7 research outputs found
A Systematic Investigation and Insight into the Formation Mechanism of Bilayers of Fatty Acid/Soap Mixtures in Aqueous Solutions
Vesicles are the most common form
of bilayer structures in fatty
acid/soap mixtures in aqueous solutions; however, a peculiar bilayer
structure called a âplanar sheetâ was found for the
first time in the mixtures. In the past few decades, considerable
research has focused on the formation theory of bilayers in fatty
acid/soap mixtures. The hydrogen bond theory has been widely accepted
by scientists to explain the formation of bilayers. However, except
for the hydrogen bond, no other driving forces were proposed systematically.
In this work, three kinds of weak interactions were investigated in
detail, which could perfectly demonstrate the formation mechanism
of bilayer structures in the fatty acid/soap mixtures in aqueous solutions.
(i) The influence of hydrophobic interaction was detected by changing
the chain length of fatty acid (C<sub><i>n</i></sub>H<sub>2<i>n</i>+1</sub>COOH), in which <i>n</i> = 10
to 18, the phase behavior was investigated, and the phase region was
presented. With the help of cryogenic transmission electron microscopy
(cryo-TEM) observations, deuterium nuclear magnetic resonance (<sup>2</sup>H NMR), and X-ray diffraction (XRD) measurements, the vesicles
and planar sheets were determined. The chain length of C<sub><i>n</i></sub>H<sub>2<i>n</i>+1</sub>COOH has an important
effect on the physical state of the hydrophobic chain, resulting in
an obvious difference in the viscoelasticity of the solution samples.
(ii) The existence of hydrogen bonds between fatty acids and their
soaps in aqueous solutions was demonstrated by Fourier transform infrared
(FT-IR) spectroscopy and molecule dynamical simulation. From the pH
measurements, the pH ranges of the bilayer formation were at the p<i>K</i><sub>a</sub> values of fatty acids, respectively. (iii)
Counterions can be embedded in the stern layer of the bilayers and
screen the electrostatic repulsion between the COO<sup>â</sup> anionic headgroups. FT-IR characterization demonstrated a bidentate
bridging coordination mode between counterions and carboxylates. The
conductivity measurements provided the degree of counterion binding
(ÎČ = 0.854), indicating the importance of the counterions
Direct Observation of CH/CH van der Waals Interactions in Proteins by NMR
van der Waals interactions are important
to protein stability and
function. These interactions are usually identified empirically based
on protein 3D structures. In this work, we performed a solution nuclear
magnetic resonance (NMR) spectroscopy study of van der Waals interactions
by detecting the through-space <sup>vdw</sup><i>J</i><sub>CC</sub>-coupling between protein aliphatic side chain groups. Specifically, <sup>vdw</sup><i>J</i><sub>CC</sub>-coupling values up to âŒ0.5
Hz were obtained between the methyl and nearby aliphatic groups in
protein GB3, providing direct experimental evidence for the van der
Waals interactions. Quantum mechanical calculations suggest that the <i>J</i>-coupling is correlated with the exchange-repulsion term
of van der Waals interaction. NMR detection of <sup>vdw</sup><i>J</i><sub>CC</sub>-coupling offers a new tool to characterize
such interactions in proteins
CarboxylâPeptide Plane Stacking Is Important for Stabilization of Buried E305 of Trichoderma reesei Cel5A
Hydrogen
bonds or salt bridges are usually formed to stabilize
the buried ionizable residues. However, such interactions do not exist
for two buried residues D271 and E305 of Trichoderma
reesei Cel5A, an endoglucanase. Mutating D271 to alanine
or leucine improves the enzyme thermostability quantified by the temperature <i>T</i><sub>50</sub> due to the elimination of the desolvation
penalty of the aspartic acid. However, the same mutations for E305
decrease the enzyme thermostability. Free energy calculations based
on the molecular dynamics simulation predict the thermostability of
D271A, D271L, and E305A (compared to WT) in line with the experimental
observation but overestimate the thermostability of E305L. Quantum
mechanical calculations suggest that the carboxylâpeptide plane
stacking interactions occurring to E305 but not D271 are important
for the carboxyl group stabilization. For the protonated carboxyl
group, the interaction energy can be as much as about â4 kcal/mol
for parallel stacking and about â7 kcal/mol for T-shaped stacking.
