Molecular-recognition properties of a water-soluble cucurbit[6]uril analogue

The molecular-recognition properties of the cucurbit[6]uril analogue (1) in aqueous buffer (sodium acetate, 50 mM, pH 4.74, 25 degrees C) toward a variety of guests including alkanediamines (6-12), aromatics (14-32), amino acids (33-36), and nucleobases (37-42) were studied by fluorescence spectroscopy. For the alkanediamines studied (H2N(CH)nNH2, n = 6, 7, 8, 9, 10, 11, 12), the association constants increase as the length of the alkane (n) is increased. Host 1 is capable of forming strong complexes with guests containing aromatic rings with association constants (Ka) ranging from 10(2) to 10(6) M(-1) as a result of the favorable pi-pi interactions that occur between host 1 and the aromatic rings of the guest when bound in the cavity of 1. Biologically relevant guests such as amino acids and nucleobases are also bound in the cavity of 1 with Ka values ranging from 10(3) to 10(6) M(-1). Consequently, cucurbit[6]uril analogue 1 functions as a versatile fluorescent sensor for the presence of a wide range of chemically and biologically important substances in aqueous solution including nitroaromatics, neurotransmitters, amino acids, and nucleobases.     

High fidelity kinetic self-sorting in multi-component systems based on guests with multiple binding epitopes

The molecular recognition platforms of natural systems often possess multiple binding epitopes, each of which has programmed functional consequences. We report the dynamic behavior of a system comprising CB[6], CB[7], and guests cyclohexanediammonium (1) and adamantanealkylammonium (2) that we refer to as a two-faced guest because it contains two distinct binding epitopes. We find that the presence of the two-faced guest--just as is observed for protein targeting in vivo--dictates the kinetic pathway that the system follows toward equilibrium. The influence of two-faced guest structure, cation concentration, cation identity, and individual rate and equilibrium constants on the behavior of the system was explored by a combination of experiment and simulation. Deconstruction of this system led to the discovery of an anomalous host-guest complex (CB[6].1) whose dissociation rate constant (k(out) = 8.5 x 10(-10) s(-1)) is approximately 100-fold slower than the widely used avidin.biotin affinity pair. This result, in combination with the analysis of previous systems which uncovered extraordinarily tight binding events (K(a) > or = 10(12) M(-1)), highlights the inherent potential of pursuing a systems approach toward supramolecular chemistry.     

Exploring Polygon Rings

Some topics in mathematics are unique because they can be explored by learners from the early grades through the advanced grades. One such topic is polygon rings. As suggested in the Curriculum and Evaluation Standards for School Mathematics ( National Council of Teachers of Mathematics. 1989 ), students can learn mathematics by actively engaging in the activities outlined in this article. The activities integrate problem solving, reasoning, and communication, and they offer a fascinating look at the beauty of the structure of mathematics.     

Obstacles with non-trivial trapping sets in higher dimensions

The new results concerning the continuity of holomorphically contractible systems treated as set functions with respect to non-monotonic sequences of sets are given. In particular, continuity properties of Kobayashi and Carathéodory pseudodistances, as well as Lempert and Green functions with respect to sequences of domains converging in the Hausdorff metric are delivered.     

Minimum L ∞ accelerations in Riemannian manifolds

Riemannian cubics are curves in Riemannian manifolds M that are critical points for the L ² norm of covariant acceleration, and are already rather well studied as elementary curves for interpolation problems in engineering. In the present paper the L ² norm is replaced by the L ^∞ norm, which may be more appropriate for some applications. However it is more difficult to derive the analogue of the Euler-Lagrange equation for the L ^∞ norm, requiring techniques from optimal control, and the resulting necessary conditions take a different form. These necessary conditions are examined when M is a sphere or a bi-invariant Lie group, and some examples are given.     

