Understanding the transport of charge across non-covalently bonded molecules is key to our understanding of many diverse systems, from oxidative damage and repair of DNA to the efficiency of organic electronic materials. Single-molecule break junctions are an important stage on which to test and develop this understanding and they have now been used by Ferdinand C. Grozema, Herre S. J. van der Zant and colleagues to show that electronic transport through a π-stacked dimer can be precisely controlled by mechanically manipulating its conformation and thus turning destructive interference effects ON or OFF. The cover shows the electrodes of a break junction connected by a π-stacked dimer, the molecular orbitals of which are also shown.Article p1099IMAGE: NICOLAS RENAUD, DELFT UNIVERSITY OF TECHNOLOGY

Just like the fish shown on the cover of this issue, tessellation with simple shapes can cover large surfaces. Recognizing that triple-helical self-assembly can be thought of as a tiling problem, Ronald T. Raines and co-workers have now demonstrated that collagen-mimetic peptide tiles can be assembled into triple helices with perfect symmetry to form synthetic collagen nanofibres that are nearly a micrometre in length.Article p1008IMAGE: H. ADAM STEINBERGCOVER DESIGN: KAREN MOORE

It is not easy to tie a molecule in knots. It requires the precise positioning of molecular fragments such that they retain the correct orientation relative to one another in the final product. Now, Yossi Weizmann and co-workers have used DNA four-way junctions to create knots and links, and have shown how these topologically non-trivial structures can be used to investigate important DNA-processing enzymes. Shown on the cover is a scene, inspired by van Gogh’s The Starry Night, in which double helical trees, symbolizing DNA, reach up to the knotted swirling clouds (topological structures) in the sky.Article p907IMAGE: DI LIUCOVER DESIGN: KAREN MOORE

Cyclic molecules in their many and varied forms have long fascinated chemists and ferrocene is undoubtedly an iconic structure in its own right. Now, a team led by Tim Albrecht and Nicholas Long have made macrocycles built up from directly linked ferrocene units. These nanorings, containing 5, 6, 7 or 9 metal centres, are found to be highly symmetric in solution and exhibit fast intramolecular electron transfer between the redox sites. The X-ray structure of the cyclo[6] compound is shown on the cover.Article p825News & Views p819IMAGE: MICHAEL S. INKPENCOVER DESIGN: KAREN MOORE

Improved understanding of assembly dynamics should lead to more rational design of supramolecular materials and better prediction of their resulting properties. However, the analytical tools routinely used to study such processes are generally indirect or are performed ex situ. Now, Itaru Hamachi and co-workers have devised a means of directly visualizing an assembly process in real time. Confocal laser microscopy, aided by fluorescent probes, reveals the growth of self-sorted supramolecular nanofibres into three-dimensional interpenetrated networks (as stylized on the cover in red and green).Article p743News & Views p737IMAGE: HAJIME SHIGEMITSU AND ITARU HAMACHICOVER DESIGN: KAREN MOORE

Quasicrystals are materials that exhibit long-range order, but lack translational symmetry. Now, David écija, Nian Lin, Johannes V. Barth and co-workers have successfully used interfacial coordination chemistry to engineer a 2D square–triangle random tiling with quasicrystalline-order characteristics and dodecagonal symmetry. The metal–organic tessellation, as revealed by high-resolution STM imaging (shown on the cover), combines europium centres with linear dicarbonitrile linkers on a smooth Au(111) substrate. By careful stoichiometric control, other metallosupramolecular networks with different tiling schemes could also be prepared.

Article p657

IMAGE: DAVID éCIJA, IMDEA NANOSCIENCE

COVER DESIGN: KAREN MOORE

Synthesizing polymers that comprise a specific sequence of building blocks requires a method to tightly control the order of monomer addition — something that is not simple when the same functional groups responsible for chain propagation are present in all of the different monomers. Now, Andrew J. Turberfield and co-workers have developed a synthetic molecular machine made from DNA that controls the sequence in which olefin or peptide bonds are formed to produce a defined oligomeric chain. The sequence of reactions is also recorded in a DNA duplex; information that can subsequently be recovered by DNA sequencing.Article p542IMAGE: WENJING MENG, DEPARTMENT OF PHYSICS, UNIVERSITY OF OXFORDCOVER DESIGN: KAREN MOORE

Frances H. Arnold, Vijay S. Pande and co-workers show, using large-scale molecular dynamics simulations, that the flexible F/G loop of a nitrating cytochrome P450 enzyme transitions between a set of disordered open-lid conformations and a set of structured closed-lid conformations. In the closed-lid conformations, the His176 residue engages in a direct interaction with the l-tryptophan substrate. A single mutation at this position not only controls loop dynamics but also, surprisingly, switches the regioselectivity of the nitration reaction.Article p419IMAGE: SHEEL DODANI AND GERT KISSCOVER DESIGN: KAREN MOORE

The regular repeating structure of covalent organic frameworks (COFs) makes these materials attractive for optoelectronic applications—in particular those made of two-dimensional covalent layers held together by π-stacking—but the necessary high crystallinity has remained difficult to engineer. Now, Florian Auras, Thomas Bein and co-workers have devised a building block (represented in yellow on the cover) with a shape that ensures successive units lock in position and promote long-range order by minimizing stacking faults and dislocations that otherwise typically occur in COF assembly (as demonstrated in green).Article p310IMAGE: CHRISTOPH HOHMANN, NANOSYSTEMS INITIATIVE MUNICHCOVER DESIGN: KAREN MOORE

The Bergman cyclization is an eponymous chemical reaction in which an enediyne is converted into a benzene derivative via a diradical intermediate. Now, Leo Gross and co-workers have shown that it is possible to use an atomic force microscope (AFM) to induce a reversible Bergman cyclization in an anthracene diradical sitting on an ultrathin NaCl film. The tricyclic anthracene framework can be converted into a bicyclic system that has a 6- and a 10-membered ring. This molecule contains an enediyne moiety that can then be cyclized to reform the original anthracene framework. The cover shows an AFM image of one of the 10-6 bicyclic molecules anchored to a NaCl step edge.Article p220IMAGE: BRUNO SCHULER, IBM RESEARCH – ZURICHCOVER DESIGN: KAREN MOORE

The mechanism by which RNA and proteins form liquid organelles is not fully understood, but William Aumiller Jr and Christine Keating have now demonstrated that charge-mediated phase separation can generate droplets that resemble a simple model of these assemblies. By controlling the phosphorylation of cationic peptides using a kinase/phosphatase enzyme pair, Aumiller and Keating were able to reversibly form and dissolve liquid droplets from the peptides and RNA. The cover image depicts the formation of liquid organelles of different sizes.Article p129IMAGE: CHRISTINE KEATING AND WILLIAM AUMILLER JRCOVER DESIGN: KAREN MOORE

Self-assembly processes can result in the formation of sophisticated structures that arise from multiple competing pathways that are often poorly understood. Now, Aliprandi, Mauro and De Cola have elucidated — and are able to control — the complex aggregation pathways of a platinum(II) compound into three different assemblies. The evolution of the system could be observed in real time because of the different morphologies and emission properties of each assembly. A snapshot of the process is pictured on the cover — the orange spheres and green needles are two different kinetically trapped metastable structures whereas the blue needles are the thermodynamically favoured product.Article p10News & Views p6IMAGE: ALESSANDRO ALIPRANDI, DE COLA LABCOVER DESIGN: KAREN MOORE