Self-sorting of crown ether/secondary ammonium ion hetero-[c2]daisy chain pseudorotaxanes

Four monomeric building blocks equipped with one crown ether and one secondary ammonium ion are synthesized and studied with respect to their ability to form daisy chain dimers. Two crown ethers with different cavity sizes – i.e. [21]crown-7 and [24]crown-8 – and two ammonium ions substituted with either a thin alkyl group or a more bulky benzyl group are used as the binding motifs. Self-sorting behaviour can be expected as the [21]crown-7/alkyl ammonium and [24]crown-8/benzyl ammonium binding motifs are orthogonal. Three homodimers are characterized by NMR, X-ray crystallography and ESI mass spectrometry. They are recognizable by the presence of signals for diastereotopic protons in the 1H NMR spectra. The formation of hetero-[c2]daisy chain dimers can be monitored by NMR spectroscopy and ESI mass spectrometry and show the expected self-sorting behaviour

Cosmopolitan career choices: a cross-cultural study of job candidates' expatriation willingness

Cosmopolitanism, i.e. openness to divergent cultural experiences, has become a desired characteristic of today's global managers. This study investigates the antecedents of cosmopolitanism and expatriation willingness as a final outcome. The results of surveys in Germany and South Korea demonstrate that cosmopolitanism is a strong predictor of job candidates' expatriation willingness. However, there are some differences in the antecedents of cosmopolitanism between the respondents from the two countries. Living and travelling abroad increase cosmopolitanism for Germans, whereas they have no effect on Koreans. For Germans, cosmopolitanism mediates the relationship between various international exposure variables and expatriation willingness. Explanations and practical implications are provided

Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes

Crown ethers are common building blocks in supramolecular chemistry and are frequently applied as cation sensors or as subunits in synthetic molecular machines. Developing switchable and specifically designed crown ethers enables the implementation of function into molecular assemblies. Seven tailor-made redox-active crown ethers incorporating tetrathiafulvalene (TTF) or naphthalene diimide (NDI) as redox-switchable building blocks are described with regard to their potential to form redox-switchable rotaxanes. A combination of isothermal titration calorimetry and voltammetric techniques reveals correlations between the binding energies and redox-switching properties of the corresponding pseudorotaxanes with secondary ammonium ions. For two different weakly coordinating anions, a surprising relation between the enthalpic and entropic binding contributions of the pseudorotaxanes was discovered. These findings were applied to the synthesis of an NDI-[2]rotaxane, which retains similar spectroelectrochemical properties compared to the corresponding free macrocycle. The detailed understanding of the thermodynamic and electrochemical properties of the tailor-made crown ethers lays the foundation for the construction of new types of molecular redox switches with emergent properties.peerReviewe

Systematic construction of progressively larger capsules from a 5-fold linking subcomponent

Biological encapsulants, such as viral capsids and ferritin protein cages, use many identical subunits to tile the surface of a polyhedron. Inspired by these natural systems, synthetic chemists have prepared an extensive series of artificial nanocages, with well-defined shapes and cavities. Rational control over the self-assembly of discrete, nanometre-scale, hollow coordination cages composed of simple components still poses considerable challenges as a result of the entropic costs associated with binding many subunits together, difficulties in the error-correction processes associated with assembly, and increasing surface energy as their size grows. Here we demonstrate the construction of a family of nanocages of increasing size derived from a single simple pentatopic pyrrole-based subcomponent. Reasoned shifts in the preferred coordination number of the metal ions employed, along with the denticity and steric hindrance of the ligands, enabled the generation of progressively larger cages, incorporating more subunits. These structural changes of the cages through these ‘mutations’ are reminiscent of differences in the folding of proteins caused by minor variations in their amino acid sequences; understanding how they impact capsule structure and thus cavity size may help to elucidate construction principles for still larger, more complex and functional capsules, capable of binding and carrying large biomolecules as cargoes

Systematic construction of progressively larger capsules from a fivefold linking pyrrole-based subcomponent

Biological encapsulants, such as viral capsids and ferritin protein cages, use many identical subunits to tile the surface of a polyhedron. Inspired by these natural systems, synthetic chemists have prepared artificial nanocages with well-defined shapes and cavities. Rational control over the self-assembly of discrete, nanometre-scale, hollow coordination cages composed of simple components remains challenging as a result of the entropic costs associated with binding many subunits together, difficulties in the error-correction processes associated with assembly, and increasing surface energy as their size grows. Here we demonstrate the construction of nanocages of increasing size derived from a single pentatopic pyrrole-based subcomponent. Reasoned shifts in the preferred coordination number of the metal ions employed, along with the denticity and steric hindrance of the ligands, enabled the generation of progressively larger cages. These structural changes of the cages are reminiscent of differences in the folding of proteins caused by minor variations in their amino acid sequences; understanding how they impact capsule structure and thus cavity size may help to elucidate construction principles for larger and functional capsules, capable of binding and carrying large biomolecules as cargoes.This study was supported by the European Research Council (695009), the UK Engineering and Physical Sciences Research Council (EPSRC, EP/P027067/1) and Astex Therapeutics Ltd. (RG73357, Sustaining Innovation Postdoctoral Training Program – AH). We thank Diamond Light Source for providing time on Beamline I24

Chelate Cooperativity and Spacer Length Effects on the Assembly Thermodynamics and Kinetics of Divalent Pseudorotaxanes

Homo- and heterodivalent crown-ammonium pseudorotaxanes with different spacers connecting the two axle ammonium binding sites have been synthesized and characterized by NMR spectroscopy and ESI mass spectrometry. The homodivalent pseudorotaxanes are investigated with respect to the thermodynamics of divalent binding and to chelate cooperativity. The shortest spacer exhibits a chelate cooperativity much stronger than that of the longer spacers. On the basis of crystal structure, this can be explained by a noninnocent spacer, which contributes to the binding strength in addition to the two binding sites. Already very subtle changes in the spacer length, i.e., the introduction of an additional methylene group, cause substantial changes in the magnitude of cooperative binding as expressed in the large differences in effective molarity. With a similar series of heterodivalent pseudorotaxanes, the spacer effects on the barrier for the intramolecular threading step has been examined with the result that the shortest spacer causes a strained transition structure and thus the second binding event occurs slower than that of the longer spacers. The activation enthalpies and entropies show clear trends. While the longer spacers reduce the enthalpic strain that is present in the transition state for the shortest member of the series, the longer spacers become entropically slightly more unfavorable because of conformational fixation of the spacer chain during the second binding event. These results clearly show the noninnocent spacers to complicate the analysis of multivalent binding. An approximate description which considers the binding sites to be connected just by a flexible chain turns out to be more a rough approximation than a good model. The second conclusion from the results presented here is that multivalency is expressed in both the thermodynamics and the kinetics in different ways. A spacer optimized for strong binding is suboptimal for fast pseudorotaxane formation