For the deprotonated carboxyl group, the largest interaction energies
for parallel stacking and T-shaped stacking are comparable, about
â7 kcal/mol. The solvation effect generally weakens the interaction,
especially for the charged system. A search of the carboxylâpeptide
plane stacking in the PDB databank indicates that parallel stacking
but not T-shaped stacking is quite common, and the most probable distance
between the two stacking fragments is close to the value predicted
by the QM calculations. This work highlights the potential role of
carboxyl amide ÏâÏ stacking in the stabilization
of aspartic acid and glutamic acid in proteins
Observation of 뱉Helical Hydrogen-Bond Cooperativity in an Intact Protein
The
presence and extent of hydrogen-bonding (H-bonding) cooperativity
in proteins remains a fundamental question, which in the past has
been studied extensively, mostly by infrared and fluorescence measurements
on model peptides. We demonstrate that such cooperativity can be studied
in an intact protein by hydrogen/deuterium exchange NMR spectroscopy.
The method is based on the fact that substitution of NH by ND in a
backbone amide group slightly weakens the NâH···Oî»C
hydrogen bond. Our results show that such substitution at position <i>i</i> in an α-helix impacts the <sup>1</sup>H and <sup>15</sup>N chemical shifts of the amide sites of residues <i>i</i> â 3 to <i>i</i> + 3. Quantum mechanical
calculations indicate that the upfield shifts of <sup>1</sup>H and <sup>15</sup>N resonances at site <i>i</i>, observed upon H/D
exchanges at sites <i>i</i> â 3, <i>i</i> + 1, <i>i</i> + 2, and <i>i</i> + 3, correspond
to a decrease of the <i>i</i>th backbone amide electric
dipole moment, which weakens its H-bonding and long-range electrostatic
interactions with other backbone amides in the α-helix. These
results provide new quantitative insights into the cooperativity of
H-bonding in protein α-helices
Artificial Light-Harvesting System with White-Light Emission in a Bicontinuous Ionic Medium
Artificial
light-harvesting systems (ALHSs), which are
closely
related to FoÌrster resonance energy transfer (FRET), are among
the most attractive scientific topics during the past few decades.
Specifically, binary ALHSs that are composed of a fluid donor and
acceptor have a simplified composition and high number density of
the donor units. However, largely due to the difficulty in obtaining
a fluid donor, investigation of these systems is still quite limited,
especially for the ionic systems. Herein, we report a new type of
binary ALHS using an ionic naphthalimide (NPI) derivative as a donor,
which shows greatly improved photoluminescence for its bicontinuous
liquid structure. When blending with an acceptor such as rhodamine
6G or trans-4-[4-(dimethylamino)styryl]-methylpyridinium
iodide, efficient FRET was confirmed by both experimental results
and molecular dynamics simulations, with an energy transfer efficiency
up to âŒ90%. Tunable color, including white-light emission,
was achieved by tuning the acceptor/donor ratio, opening the door
for a variety of applications such as light-emitting diodes and photoluminescent
inks
Protein Allostery Study in Cells Using NMR Spectroscopy
Protein
allostery is commonly observed in vitro. But how protein
allostery behaves in cells is unknown. In this work, a protein monomerâdimer
equilibrium system was built with the allosteric effect on the binding
characterized using NMR spectroscopy through mutations away from the
dimer interface. A chemical shift linear fitting method was developed
that enabled us to accurately determine the dissociation constant.
A total of 28 allosteric mutations were prepared and grouped to negative
allosteric, nonallosteric, and positive allosteric modulators. âŒ
50% of mutations displayed the allosteric-state changes when moving
from a buffered solution into cells. For example, there were no positive
allosteric modulators in the buffered solution but eight in cells.
The change in protein allostery is correlated with the interactions
between the protein and the cellular environment. These interactions
presumably drive the surrounding macromolecules in cells to transiently
bind to the monomer and dimer mutational sites and change the free
energies of the two species differently which generate new allosteric
effects. These surrounding macromolecules create a new protein allostery
pathway that is only present in cells