible water-free ILs. These systems remain of great
interest because of the potential for using ILs as designer solvents
for biocatalytic processes. Increasing evidence demonstrates that
aqueous solutions of water-miscible ILs, such as the well-studied
1-alkyl-3-methylimidazolium ILs, disrupt the native fold of proteins
and can drive the formation of non-native aggregates that could negatively
impact catalytic function. Here, we present a study comparing the
thermal unfolding of human serum albumin (HSA) in a 1 M solution of
the protein denaturant guanidine hydrochloride with two 1 M aqueous
solutions of 1-butyl-3-methylimidazolium ILs, namely the chloride
and the acetate. Small-angle neutron scattering (SANS) measurements
found qualitative agreement between the thermally driven unfolding
process for the three denaturants, as well as with a Tris buffer solution.
HSA irreversibly aggregates and unfolds in the three denaturant solutions
upon heating to temperatures below that required to drive the same
process in a simple Tris buffer solution. The results reveal subtle
differences in the interaction of the ILs and guanidine hydrochloride
with the protein, although the final states of the protein were similar
in all cases. The results indicate that the ions of water-miscible
ILs and guanidine hydrochloride have specific roles in disrupting
protein structure and driving aggregation. The experimental approach
employed has the potential to provide new insights into protein interactions
with ionic liquids that may aid in the search for more biocompatible
ionic liquids
Alamethicin Disrupts the Cholesterol Distribution in Dimyristoyl Phosphatidylcholine–Cholesterol Lipid Bilayers
Cell membranes are complex mixtures
of lipids, proteins, and other
molecules that serve as active, semipermeable barriers between cells,
as well as between their internal organelles, and the surrounding
medium. Their compositions and structures are tightly regulated to
ensure proper function. Cholesterol is a key component in mammalian
cellular membranes, where it serves to maintain membrane fluidity
and permeability. Here, the interaction of alamethicin, a 20 amino
acid residue peptide that creates transmembrane pores in lipid bilayer
membranes in a concentration-dependent manner, with bilayer membranes
composed of dimyristoyl phosphatidylcholine (DMPC) and cholesterol
(Chol) was studied. Small-angle neutron scattering (SANS) data demonstrate
that a low concentration of alamethicin (peptide-to-lipid ratio of
1/200) disrupts a lateral inhomogeneity seen in peptide-free DMPC:Chol
vesicles, which analysis of the SANS data indicates are Chol-rich
and Chol-poor phases having different thicknesses. Alamethicin disrupts
this structure, producing laterally homogeneous bilayers that are
thinner than either phase of the peptide-free bilayers, and possess
a strong asymmetry in the Chol content of the inner and outer bilayer
leaflets. The results suggest that a secondary membrane disruption
mechanism exists in parallel with the well-understood cytotoxic membrane
permeabilization that results when alamethcin forms transmembrane
pores. Specifically, the peptide can disrupt laterally organized lipidic
structures in cell membranes, as well as significantly perturb the
compositions of the inner and outer leaflets of the membrane. The
existence of a secondary mechanism of action against cellular membranes
for alamethicin raises the possibility that other membrane-active
peptides function similarly
Multicompartmental Microcapsules from Star Copolymer Micelles
We present the layer-by-layer (LbL) assembly of amphiphilic
heteroarm
pH-sensitive star-shaped polystyrene-poly(2-pyridine) (PS<sub><i>n</i></sub>P2VP<sub><i>n</i></sub>) block copolymers
to fabricate porous and multicompartmental microcapsules. Pyridine-containing
star molecules forming a hydrophobic core/hydrophilic corona unimolecular
micelle in acidic solution (pH 3) were alternately deposited with
oppositely charged linear sulfonated polystyrene (PSS), yielding microcapsules
with LbL shells containing hydrophobic micelles. The surface morphology
and internal nanopore structure of the hollow microcapsules were comparatively
investigated for shells formed from star polymers with a different
numbers of arms (9 versus 22) and varied shell thickness (5, 8, and
11 bilayers). The successful integration of star unimers into the
LbL shells was demonstrated by probing their buildup, surface segregation
behavior, and porosity. The larger arm star copolymer (22 arms) with
stretched conformation showed a higher increment in shell thickness
due to the effective ionic complexation whereas a compact, uniform
grainy morphology was observed regardless of the number of deposition
cycles and arm numbers. Small-angle neutron scattering (SANS) revealed
that microcapsules with hydrophobic domains showed different fractal
properties depending upon the number of bilayers with a surface fractal
morphology observed for the thinnest shells and a mass fractal morphology
for the completed shells formed with the larger number of bilayers.
