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Volume 10 Issue 11, November 2018

Trefoiled again!

Knotted molecular chains can be found in DNA, proteins and polymers, and can have a significant influence on their properties. Most small-molecule knots reported so far are 'prime' knots; much rarer are 'composite' knots, which consist of two or more prime knots that have been ring-opened and the loose ends connected to one another. Now, a team led by David Leigh has used a circular metal helicate to make a nine-crossing composite knot that contains three trefoil tangles (as shown schematically on the cover). The backbone of the knot is a single molecular chain — a 324-membered ring — wound around six Fe(II) ions that all have the same chirality in any given knot (either all Δ or all Λ).

See Leigh et al.

Image: David Leigh, University of Manchester, and AXS Studio Inc. Cover Design: Tulsi Voralia.


  • Bruce Gibb focuses on fatty acids and wonders whether we’ll all be eating cyanobacteria before too long.

    • Bruce C. Gibb


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News & Views

  • Knots have been rigorously studied since the 1860s, but only in the past 30 years have they been made in the laboratory in molecular form. Now, the most complex small-molecule examples so far — a composite knot and an isomeric link, each with nine crossings — have been prepared.

    • Edward E. Fenlon
    News & Views
  • The applicability of metal-organic frameworks (MOFs) — in spite of their obvious potential — is hindered by stability issues, in particular towards water. Now, a ‘crumple zone’ concept has been proposed in which the presence of sacrificial bonds protects a MOF without significantly altering its structure or functionality.

    • Jürgen Senker
    News & Views
  • The preparation of three-dimensional frameworks with multiple stereocentres from simple acyclic hydrocarbons represents a challenging transformation. Now, starting from simple and readily available reagents, formation of these complex targets can be achieved in just three catalytic transformations with high levels of stereocontrol.

    • Laura Castoldi
    • Vittorio Pace
    News & Views
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  • A composite knot with nine crossings of the same handedness has been prepared from a hexameric circular helicate in 41% yield in a two-step synthesis. An isomeric cyclic [3]catenane topologically constrained to always have at least three twists within the links is also formed. Both topologies have a high degree of writhe, analogous to that of supercoiled DNA.

    • Liang Zhang
    • Alexander J. Stephens
    • David A. Leigh
  • Aryl functionalization of carbon nanotubes generates sp3 defects capable of quantum light emission. A multiplicity of possible binding configurations, however, leads to spectrally diverse emission bands. Now, it is shown that the structural symmetry of zigzag nanotubes and a high chemical selectivity for ortho configurations results in defect-state emission from a single narrow band.

    • Avishek Saha
    • Brendan J. Gifford
    • Stephen K. Doorn
  • The promise shown by metal–organic frameworks for various applications is somewhat dampened by their instability towards water. Now, an activated MOF has shown good hydrolytic stability owing to the presence of weak, sacrificial coordination bonds that act as a ‘crumple zone’. On hydration, these weak bonds are cleaved preferentially to stronger coordination bonds that hold the MOF together.

    • Lauren N. McHugh
    • Matthew J. McPherson
    • Russell E. Morris
  • New natural-product-inspired molecules are often limited by their only partial coverage of biologically relevant chemical space. Combining fragments of natural products has now been shown to yield pseudo natural products, which — while still being inspired by natural products — populate previously unexplored areas of chemical space and have novel biological activities.

    • George Karageorgis
    • Elena S. Reckzeh
    • Herbert Waldmann
  • On-surface polymerization is a promising technique to prepare organic functional nanomaterials, but it has remained difficult to carry out on insulating surfaces. Now, the photoinitiated radical polymerization of dimaleimide on KCl, initiated from a two-dimensional gas phase and guided by molecule–substrate interactions, has led to polymer fibres up to 1 μm long.

    • Franck Para
    • Franck Bocquet
    • Matthew B. Watkins
  • Lipid membranes—which separate cells and organelles from their environment—experience tension during various cell processes; however, measuring membrane tension is notoriously difficult. Now, a new fluorescent, mechanosensitive membrane probe called FliptR has been developed. FliptR enables simple, direct membrane tension measurements in cellular and artificial membranes.

    • Adai Colom
    • Emmanuel Derivery
    • Aurélien Roux
  • Traditionally, strong-bond activation by transition metals has been achieved through an oxidative addition pathway. Now, a redox-neutral palladium(ii)-catalysed β-elimination strategy has been shown to activate alkyl C–O, N, C, F and S bonds to give an alkene that can be trapped with various nucleophiles. This functional group metathesis allows upgrading of amino acid derivatives and ring-opening of saturated heterocycles.

    • Van T. Tran
    • John A. Gurak Jr
    • Keary M. Engle
  • A paradoxical case of a well-defined diradicaloid that has an unusually large singlet–triplet energy gap (ΔES-T) imparted by the thiophene sulfur atom is reported. Quantum chemistry, organic synthesis, molecular spectroscopies, X-ray crystal analysis and high-temperature magnetic measurements help account for the dichotomy between the large diradical character and large ΔEST.

    • Justin J. Dressler
    • Mitsuru Teraoka
    • Michael M. Haley
  • The simplest sugar—glycolaldehyde—has recently been detected in space and now a mechanistic rationale for its formation is presented, which includes its onward reaction to the next higher aldose, glyceraldehyde. The key species in the chemistry at play is the formaldehyde isomer hydroxymethylene, which reacts with the carbonyl component in an essentially barrierless carbonyl–ene-type reaction.

    • André K. Eckhardt
    • Michael M. Linden
    • Peter R. Schreiner
  • Measurements of vector correlations provide insight into the forces acting during molecular collisions, and are a stringent test of electronic-structure calculations. Now, non-intuitive dynamics of molecular collisions have been revealed by measuring the correlation between the relative velocities of the colliders and the molecular rotational angular momentum—before and after the collision—for NO(A 2Σ+) + Ne.

    • Thomas R. Sharples
    • Joseph G. Leng
    • Matthew L. Costen
  • Organoclay/DNA semipermeable microcapsules with catalase-powered oxygen gas bubble-dependent buoyancy are prepared and exploited as synthetic protocells capable of programmed motility and sustained oscillatory movement.

    • B. V. V. S. Pavan Kumar
    • Avinash J. Patil
    • Stephen Mann
  • A rapid, modular, stereodivergent and diversity-oriented strategy for constructing acyclic molecular frameworks bearing up to four contiguous and congested stereogenic elements has been developed. This approach can yield the target compounds with remarkably high levels of stereocontrol in only three catalytic steps from commercially available alkynes.

    • Jeffrey Bruffaerts
    • David Pierrot
    • Ilan Marek
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