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Elucidating the Molecular Behavior on the pH Responsiveness of Chitosan

Written by Tianyu Liu, University of California, Santa Cruz

Chitosan is a polysaccharide derived from chitin, a second most abundant bio-polymer on earth that consists of the shells of crustaceans (such as shrimps and crabs). It is a commonly used bio-compatible material for bio-medical applications such as drug delivery.

Previous studies have shown that chitosan is pH sensitive in water: it forms a viscous solution at low pH values and becomes insoluble at high pH values. Preliminary investigations suggest that such pH responsiveness is associated with the protonation and de-protonation processes of the amine groups on the chitosan polymer chain (the red box in Figure a). However, there still lacks fundamental understanding on how the pH affects the behavior of chitosan.

Now writing in Chem. Commun., Xu and Matysiak from University of Maryland, USA provided us new insights on the chitosan’s pH responsiveness at the molecular level. They adopted a method called “coarse-grained molecular simulation” to illustrate the self-assembly behaviors of chitosan polymer chains at different pH values. Unlike atomistic molecular simulations that focus on individual atoms of a molecule, the coarse-grained molecular simulation treats a group of atoms as one ensemble and probes the collective behavior of each ensemble (Figure a and b). This simulation technique demands less time than the atomistic counterparts without significantly reducing the simulation accuracy. It is suitable for characterizing polymers composed of thousands of atoms, such as chitosan.

The key discovery of this work is that the chitosan polymer chains can adopt different configurations at different pH values. At high pH values, each chain tends to crosslink perpendicularly with adjacent chains. The crosslinking reaction propagates and eventually builds up a three-dimensional dense chain network (Figure c). At low pH values, the protonated amine groups favor parallel crosslinking. Thus, each chain aligns in parallel with each other, which leads to a loosely-packed structure (Figure d). The perpendicularly cross-linked configuration reduces the solubility of chitosan in water but renders robustness and elasticity of the chitosan networks. The parallel cross-linked morphology increases water solubility but decreases the elasticity of the chitosan assembly. These conclusions obtained by the simulation are consistent with experimental results.

To find out more please read:
Effect of pH on Chitosan Hydrogel Polymer Network Structure
Hongcheng Xu and Silvina Matysiak
DOI: 10.1039/C7CC01826F

About the author:
Tianyu Liu is a Ph.D. in chemistry from University of California-Santa Cruz. He is passionate about scientific communication to introduce cutting-edge researches to both the general public and the scientists with diverse research expertise. He is a web writer for the Chem. Commun. and Chem. Sci. blog websites. More information about him can be found at http://liutianyuresearch.weebly.com/

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Siliconrhodamine Probes Enable Bio-imaging with Super-resolution

Written by Tianyu Liu, University of California, Santa Cruz

Intracellular imaging is used to reveal fine details of live organisms. It is an indispensable component for the exploration of biomolecular processes in living cells. Super-resolution microscopy (SRM) is an emerging intracellular imaging technique which can acquire images of much higher resolution than those collected by conventional optical microscopy. Currently, the greatest challenge facing SRM is to develop imaging probes that are suitable for site-specific tagging of intracellular biomolecules. Such probes must be biocompatible, membrane-permeable, intensively fluorescent and photo-stable.

Writing in ChemComm., Dr. Peter Kele and coworkers at Research Center for Natural Sciences, Hungarian Academy of Sciences have developed a group of siliconrhodamine probes that permit the labelling of intracellular proteins with excellent selectivity as well as fast response time (within 10 min).

The synthesized siliconrhodamine probes consist of a siliconrhodamine backbone anchored with a carboxyl group. The carboxyl group is responsible for the polarity-responsive property of the probes. When bound to polar protein surfaces, the probes exist in a fluorescent form. While upon non-specific binding to hydrophobic surfaces, the probes change their configurations and consequently, the fluorescence is lost. This conversion process is based on an intra-molecular Diels-Alder reaction (Figure below) that can be readily initiated by a polarity change without interrupting native biochemical processes in cells. Such a mechanism provides the probe biocompatibility and fast response characteristics.

