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David A Leigh

Publications and source records attributed to David A Leigh.

At least 19 recordsLinked to original sources
PubMed2004

The mechanism of formation of amide-based interlocked compounds: prediction of a new rotaxane-forming motif.

Molecular modeling of four different reagent systems shows that the (free) energies of supramolecular interactions in the gas phase and in solution can explain the different reaction products (i.e., various sized macrocycles, catenanes, and linear oligomers) that are formed in classic amide-catenane-forming reactions. Self-assembly of the catenanes requires the formation of ordered intertwined chains and is driven by bifurcated hydrogen bonds, with pi stacking only playing a lesser role. The understanding gained from the computational study was used to predict the possibility of a new rotaxane-forming system that does not permit catenane formation. The predictions were confirmed by the successful synthesis and characterization (including X-ray crystallography) of two novel rotaxanes.

Journal Article↗
PubMed2004

A generic basis for some simple light-operated mechanical molecular machines.

A novel type of mechanical switch is described in which light-induced translation of a macrocycle in a [2]rotaxane quenches anthracene fluorescence. Features of the system include the remarkable 200:1 difference in fluorescence intensity between the two positional states of the molecule ( approximately 85:1 between one isomer and the photostationary state). In principle the same concept could be used for mechanically switching virtually any property that can be influenced by functional group proximity effects.

Amides↗
PubMed2003

Chiroptical switching in a bistable molecular shuttle.

Although various methods for switching the positions of macrocycles in bistable rotaxane-based molecular shuttles have been developed, exploiting such movements to trigger property changes has thus far received little attention. Here we describe one of the first examples of a property change achieved through a controlled large-amplitude translational motion in a rotaxane; a novel type of chiroptical switch is described, in which light-induced translation of the macrocycle along the thread of a [2]rotaxane produces a strong induced circular dichroism (ICD) response only when the macrocycle is hydrogen-bonded to a chiral peptide station.

Journal Article↗
PubMed2003

Electrochemically switchable hydrogen-bonded molecular shuttles.

A series of [2]rotaxanes containing succinamide and naphthalimide hydrogen-bonding stations for a benzylic amide macrocycle is described. Electrochemical reduction and oxidation of the naphthalimide group alters its ability to form hydrogen bonds to the macrocycle to such a degree that redox processes can be used to switch the relative macrocycle-binding affinities of the two stations in a rotaxane by over 8 orders of magnitude. The structure of the neutral [2]rotaxane in solution is established by (1)H NMR spectroscopy and shows that the macrocycle exhibits remarkable positional integrity for the succinamide station in a variety of solvents. Cyclic voltammetry experiments allow the simultaneous stimulation and observation of a redox-induced dynamic process in the rotaxane which is both reversible and cyclable. Model compounds in which various conformational and co-conformational changes are prohibited demonstrate unequivocally that the redox response is the result of shuttling of the macrocycle between the two stations. At room temperature in tetrahydrofuran the electrochemically induced movement of the macrocycle between the two stations takes approximately 50 micros.

Journal Article↗
PubMed2003

Unidirectional rotation in a mechanically interlocked molecular rotor.

Molecular motor proteins are ubiquitous in nature and have inspired attempts to create artificial machines that mimic their ability to produce controlled motion on the molecular level. A recent example of an artificial molecular rotor is a molecule undergoing a unidirectional 120 degrees intramolecular rotation around a single bond; another is a molecule capable of repetitive unimolecular rotation driven by multiple and successive isomerization of its central double bond. Here we show that sequential and unidirectional rotation can also be induced in mechanically interlocked assemblies comprised of one or two small rings moving around one larger ring. The small rings in these [2]- and [3]catenanes move in discrete steps between different binding sites located on the larger ring, with the movement driven by light, heat or chemical stimuli that change the relative affinity of the small rings for the different binding sites. We find that the small ring in the [2]catenane moves with high positional integrity but without control over its direction of motion, while the two rings in the [3]catenane mutually block each other's movement to ensure an overall stimuli-induced unidirectional motion around the larger ring.

Journal Article↗
PubMed2003

Hydrogen bond-assembled fullerene molecular shuttle.

A novel [2]rotaxane has been prepared in which fullerene C(60) behaves as both a stopper and a photoactive unit. The amphiphilic nature of the rotaxane thread can be used to shuttle the macrocycle from close to the fullerene spheroid (in nonpolar solvents) to far away (in polar solvents). The differing location of the macrocycle in dichloromethane and dimethyl sulfoxide gives rise to effects detectable by (1)H NMR and time-resolved spectroscopy.

Journal Article↗
PubMed2003

Mechanically linked polycarbonate.

The synthesis, by solid-state copolymerization, and characterization of the first polycatenanes based on a commercial polymer are reported. Various amounts of a benzylic amide [2]catenane, the corresponding macrocycle, and a rigid bisphenol fluorene derivative have been quantitatively and homogeneously incorporated into bisphenol A polycarbonate. The resulting copolymers were characterized by size exclusion chromatography coupled with viscosimetry, (1)H NMR, differential scanning calorimetry, and dynamic mechanical analysis. The unexpectedly small influence of [2]catenane incorporation on the glass transition temperature of the copolymers points to remarkable internal mobility of the catenane comonomer rings. A new relaxation linked to the flexible catenane units is also observed. The studies represent a detailed structural characterization of a polymer containing small amounts of mechanical linkages in its backbone and demonstrate that significant effects can be induced by doping conventional polymers with small percentages (2-6% of repeat units) of flexible catenanes.

Journal Article↗
PubMed2003

Summing up ligand binding interactions.

Artificial molecular recognition systems show negligible positive binding cooperativity compared to biological systems, possibly because rigid ligands cannot accommodate numerous partly bound states that comprise overall ligand binding affinity. This directly correlates with the phenomenon of enthalpy-entropy compensation.

Allosteric Regulation↗