Moreover, SANS provides support for the presence of relatively large
pores (about 25 nm across) for the thinnest shells as suggested from
permeability experiments. The formation of robust microcapsules with
nanoporous shells composed of a hydrophilic polyelectrolyte with a
densely packed hydrophobic core based on star amphiphiles represents
an intriguing and novel case of compartmentalized microcapsules with
an ability to simultaneously store different hydrophilic, charged,
and hydrophobic components within shells
Synergistic Role of Temperature and Salinity in Aggregation of Nonionic Surfactant-Coated Silica Nanoparticles
The adsorption of nonionic surfactants onto hydrophilic
nanoparticles
(NPs) is anticipated to increase their stability in aqueous medium.
While nonionic surfactants show salinity- and temperature-dependent
bulk phase behavior in water, the effects of these two solvent parameters
on surfactant adsorption and self-assembly onto NPs are poorly understood.
In this study, we combine adsorption isotherms, dispersion transmittance,
and small-angle neutron scattering (SANS) to investigate the effects
of salinity and temperature on the adsorption of pentaethylene glycol
monododecyl ether (C12E5) surfactant on silica
NPs. We find an increase in the amount of surfactant adsorbed onto
the NPs with increasing temperature and salinity. Based on SANS measurements
and corresponding analysis using computational reverse-engineering
analysis of scattering experiments (CREASE), we show that the increase
in salinity and temperature results in the aggregation of silica NPs.
We further demonstrate the non-monotonic changes in viscosity for
the C12E5–silica NP mixture with increasing
temperature and salinity and correlate the observations to the aggregated
state of NPs. The study provides a fundamental understanding of the
configuration and phase transition of the surfactant-coated NPs and
presents a strategy to manipulate the viscosity of such dispersion
using temperature as a stimulus
Polymer Chain Shape of Poly(3-alkylthiophenes) in Solution Using Small-Angle Neutron Scattering
The chain shape of polymers affects many aspects of their
behavior and is governed by their intramolecular interactions. Delocalization
of electrons along the backbone of conjugated polymers has been shown
to lead to increased chain rigidity by encouraging a planar conformation.
Poly(3-hexylthiophene) and other poly(3-alkylthiophenes) (P3ATs) are
interesting for organic electronics applications, and it is clear
that a hierarchy of structural features in these polymers controls
charge transport. While other conjugated polymers are very rigid,
the molecular structure of P3AT allows for two different planar conformations
and a significant degree of torsion at room temperature. It is unclear,
however, how their chain shape depends on variables such as side chain
chemistry or regioregularity, both of which are key aspects in the
molecular design of organic electronics. Small-angle neutron scattering
from dilute polymer solutions indicates that the chains adopt a random
coil geometry with a semiflexible backbone. The measured persistence
length is shorter than the estimated conjugation length due to the
two planar conformations that preserve conjugation but not backbone
correlations. The persistence length of regioregular P3HT has been
measured to be 3 nm at room temperature and decreases at higher temperatures.
Changes in the regioregularity, side chain chemistry, or synthetic
defects decrease the persistence length by 60–70%
Observing Framework Expansion of Ordered Mesoporous Hard Carbon Anodes with Ionic Liquid Electrolytes via in Situ Small-Angle Neutron Scattering
The
reversible capacity of materials for energy storage, such as
battery electrodes, is deeply connected with their microstructure.
Here, we address the fundamental mechanism by which hard mesoporous
carbons, which exhibit high capacities versus Li, achieve stable cycling
during the initial “break-in” cycles with ionic liquid
electrolytes. Using in situ small-angle neutron scattering we show
that hard carbon anodes that exhibit reversible Li<sup>+</sup> cycling
typically expand in volume up to 15% during the first discharge cycle,
with only relatively minor expansion and contraction in subsequent
cycles after a suitable solid electrolyte interphase (SEI) has formed.
