Benzobis(imidazolium)-cucurbit[8]uril complexes for binding and sensing aromatic compounds in aqueous solution

The utilities of benzobis(imidazolium) salts (BBIs) as stable and fluorescent components of supramolecular assemblies involving the macrocyclic host, cucurbit[8]uril (CB[8]), are described. CB[8] has the unusual ability to bind tightly and selectively to two different guests in aqueous media, typically methyl viologen (MV) as the first guest, followed by an indole, naphthalene, or catechol-containing second guest. Based on similar size, shape, and charge, tetramethyl benzobis(imidazolium) (MBBI) was identified as a potential alternative to MV that would increase the repertoire of guests for cucurbit[8]uril. Isothermal titration calorimetry (ITC) studies showed that MBBI binds to CB[8] in a 1:1 ratio with an equilibrium association constant (K(a)) value of 5.7×10(5) M(-1), and that the resulting MBBI·CB[8] complex binds to a series of aromatic second guests with K(a) values ranging from 10(3) to 10(5) M(-1). These complexation phenomena were supported by mass spectrometry, which confirmed complex formation, and a series of NMR studies that showed the expected upfield perturbation of aromatic peaks and of the MBBI methyl peaks. Surprisingly, the binding behavior of MBBI is strikingly similar to that of MV, and yet MBBI offers a number of substantial advantages for many applications, including intrinsic fluorescence, high chemical stability, and broad synthetic tunability. Indeed, the intense fluorescence emission of the MBBI·CB[8] complex was quenched upon binding to the second guests, thus demonstrating the utility of MBBI as a component for optical sensing. Building on these favorable properties, the MBBI·CB[8] system was successfully applied to the sequence-selective recognition of peptides as well as the controlled disassembly of polymer aggregates in water. These results broaden the available guests for the cucurbit[n]uril family and demonstrate potentially new applications.     

Operators That Achieve the Norm

In this paper we study the theory of operators on complex Hilbert spaces, which achieve the norm in the unit sphere. We prove important results concerning the characterization of the AN{\mathcal{AN} } operators, see Definition 1.2. The class of the AN{\mathcal{AN}} operators contains the algebra of the compact ones.     

Effect of a neutral water-soluble polymer on the lamellar phase of a zwitterionic surfactant system

We have studied the effect of adding a water-soluble polymers (PEG) to the lamellar phases of the ternary system tetradecyldimethylaminoxide (C14DMAO)-hexanol-water. The results of Freeze-Fracture Electron Microscopy (FFEM) and Small Angle X-ray Scattering (SAXS) experiments show that the addition of the polymer induces the spontaneous formation of highly monodisperse multilayered vesicles above a threshold polymer concentration.     

Barbilian spaces: The history of a geometric idea

Barbilian spaces are metric spaces with a metric induced by a special procedure of metrization that is inspired by the study of the models of non-Euclidean geometry. In this note we discuss the history of Barbilian spaces and the evolution of the theory. We point out that some of the current references to work done in Barbilian spaces refer to Barbilian's contribution from 1934, while his construction has been greatly extended in four works published in Romanian in 1959–1962.     

Investigation of the stability of a natural circulation two-pass boiler

In this paper, the results of a theoretical analysis of the static flow instability, namely the reverse flow, for a two-pass steam generator (see Fig. 1) will be presented. The aim of the work was the development of design criteria to avoid the reversal of flow. To determine the critical heat absorption ratio between the two riser systems of a two-pass boiler several dynamic simulations of a warm start-up of the two-pass boiler with different heat absorption ratios, different heat profiles along the lower heated riser system 1 and modified geometry of the boiler (influence of the flow resistance) are done. On the basis of the results of this investigations it can be shown that for every natural circulation system with unequally heated risers and a common downcomer a special critical heat absorption ratio exists. The investigations clearly indicate that such types of steam generators should be designed only in that way that the operation below the critical heat absorption ratio is possible. The modification of the flow resistance in the riser systems can help to extend the range of stable operation.     