The probe has been demonstrated for site-specific super-resolution imaging for live cells. The figure below depicts the experimental results collected using a mammalian cell. The cyan colored image (left) presents the actual cell image (as the reference). The middle magenta colored image was obtained by using one of the synthesized imaging probes. The overlay image (right) exhibits near-perfect co-localization of the reference and labelling images, indicating the probe’s excellent selectivity. Moreover, the labelling process is efficient with the probe concentration as low as 1.5 μM, and the duration as short as 10 min.

These stable, efficient, and biocompatible probes could profoundly advance super-resolution imaging of various intracellular structures.

To find out more please read:

Bioorthogonal Double-Fluorogenic Siliconrhodamine Probes for Intracellular Super-resolution Microscopy
Eszter Kozma, Gemma Estrada Girona, Giulia Paci, Edward A Lemke and Peter Kele
DOI: 10.1039/C7CC02212C

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Destruction and Reconstruction of Nanorods Controlled by Visible Light

Written by Tianyu Liu, University of California, Santa Cruz

Supramolecular smart materials are a family of materials composed of several molecules. They have the ability to change their configurations in response to external stimuli such as the presence of enzymes, light irradiation, and changes in pH. This property can be manipulated for a variety of applications including drug delivery and tissue engineering.

In recent years, pH-responsive supramolecular smart materials have been intensively investigated due to the simplicity of pH alteration. However, adjusting pH can have undesired consequences. First, chemical species other than the supramolecular materials (e.g., acid and base) are needed for tuning pH. The involvement of external reagents hinders the readiness of operation. Additionally, the use of acid and base inevitably introduces waste products, which could eventually suppress the stimulus-response activity of the smart materials. Therefore, developing alternative ways to initiate the configuration modification of the supramolecular smart materials is highly desirable.

In a recent ChemComm. publication, Professor Heng-Yi Zhang, Professor Yu Liu and coworkers from Nankai University, China have developed supramolecular smart nanorods consisting of β-cyclodextrin (β-CD) and 4,4’-bipyridine-coordinated zinc ions. In the presence of protonated merocyanine (MEH) in water, the nanorods are able to dissociate upon visible light illumination and reconstruct themselves when placed in the dark (Figure above).

The method by which these structures can reconfigure involves a light-driven proton transfer process (Figure below). MEH molecules absorb energy from visible light and subsequently release their protons to the surroundings. These free protons then combine with the 4,4’-bipyridine (DPD). The protonated DPD molecules lose their coordination ability and disassemble with zinc ions. As a result, the entire nanorod structure collapses. When no light is present, the aforementioned proton transfer process is reversed and the nanorods are reformed. Such a process is highly reversible with no observable light-responsive activity loss for at least five cycles.

The demonstrated light-responsive supramolecular nanorods enable facile operations with no additional chemicals. This technology opens up endless new opportunities in remote control of light-responsive processes.

To find out more please see:

Light-controlled reversible self-assembly of nanorod suprastructures

Jie Guo, Heng-Yi Zhang, Yan Zhou and Yu Liu

DOI:10.1039/C7CC03280C

 

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Improving Sodium-Ion Batteries for Large-Scale Energy Storage

One of the greatest global challenges is the ever-growing demand for reliable, large-scale energy production.

The depletion of cost-effective fossil fuels and concerns about climate change are driving the need for clean energy sources derived from renewable technologies. Wind and solar power show significant potential as sustainable alternatives however, both solar photovoltaics and wind energy display intermittent output which has led to concerns regarding reliability for global energy production. As a result, there has been an increased demand for the development of large-scale energy storage.

Among energy storage technologies, lithium-ion batteries (LIBs) predominate however lithium’s high cost, abundance, unevenglobal distribution and safety concerns have limited its widespread application. In recent years, researchers have become interested in high energy sodium-ion batteries (SIBs) as a safer and less expensive alternative. Nevertheless, their inferior electrochemical performances, due to the larger size and heavier mass of sodium ions, has become a major hurdle in the development and implementation of SIBs.

