No abstract

The ability of highly cross-linked thermoset materials to relax stresses by network chain segment mobility above or just below T g is generally limited. We describe materials in which some of the crosslinks have been replaced by dimers of quadruple hydrogen-bonding ureidopyrimidinone (UPy) moieties, which act as reversible cross-links. Materials based on mixtures of ε-caprolactone and L-lactic acid, and containing different ratios of covalent and UPy dimer cross-links, were synthesized, and their thermal and mechanical properties were tested by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The presence of reversible cross-links results in superior relaxation of stresses even below T g , as witnessed by the creep and stress relaxation behavior. These properties bode well for potential applications, e.g., in coatings technology.

We report herein the complexation of a string of buckminsterfullerenes in chloroform by our recently discovered selfassembled, helical nanotubes.[1] The poorly soluble C 60 molecules are solubilized by the nanotubes and they also experience the helicity of the environment, as demonstrated by induced circular dichroism (ICD). Previously reported C 60 receptors-mostly aza-crown ethers, calixarenes, and porphyrins [2] -require lengthy syntheses and only bind isolated fullerenes. Our polymeric, dynamic receptor is prepared in one step and represents an entirely new class of C 60 -complexing structures; it also allows the hitherto unknown formation of a one-dimensional C 60 array.We recently reported the synthesis [3] of a-amino acid functionalized naphthalenediimides (Scheme 1) and their self-assembly into hydrogen-bonded nanotubes.[1] The crystal structure of these helical superstructures shows the presence of tubular cavities with a mean diameter of 12.4 , which is well-suited to accommodate C 60 molecules with a van der Waals (vdW) radius of about 10.3 . [4] Colorimetric studies were carried out to test the C 60 complexation ability of the helical nanotubes. Solutions of l-and d-1-3 as well as 5 in chloroform were left to stand over solid C 60 . A drastic color change from pale yellow to dark orange or brown was seen within a few minutes. This color change is quite different from that observed upon mere addition of a saturated solution of C 60 in chloroform (pale purple) to the initial solution (Figure 1, inset). The visible region of the absorption spectrum (Figure 1, inset) is marked by the appearance of the absorbance bands characteristic of C 60 , as well as an additional, broad band centered around 452 nm (identified with * ). This band, best visualized by subtracting the naphthalenediimide (NDI) spectrum from the spectrum of the complex (Figure 1, blue trace), is very similar to an absorbance normally only seen in C 60 films [6] and aggregates; [7] it has been attributed to interactions between fullerenes.[8] This proximity effect, which has not previously been reported [4] for C 60 constrained within channels, indicates that the fullerenes are tightly packed within our system, although we cannot rule out a contribution from NDI-C 60 interactions.The uptake of C 60 corresponds to the appearance of the bands at 258 and 328 nm in the UV region of the absorption spectrum of 1 + C 60 . Comparison of a solution of 1 + C 60 in chloroform with known NDI concentration and a saturated solution of C 60 in chloroform allows calculation of the C 60 uptake. We found that depending on NDI concentration, and, to a lesser degree, on the amino acid side chain, the C 60 concentration increased up to 16-fold in the presence of NDI nanotubes relative to the solubility of C 60 in chloroform.The amount of C 60 taken up into the nanotubes is in good agreement with C 60 being contained inside the nanotubular core rather than interacting with the outside of the tubes. To quantify the ratio of NDI to C 60 molecules, we recorded the a...

The search for small-molecule ligands of biological targets remains a challenge with major implications for both fundamental studies and drug discovery. [1] We are interested in the discovery of small molecules that specifically interact with regulatory nucleic acid elements. Such molecules have the potential to alter the expression of particular genes and thus influence cellular functions.Certain guanine-rich (G-rich) regions in genomic DNA can form four-stranded structures, called G quadruplexes, which have emerged as biologically important elements.[2] Gquadruplex formation has been linked to cancer-related biology, most notably by remodeling of the telomere structure or by the regulation of oncogenic expression.[3] The two key challenges in the design of small-molecule[4] ligands for quadruplex DNA are: 1) to attain specificity for G-quadruplex-forming sequences over duplex DNA and 2) to achieve specificity for a given G-quadruplex structure and/or G-quadruplex-forming sequence. The latter criterion has become more important in the light of the recently revealed prevalence of G-quadruplex-forming sequences in the human genome,[5a,b] and particularly in promoter regions. [5c] Although G quadruplexes all contain G quartets, there is considerable scope for structural variations within the loop and groove regions, [6] suggesting that specificity in the molecular recognition of a quadruplex is attainable. However, the rational design of quadruplexbinding molecules requires a good understanding of the interactions between the ligand and its host. Owing to the paucity of structural data and the dynamic nature of G quadruplexes, combinatorial searches are appealing.Herein, we report on a study that employs a dynamic combinatorial approach to explore the differential recognition of G-quadruplex targets by closely related small molecules. Dynamic combinatorial chemistry (DCC) is a powerful approach for the rapid identification of binders for small molecules and biological targets. [7] Owing to its adaptive nature, small changes in the composition of a dynamic combinatorial library (DCL) upon introduction of a ** This study was supported by the Cancer Research UK, the EU, and EPSRC. We thank the EPSRC Mass Spectrometry Service for mass analysis.

The self-assembly of two tripodal porphyrin hosts in the presence of C(60), in the solid state, has been studied using synchrotron X-ray crystallography, and in solution by using (1)H NMR and fluorescence spectroscopies. The binding affinities, stoichiometries and geometries strongly depend on the size of the porphyrin host. Intramolecular and/or intermolecular porphyrin-fullerene interactions are observed in the co-crystallites and in each case, the trimer exhibits a "tweezers-like" structural motif. The solid-state structures of the trimer-fullerene co-crystallites reveal close fullerene-porphyrin and fullerene-fullerene contacts.