Monica Melegari

Large and stable transmembrane pores from guanosine-bile acid conjugates.

United States Department of Energy, Office of Basic Energy Sciences and the Maryland Department of Economic and Business Development for support from the Maryland Nano-Biotechnology Initiative.

Supramolecular Sensing with Phosphonate Cavitands

Phosphonate cavitands are an emerging class of synthetic receptors for supramolecular sensing. The molecular recognition properties of the third-generation tetraphosphonate cavitands toward alcohols and water at the gas-solid interface have been evaluated by means of three complementary techniques and compared to those of the parent mono- and diphosphonate cavitands. The combined use of ESI-MS and X-ray crystallography defined precisely the host-guest association at the interface in terms of type, number, strength, and geometry of interactions. Quartz crystal microbalance (QCM) measurements then validated the predictive value of such information for sensing applications. The importance of energetically equivalent multiple interactions on sensor selectivity and sensitivity has been demonstrated by comparing the molecular recognition properties of tetraphosphonate cavitands with those of their mono- and diphosphonate counterparts.

Supramolecular Control of Single‐Crystal‐to‐Single‐Crystal Transformation through Selective Guest Exchange

Guest-switchable crystals: A solid-state guest exchange of the tetraphosphonate cavitand Tiiii[H,CH(3),Ph] as host promotes single-crystal-to-single-crystal transformations (see graphic). The strong preference for methanol over water is observed in all three phases (gas, liquid, solid), thus demonstrating the fundamental role played by the preorganized cavity through synergistic H-bonds and C-H⋅⋅⋅π interactions.

CO2 Capture by Multivalent Amino-Functionalized Calix[4]arenes: Self-Assembly, Absorption, and QCM Detection Studies

The reactivity of CO(2) with polyamino substrates based on calix[4]arenes and on a difunctional, noncyclic model has been studied. All the compounds react with CO(2) in chloroform to form ammonium carbamate salts. However, the number, topology, and conformational features of the amino-functionalized arms present on the multivalent scaffold have a remarkable influence on the reaction efficiency and on the product composition. Tetraaminocalix[4]arenes 1-3 rapidly and efficiently react with 2 equiv of CO(2), yielding highly stable hydrogen-bonded dimers formed by the self-assembly of two bis-ammonium bis-carbamate intramolecular salts. 1,3-Diaminocalix[4]arene 4 absorbs 1 mol of CO(2), affording less stable zwitterionic ammonium carbamates. Gemini compound 5 reacts with CO(2) in a 1:1 stoichiometry, forming hydrogen-bonded dimers of ammonium carbamate derivatives of moderate stability. For upper rim 1,3-diaminocalix[4]arene 6, in addition to the labile intramolecular salt, the presence of a self-assembled polymer was also detected. These systems were fully characterized in solution by (1)H and (13)C NMR spectroscopy, whereas the corresponding gas-solid reactions were further investigated by QCM measurements. Interestingly, the high affinity and reversibility of CO(2) uptake shown by 1,3-diamino calix[4]arene 4 enabled us to attain a promising QCM device for carbon dioxide sensing.

Tetraphosphonate cavitands: interplay between metal coordination and H-bonding in the formation of dimeric capsules

Three novel dimeric capsules formed by a tetraphosphonate cavitand with barium, calcium and zinc salts have been synthesized and structurally characterized. The role of cation dimensions and of coordinated water molecules in determining different architectures is also described.

Selectivity assessment in host–guest complexes from single-crystal X-ray diffraction data: the cavitand–alcohol case

The determination of selectivity is central to the development of molecular receptors. Competition binding experiments based on the relative binding ratios are commonly used to evaluate relative binding constants. When more than one species binds to the same active site of a receptor, the crystallographic evaluation of the occupancy factors of each ligand could be very informative about their relative affinity. However, in the presence of overlapped electron densities, the statistical occupancy factors are hard to retrieve using conventional crystal structure refinement. Here, we present an original method to evaluate the relative binding constants based on the direct treatment of the diffraction intensities obtained from isomorphous single crystals grown in the presence of binary mixtures of competitive ligands. This method was developed and first applied to evaluate the affinity of a tetraphosphonate cavitand receptor towards short-chain alcohols. In the easier cases, wherein the electron densities of the two alcohols are less overlapped, the occupancy factors for guest molecules obtained in conventional structural refinement are in good agreement with the values calculated from the direct comparison of diffraction intensities. The affinity constants were estimated from the calculated occupancy factors, considering the molar ratios of alcohols used in the competition experiments. The congruence of the method has been tested on several binary mixtures using different concentration ratios of alcoholic guests. In general, for a given alcoholic pair, the different molar ratios of alcohols, used in the crystallization batch, produce a trend of the occupancy factors in agreement with the binding constant ratios of the two alcohols. From 32 data sets collected at the Elettra synchrotron for six short alkylic chain guest molecules, we evaluated the binding constant ratios with a good internal consistency. The relative binding constants for these six alcohols were evaluated from the entire statistical sample (EtOH, 8.8 > 1-PrOH, 2.2 > MeOH, 1.3 > 2-PrOH, 1.00 > 2-BuOH, 0.32 > 1-BuOH, 0.11, using 2-PrOH as reference with an arbitrary value of 1) and are in good agreement with the structural parameters of host–guest interactions observed in the corresponding crystal structures. In particular, the binding constant decreases with the increase in the host–guest H-bond distance, which follows the increase in the length of the alkyl chain of the alcoholic guest. Moreover, quartz crystal microbalance (QCM) measurements and fluorescence data have been compared and discussed with respect to the relative affinity scale obtained by crystallography.

Introduction of Water-Solubilizing Groups at the Lower Rim of Tolylpyridine-Bridged Cavitands

In this paper the preparation of water-soluble methylene-bridged cavitands presenting either positively or negatively charged groups at the lower rim following three different synthetic routes is reported. Moreover, four anionic sulphate functions have been successfully inserted on a tolylpyridine-bridged cavitand in order to carry out the self-assembly of coordination cages in water.

Novel Vacuum Evaporated Cavitand Sensors for Detecting Very Low Alcohol Concentrations

High vacuum evaporation (VE) is used for the first time to grow thin films of novel tetraphosphonate, Tiiii[H, CH3, Ph], and tetrathiophosphonate, TSiiii[H, CH3, Ph], cavitands for gas sensing applications. The sensing capabilities of the samples were investigated by exposing Tiiii- and TSiiii-coated QCMs to very low concentrations of ethyl alcohol. The sensitivity, the speed (t50 = 5 s for both the samples) and the detection limit (0.4 ppm for Tiiii and 2.5 ppm for TSiiii) of the samples were determined, indicating highly competitive sensing capabilities.