Social self-sorting in aqueous solution

Self-sorting-the ability to efficiently distinguish between self and nonself-is common in nature but is still relatively rare in synthetic supramolecular systems. We report a 12-component mixture comprising 1-11 and KCl that undergoes thermodynamically controlled self-sorting in aqueous solution based on metal-ligand, ion-dipole, electrostatic, charge-transfer interactions, as well as the hydrophobic effect. We refer to this molecular ensemble-characterized by high-fidelity host-guest interactions between components-as a social self-sorting system to distinguish it from narcissistic self-sorting systems based on self-association processes. The influence of several key variables-temperature, pH, concentration, and host/guest stoichiometry-was explored by a combination of simulation and experiment. Variable temperature NMR experiments, for example, revealed a kinetically controlled irreversible process upon cycling from 298 to 338 K, which is an emergent property of this molecular ensemble. Variable pH and concentration experiments, in contrast, did not reveal any emergent properties of the molecular ensemble. Simulations of a four-component mixture establish that by proper control of the relative magnitude of the various equilibrium constants, it is possible to prepare socially self-sorted mixtures that are responsive (irresponsive) to host/guest stoichiometry over narrow (broad) ranges. The 12-component mixture is relatively irresponsive to host/guest stoichiometry. Such social self-sorting systems, like their natural counterparts, have potential applications as chemical sensors, as artificial regulatory elements, and in the preparation of biomimetic systems.     

Synthesis of two novel ring systems via photocyclization: Thieno[2′,3′:4,5]thieno[2,3-c][1,10]phenanthroline and thieno[3′,2′:4,5]thieno[2,3-c][1,10]phenanthroline

The synthesis of thieno[2′,3′:4,5]thieno[2,3-c][1,10]phenanthroline ( 5 ) and thieno[3′,2′:4,5]thieno-[2,3-c][1,10]phenanthroline ( 10 ) are described. Each compound was obtained in four steps from known starting materials. The basic skeleton of the molecule and of the phenanthroline ring were formed via photocyclization. The total assignment of ¹H-nmr spectra was accomplished with the aid of two-dimensional nmr methods.     

Cucurbit[n]uril analogues: synthetic and mechanistic studies

[reaction: see text] The synthesis of cucurbit[n]uril analogues (18, 19, (+/-)-20, 33, 34, 35, 36, and 37) is presented. These CB[5], CB[6], and CB[7] analogues all contain bis(phthalhydrazide) walls that are incorporated into the macrocycle. The tailor-made synthesis of these CB[n] analogues proceeds by the condensation of the appropriate bis(electrophile) (4, 7, or 9) with bis(phthalhydrazide) (17), which delivers the CB[6] and CB[7] analogues in good yield, whereas the CB[5] analogue is formed in low yield. To improve the solubility characteristics of the CB[n] analogues for recognition studies in water or organic solution, the CO2Et groups were transformed to CO2H and CO2(CH2)9CH3 groups. On the basis of the results of product resubmission experiments, we conclude that these macrocycles are kinetic products. To help rationalize the good yields obtained in the CB[6] and CB[7] analogue macrocyclization reactions, we performed mechanistic studies of model methylene bridged glycoluril dimers, which suggest an intramolecular isomerization during CB[n] analogue formation.     

Muon spin rotation in GdSr2Cu2RuO8: Implications

Muon spin rotation measurements are reported for GdSr₂Cu₂RuO₈, a material with an onset temperature for superconductivity of about 45 K (which is virtually the same for its superconducting sister compounds Sr₂YRu_{1? u} Cu _u O₆ and Gd_{2? z} Ce _z Sr₂Cu₂RuO₁₀). The data indicate two magnetic ordering transitions, at about 15 K and about 130 K, in addition to the Gd ordering transition known to occur at about 2.6 K. We tentatively attribute the 130 K transition to Ru and the 15 K transition to Cu ordering, effectively ruling out any superconducting mechanism based on fluctuating magnetic moments, which are frozen below about 15 K. If there is only one mechanism of high-temperature superconductivity, then the three facts that (i) all three sister compounds have essentially the same onset T _c for superconductivity and (ii) all three of these compounds contain SrO layers but (iii) only two of the three compounds, GdSr₂Cu₂RuO₈ and Gd_{2? z} Ce _z Sr₂Cu₂RuO₁₀ (and not Sr₂YRu_{1? u} Cu _u O₆) contain cuprate planes imply that the superconducting layers of all three compounds must be the common SrO layers, and not the cuprate planes (which do not occur in Cu-doped Sr₂YRuO₆). Otherwise the coincidence of onset temperatures must be an accident, and there must be at least two mechanisms of high-T _c superconductivity: one for doped Sr₂YRuO₆ and another for cuprate-plane superconductivity.