While a largely irreversible framework expansion is observed in the
first cycle for the 1-methyl-1-propypyrrolidinium bis(trifluoromethanesulfonyl)imide
(MPPY.TFSI) electrolyte, reversible expansion is observed in the electrolyte
lithium bis(trifluoro-methanesulfonyl)imide (LiTFSI)/1-ethyl-3-methyl-imidazolium
bis(trifluoromethanesulf-onyl)imide (EMIM.TFSI) related to EMIM<sup>+</sup> intercalation and deintercalation before a stable SEI is
formed. We find that irreversible framework expansion in conjunction
with SEI formation is essential for the stable cycling of hard carbon
electrodes
Small-angle neutron scattering study of specific interaction and coordination structure formed by mono-acetyl-substituted dibenzo-20-crown-6-ether and cesium ions
<p>This study uses small-angle neutron scattering (SANS) to elucidate the coordination structure of the complex of mono-acetyl-substituted dibenzo-20-crown-6-ether (ace-DB20C6) with cesium ions (Cs<sup>+</sup>). SANS profiles obtained for the complex of ace-DB20C6 and Cs<sup>+</sup> (ace-DB20C6/Cs) in deuterated dimethyl sulfoxide indicated that Cs<sup>+</sup> coordination resulted in a more compact structure than the free ace-DB20C6. The data were fitted well with SANS profiles calculated using Debye function for scattering on an absolute scattering intensity scale. For this theoretical calculation of the scattering profiles, the coordination structure proposed based on density functional theory calculation was used. Consequently, we conclude that the SANS analysis experimentally supports the proposed coordination structure of ace-DB20C6/Cs and suggests the following: (1) the complex of ace-DB20C6 and Cs<sup>+</sup> is formed with an ace-DB20C6/Cs molar ratio of 1/1 and (2) the two benzene rings of ace-DB20C6 fold around Cs<sup>+</sup> above the center of the crown ether ring of ace-DB20C6.</p
Metal-Free cAMP-Dependent Protein Kinase Can Catalyze Phosphoryl Transfer
X-ray structures of several ternary
product complexes of the catalytic
subunit of cAMP-dependent protein kinase (PKAc) have been determined
with no bound metal ions and with Na<sup>+</sup> or K<sup>+</sup> coordinated
at two metal-binding sites. The metal-free PKAc and the enzyme with
alkali metals were able to facilitate the phosphoryl transfer reaction.
In all studied complexes, the ATP and the substrate peptide (SP20)
were modified into the products ADP and the phosphorylated peptide.
The products of the phosphotransfer reaction were also found when
ATP-γS, a nonhydrolyzable ATP analogue, reacted with SP20 in
the PKAc active site containing no metals. Single turnover enzyme
kinetics measurements utilizing <sup>32</sup>P-labeled ATP confirmed
the phosphotransferase activity of the enzyme in the absence of metal
ions and in the presence of alkali metals. In addition, the structure
of the <i>apo</i>-PKAc binary complex with SP20 suggests
that the sequence of binding events may become ordered in a metal-free
environment, with SP20 binding first to prime the enzyme for subsequent
ATP binding. Comparison of these structures reveals conformational
and hydrogen bonding changes that might be important for the mechanism
of catalysis
Thermally Responsive Hyperbranched Poly(ionic liquid)s: Assembly and Phase Transformations
A library of linear and branched
amphiphilic poly(ionic liquid)s
based on hydrophobic cores and peripheral thermally sensitive shells
was synthesized and studied with regard to their ability to form stimuli-responsive,
organized assemblies in aqueous media. The thermally responsive derivatives
of poly(ionic liquid)s were synthesized by neutralizing 32 terminal
carboxyl groups of functionalized polyester cores by amine-terminated
poly(<i>N</i>-isopropylacrylamide)s (PNIPAM) (50%
and 100%). We observed that these hyperbranched poly(ionic liquid)s
possessed a narrow low critical solution transition (LCST) window
with LCST for hyperbranched compounds being consistently lower than
that for linear PNIPAM containing counterparts. We found that the
poly(ionic liquid)s form spherical micellar assemblies with diverse
morphologies, such as micelles and their aggregates, depending on
the terminal compositions with reduced sizes for hyperbranched poly(ionic
liquid)s. Increasing temperature above LCST promoted formation of
network-like aggregates, large vesicles, and spherical micelles. Moreover,
all PNIPAM-terminated compounds exhibited distinct unimolecular prolate
nanodomain morphology in contrast to common spherical domains of initial
cores. We proposed a multilength scale organized morphology to describe
the thermoresponsive poly(ionic liquid)s micellar assemblies and discussed
their morphological transformations during phase transitions associated
with changes in hydrophobic–hydrophilic balance of poly(ionic
liquid)s with distinct hydrophobic cores and variable peripheral shells
CO<sub>2</sub>‑Reactive Ionic Liquid Surfactants for the Control of Colloidal Morphology
This article reports
on a new class of stimuli-responsive surfactant
generated from commercially available amphiphiles such as dodecyltrimethylammmonium
bromide (DTAB) by substitution of the halide counterion with counterions
such as 2-cyanopyrrolide, 1,2,3-triazolide, and <i>L</i>-proline that complex reversibly with CO<sub>2</sub>. Through
a combination of small-angle neutron scattering (SANS), electrical
conductivity measurements, thermal gravimetric analysis, and molecular
dynamics simulations, we show how small changes in charge reorganization
and counterion shape and size induced by complexation with CO<sub>2</sub> allow for fine-tunability of surfactant properties. We then
use these findings to demonstrate a range of potential practical uses,
from manipulating microemulsion droplet morphology to controlling
micellar and vesicular aggregation. In particular, we focus on the
binding of these surfactants to DNA and the reversible compaction
of surfactant–DNA complexes upon alternate bubbling of the
solution with CO<sub>2</sub> and N<sub>2</sub>