Polymersomes with Asymmetric Membranes Based on Readily Accessible Di- and Triblock Copolymers Synthesized via SET-LRP

One of the hallmarks of nature is compartmentalization, and natural cell membranes are often asymmetric in terms of the inner and outer side. This communication describes work toward synthesizing such an asymmetric membrane from the bottom-up. A family of amphiphilic di- and triblock copolymers were synthesized via Cu(0)-mediated single electron transfer-living radical polymerization with the aim to generate polymer vesicles, or polymersomes, with an asymmetric membrane. Self-assembly of these polymeric amphiphiles in aqueous media was investigated using asymmetric field-flow fractionation and cryo-electron microscopy. Utilizing mixtures of diblock copolymers with differing hydrophilic moieties resulted in the formation of vesicles with an asymmetric segregation of charge between the inner and outer leaflet, confirmed by zeta potential measurements. These polymers, synthesized in good yields and using a biologically compatible method to induce self-assembly, have a promising range of applications from nanomedicine to synthetic cell research

Cooperativity and Complexity in the Binding of Anions and Cations to a Tetratopic Ion-Pair Host

Cooperative interactions play a very important role in both natural and synthetic supramolecular systems. We report here on the cooperative binding properties of a tetratopic ion-pair host <b>1</b>. This host combines two isophthalamide anion recognition sites with two unusual “half-crown/two carbonyl” cation recognition sites as revealed by the combination of single-crystal X-ray analysis of the free host and the 1:2 host:calcium cation complex, together with two-dimensional NMR and computational studies. By systematically comparing all of the binding data to several possible binding models and focusing on four different variants of the 1:2 binding model, it was in most cases possible to quantify these complex cooperative interactions. The data showed strong negative cooperativity (α = 0.01–0.05) of <b>1</b> toward chloride and acetate anions, while for cations the results were more variable. Interestingly, in the competitive (CDCl₃/CD₃OD (9:1, v/v)) solvent, the addition of calcium cations to the tetratopic ion-pair host <b>1</b> allosterically switched “on” chloride binding that is otherwise not present in this solvent system. The insight into the complexity of cooperative interactions revealed in this study of the tetratopic ion-pair host <b>1</b> can be used to design better cooperative supramolecular systems for information transfer and catalysis

Step-by-step DFT analysis of the cooperativity in the binding of cations and anions to a tetratopic ion-pairing host

<div><p>Computational techniques allow insights into chemical processes that are normally experimentally difficult to access (e.g. transition states). Here, we report on the computational modelling of a recently reported supramolecular system that experimentally displays a complex mixture of cooperative, allosteric, homotropic and heterotropic binding behaviour in the interactions of two calcium cations and two chloride anions with a tetratopic ion-pairing host (bis-isophthalamide-crown-6) <b>H</b>. Using DFT, we are able to predict the structural conformations and binding energies of the multi-step thermodynamic binding cycle in both directions for the formation of the quinary 2Ca²⁺·<b>H</b>·2Cl⁻ complex in both vacuum and solvent environments with several key steps validated against experimental X-ray and binding study data. This study revealed unexpected conformational changes in the free host upon binding two Cl⁻ anions, with a tetra-coordinated (four hydrogen-bonds) 1:1 <b>H</b>·Cl⁻ complex forming before the expected bis(isopthhalimide-Cl⁻) 1:2 <b>H</b>·2Cl⁻ complex with two hydrogen bonds is formed. The results here show that given the appropriate method (basis set, solvent environment) DFT calculations can both predict and give unique insight into multi-ion host supramolecular interactions like those encountered in ion-pair host–guest studies.</p></div

Site-level strategies for managing secondary forests

This chapter sets out the possible management objectives and technical options for managing secondary forests as part of an forest landscape rehabilitation (FLR) program. The two main alternative strategies – managing improved fallows without compromising agricultural production, and managing forests for production or conservation purposes – are discussed, together with the types of conditions that favour one above the other. There is considerable ambiguity and confusion in the current use of the term ‘secondary forest’ both in the literature and in people’s perceptions. The term has been applied to numerous types of forests with different characteristics and arising from many different processes. ITTO (2002) defines it as: woody vegetation regrowing on land that was largely cleared of its original forest cover (ie carried less than 10% of the original forest cover)

Kinetically Controlled Lifetimes in Redox-Responsive Transient Supramolecular Hydrogels

It remains challenging to program soft materials to show dynamic, tunable time-dependent properties. In this work, we report a strategy to design transient supramolecular hydrogels based on kinetic control of competing reactions. Specifically, the pH-triggered self-assembly of a redox-active supramolecular gelator, <i>N</i>,<i>N</i>′-dibenzoyl-l-cystine (DBC) in the presence of a reducing agent, which acts to disassemble the system. The lifetimes of the transient hydrogels can be tuned simply by pH or reducing agent concentration. We find through kinetic analysis that gel formation hinders the ability of the reducing agent and enables longer transient hydrogel lifetimes than would be predicted. The transient hydrogels undergo clean cycles, with no kinetically trapped aggregates observed. As a result, multiple transient hydrogel cycles are demonstrated and can be predicted. This work contributes to our understanding of designing transient assemblies with tunable temporal control