Nonlinear interferometric vibrational imaging for fast label-free visualization of molecular domains in skin

The most prevalent molecular constituents of skin are spatially mapped by the use of nonlinear interferometric vibrational imaging, a coherent anti-Stokes Raman scattering (CARS)-based technique. Raman-like profiles over the range from 2,800 to 3,000 cm⁻¹ are acquired by means of completely suppressing the non-resonant background, allowing the generation of images based on the molecule-specific spectral profiles over the probed region with high spatial resolution. A simple algorithm that maps spectral content to color allows the visualization of histology in a manner analogous to that obtained with more conventional staining procedures (e.g., hematoxylin-eosin), but faster and with the benefit of having access to localized spectra, which could further enhance the potential for diagnosis of diseases, especially during the early stages of development.     

Determining protease substrate selectivity and inhibition by label-free supramolecular tandem enzyme assays

An analytical method has been developed for the continuous monitoring of protease activity on unlabeled peptides in real time by fluorescence spectroscopy. The assay is enabled by a reporter pair comprising the macrocycle cucurbit[7]uril (CB7) and the fluorescent dye acridine orange (AO). CB7 functions by selectively recognizing N-terminal phenylalanine residues as they are produced during the enzymatic cleavage of enkephalin-type peptides by the metalloendopeptidase thermolysin. The substrate peptides (e.g., Thr-Gly-Ala-Phe-Met-NH(2)) bind to CB7 with moderately high affinity (K ≈ 10(4) M(-1)), while their cleavage products (e.g., Phe-Met-NH(2)) bind very tightly (K > 10(6) M(-1)). AO signals the reaction upon its selective displacement from the macrocycle by the high affinity product of proteolysis. The resulting supramolecular tandem enzyme assay effectively measures the kinetics of thermolysin, including the accurate determination of sequence specificity (Ser and Gly instead of Ala), stereospecificity (d-Ala instead of l-Ala), endo- versus exopeptidase activity (indicated by differences in absolute fluorescence response), and sensitivity to terminal charges (-CONH(2) vs -COOH). The capability of the tandem assay to measure protease inhibition constants was demonstrated on phosphoramidon as a known inhibitor to afford an inhibition constant of (17.8 ± 0.4) nM. This robust and label-free approach to the study of protease activity and inhibition should be transferable to other endo- and exopeptidases that afford products with N-terminal aromatic amino acids.     

Revisiting the foundations of Barbilian?s metrization procedure

In the present work we prove that one of Barbilian?s theorems from 1960 regarding the metrization procedure in the plane admits a natural extension depending on a bilinear form and the relative position of two Apollonian hyperspheres. This result allows us to pursue two fundamental ideas. First, that all the distances with constant curvature can be described by Barbilian?s metrization principle. Secondly, that all the Riemannian metric corresponding to these distances can be obtained with the same unique procedure derived from the main theorem in the text (Theorem 2.5). We show how the hyperbolic metric of the disk, the hyperbolic metric on the exterior of the disk and the hyperbolic metric on the half-plane can be obtained in the same way using Theorem 2.5, which appears here for the first time and is an extension of a Barbilian classical result (Barbilian, 1960 [7]). Furthermore, we obtain metrics corresponding to quadratic forms with signature that includes minus. By considering the norms provided by either Lorentz or Minkowski (pseudo-)inner product as influence functions, two oscillant distances can be generated in some subsets of Lorentz or Minkowski plane. The extension of 1960 Barbilian?s theorem mentioned above allow us to obtain the metrics attached to these two Barbilian distances on corresponding subsets of Lorentz and Minkowski 2-dimensional spaces. The geometric study concludes that these metrics are generalized Lagrange metrics. A result concerning the distance induced by a Riemannian metric as a local Barbilian distance is also proved.     

KF/Al2O3 and PEG-400 as a recyclable medium for the selective α-selenation of aldehydes and ketones. Preparation of potential antimicrobial agents

2-Phenylseleno aldehydes and ketones were selectively obtained using solid-supported catalyst (KF/Al₂O₃) and PEG-400 as clean, recyclable medium in good to excellent yields. The method was applied in the preparation of highly functionalized 2-phenylseleno citronellal and citronellol, potential bactericide agents. The catalytic system KF/Al₂O₃ and PEG-400 can be re-used for four times without previous treatment.