Coordination Chemistry of a Molecular Pentafoil Knot.

The binding of Zn(II) cations to a pentafoil (51) knotted ligand allows the synthesis of otherwise inaccessible metalated molecular pentafoil knots via transmetalation, affording the corresponding "first-sphere" coordination Co(II), Ni(II), and Cu(II) pentanuclear knots in good yields (≥85%). Each of the knot complexes was characterized by mass spectrometry, the diamagnetic (zinc) knot complex was characterized by 1H and 13C NMR spectroscopy, and the zinc, cobalt, and nickel pentafoil knots afforded single crystals whose structures were determined by X-ray crystallography. Lehn-type circular helicates generally only form with tris-bipy ligand strands and Fe(II) (and, in some cases, Ni(II) and Zn(II)) salts, so such architectures become accessible for other metal cations only through the use of knotted ligands. The different metalated knots all exhibit "second-sphere" coordination of a single chloride ion within the central cavity of the knot through CH···Cl- hydrogen bonding and electrostatic interactions. The chloride binding affinities were determined in MeCN by isothermal titration calorimetry, and the strength of binding was shown to vary over 3 orders of magnitude for the different metal-ion-knotted-ligand second-sphere coordination complexes.

Strong and Selective Anion Binding within the Central Cavity of Molecular Knots and Links.

A molecular pentafoil knot and doubly and triply entwined [2]catenanes based on circular Fe(II) double helicate scaffolds bind halide anions in their central cavities through electrostatic and CH···X(-) hydrogen-bonding interactions. The binding is up to (3.6 ± 0.2) × 10(10) M(-1) in acetonitrile (for pentafoil knot [2·Cl](PF6)9), making these topologically complex host molecules some of the strongest synthetic noncovalent binders of halide anions measured to date, comparable in chloride ion affinity to silver salts.

Catenanes: fifty years of molecular links.

Half a century after Schill and Lüttringhaus carried out the first directed synthesis of a [2]catenane, a plethora of strategies now exist for the construction of molecular Hopf links (singly interlocked rings), the simplest type of catenane. The precision and effectiveness with which suitable templates and/or noncovalent interactions can arrange building blocks has also enabled the synthesis of intricate and often beautiful higher order interlocked systems, including Solomon links, Borromean rings, and a Star of David catenane. This Review outlines the diverse strategies that exist for synthesizing catenanes in the 21st century and examines their emerging applications and the challenges that still exist for the synthesis of more complex topologies.

Beyond Just Peptide Antigens: The Complex World of Peptide-Based Cancer Vaccines.

Peptide-based cancer vaccines rely upon the strong activation of the adaptive immune response to elicit its effector function. They have shown to be highly specific and safe, but have yet to prove themselves as an efficacious treatment for cancer in the clinic. This is for a variety of reasons, including tumour heterogeneity, self-tolerance, and immune suppression. Importance has been placed on the overall design of peptide-based cancer vaccines, which have evolved from simple peptide derivatives of a cancer antigen, to complex drugs; incorporating overlapping regions, conjugates, and delivery systems to target and stimulate different components of antigen presenting cells, and to bolster antigen cross-presentation. Peptide-based cancer vaccines are increasingly becoming more personalised to an individual's tumour antigen repertoire and are often combined with existing cancer treatments. This strategy ultimately aids in combating the shortcomings of a more generalised vaccine strategy and provides a comprehensive treatment, taking into consideration cancer cell variability and its ability to avoid immune interrogation.

Stereoselective synthesis of a composite knot with nine crossings.

The simultaneous synthesis of a molecular nine-crossing composite knot that contains three trefoil tangles of the same handedness and a [Formula: see text] link (a type of cyclic [3]catenane topologically constrained to always have at least three twists within the links) is reported. Both compounds contain high degrees of topological writhe (w = 9), a structural feature of supercoiled DNA. The entwined products are generated from the cyclization of a hexameric Fe(II) circular helicate by ring-closing olefin metathesis, with the mixture of topological isomers formed as a result of different ligand connectivity patterns. The metal-coordinated composite knot was isolated by crystallization, the topology unambiguously proven by tandem mass spectrometry, with X-ray crystallography confirming that the 324-atom loop crosses itself nine times with matching handedness (all Δ or all Λ) at every metal centre within each molecule. Controlling the connectivity of the ligand end groups on circular metal helicate scaffolds provides an effective synthetic strategy for the stereoselective synthesis of composite knots and other complex molecular topologies.

Braiding a molecular knot with eight crossings.

Knots may ultimately prove just as versatile and useful at the nanoscale as at the macroscale. However, the lack of synthetic routes to all but the simplest molecular knots currently prevents systematic investigation of the influence of knotting at the molecular level. We found that it is possible to assemble four building blocks into three braided ligand strands. Octahedral iron(II) ions control the relative positions of the three strands at each crossing point in a circular triple helicate, while structural constraints on the ligands determine the braiding connections. This approach enables two-step assembly of a molecular 819 knot featuring eight nonalternating crossings in a 192-atom closed loop ~20 nanometers in length. The resolved metal-free 819 knot enantiomers have pronounced features in their circular dichroism spectra resulting solely from topological chirality.

Allosteric initiation and regulation of catalysis with a molecular knot.

Molecular knots occur in DNA, proteins, and other macromolecules. However, the benefits that can potentially arise from tying molecules in knots are, for the most part, unclear. Here, we report on a synthetic molecular pentafoil knot that allosterically initiates or regulates catalyzed chemical reactions by controlling the in situ generation of a carbocation formed through the knot-promoted cleavage of a carbon-halogen bond. The knot architecture is crucial to this function because it restricts the conformations that the molecular chain can adopt and prevents the formation of catalytically inactive species upon metal ion binding. Unknotted analogs are not catalytically active. Our results suggest that knotting molecules may be a useful strategy for reducing the degrees of freedom of flexible chains, enabling them to adopt what are otherwise thermodynamically inaccessible functional conformations.

A Star of David catenane.

We describe the synthesis of a [2]catenane that consists of two triply entwined 114-membered rings, a molecular link. The woven scaffold is a hexameric circular helicate generated by the assembly of six tris(bipyridine) ligands with six iron(II) cations, with the size of the helicate promoted by the use of sulfate counterions. The structure of the ligand extension directs subsequent covalent capture of the catenane by ring-closing olefin metathesis. Confirmation of the Star of David topology (two rings, six crossings) is provided by NMR spectroscopy, mass spectrometry and X-ray crystallography. Extraction of the iron(II) ions with tetrasodium ethylenediaminetetraacetate affords the wholly organic molecular link. The self-assembly of interwoven circular frameworks of controlled size, and their subsequent closure by multiple directed covalent bond-forming reactions, provides a powerful strategy for the synthesis of molecular topologies of ever-increasing complexity.