In a recent ChemComm publication, Prof. Jun Chen of Nankai University has demonstrated the improved capabilities of SIBs using a manganite hydroxide (MnOOH)-based anode.

In the past, transition metal oxides, such as MnOx-based materials, have been used in LIBs as they possess a high theoretical capacity and—in some cases—improved conductivity. In this study by Chen and co-workers, MnOOH nanorods (figure, top) were synthesized, and were shown to display a higher initial Coulombic efficiency and rate performance compared to MnO2 (a common anode material in LIBs). Cyclic voltammetry (figure, bottom) and various other spectroscopic techniques were used to investigate the electrochemical properties and storage behaviour of MnOOH-SIBs. These experiments showed improvements in charge capacity and overall rate performance when compared to other transition metal oxides and sulfides.

The results of this work show promise toward the fabrication of high-performance SIBs which are encouraging alternatives for energy storage due to sustainable cost, improved thermal stability and transport safety. The performance of SIBs still lags behind that of LIBs but this study, among others, demonstrates that new electrode materials need to be explored in the development of SIBs and solving large-scale energy storage challenges.

To find out more see:

MnOOH nanorods as high-performance anodes for sodium ion batteries
Lianyi Shao, Qing Zhao and Jun Chen
DOI: 10.1039/C7CC00087A


Victoria Corless is currently completing her Ph.D. in organic chemistry with Prof. Andrei Yudin at The University of Toronto. Her research is centred on the synthesis of kinetically amphoteric molecules, which offer a versatile platform for the development of chemoselective transformations with particular emphasis on creating novel biologically active molecules.

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Hydrogen bonds in water clusters catalyse acid rain formation

Hydrogen bonds in water clusters help catalyse acid rain formation via a mechanism more typically found in organic synthesis, new research shows.

Burning fossil fuels, volcanic eruptions and soil bacteria release oxides of sulfur and nitrogen into the air. High in the atmosphere, these oxides transform into sulfuric acid and nitric acid – which falls as acid rain.

Source: © Royal Society of Chemistry
Comparison between a typical bifunctional catalyst in synthetic organic chemistry (left) and the embedded water molecules in the supramolecular complexes (H2O)2⋯SO3 (middle) and (H2O)3⋯SO3 (right). Red = oxygen, grey = carbon, blue = nitrogen, yellow = sulfur, white = hydrogen

 

Sulfuric acid, in particular, forms when sulfur trioxide reacts with atmospheric water. During the reaction, hydrogen bonds organise sulfur trioxide and water into a stable supramolecular complex called an adduct, which facilitates an unusual nucleophilic attack by water. However, the precise mechanism behind this nucleophilic behaviour has long been unclear.

 

Read the full story by Thomas Foley in Chemistry World.


This article is free to access until 17 April 2017.

E Romero-Montalvo et al., Chem. Commun., 2017, DOI: 10.1039/c6cc09616f

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Molecular structure is teixobactin’s pièce de résistance

Study builds scientists’ arsenal against drug-resistant superbugs

Scientists in the UK, Belgium and the Netherlands have gained a crucial understanding of the structure–activity relationship of new antibiotic, teixobactin. Since reports of its discovery in early 2015, researchers have shown it can kill a number of pathogens without them developing resistance to it.

The University of Lincoln’s Ishwar Singh explains that there are several reasons for teixobactin’s potency: ‘It uses multiple modes of action to kill resistant bacteria, this makes it very attractive since, if it worked by only one mode, bacteria could modify more easily. It is much more challenging for bacteria to mutate on multiple levels.’ Teixobactin also targets lipids in the bacteria’s cell walls, which are considered to be less able to mutate and develop resistance.

Read the full story by Hannah Dunckley on Chemistry World.

