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Julius Rebek

Publications and source records attributed to Julius Rebek.

At least 55 records · Page 3Linked to original sources
PubMed2004

Helical folding of alkanes in a self-assembled, cylindrical capsule.

The reversible encapsulation of a series of normal alkane guests in a cylindrical host was studied by NMR methods. For small hydrocarbons such as n-pentane or n-hexane, two guests enter the host, and they move freely within. With n-heptane no encapsulation takes place. For longer alkanes such as n-decane, a single guest enters and the aromatic walls of the host are seen to twist to avoid empty spaces and increase favorable interactions with the hydrocarbon. The best guest (n-undecane) adopts a conformation with minimal gauche interactions. The longest alkane accommodated, n-tetradecane, adopts a helical conformation to fit in the cavity, a shape that maximizes CH/pi interactions with the aromatic walls of the receptor. These reciprocal conformational changes are discussed in terms of optimal host/guest interactions.

Alkanes↗
PubMed2004

The ins and outs of molecular encapsulation.

Molecular capsules can act as hosts for appropriate guests, and bring them into well-defined nanoenvironments. Various spectroscopic methods have been used to deduce the mechanism of guest exchange in such systems. Generally, the guests get in and out of capsules through the opening and closing of host "flaps" but smaller capsules can exchange simply by dissociation of the host subunits.

Calixarenes↗
PubMed2004

Preferred dimerization of tetra-tolyl- and tetra-tosylurea derivatives of flexible and rigidified calix[4]arenes.

The dimerization of tetratolyl- and tetratosyl-urea derivatives 1 and 2, derived from a tetrapentoxy calix[4]arene in the cone conformation and of the corresponding tetra-urea derivatives 3 and 4, in which the cone conformation is rigidified by the two crown-3 tethers, have been studied. All six possible equimolar mixtures were examined by 1H NMR using CDCl3 and CD2Cl2 as solvents. While no heterodimers are found for the combinations 1/3 and 2/4 in either solvent, all remaining combinations lead to the (exclusive) formation of heterodimers in CD2Cl2. In CDCl3 heterodimers are only observed for the combinations of 3 with 2 or 4. These results are discussed in terms of entropic and enthalpic contributions and compared with MD-simulations in a box of chloroform solvent molecules.

Calixarenes↗
PubMed2004

Equilibrium isotope effects as a probe of nonbonding attractions.

The equilibrium isotope effects (EIEs) in the capsule complex (12 + p-xylene + CCl4) were computed with two simple models that mimic two types of contacts, -CH3.Ar and -CH3.CCl4. By using the MP2/6-311++G(d,p) method, the calculated equilibrium constants are consistent with the reported experimental data. Harmonic frequency analyses indicate that vibrational frequency changes caused by nonbonding attractions lead to the observed EIE.

Benzene↗
PubMed2004

Enhanced thermodynamic and kinetic stability of calix[4]arene dimers locked in the cone conformation.

Wide rim tetraurea derivatives (2a,b) have been prepared from a calix[4]arene rigidified in the cone conformation by two diethyleneglycol ether bridges between adjacent oxygens. In comparison to the analogous tetraurea derivatives (3a,b) of a tetrapentoxy calix[4]arene, 2a,b show an increased thermodynamic stability in mixtures of CDCl(3) and DMSO-d(6). Their kinetic stability as expressed by the rate of guest exchange (benzene or cyclohexane against the solvent benzene-d(6)) is also strongly increased by factors of 30-38. Noticeable differences for the inclusion of selected guests are found.

Calixarenes↗
PubMed2004

Single molecule solvation and its effects on tautomeric equilibria in a self-assembled capsule.

A self-assembled cylindrical capsule provides a nanoscale environment that affects keto-enol equilibria. The equilibrium constants for encapsulated beta-ketoesters show values that differ by an order of magnitude from that of the free tautomers in solution. For complexes with a single, large encapsulated guest, the inner surfaces of the capsule and the seam of the hydrogen bonds influence the equilibrium between the encapsulated keto and enol forms. For complexes of smaller beta-ketoesters, the coencapsulated solvent influences the equilibria. The solvent reduces the space available and affects the positioning of the ester in the capsule.

Alcohols↗
PubMed2004

Galmic, a nonpeptide galanin receptor agonist, affects behaviors in seizure, pain, and forced-swim tests.