Unravelling the interaction between α-cyclodextrin with the thaumatin protein and a peptide mimic

<div><p>G-protein-coupled receptors (GPCRs) are responsible for signal transduction; through these transmembrane proteins, our senses are evoked: sight, smell and taste. Thaumatin is a natural sweet-tasting protein that is 100,000 times sweeter than sucrose but its use in food products has been hampered due to a liquorice aftertaste. Thaumatin has been shown to bind to a class C GPCR and the active binding site of the thaumatin protein is known. Here, we report on the binding of a well-known food grade host: α-cyclodextrin to thaumatin. We show through a combination of one- and two-dimensional NMR experiments that α-cyclodextrin binds to aromatic residues on thaumatin with <i>K</i>_a = 8.5 ± 2.4 M^− 1. We also synthesise a heptapeptide KTGDRGF that mimics the active binding site of thaumatin and show that α-cyclodextrin binds to the C-terminal solvent accessible phenylalanine residue of this peptide with <i>K</i>_a = 8.8 ± 3.1 M^− 1. This indicates that α-cyclodextrin may interact with the active binding site on thaumatin, suggesting that α-cyclodextrin could be used to modify the interaction of thaumatin with GPCRs and hence its sweet-taste profile.</p></div

A Capped Dipeptide Which Simultaneously Exhibits Gelation and Crystallization Behavior

Short peptides capped at their N-terminus are often highly efficient gelators, yet notoriously difficult to crystallize. This is due to strong unidirectional interactions within fibers, resulting in structure propagation only along one direction. Here, we synthesize the N-capped dipeptide, benzimidazole-diphenylalanine, which forms both hydrogels and single crystals. Even more remarkably, we show using atomic force microscopy the coexistence of these two distinct phases. We then use powder X-ray diffraction to investigate whether the single crystal structure can be extrapolated to the molecular arrangement within the hydrogel. The results suggest parallel β-sheet arrangement as the dominant structural motif, challenging existing models for gelation of short peptides, and providing new directions for the future rational design of short peptide gelators

A Capped Dipeptide Which Simultaneously Exhibits Gelation and Crystallization Behavior

Short peptides capped at their N-terminus are often highly efficient gelators, yet notoriously difficult to crystallize. This is due to strong unidirectional interactions within fibers, resulting in structure propagation only along one direction. Here, we synthesize the N-capped dipeptide, benzimidazole-diphenylalanine, which forms both hydrogels and single crystals. Even more remarkably, we show using atomic force microscopy the coexistence of these two distinct phases. We then use powder X-ray diffraction to investigate whether the single crystal structure can be extrapolated to the molecular arrangement within the hydrogel. The results suggest parallel β-sheet arrangement as the dominant structural motif, challenging existing models for gelation of short peptides, and providing new directions for the future rational design of short peptide gelators

Redox- and pH-Responsive Orthogonal Supramolecular Self-Assembly: An Ensemble Displaying Molecular Switching Characteristics

Two heteroditopic monomers, namely a thiopropyl-functionalized tetrathiafulvalene-annulated calix[4]­pyrrole (SPr-TTF-C[4]P <b>1</b>) and phenyl C₆₁ butyric acid (PCBA <b>2</b>), have been used to assemble a chemically and electrochemically responsive supramolecular ensemble. Addition of an organic base initiates self-assembly of the monomers via a molecular switching event. This results in the formation of materials that may be disaggregated via the addition of an organic acid or electrolysis

One-Pot Synthesis of High Molecular Weight Synthetic Heteroprotein Dimers Driven by Charge Complementarity Electrostatic Interactions

Despite the importance of protein dimers and dimerization in biology, the formation of protein dimers through synthetic covalent chemistry has not found widespread use. In the case of maleimide–cysteine-based dimerization of proteins, we show here that when the proteins have the same charge, dimerization appears to be inherently difficult with yields around 1% or less, regardless of the nature of the spacer used or whether homo- or heteroprotein dimers are targeted. In contrast, if the proteins have opposing (complementary) charges, the formation of heteroprotein dimers proceeds much more readily, and in the case of one high molecular weight (>80 kDa) synthetic dimer between cytochrome <i>c</i> and bovine serum albumin, a 30% yield of the purified, isolated dimer was achieved. This represents at least a 30-fold increase in yield for protein dimers formed from proteins with complementary charges, compared to when the proteins have the same charge, under otherwise similar conditions. These results illustrate the role of ionic supramolecular interactions in controlling the reactivity of proteins toward bis-functionalized spacers. The strategy here for effective synthetic dimerization of proteins could be very useful for developing novel approaches to study the important role of protein–protein interactions in chemical biology