Source: © Royal Society of Chemistry
Structure of teixobactin and with the D-amino acids highlighted in red

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Sunset for perovskites?

Perovskites have arguably transformed solar energy more in the last few years than other technologies have in decades. But British researchers have called into question optimistic predictions of undiscovered perovskites.

© Shutterstock

Hybrid perovskites are a mix of organic and inorganic ions with the same crystal structure as calcium titanium oxide (CaTiO3). Halide perovskites are a subset of these structures containing halide ions such as fluoride or chloride. Iodide perovskites such as methylammonium lead iodide (CH3NH3PbI3) can convert sunlight to electricity.

Researchers use a decades-old geometric ‘tolerance factor’ to propose new combinations of ions that will form stable perovskites. Now, Robert Palgrave and his team at University College London, UK, have reassessed the validity of the tolerance factor in predicting new hybrid perovskite structures. Read the full article in Chemistry World»


Read the original journal article in Chemical Science – it’s open access:
On the application of the tolerance factor to inorganic and hybrid halide perovskites: a revised system
W. Travis, E. N. K. Glover, H. Bronstein, D. O. Scanlon and R. G. Palgrave 
Chem. Sci., 2016, Advance Article, DOI: 10.1039/C5SC04845A, Edge Article

 
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Iron-rich silicate plays cosmic matchmaker

Computational chemists in Spain have discovered that iron in cosmic dust grains helps turn hydrogen atoms into molecular hydrogen (H2).

The average density of the interstellar medium is several billion times less dense than even the best vacuum chambers on Earth. Collisions between hydrogen atoms are therefore rare, and when they do occur, only one out of every 100,000 creates H2. Read the full article in Chemistry World»


Read the original journal article in ChemComm – it’s open access:
Does Fe2+ in olivine-based interstellar grains play any role in the formation of H2? Atomistic insights from DFT periodic simulations
J. Navarro-Ruiz, P. Ugliengo, M. Sodupe and A. Rimola 
Chem. Commun., 2016, Advance Article, DOI: 10.1039/C6CC02313D, Communication

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Caging chemical weapons

Scientists in the UK have developed supramolecular cages that can trap chemical weapon simulants using the hydrophobic effect.

Organophosphorous chemical weapons, such as sarin and soman, interfere with signals between nerve cells, and have recently been used to deadly effect in places such as Syria. Researchers are therefore trying to develop techniques that detect these chemical weapons in the environment, and destroy them. Read the full article in Chemistry World»


Read the original journal article in ChemComm – it’s open access:
Binding of chemical warfare agent simulants as guests in a coordination cage: contributions to binding and a fluorescence-based response
Christopher G. P. Taylor, Jerico R. Piper and Michael D. Ward 
DOI: 10.1039/C6CC02021F, Communication

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Macrocyle aromaticity switch is all about that base

Researchers have discovered a macrocyle that they can render aromatic, non-aromatic or anti-aromatic by altering the amount of base they add.

 

Meso-aryl expanded porphyrins are usually exceedingly twisted structures due to strong hydrogen bonds within them. Even though they contain many conjugated bonds, this twisted structure means that most of these porphyrins are non-aromatic (to be aromatic, a molecule not only needs conjugation, but must also be flat). Previously scientists had added hydrogen ions to disturb hydrogen bonding in porphyrins, causing them to untwist and become aromatic. Here, a team led by Dongho Kim from Yonsei University, Korea, have flattened a porphyrin by removing hydrogen ions. Read the full article in Chemistry World»


Read the original research in ChemComm – it’s free to read until 20 May 2016:
Multifaceted [36]octaphyrin(1.1.1.1.1.1.1.1): deprotonation-induced switching among nonaromatic, Möbius aromatic, and Hückel antiaromatic species
Won-Young Cha, Takanori Soya, Takayuki Tanaka, Hirotaka Mori, Yongseok Hong, Sangsu Lee, Kyu Hyung Park, Atsuhiro Osuka and Dongho Kim 
DOI: 10.1039/C6CC02051H, Communication

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