The pharmacological exploitation of the galanin receptors as drug targets for treatment of epilepsy, depression, and pain has been hampered by the lack of workable compounds for medicinal chemists from random screening of large chemical libraries. The present work uses the tripeptidomimetic galnon and displays its presumed pharmacophores on a rigid molecular scaffold. The scaffold is related to marine natural products and presents three functional groups near one another in space, in a manner reminiscent of a protein surface. An active compound, Galmic, was identified from a small synthetic library and tested in vitro and in vivo for its affinity and efficacy at galanin receptors. Galmic has micromolar affinity for GalR1 receptors (Ki = 34.2 microM) and virtually no affinity for GalR2 receptors. In vitro, Galmic, like galanin, suppresses long-term potentiation in the dentate gyrus; it blocks status epilepticus when injected intrahippocampally or administered i.p. Galmic applied i.p. shows antidepressant-like effects in the forced-swim test, and it is a potent inhibitor of flinching behavior in the inflammatory pain model induced by formalin injection. These data further implicate brain and spinal cord galanin receptors as drug targets and provide an example of a systemically active compound based on a scaffold that mimics protein surfaces.

Animals↗
PubMed2004

A functionalized, deep cavitand catalyzes the aminolysis of a choline derivative.

The aminolysis of choline p-nitrophenyl carbonate is catalyzed with turnover by a deep cavitand bearing an introverted pyridone function. The synergy of action between the recognition of the guest in the binding pocket and the catalytic activity brought to bear by the pyridone is responsible for the high substrate specificity observed.

Amines↗
PubMed2004

Some got away, but others didn't.

A history of research projects undertaken in the group-ranging from mechanistic investigations to biomimetic chemistry to molecular devices and synthesis-is presented.

Biological Factors↗
PubMed2004

Kinetics and thermodynamics of hexameric capsule formation.

Resorcinarene 1b forms a hexameric assembly in water-saturated CDCl(3) that encapsulates one tetraalkylammonium salt (2(+)Br(-)). The remaining space is occupied by coencapsulated solvent molecules. A maximum of three and minimum of one CHCl(3) molecule were found inside of capsules with tetrapropyl- and tetraheptylammonium bromide, respectively. The encapsulation of the salt is endothermic and entropically favored by the liberation of the solvent molecules. The stabilities of the encapsulation complexes and the rates of guest exchange decrease for larger cations. The higher activation barriers for in/out exchange of the larger guests suggest tight conformational restraints in the transition state. It is likely that complete dissociation of one resorcinarene molecule is necessary for the exchange of the alkylammonium salts.

Calixarenes↗
PubMed2004

Kinetically stable complexes in water: the role of hydration and hydrophobicity.

We describe here the synthesis and characterization of a molecular receptor that forms kinetically and thermodynamically stable host-guest complexes in water. This cavitand-based host is composed of a preorganized aromatic pocket whose rim is decorated with four negatively charged carboxylate groups. (1)H NMR and isothermal titration calorimetry have been used to characterize the behavior of the resulting complexes in response to changes in pH, buffer identity, and salt concentration and in the presence of sodium dodecyl sulfate micelles.

Benzimidazoles↗
PubMed2004

Stereochemistry in self-assembled encapsulation complexes: constellational isomerism.

A previously uncharacterized form of stereochemistry, constellational isomerism, is described. The isomerism arises from different arrangements of small-molecule guests in the space of a self-assembled, cylindrical host. The cylindrical host detains three molecules each of CHCl(3), 1,2-dichloroethane, or isopropyl chloride. The exchange of guests in and out of the host is slow on the NMR time scale. The dimensions of the capsular host and the sizes of the guests hinder the mobility of molecules inside, and separate NMR signals are seen for guests at the ends of the capsule and those near its center. When two different guests are encapsulated, the spectra show up to four additional species: two sets of constellational isomers. In every pairwise combination of the three guests, all isomers could be identified. The equilibrium distributions of isomers depended on the concentrations of the guests in the bulk solution. The relative stability of the constellational isomers was a function of the polarity of the guest molecule and its ability to interact with the components of the capsule. The different arrangements represent information, and some possibilities for their use in data storage are proposed.

Chloroform↗