Rationales Design molekularer Wirte für Anionen durch Feinabstimmung energetischer Einflüsse

Fingerprinting the Nature of Anions in Pyrylium Complexes: Dual Binding Mode for Anion-π Interactions

The interplay between noncovalent interactions that involve oxygenated heteroaromatic rings have been studied for the first time in this work. In particular, we report an advance in knowledge-based anion-π interactions together with (C-H)⁺ ⋅⋅⋅anion contacts. To understand how the anion modulates these interactions, the synthesis of pyrylium salts with a variety of anions was performed by using an anionic metathesis methodology. The synthesized pyrylium complexes were classified in series, for example, anions derived from halogens, from oxoacids, from p-block elements, and from transition metals. Crystallographic data, DFT calculations, and NMR spectroscopy methods provided access to an overall insight into the noncovalent behavior of the anion in this kind of system. Based on the DFT calculations and ¹ H NMR spectroscopy, pyrylium protons can be used as chemical tags to detect noncovalent interactions in this type of compound.

A highly selective fluorescent chemosensor for guanosine-5'-triphosphate via excimer formation in aqueous solution of physiological pH

A new water-soluble and fluorescent imidazolium-anthracene cyclophane 1 effectively recognizes and differentiates the biologically important GTP and ATP in 100% aqueous solution of physiological pH 7.4. Fluorescence, (1)H-NMR spectra and ab initio calculations demonstrate that excimer formation and fluorescence enhancement occur upon GTP and ATP binding, respectively, through (C-H)(+)···A(-) hydrogen bond interactions.

Fluorescent imidazolium-based cyclophane for detection of guanosine-5'-triphosphate and I(-) in aqueous solution of physiological pH

A new water-soluble and fluorescent imidazolium-anthracene cyclophane (1) effectively recognizes the biologically important GTP and I(-) over other anions in a 100% aqueous solution of physiological pH 7.4. Fluorescence and (1)H NMR spectra and ab initio calculations demonstrate that emission arises from the formation of an excimer state and quenching occurs upon GTP/I(-) binding through (C-H)(+)···A(-) hydrogen bond interactions.

Aromatic and aliphatic CH hydrogen bonds fight for chloride while competing alongside ion pairing within triazolophanes

Triazolophanes are used as the venue to compete an aliphatic propylene CH hydrogen-bond donor against an aromatic phenylene one. Longer aliphatic C-H...Cl(-) hydrogen bonds were calculated from the location of the chloride within the propylene-based triazolophane. The gas-phase energetics of chloride binding (ΔG(bind) , ΔH(bind) , ΔS(bind) ) and the configurational entropy (ΔS(config) ) were computed by taking all low-energy conformations into account. Comparison between the phenylene- and propylene-based triazolophanes shows the computed gas-phase free energy of binding decreased from ΔG(bind) =-194 to -182 kJ mol(-1) , respectively, with a modest enthalpy-entropy compensation. These differences were investigated experimentally. An (1) H NMR spectroscopy study on the structure of the propylene triazolophane's 1:1 chloride complex is consistent with a weaker propylene CH hydrogen bond. To quantify the affinity differences between the two triazolophanes in dichloromethane, it was critical to obtain an accurate binding model. Four equilibria were identified. In addition to 1:1 complexation and 2:1 sandwich formation, ion pairing of the tetrabutylammonium chloride salt (TBA(+) ⋅Cl(-) ) and cation pairing of TBA(+) with the 1:1 triazolophane-chloride complex were observed and quantified. Each complex was independently verified by ESI-MS or diffusion NMR spectroscopy. With ion pairing deconvoluted from the chloride-receptor binding, equilibrium constants were determined by using (1) H NMR (500 μM) and UV/Vis (50 μM) spectroscopy titrations. The stabilities of the 1:1 complexes for the phenylene and propylene triazolophanes did not differ within experimental error, ΔG=(-38±2) and (-39±1) kJ mol(-1) , respectively, as verified by an NMR spectroscopy competition experiment. Thus, the aliphatic CH donor only revealed its weaker character when competing with aromatic CH donors within the propylene-based triazolophane.

Carboxylate binding in polar solvents using pyridylguanidinium salts

A series of thiourea and guanidinium derivatives have been prepared and their ability to bind a carboxylate group has been investigated. Guanidinium 33, featuring two additional amides and a pyridine moiety, proved to be the most potent carboxylate binding site and was able to bind acetate in aqueous solvent systems (K(ass) = 480 M(-1) in 30% H(2)O-DMSO). The pyridine moiety is critical to obtaining strong binding, and comparison with the binding properties of analogous compounds in which the pyridine is replaced by a benzene ring provides a striking example of enthalpy-entropy compensation.

Probing Binding-Mode Diversity in Guanidinium-Oxoanion Host-Guest Systems

An attempt to experimentally estimate the role of binding-mode diversity (structural fuzziness) on the molecular recognition seen in the prominent guanidinium-oxoanion host-guest pair is described. The global heat response as measured by isothermal titration calorimetry in acetonitrile, which was obtained from the interaction of five different but structurally closely related guanidinium hosts with three rigid phosphinate guests of decreasing accessibility of their binding sites, is correlated to provide a trend analysis. All host-guest associations of 1:1 stoichiometry in this series are strongly enthalpy-driven. The change in complexation entropy can be related to the tightness of the mutual fit of the host-guest partners, which approaches a minimum limit and is interpreted as the unique lock-and-key binding mode. The ordinary host-guest complexation in this ensemble features substantial positive entropy changes that correlate inversely with the binding interface area. This finding excludes desolvation effects as the major cause of entropy production, and provides evidence for the existence of a broad variety of complex configurations rather than a single binding mode to represent the associated host-guest pair. This result bears on the molecular design of systems that vitally depend on structural fidelity, such as nanoassemblies or homogeneous catalysis.

A molecular oyster: a neutral anion receptor containing two cyclopeptide subunits with a remarkable sulfate affinity in aqueous solution

An artificial anion receptor is presented, in which two cyclohexapeptide subunits containing l-proline and 6-aminopicolinic acid subunits in an alternating sequence are connected via an adipinic acid spacer. This compound was devised to stabilize the 2:1 sandwich-type anion complexes that are observed when the two cyclopeptide moieties are not covalently connected and to obtain a 1:1 stoichiometry for these aggregates. Electrospray ionization mass spectrometry and NMR spectroscopic investigations showed that the bridged bis(cyclopeptide) does indeed form defined 1:1 complexes with halides, sulfate, and nitrate. ROESY NMR spectroscopy and molecular modeling allowed a structural assignment of the sulfate complex in solution. The stabilities of various anion complexes were determined by means of NMR titrations and isothermal titration microcalorimetry in 50% water/methanol. Both methods gave essentially the same quantitative results, namely stability constants that varied in the range 105-102 M-1 and decreased in the order SO42- > I- > Br- > Cl- > NO3-. This order was rationalized in terms of the size of the anions with the larger anions forming the more stable complexes because they better fit into the cavity of the host. The ability of sulfate to form stronger hydrogen bonds to the NH groups of the receptor, in addition to its slightly larger ionic radius with respect to iodide, causes the higher stability of the sulfate complex. No significant effect of the countercation on complex stability was observed. Furthermore, complex stability is enthalpically as well as entropically favored. A comparison of the iodide and sulfate complex stabilities of the ditopic receptor with those of a cyclopeptide that forms 1:1 anion complexes in solution showed that the presence of a second binding site increases complex stability by a factor of 100-350.